text:
Wendy Mao
Professor of Earth and Planetary Sciences, of Photon Science and, by courtesy, of Geophysics
Professor, Geological Sciences
Professor, Photon Science Directorate
Professor (By courtesy), Geophysics
Principal Investigator, Stanford Institute for Materials and Energy Sciences
Member, Stanford PULSE Institute
Administrative Appointments
Professor, Stanford University (2019 - Present)
Associate Professor, Stanford University (2014 - 2019)
Assistant Professor of Geophysics (by courtesy), Stanford University (2009 - Present)
Assistant Professor, Stanford University (2007 - 2014)
J. R. Oppenheimer Post-doctoral Fellow, Los Alamos National Laboratory (2005 - 2007)
Fellow, American Geophysical Union (2021)
Fellow, Geochemical Society (2021)
Award Recipient, Mineralogical Society of America (2013)
NSF CAREER Award, National Science Foundation (2011)
Fellow, Frederick E. Terman Fellowship (2009 - Present)
COMPRES Distinguished Lecturer, Stanford University (2008-2009)
Mineral and Rocks Physics Group Student Research Award, University of Chicago (2006)
Rosalind Franklin Young Investigator Award, University of Chicago (2006)
Fellow, J. R. Oppenheimer Fellowship (2005 - 2007)
Phi Beta Kappa, Massachusetts Institute of Technology (1998)
Boards, Advisory Committees, Professional Organizations
Advisory Group on Women at SLAC, SLAC National Accelerator Laboratory (2015 - Present)
CSEDI Steering Committee, NSF (2015 - Present)
Chair, GES Graduate Admissions Committee, Stanford University (2015 - Present)
DIF Advisory Committee, Stanford University (2015 - Present)
GES Communications Committee, Stanford University (2014 - Present)
Co-chair of Extreme Physics and Chemistry Directorate, Deep Carbon Observatory (2013 - Present)
GES Graduate Admissions Committee, Stanford University (2012 - Present)
Photon Science Integration Committee, SLAC National Accelerator Laboratory (2012 - 2013)
Scientific Steering Committee for the Extreme Physics and Chemistry Directorate, Deep Carbon Observatory (2011 - Present)
LCLS Users' Executive Committee, SLAC National Accelerator Laboratory (2011 - 2014)
Associate Editor, American Mineralogist (2010 - Present)
GES representative on SES Educational Outreach Committee, Stanford University (2010 - Present)
Pre-Majors Advisor, Stanford University (2010 - Present)
SNAP Instrument Design Team - Spallation Neutron Source, ORNL, Oak Ridge National Laboratory (2010 - Present)
Chair, Award Committee, Rosalind Frankin Young Investigator Asard (2010 - 2010)
COMPRES Facilities Committee, Consortium for Materials Properties Research in Earth Sciences (2009 - Present)
Member of APS Users Organization Steering Committee, Advanced Photon Source, Argonne National Laboratory (2009 - Present)
GES Dept Seminar Coordinator (w/ Maher), Stanford University (2009 - 2011)
Search Committee for Geochronology, Petrology, Geodynamics position, Stanford University (2008 - 2009)
GES Long Range Planning Committee, Stanford University (2007 - 2008)
West Coast High Pressure Facilities Review Committee, Advanced Light Source, Lawrence Berkeley National Laboratory (2006 - Present)
Ph.D., University of Chicago, Geophysical Sciences (2005)
B.S., Massachusetts Institute of Technology, Materials Science and Engineering (1998)
wmao@stanford.edu
University - Faculty Department:&nbspDepartment of Geological Sciences Position: Professor
Green Earth Sciences Building
Stanford, California 94305
Curriculum Vitae PDF
Research Group Site
Current Research and Scholarly Interests
Pressure induces dramatic changes in materials. I study the behavior of materials under compression which often leads to the discovery of novel phases and new phenomena. This research has a wide variety of applications including improving our understanding the interiors of Earth and other planetary bodies, providing insight into the condensation and evolution of volatiles in planetary systems, and providing guidance for developing new materials for energy related applications like hydrogen fuel storage and advanced batteries.
I teach classes on understanding the Earth's interior, mineralogy, and a freshman seminar on diamonds.
2022-23 Courses
Chemistry of the Earth and Planets
EARTHSYS 2, GEOLSCI 2 (Aut)
Independent Studies (8)
PHYSICS 291 (Sum)
Directed Reading with Geological Sciences Faculty
GEOLSCI 292 (Win)
GEOLSCI 400 (Aut, Win, Spr, Sum)
GEOPHYS 198 (Aut)
Practical Experience in the Geosciences
GEOLSCI 385 (Sum)
PHYSICS 490 (Aut, Win, Spr, Sum)
Teaching in Geological Sciences
GEOLSCI 398 (Aut, Win, Spr)
Undergraduate Research in Geophysics
GEOPHYS 196 (Win, Spr)
Prior Year Courses
GEOLSCI 2 (Spr)
GEOLSCI 107, GEOPHYS 184 (Win)
GEOLSCI 2 (Aut)
GEOLSCI 40N (Spr)
Stanford Advisees
Postdoctoral Faculty Sponsor
Anna Celeste, Yanyao Zhang
Doctoral Dissertation Co-Advisor (AC)
Dara Hok
Doctoral (Program)
Minkyung Han, Hong Yang
Making the most of metastability. Science (New York, N.Y.) Mao, W. L., Lin, Y. 2022; 377 (6608): 814-815
Researchers seek to preserve materials that are formed at high pressure.
View details for DOI 10.1126/science.add5433
View details for PubMedID 35981027
Preservation of high-pressure volatiles in nanostructured diamond capsules. Nature Zeng, Z., Wen, J., Lou, H., Zhang, X., Yang, L., Tan, L., Cheng, B., Zuo, X., Yang, W., Mao, W. L., Mao, H., Zeng, Q. 2022; 608 (7923): 513-517
High pressure induces dramatic changes and novel phenomena in condensed volatiles1,2 that are usually not preserved after recovery from pressure vessels. Here we report a process that pressurizes volatiles into nanopores of type1 glassy carbon precursors, converts glassy carbon into nanocrystalline diamond by heating and synthesizes free-standing nanostructured diamond capsules (NDCs) capable of permanently preserving volatiles at high pressures, even after release back to ambient conditions for various vacuum-based diagnostic probes including electron microscopy. As a demonstration, we perform a comprehensive study of a high-pressure argon sample preserved in NDCs. Synchrotron X-ray diffraction and high-resolution transmission electron microscopy show nanometre-sized argon crystals at around 22.0gigapascals embedded in nanocrystalline diamond, energy-dispersive X‑ray spectroscopy provides quantitative compositional analysis and electron energy-loss spectroscopy details the chemical bonding nature of high-pressure argon. The preserved pressure of the argon sample inside NDCs can be tuned by controlling NDC synthesis pressure. To test the general applicability of the NDC process, we show that high-pressure neon can also be trapped in NDCs and that type2 glassy carbon can be used as the precursor container material. Further experiments on other volatiles and carbon allotropes open the possibility of bringing high-pressure explorations on a par with mainstream condensed-matter investigations and applications.
View details for DOI 10.1038/s41586-022-04955-z
Ultrafast structural response of shock-compressed plagioclase METEORITICS & PLANETARY SCIENCE Gleason, A. E., Park, S., Rittman, D. R., Ravasio, A., Langenhorst, F., Bolis, R. M., Granados, E., Hok, S., Kroll, T., Sikorski, M., Weng, T., Lee, H., Nagler, B., Sisson, T., Xing, Z., Zhu, D., Giuli, G., Mao, W. L., Glenzer, S. H., Sokaras, D., Alonso-Mori, R. 2022
View details for DOI 10.1111/maps.13785
View details for Web of Science ID 000755921800001
Engineering Bright and Mechanosensitive Alkaline-Earth Rare-Earth Upconverting Nanoparticles. The journal of physical chemistry letters McLellan, C. A., Siefe, C., Casar, J. R., Peng, C. S., Fischer, S., Lay, A., Parakh, A., Ke, F., Gu, X. W., Mao, W., Chu, S., Goodman, M. B., Dionne, J. A. 2022: 1547-1553
Upconverting nanoparticles (UCNPs) are an emerging platform for mechanical force sensing at the nanometer scale. An outstanding challenge in realizing nanometer-scale mechano-sensitive UCNPs is maintaining a high mechanical force responsivity in conjunction with bright optical emission. This Letter reports mechano-sensing UCNPs based on the lanthanide dopants Yb3+ and Er3+, which exhibit a strong ratiometric change in emission spectra and bright emission under applied pressure. We synthesize and analyze the pressure response of five different types of nanoparticles, including cubic NaYF4 host nanoparticles and alkaline-earth host materials CaLuF, SrLuF, SrYbF, and BaLuF, all with lengths of 15 nm or less. By combining optical spectroscopy in a diamond anvil cell with single-particle brightness, we determine the noise equivalent sensitivity (GPa/√Hz) of these particles. The SrYb0.72Er0.28F@SrLuF particles exhibit an optimum noise equivalent sensitivity of 0.26 ± 0.04 GPa/√Hz. These particles present the possibility of robust nanometer-scale mechano-sensing.
View details for DOI 10.1021/acs.jpclett.1c03841
Femtosecond Visualization of hcp-Iron Strength and Plasticity under Shock Compression. Physical review letters Merkel, S., Hok, S., Bolme, C., Rittman, D., Ramos, K. J., Morrow, B., Lee, H. J., Nagler, B., Galtier, E., Granados, E., Hashim, A., Mao, W. L., Gleason, A. E. 2021; 127 (20): 205501
Iron is a key constituent of planets and an important technological material. Here, we combine insitu ultrafast x-ray diffraction with laser-induced shock compression experiments on Fe up to 187(10)GPa and 4070(285)K at 10^{8}s^{-1} in strain rate to study the plasticity of hexagonal-close-packed (hcp)-Fe under extreme loading states. {101[over ]2} deformation twinning controls the polycrystalline Fe microstructures and occurs within 1ns, highlighting the fundamental role of twinning in hcp polycrystals deformation at high strain rates. The measured deviatoric stress initially increases to a significant elastic overshoot before the onset of flow, attributed to a slower defect nucleation and mobility. The initial yield strength of materials deformed at high strain rates is thus several times larger than their longer-term flow strength. These observations illustrate how time-resolved ultrafast studies can reveal distinctive plastic behavior in materials under extreme environments.
View details for DOI 10.1103/PhysRevLett.127.205501
Pressure-induced suppression of Jahn-Teller distortions and enhanced electronic properties in high-entropy oxide (Mg0.2Ni0.2Co0.2Zn0.2Cu0.2)O APPLIED PHYSICS LETTERS Yan, J., Zhang, L., Liu, J., Li, N., Tamura, N., Chen, B., Lin, Y., Mao, W. L., Zhang, H. 2021; 119 (15)
View details for DOI 10.1063/5.0067432
Sub-10-nm graphene nanoribbons with atomically smooth edges from squashed carbon nanotubes NATURE ELECTRONICS Chen, C., Lin, Y., Zhou, W., Gong, M., He, Z., Shi, F., Li, X., Wu, J., Lam, K., Wang, J., Yang, F., Zeng, Q., Guo, J., Gao, W., Zuo, J., Liu, J., Hong, G., Antaris, A. L., Lin, M., Mao, W. L., Dai, H. 2021
View details for DOI 10.1038/s41928-021-00633-6
Characteristics and implications of podiform-chromite hosted silicate inclusions in the Zedang ophiolite, Southern Tibet LITHOS Guo, G., Mao, W. L., Zhang, R. Y., Liou, J. G., Ernst, W. G., Yang, J., Liu, X., Xu, X., Zhang, Y., Wu, B. 2021; 396
View details for DOI 10.1016/j.lithos.2021.106218
Ultrafast X-ray Diffraction Study of a Shock-Compressed Iron Meteorite above 100 GPa MINERALS Tecklenburg, S., Colina-Ruiz, R., Hok, S., Bolme, C., Galtier, E., Granados, E., Hashim, A., Lee, H., Merkel, S., Morrow, B., Nagler, B., Ramos, K., Rittman, D., Walroth, R., Mao, W. L., Gleason, A. E. 2021; 11 (6)
View details for DOI 10.3390/min11060567
Mineralogy of the deep lower mantle in the presence of H2O. National science review Hu, Q., Liu, J., Chen, J., Yan, B., Meng, Y., Prakapenka, V. B., Mao, W. L., Mao, H. K. 2021; 8 (4): nwaa098
Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800-2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H2O to form a previously unknown (Mg, Fe)O2H x (x ≤ 1) phase. The (Mg, Fe)O2H x has a pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg0.6Fe0.4)O or beyond 2300 km for (Mg0.7Fe0.3)O. The (Mg, Fe)O2H x is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg2+, Fe3+ and H+). This important phase has a number of far-reaching implications including extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM and an internal source for modulating oxygen in the atmosphere.
View details for DOI 10.1093/nsr/nwaa098
View details for PubMedCentralID PMC8288427
Evidence for oxygenation of Fe-Mg oxides at mid-mantle conditions and the rise of deep oxygen. National science review Liu, J., Wang, C., Lv, C., Su, X., Liu, Y., Tang, R., Chen, J., Hu, Q., Mao, H. K., Mao, W. L. 2021; 8 (4): nwaa096
As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D" layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+ δ (0 < δ < 1, denoted as 'OE-phase'). It forms at pressures greater than 40 gigapascal when (Mg, Fe)-bearing hydrous materials are heated over 1500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2Hx which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1000 km in the Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial or subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.
Probing the Electronic Band Gap of Solid Hydrogen by Inelastic X-Ray Scattering up to 90GPa. Physical review letters Li, B., Ding, Y., Kim, D. Y., Wang, L., Weng, T., Yang, W., Yu, Z., Ji, C., Wang, J., Shu, J., Chen, J., Yang, K., Xiao, Y., Chow, P., Shen, G., Mao, W. L., Mao, H. 2021; 126 (3): 036402
Metallization of hydrogen as a key problem in modern physics is the pressure-induced evolution of the hydrogen electronic band from a wide-gap insulator to a closed gap metal. However, due to its remarkably high energy, the electronic band gap of insulating hydrogen has never before been directly observed under pressure. Using high-brilliance, high-energy synchrotron radiation, we developed an inelastic x-ray probe to yield the hydrogen electronic band information insitu under high pressures in a diamond-anvil cell. The dynamic structure factor of hydrogen was measured over a large energy range of 45eV. The electronic band gap was found to decrease linearly from 10.9 to 6.57eV, with an 8.6 times densification (rho/rho_{0}8.6) from zero pressure up to 90GPa.
Pressure-induced excimer formation and fluorescence enhancement of an anthracene derivative JOURNAL OF MATERIALS CHEMISTRY C Dai, Y., Liu, H., Geng, T., Ke, F., Niu, S., Wang, K., Qi, Y., Zou, B., Yang, B., Mao, W. L., Lin, Y. 2021; 9 (3): 934–38
View details for DOI 10.1039/d0tc04677a
Polyamorphism in a solute-lean Al-Ce metallic glass JOURNAL OF APPLIED PHYSICS Yin, Z., Lou, H., Sheng, H., Zeng, Z., Mao, W. L., Zeng, Q. 2021; 129 (2)
Preserving a robust CsPbI3 perovskite phase via pressure-directed octahedral tilt. Nature communications Ke, F. n., Wang, C. n., Jia, C. n., Wolf, N. R., Yan, J. n., Niu, S. n., Devereaux, T. P., Karunadasa, H. I., Mao, W. L., Lin, Y. n. 2021; 12 (1): 461
Functional CsPbI3 perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite δ phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI3 perovskite structure to room temperature by subjecting the δ phase to pressures of 0.1 - 0.6 GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic γ-CsPbI3. Once formed, γ-CsPbI3 could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI6]4- octahedra which in turn tune the energy difference between δ- and γ-CsPbI3, leading to the preservation of γ-CsPbI3. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality.
Revealing Local Disorder in a Silver-Bismuth Halide Perovskite upon Compression. The journal of physical chemistry letters Girdzis, S. P., Lin, Y., Leppert, L., Slavney, A. H., Park, S., Chapman, K. W., Karunadasa, H. I., Mao, W. L. 2020: 532–36
The halide double perovskite Cs2AgBiBr6 has emerged as a promising nontoxic alternative to the lead halide perovskites APbX3 (A = organic cation or Cs; X = I or Br). Here, we perform high-pressure synchrotron X-ray total scattering on Cs2AgBiBr6 and discover local disorder that is hidden from conventional Bragg analysis. While our powder diffraction data show that the average structure remains cubic up to 2.1 GPa, analysis of the X-ray pair distribution function reveals that the local structure is better described by a monoclinic space group, with significant distortion within the Ag-Br and Bi-Br octahedra and off-centering of the Cs atoms. By tracking the distribution of interatomic Cs-Br distances, we find that the local disorder is enhanced upon compression, and we corroborate these results with molecular dynamics simulations. The observed local disorder affords new understanding of this promising material and potentially offers a new parameter to tune in halide perovskite lattices.
Synthesis of Atomically Thin Hexagonal Diamond with Compression. Nano letters Ke, F., Zhang, L., Chen, Y., Yin, K., Wang, C., Tzeng, Y., Lin, Y., Dong, H., Liu, Z., Tse, J. S., Mao, W. L., Wu, J., Chen, B. 2020
Atomically thin diamond, also called diamane, is a two-dimensional carbon allotrope and has attracted considerable scientific interest because of its potential physical properties. However, the successful synthesis of a pristine diamane has up until now not been achieved. We demonstrate the realization of a pristine diamane through diamondization of mechanically exfoliated few-layer graphene via compression. Resistance, optical absorption, and X-ray diffraction measurements reveal that hexagonal diamane (h-diamane) with a bandgap of 2.8 ± 0.3 eV forms by compressing trilayer and thicker graphene to above 20 GPa at room temperature and can be preserved upon decompression to 1.0 GPa. Theoretical calculations indicate that a (-2110)-oriented h-diamane is energetically stable and has a lower enthalpy than its few-layer graphene precursor above the transition pressure. Compared to gapless graphene, semiconducting h-diamane offers exciting possibilities for carbon-based electronic devices.
View details for DOI 10.1021/acs.nanolett.0c01872
Origin of Plasticity in Nanostructured Silicon. Physical review letters Zeng, Z., Zeng, Q., Ge, M., Chen, B., Lou, H., Chen, X., Yan, J., Yang, W., Mao, H. K., Yang, D., Mao, W. L. 2020; 124 (18): 185701
The mechanism of plasticity in nanostructured Si has been intensively studied over the past decade but still remains elusive. Here, we used in situ high-pressure radial x-ray diffraction to simultaneously monitor the deformation and structural evolution of a large number of randomly oriented Si nanoparticles (SiNPs). In contrast to the high-pressure β-Sn phase dominated plasticity observed in large SiNPs (∼100 nm), small SiNPs (∼9 nm) display a high-pressure simple hexagonal phase dominated plasticity. Meanwhile, dislocation activity exists in all of the phases, but significantly weakens as the particle size decreases and only leads to subtle plasticity in the initial diamond cubic phase. Furthermore, texture simulations identify major active slip systems in all of the phases. These findings elucidate the origin of plasticity in nanostructured Si under stress and provide key guidance for the application of nanostructured Si.
Origin of Plasticity in Nanostructured Silicon PHYSICAL REVIEW LETTERS Zeng, Z., Zeng, Q., Ge, M., Chen, B., Lou, H., Chen, X., Yan, J., Yang, W., Mao, H., Yang, D., Mao, W. L. 2020; 124 (18)
Key problems of the four-dimensional Earth system MATTER AND RADIATION AT EXTREMES Mao, H., Mao, W. L. 2020; 5 (3)
Crystallography of low Z material at ultrahigh pressure: Case study on solid hydrogen MATTER AND RADIATION AT EXTREMES Ji, C., Li, B., Liu, W., Smith, J. S., Bjorling, A., Majumdar, A., Luo, W., Ahuja, R., Shu, J., Wang, J., Sinogeikin, S., Meng, Y., Prakapenka, V. B., Greenberg, E., Xu, R., Huang, X., Ding, Y., Soldatov, A., Yang, W., Shen, G., Mao, W. L., Mao, H. 2020; 5 (3)
Facile diamond synthesis from lower diamondoids. Science advances Park, S. n., Abate, I. I., Liu, J. n., Wang, C. n., Dahl, J. E., Carlson, R. M., Yang, L. n., Prakapenka, V. B., Greenberg, E. n., Devereaux, T. P., Jia, C. n., Ewing, R. C., Mao, W. L., Lin, Y. n. 2020; 6 (8): eaay9405
Carbon-based nanomaterials have exceptional properties that make them attractive for a variety of technological applications. Here, we report on the use of diamondoids (diamond-like, saturated hydrocarbons) as promising precursors for laser-induced high-pressure, high-temperature diamond synthesis. The lowest pressure and temperature (P-T) conditions that yielded diamond were 12 GPa (at ~2000 K) and 900 K (at ~20 GPa), respectively. This represents a substantially reduced transformation barrier compared with diamond synthesis from conventional (hydro)carbon allotropes, owing to the similarities in the structure and full sp3 hybridization of diamondoids and bulk diamond. At 20 GPa, diamondoid-to-diamond conversion occurs rapidly within <19 μs. Molecular dynamics simulations indicate that once dehydrogenated, the remaining diamondoid carbon cages reconstruct themselves into diamond-like structures at high P-T. This study is the first successful mapping of the P-T conditions and onset timing of the diamondoid-to-diamond conversion and elucidates the physical and chemical factors that facilitate diamond synthesis.
View details for DOI 10.1126/sciadv.aay9405
Diamondoids Under Pressure CARBON IN EARTH'S INTERIOR Park, S., Lin, Y., Mao, W. L., Manning, C. E., Lin, J. F., Mao, W. L. 2020; 249: 341-349
View details for DOI 10.1002/9781119508229.ch27
Nitrogen in black phosphorus structure. Science advances Ji, C. n., Adeleke, A. A., Yang, L. n., Wan, B. n., Gou, H. n., Yao, Y. n., Li, B. n., Meng, Y. n., Smith, J. S., Prakapenka, V. B., Liu, W. n., Shen, G. n., Mao, W. L., Mao, H. K. 2020; 6 (23): eaba9206
Group V elements in crystal structure isostructural to black phosphorus with unique puckered two-dimensional layers exhibit exciting physical and chemical phenomena. However, as the first element of group V, nitrogen has never been found in the black phosphorus structure. Here, we report the synthesis of the black phosphorus-structured nitrogen at 146 GPa and 2200 K. Metastable black phosphorus-structured nitrogen was retained after quenching it to room temperature under compression and characterized in situ during decompression to 48 GPa, using synchrotron x-ray diffraction and Raman spectroscopy. We show that the original molecular nitrogen is transformed into extended single-bonded structure through gauche and trans conformations. Raman spectroscopy shows that black phosphorus-structured nitrogen is strongly anisotropic and exhibits high Raman intensities in two Ag normal modes. Synthesis of black phosphorus-structured nitrogen provides a firm base for exploring new type of high-energy-density nitrogen and a new direction of two-dimensional nitrogen.
View details for DOI 10.1126/sciadv.aba9206
In situ X-ray diffraction of silicate liquids and glasses under dynamic and static compression to megabar pressures. Proceedings of the National Academy of Sciences of the United States of America Morard, G. n., Hernandez, J. A., Guarguaglini, M. n., Bolis, R. n., Benuzzi-Mounaix, A. n., Vinci, T. n., Fiquet, G. n., Baron, M. A., Shim, S. H., Ko, B. n., Gleason, A. E., Mao, W. L., Alonso-Mori, R. n., Lee, H. J., Nagler, B. n., Galtier, E. n., Sokaras, D. n., Glenzer, S. H., Andrault, D. n., Garbarino, G. n., Mezouar, M. n., Schuster, A. K., Ravasio, A. n. 2020
Properties of liquid silicates under high-pressure and high-temperature conditions are critical for modeling the dynamics and solidification mechanisms of the magma ocean in the early Earth, as well as for constraining entrainment of melts in the mantle and in the present-day core-mantle boundary. Here we present in situ structural measurements by X-ray diffraction of selected amorphous silicates compressed statically in diamond anvil cells (up to 157 GPa at room temperature) or dynamically by laser-generated shock compression (up to 130 GPa and 6,000 K along the MgSiO3 glass Hugoniot). The X-ray diffraction patterns of silicate glasses and liquids reveal similar characteristics over a wide pressure and temperature range. Beyond the increase in Si coordination observed at 20 GPa, we find no evidence for major structural changes occurring in the silicate melts studied up to pressures and temperatures exceeding Earth's core mantle boundary conditions. This result is supported by molecular dynamics calculations. Our findings reinforce the widely used assumption that the silicate glasses studies are appropriate structural analogs for understanding the atomic arrangement of silicate liquids at these high pressures.
View details for DOI 10.1073/pnas.1920470117
Tuning Emission and Electron-Phonon Coupling in Lead-Free Halide Double Perovskite Cs2AgBiCl6 under Pressure ACS ENERGY LETTERS Zhang, L., Fang, Y., Sui, L., Yan, J., Wang, K., Yuan, K., Mao, W. L., Zou, B. 2019; 4 (12): 2975–82
View details for DOI 10.1021/acsenergylett.9b02155
Ultrahigh-pressure isostructural electronic transitions in hydrogen. Nature Ji, C., Li, B., Liu, W., Smith, J. S., Majumdar, A., Luo, W., Ahuja, R., Shu, J., Wang, J., Sinogeikin, S., Meng, Y., Prakapenka, V. B., Greenberg, E., Xu, R., Huang, X., Yang, W., Shen, G., Mao, W. L., Mao, H. 2019; 573 (7775): 558–62
High-pressure transitions are thought to modify hydrogen molecules to a molecular metallic solid and finally to an atomic metal1, which is predicted to have exotic physical properties and the topology of a two-component (electron and proton) superconducting superfluid condensate2,3. Therefore, understanding such transitions remains an important objective in condensed matter physics4,5. However, measurements of the crystal structure of solid hydrogen, which provides crucial information about the metallization of hydrogen under compression, are lacking for most high-pressure phases, owing to the considerable technical challenges involved in X-ray and neutron diffraction measurements under extreme conditions. Here we present a single-crystal X-ray diffraction study of solid hydrogen at pressures of up to 254 gigapascals that reveals the crystallographic nature of the transitions from phase I to phases III and IV. Under compression, hydrogen molecules remain in the hexagonal close-packed (hcp) crystal lattice structure, accompanied by a monotonic increase in anisotropy. In addition, the pressure-dependent decrease of the unit cell volume exhibits a slope change when entering phase IV, suggesting a second-order isostructural phase transition. Our results indicate that the precursor to the exotic two-component atomic hydrogen may consist of electronic transitions caused by a highly distorted hcp Brillouin zone and molecular-symmetry breaking.
View details for DOI 10.1038/s41586-019-1565-9
Pressure-Induced Emission (PIE) and Phase Transition of a Two-dimensional Halide Double Perovskite (BA)4AgBiBr8 (BA = CH3(CH2)3NH3+). Angewandte Chemie (International ed. in English) Fang, Y., Zhang, L., Wu, L., Yan, J., Lin, Y., Wang, K., Mao, W. L., Zou, B. 2019
Two-dimensional (2D) halide perovskites have attracted significant attention due in part to their compositional flexibility and electronic diversity. Understanding the structure property relationships in 2D double perovskites is essential for their development for optoelectronic applications. In this work, we observed the emergence of pressure-induced emission (PIE) at 2.5 GPa with a broad emission band and large Stokes shift from initially nonfluorescent (BA)4AgBiBr8 (BA = CH3(CH2)3NH3+). The emission intensity increased significantly upon further compression up to 8.2 GPa. Moreover, the bandgap narrowed from the starting 2.61 eV to 2.19 eV at 25.0 GPa accompanied by a color change from light yellow to dark yellow. Analysis of combined in situ high-pressure photolu-minescence, absorption, and angle-dispersive X-ray diffraction data indicates that the observed PIE can be attributed to the emission from self-trapped excitons. This coincides with [AgBr6]5- and [BiBr6]3- inter-octahedral tilting which cause a structural phase transition. High-pressure study on (BA)4AgBiBr8 sheds light upon the relationship between the structure and optical properties that may improve the material's potential applications in the fields of pressure sensing, information storage and trademark security.
View details for DOI 10.1002/anie.201906311
Halide perovskites under pressure Jaffe, A., Lin, Y., Mao, W., Karunadasa, H. AMER CHEMICAL SOC. 2019
Electronic spin transition in FeO2: Evidence for Fe(II) with peroxide O-2(2-) PHYSICAL REVIEW B Jang, B., Liu, J., Hu, Q., Haule, K., Mao, H., Mao, W. L., Kim, D., Shim, J. 2019; 100 (1)
View details for DOI 10.1103/PhysRevB.100.014418
Applications for Nanoscale X-ray Imaging at High Pressure ENGINEERING Mao, W. L., Lin, Y., Liu, Y., Liu, J. 2019; 5 (3): 479–89
View details for DOI 10.1016/j.eng.2019.01.006
Structure-Controlled Oxygen Concentration in Fe2O3 and FeO2 INORGANIC CHEMISTRY Zhu, S., Liu, J., Hu, Q., Mao, W. L., Meng, Y., Zhang, D., Mao, H., Zhu, Q. 2019; 58 (9): 5476–82
View details for DOI 10.1021/acs.inorgchem.8b02764
Anomalous behavior of nonequilibrium excitations in UO2 PHYSICAL REVIEW B Rittman, D. R., Teitelbaum, S. W., Reis, D. A., Mao, W. L., Ewing, R. C. 2019; 99 (13)
View details for DOI 10.1103/PhysRevB.99.134307
Tuning Optical and Electronic Properties in Low-Toxicity Organic-Inorganic Hybrid (CH3NH3)(3)Bi2I9 under High Pressure JOURNAL OF PHYSICAL CHEMISTRY LETTERS Zhang, L., Liu, C., Lin, Y., Wang, K., Ke, F., Liu, C., Mao, W. L., Zou, B. 2019; 10 (8): 1676–83
View details for DOI 10.1021/acs.jpclett.9b00595
Phase transformations of Al-bearing high-entropy alloys AlxCoCrFeNi (x=0, 0.1, 0.3, 0.75, 1.5) at high pressure APPLIED PHYSICS LETTERS Wang, C., Tracy, C. L., Park, S., Liu, J., Ke, F., Zhang, F., Yang, T., Xia, S., Li, C., Wang, Y., Zhang, Y., Mao, W. L., Ewing, R. C. 2019; 114 (9)
Altered chemistry of oxygen and iron under deep Earth conditions NATURE COMMUNICATIONS Liu, J., Hu, Q., Bi, W., Yang, L., Xiao, Y., Chow, P., Meng, Y., Prakapenka, V. B., Mao, H., Mao, W. L. 2019; 10
Diffusion-controlled alloying of single-phase multi-principal transition metal carbides with high toughness and low thermal diffusivity APPLIED PHYSICS LETTERS Peng, C., Gao, X., Wang, M., Wu, L., Tang, H., Li, X., Zhang, Q., Ren, Y., Zhang, F., Wang, Y., Zhang, B., Gao, B., Zou, Q., Zhao, Y., Yang, Q., Tian, D., Xiao, H., Gou, H., Yang, W., Bai, X., Mao, W. L., Mao, H. 2019; 114 (1)
High Compression-Induced Conductivity in a Layered Cu-Br Perovskite. Angewandte Chemie (International ed. in English) Jaffe, A. n., Mack, S. A., Lin, Y. n., Mao, W. n., Neaton, J. B., Karunadasa, H. n. 2019
We show that the onset pressure for appreciable conductivity in layered copper-halide perovskites can decrease by ca. 50 GPa upon replacement of Cl with Br. Layered Cu-Cl perovskites require pressures >50 GPa to show a conductivity of 10-4 S·cm-1, whereas here a Cu-Br congener, (EA)2CuBr4 (EA = ethylammonium), exhibits conductivity as high as 2 × 10-3 S·cm-1 at only 2.6 GPa, and 0.17 S·cm-1 at 59 GPa. Substitution of higher-energy Br 4p for Cl 3p orbitals lowers the charge-transfer bandgap of the perovskite by 0.9 eV. This 1.7 eV bandgap decreases to 0.3 eV at 65 GPa. High-pressure X-ray diffraction, optical absorption, and transport measurements, and density functional theory calculations allow us to track compression-induced structural and electronic changes. The notable enhancement of the Br perovskite's electronic response to pressure may be attributed to more diffuse Br valence orbitals relative to Cl orbitals. This work brings the compression-induced conductivity of Cu-halide perovskites to more technologically accessible pressures.
Superconducting transition temperatures in the electronic and magnetic phase diagrams of Sr<sub>2</sub>VFeAsO<sub>3-<i>delta</i></sub>, a superconductor. Journal of physics. Condensed matter : an Institute of Physics journal Tojo, Y., Shibuya, T., Nakamura, T., Shoji, K., Fujioka, H., Matoba, M., Yasui, S., Itoh, M., Iimura, S., Hiramatsu, H., Hosono, H., Hirai, S., Mao, W. L., Kitao, S., Seto, M., Kamihara, Y. 2018
We unveil magnetic phases and superconducting transition temperatures (<i>T</i><sub>c</sub>) in an iron-based superconductor with a thick-blocking layer of a perovskite-related transition metal oxide, Sr<sub>2</sub>VFeAsO<sub>3-delta</sub> (21113V). 21113V exhibits a superconducting phase in 0.031 ≦ <i>delta</i> ≦ 0.145 at temperatures (<i>T</i>) < 37.1 K. Antiferromagnetic (AFM) iron sublattice are observed in 0.267 ≦ <i>delta</i> ≦ 0.664. A mixed valent vanadium exhibits a dominant AFM phase in 0.031 ≦ <i>delta</i> ≦ 0.088, while a partial ferrimagnetic (Ferri.) phase of the vanadium appears in 0.124 ≦ <i>delta</i> ≦ 0.664. The partial Ferri. phase becomes the most dominant for <i>delta</i>~0.267, in which the Fe shows AFM phase at <i>T</i> < 20 K. A volume fraction of the superconducting phase is suppressed by increasing of spontaneous magnetic moments due to the partial Ferri. vanadium; i.e the magnetic phase of the vanadium dominates superconductivity in 21113V. The <i>T</i><sub>c</sub>-<i>delta</i> curve shows two maxima. The lower maximum of Tc are observed for <i>delta</i> = 0.073. It is noted that the highest Tc of 21113V appears for <i>delta</i> = 0.145, which exists in a phase boundary between AFM and the partial Ferri. phases of the vanadium. 21113V is a platform to verify new mechanism for enhancing <i>T</i><sub>c</sub> in iron-based superconductors.
View details for DOI 10.1088/1361-648X/aaf7e0
The effect of nickel on the strength of iron nickel alloys: Implications for the Earth's inner core PHYSICS OF THE EARTH AND PLANETARY INTERIORS Reagan, M. M., Gleason, A. E., Liu, J., Krawczynski, M. J., Van Orman, J. A., Mao, W. L. 2018; 283: 43–47
View details for DOI 10.1016/j.pepi.2018.08.003
Mechanosensitive upconverting nanoparticles for visualizing mechanical forces in vivo Lay, A., Siefe, C., Fischer, S., Mehlenbacher, R., Das, A., Nekimken, A., Ke, F., Mao, W., Pruitt, B., Cohen, B., Alivisatos, P., Goodman, M., Dionne, J. AMER CHEMICAL SOC. 2018
Revealing the formation mechanism of ultrahard nanotwinned diamond from onion carbon CARBON Tang, H., Yuan, X., Yu, P., Hu, Q., Wang, M., Yao, Y., Wu, L., Zou, Q., Ke, Y., Zhao, Y., Wang, L., Li, X., Yang, W., Gou, H., Mao, H., Mao, W. L. 2018; 129: 159–67
View details for DOI 10.1016/j.carbon.2017.12.027
Radiation-induced disorder in compressed lanthanide zirconates PHYSICAL CHEMISTRY CHEMICAL PHYSICS Park, S., Tracy, C. L., Zhang, F., Park, C., Trautmann, C., Tkachev, S. N., Lang, M., Maoah, W. L., Ewing, R. C. 2018; 20 (9): 6187–97
The effects of swift heavy ion irradiation-induced disordering on the behavior of lanthanide zirconate compounds (Ln2Zr2O7 where Ln = Sm, Er, or Nd) at high pressures are investigated. After irradiation with 2.2 GeV 197Au ions, the initial ordered pyrochlore structure (Fd3[combining macron]m) transformed to a defect-fluorite structure (Fm3[combining macron]m) in Sm2Zr2O7 and Nd2Zr2O7. For irradiated Er2Zr2O7, which has a defect-fluorite structure, ion irradiation induces local disordering by introducing Frenkel defects despite retention of the initial structure. When subjected to high pressures (>29 GPa) in the absence of irradiation, all of these compounds transform to a cotunnite-like (Pnma) phase, followed by sluggish amorphization with further compression. However, if these compounds are irradiated prior to compression, the high pressure cotunnite-like phase is not formed. Rather, they transform directly from their post-irradiation defect-fluorite structure to an amorphous structure upon compression (>25 GPa). Defects and disordering induced by swift heavy ion irradiation alter the transformation pathways by raising the energetic barriers for the transformation to the high pressure cotunnite-like phase, rendering it inaccessible. As a result, the high pressure stability field of the amorphous phase is expanded to lower pressures when irradiation is coupled with compression. The responses of materials in the lanthanide zirconate system to irradiation and compression, both individually and in tandem, are strongly influenced by the specific lanthanide composition, which governs the defect energetics at extreme conditions.
Sterically controlled mechanochemistry under hydrostatic pressure NATURE Yan, H., Yang, F., Pan, D., Lin, Y., Hohman, J., Solis-Ibarra, D., Li, F., Dahl, J. P., Carlson, R. K., Tkachenko, B. A., Fokin, A. A., Schreiner, P. R., Galli, G., Mao, W. L., Shen, Z., Melosh, N. A. 2018; 554 (7693): 505-+
Mechanical stimuli can modify the energy landscape of chemical reactions and enable reaction pathways, offering a synthetic strategy that complements conventional chemistry. These mechanochemical mechanisms have been studied extensively in one-dimensional polymers under tensile stress using ring-opening and reorganization, polymer unzipping and disulfide reduction as model reactions. In these systems, the pulling force stretches chemical bonds, initiating the reaction. Additionally, it has been shown that forces orthogonal to the chemical bonds can alter the rate of bond dissociation. However, these bond activation mechanisms have not been possible under isotropic, compressive stress (that is, hydrostatic pressure). Here we show that mechanochemistry through isotropic compression is possible by molecularly engineering structures that can translate macroscopic isotropic stress into molecular-level anisotropic strain. We engineer molecules with mechanically heterogeneous components-a compressible ('soft') mechanophore and incompressible ('hard') ligands. In these 'molecular anvils', isotropic stress leads to relative motions of the rigid ligands, anisotropically deforming the compressible mechanophore and activating bonds. Conversely, rigid ligands in steric contact impede relative motion, blocking reactivity. We combine experiments and computations to demonstrate hydrostatic-pressure-driven redox reactions in metal-organic chalcogenides that incorporate molecular elements that have heterogeneous compressibility, in which bending of bond angles or shearing of adjacent chains activates the metal-chalcogen bonds, leading to the formation of the elemental metal. These results reveal an unexplored reaction mechanism and suggest possible strategies for high-specificity mechanosynthesis.
A(2)TiO(5) (A = Dy, Gd, Er, Yb) at High Pressure INORGANIC CHEMISTRY Park, S., Rittman, D. R., Tracy, C. L., Chapman, K. W., Zhang, F., Park, C., Tkachev, S. N., O'Quinn, E., Shamblin, J., Lang, M., Mao, W. L., Ewing, R. C. 2018; 57 (4): 2269–77
The structural evolution of lanthanide A2TiO5 (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (Pnma) phase for a certain pressure range. Orthorhombic Dy2TiO5 and Gd2TiO5 form P21am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type Dy2TiO5 and Er2TiO5 as well as defect-fluorite-type Yb2TiO5 form Pnma at ∼21 GPa, followed by Im3̅m. Hexagonal Dy2TiO5 forms Pnma directly, although a small amount of remnants of hexagonal Dy2TiO5 is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy2TiO5 phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (Im3̅m) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er2TiO5. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A2BO5 composition at high pressure.
Swift-heavy ion irradiation response and annealing behavior of A(2)TiO(5) (A = Nd, Gd, and Yb) JOURNAL OF SOLID STATE CHEMISTRY Park, S., Tracy, C. L., Zhang, F., Palomares, R. I., Park, C., Trautmann, C., Lang, M., Mao, W. L., Ewing, R. C. 2018; 258: 108–16
View details for DOI 10.1016/j.jssc.2017.09.028
Phase transformation pathways of ultrafast-laser-irradiated Ln(2)O(3) (Ln = Er-Lu) PHYSICAL REVIEW B Rittman, D. R., Tracy, C. L., Chen, C., Solomon, J. M., Asta, M., Mao, W. L., Yalisove, S. M., Ewing, R. C. 2018; 97 (2)
Bright, Mechanosensitive Upconversion with Cubic-Phase Heteroepitaxial Core-Shell Nanoparticles. Nano letters Lay, A. n., Siefe, C. n., Fischer, S. n., Mehlenbacher, R. D., Ke, F. n., Mao, W. L., Alivisatos, A. P., Goodman, M. B., Dionne, J. A. 2018
Lanthanide-doped nanoparticles are an emerging class of optical sensors, exhibiting sharp emission peaks, high signal-to-noise ratio, photostability, and a ratiometric color response to stress. The same centrosymmetric crystal field environment that allows for high mechanosensitivity in the cubic-phase (α), however, contributes to low upconversion quantum yield (UCQY). In this work, we engineer brighter mechanosensitive upconverters using a core-shell geometry. Sub-25 nm α-NaYF4:Yb,Er cores are shelled with an optically inert surface passivation layer of ∼4.5 nm thickness. Using different shell materials, including NaGdF4, NaYF4, and NaLuF4, we study how compressive to tensile strain influences the nanoparticles' imaging and sensing properties. All core-shell nanoparticles exhibit enhanced UCQY, up to 0.14% at 150 W/cm2, which rivals the efficiency of unshelled hexagonal-phase (β) nanoparticles. Additionally, strain at the core-shell interface can tune mechanosensitivity. In particular, the compressive Gd shell results in the largest color response from yellow-green to orange or, quantitatively, a change in the red to green ratio of 12.2 ± 1.2% per GPa. For all samples, the ratiometric readouts are consistent over three pressure cycles from ambient to 5 GPa. Therefore, heteroepitaxial shelling significantly improves signal brightness without compromising the core's mechano-sensing capabilities and further, promotes core-shell cubic-phase nanoparticles as upcoming in vivo and in situ optical sensors.
Lanthanide stannate pyrochlores (Ln(2)Sn(2)O(7); Ln = Nd, Gd, Er) at high pressure JOURNAL OF PHYSICS-CONDENSED MATTER Turner, K. M., Tracy, C. L., Mao, W. L., Ewing, R. C. 2017; 29 (50)
View details for DOI 10.1088/1361-648X/aa9960
Lanthanide stannate pyrochlores (Ln2Sn2O7; Ln = Nd, Gd, Er) at high pressure. Journal of physics. Condensed matter : an Institute of Physics journal Turner, K. M., Tracy, C. L., Mao, W. L., Ewing, R. C. 2017; 29 (50): 504005
Lanthanide stannate pyrochlores (Ln2Sn2O7; Ln = Nd, Gd, and Er) were investigated in situ to 50 GPa in order to determine their structural response to compression and compare their response to that of lanthanide titanate, zirconate, and hafnate pyrochlores. The cation radius ratio of A3+/B4+ in pyrochlore oxides (A2B2O7) is thought to be the dominant feature that influences their response on compression. The ionic radius of Sn4+ is intermediate to that of Ti4+, Zr4+, and Hf4+, but the 〈Sn-O〉 bond in stannate pyrochlore is more covalent than the 〈B-O〉 bonds in titanates, zirconate, and hafnates. In stannates, based on in situ Raman spectroscopy, pyrochlore cation and anion sublattices begin to disorder with the onset of compression, first measured at 0.3 GPa. The extent of sublattice disorder versus pressure is greater in stannates with a smaller Ln3+ cation. Stannate pyrochlores (Fd-3m) begin a sluggish transformation to an orthorhombic, cotunnite-like structure at ~28 GPa; similar transitions have been observed in titanate, zirconate, and hafnate pyrochlores at varying pressures (18-40 GPa) with cation radius ratio. The extent of the phase transition versus pressure varies directly with the size of the Ln3+ cation. Post-decompression from ~50 GPa, Er2Sn2O7 and Gd2Sn2O7 adopt a pyrochlore structure, rather than the multi-scale defect-fluorite + weberite-type structure adopted by Nd2Sn2O7 that is characteristic of titanate, zirconate, and hafnate pyrochlores under similar conditions. Like pyrochlore titanates, zirconates, and hafnates, the bulk modulus, B 0, of stannates varies linearly and inversely with cation radius ratio from 1 1 1 GPa (Nd2Sn2O7) to 251 GPa (Er2Sn2O7). The trends of bulk moduli in stannates in this study are in excellent agreement with previous experimental studies on stannates and suggest that the size of the Ln3+ cation is the primary determining factor of B 0. Additionally, when normalized to r A/r B, the bulk moduli of stannates are comparable to those of zirconates and hafnates, which vary from titanates. Our results suggest that the cation radius ratio strongly influences the bulk moduli of stannates, as well as their overall compression response.
Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones NATURE Liu, J., Hu, Q., Kim, D., Wu, Z., Wang, W., Xiao, Y., Chow, P., Meng, Y., Prakapenka, V. B., Mao, H., Mao, W. L. 2017; 551 (7681): 494-+
Ultralow-velocity zones (ULVZs) at Earth's core-mantle boundary region have important implications for the chemical composition and thermal structure of our planet, but their origin has long been debated. Hydrogen-bearing iron peroxide (FeO2Hx) in the pyrite-type crystal structure was recently found to be stable under the conditions of the lowermost mantle. Using high-pressure experiments and theoretical calculations, we find that iron peroxide with a varying amount of hydrogen has a high density and high Poisson ratio as well as extremely low sound velocities consistent with ULVZs. Here we also report a reaction between iron and water at 86 gigapascals and 2,200 kelvin that produces FeO2Hx. This would provide a mechanism for generating the observed volume occupied by ULVZs through the reaction of about one-tenth the mass of Earth's ocean water in subducted hydrous minerals with the effectively unlimited reservoir of iron in Earth's core. Unlike other candidates for the composition of ULVZs, FeO2Hx synthesized from the superoxidation of iron by water would not require an extra transportation mechanism to migrate to the core-mantle boundary. These dense FeO2Hx-rich domains would be expected to form directly in the core-mantle boundary region and their properties would provide an explanation for the many enigmatic seismic features that are observed in ULVZs.
Lanthanide stannate pyrochlores (Ln<sub>2</sub>Sn<sub>2</sub>O<sub>7</sub>; Ln = Nd, Gd, Er) at high pressure. Journal of physics. Condensed matter : an Institute of Physics journal Turner, K. M., Tracy, C. L., Mao, W. L., Ewing, R. C. 2017
Lanthanide stannate pyrochlores (Ln2Sn2O7; Ln=Nd, Gd, and Er) were investigated in situ to 50 GPa in order to determine their structural response to compression and compare it to that of lanthanide titanate, zirconate, and hafnate pyrochlores. The cation radius ratio of A3+/B4+ in pyrochlore oxides (A2B2O7) is thought to be the dominant property that influences their compression response. The ionic radius of Sn4+ is intermediate to that of Ti4+, Zr4+, and Hf4+, but the <Sn-O> bond in stannate pyrochlore is more covalent than the <B-O> bonds in titanates, zirconate, and hafnates. In stannates, the pyrochlore cation and anion sublattices begin to disorder at 0.3 GPa. The extent of sublattice disorder vs. pressure is greater in stannates with a smaller Ln3+ cation. Stannate pyrochlores (Fd-3m) begin a sluggish transformation to a cotunnite-like structure (Pnma) at ~28 GPa; similar transitions have been observed in titanate, zirconate, and hafnate pyrochlore at varying pressures with cation radius ratio. The extent of the phase transition vs. pressure varies directly with the size of the Ln3+ cation. Post-decompression from ~50 GPa, Er2Sn2O7 and Gd2Sn2O7 adopt a pyrochlore structure, rather than the multiscale defect-fluorite + weberite structure adopted by Nd2Sn2O7 that is characteristic of titanate, zirconate, and hafnate pyrochlore treated to similar conditions. Like pyrochlore titanates, zirconates, and hafnates, the bulk modulus, B0, of stannates varies linearly and inversely with cation radius ratio. The trends of bulk moduli in stannates in this study are in excellent agreement with previous experimental studies on stannates, and suggest that the size of the Ln3+ cation is a primary determining factor of B0. Additionally, when normalized to rA/rB, the bulk moduli of stannates are comparable to those of zirconates and hafnates, which vary from titanates. Our results suggest that the cation radius ratio strongly influences the bulk moduli of stannates as well as their overall compression response.
When water meets iron at Earth's core-mantle boundary NATIONAL SCIENCE REVIEW Mao, H., Hu, Q., Yang, L., Liu, J., Kim, D., Meng, Y., Zhang, L., Prakapenka, V. B., Yang, W., Mao, W. L. 2017; 4 (6): 870–78
View details for DOI 10.1093/nsr/nwx109
Hydrogen-Bond Symmetrization Breakdown and Dehydrogenation Mechanism of FeO2H at High Pressure. Journal of the American Chemical Society Zhu, S. C., Hu, Q., Mao, W. L., Mao, H. K., Sheng, H. 2017; 139 (35): 12129-12132
The cycling of hydrogen plays an important role in the geochemical evolution of our planet. Under high-pressure conditions, asymmetric hydroxyl bonds tend to form a symmetric O-H-O configuration in which H is positioned at the center of two O atoms. The symmetrization of O-H bonds improves their thermal stability and as such, water-bearing minerals can be present deeper in the Earth's lower mantle. However, how exactly H is recycled from the deep mantle remains unclear. Here, we employ first-principles free-energy landscape sampling methods together with high pressure-high temperature experiments to reveal the dehydrogenation mechanism of a water-bearing mineral, FeO2H, at deep mantle conditions. Experimentally, we show that ∼50% H is released from symmetrically hydrogen-bonded ε-FeO2H upon transforming to a pyrite-type phase (Py-phase). By resolving the lowest-energy transition pathway from ε-FeO2H to the Py-phase, we demonstrate that half of the O-H bonds in the mineral rupture during the structural transition, leading toward the breakdown of symmetrized hydrogen bonds and eventual dehydrogenation. Our study sheds new light on the stability of symmetric hydrogen bonds during structural transitions and provides a dehydrogenation mechanism for hydrous minerals existing in the deep mantle.
View details for DOI 10.1021/jacs.7b06528
Synthesis of quenchable amorphous diamond NATURE COMMUNICATIONS Zeng, Z., Yang, L., Zeng, Q., Lou, H., Sheng, H., Wen, J., Miller, D. J., Meng, Y., Yang, W., Mao, W. L., Mao, H. 2017; 8: 322
Diamond owes its unique mechanical, thermal, optical, electrical, chemical, and biocompatible materials properties to its complete sp 3-carbon network bonding. Crystallinity is another major controlling factor for materials properties. Although other Group-14 elements silicon and germanium have complementary crystalline and amorphous forms consisting of purely sp 3 bonds, purely sp 3-bonded tetrahedral amorphous carbon has not yet been obtained. In this letter, we combine high pressure and in situ laser heating techniques to convert glassy carbon into "quenchable amorphous diamond", and recover it to ambient conditions. Our X-ray diffraction, high-resolution transmission electron microscopy and electron energy-loss spectroscopy experiments on the recovered sample and computer simulations confirm its tetrahedral amorphous structure and complete sp 3 bonding. This transparent quenchable amorphous diamond has, to our knowledge, the highest density among amorphous carbon materials, and shows incompressibility comparable to crystalline diamond.Diamond's properties are dictated by its crystalline, fully tetrahedrally bonded structure. Here authors synthesize a bulk sp 3-bonded amorphous form of carbon under high pressure and temperature, show that it has bulk modulus comparable to crystalline diamond and that it can be recovered under ambient conditions.
Pressure-induced structural modifications of rare-earth hafnate pyrochlore. Journal of physics. Condensed matter : an Institute of Physics journal Turner, K. M., Rittman, D. R., Heymach, R. A., Tracy, C. L., Turner, M. L., Fuentes, A. F., Mao, W. L., Ewing, R. C. 2017; 29 (25): 255401-?
Complex oxides with the pyrochlore (A2B2O7) and defect-fluorite ((A,B)4O7) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (A2Hf2O7) form the pyrochlore structure for A = La-Tb and the defect-fluorite structure for A = Dy-Lu. High-pressure transformations in A2Hf2O7 pyrochlore (A = Sm, Eu, Gd) and defect-fluorite (A = Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs. <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. Dy2Hf2O7 is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr(4+) and Hf(4+), rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.
High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi NATURE COMMUNICATIONS Tracy, C. L., Park, S., Rittman, D. R., Zinkle, S. J., Bei, H., Lang, M., Ewing, R. C., Mao, W. L. 2017; 8
High-entropy alloys, near-equiatomic solid solutions of five or more elements, represent a new strategy for the design of materials with properties superior to those of conventional alloys. However, their phase space remains constrained, with transition metal high-entropy alloys exhibiting only face- or body-centered cubic structures. Here, we report the high-pressure synthesis of a hexagonal close-packed phase of the prototypical high-entropy alloy CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed to suppression of the local magnetic moments, destabilizing the initial fcc structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the transformation is sluggish, occurring over a range of >40 GPa. However, the behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This demonstrates a means of tuning the structures and properties of high-entropy alloys in a manner not achievable by conventional processing techniques.
View details for DOI 10.1038/ncomms15634
Strain engineered pyrochlore at high pressure. Scientific reports Rittman, D. R., Turner, K. M., Park, S., Fuentes, A. F., Park, C., Ewing, R. C., Mao, W. L. 2017; 7 (1): 2236-?
Strain engineering is a promising method for next-generation materials processing techniques. Here, we use mechanical milling and annealing followed by compression in diamond anvil cell to tailor the intrinsic and extrinsic strain in pyrochlore, Dy2Ti2O7 and Dy2Zr2O7. Raman spectroscopy, X-ray pair distribution function analysis, and X-ray diffraction were used to characterize atomic order over short-, medium-, and long-range spatial scales, respectively, under ambient conditions. Raman spectroscopy and X-ray diffraction were further employed to interrogate the material in situ at high pressure. High-pressure behavior is found to depend on the species and concentration of defects in the sample at ambient conditions. Overall, we show that defects can be engineered to lower the phase transformation onset pressure by ~50% in the ordered pyrochlore Dy2Ti2O7, and lower the phase transformation completion pressure by ~20% in the disordered pyrochlore Dy2Zr2O7. These improvements are achieved without significantly sacrificing mechanical integrity, as characterized by bulk modulus.
The structure and unconventional dihydrogen bonding of a pressure-stabilized hydrogen-rich (NH3BH3)(H-2)(x) (x=1.5) compound JOURNAL OF MATERIALS CHEMISTRY A Lin, Y., Welchman, E., Thonhauser, T., Mao, W. L. 2017; 5 (15): 7111-7117
View details for DOI 10.1039/c7ta01005b
Pressure-Induced Metallization of the Halide Perovskite (CH3NH3)PbI3 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Jaffe, A., Lin, Y., Mao, W. L., Karunadasa, H. I. 2017; 139 (12): 4330-4333
Dehydrogenation of goethite in Earth's deep lower mantle. Proceedings of the National Academy of Sciences of the United States of America Hu, Q., Kim, D. Y., Liu, J., Meng, Y., Yang, L., Zhang, D., Mao, W. L., Mao, H. 2017; 114 (7): 1498-1501
The cycling of hydrogen influences the structure, composition, and stratification of Earth's interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO2Hx Using a combination of theoretical simulations and high-pressure-temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositional range of P phase of 0.39 < x < 0.81, corresponding to 19-61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.
High-pressure behavior of A(2)B(2)O(7) pyrochlore (A=Eu, Dy; B=Ti, Zr) JOURNAL OF APPLIED PHYSICS Rittman, D. R., Turner, K. M., Park, S., Fuentes, A. F., Yan, J., Ewing, R. C., Mao, W. L. 2017; 121 (4)
Dynamic Optical Tuning of Interlayer Interactions in the Transition Metal Dichalcogenides. Nano letters Mannebach, E. M., Nyby, C. n., Ernst, F. n., Zhou, Y. n., Tolsma, J. n., Li, Y. n., Sher, M. J., Tung, I. C., Zhou, H. n., Zhang, Q. n., Seyler, K. L., Clark, G. n., Lin, Y. n., Zhu, D. n., Glownia, J. M., Kozina, M. E., Song, S. n., Nelson, S. n., Mehta, A. n., Yu, Y. n., Pant, A. n., Aslan, O. B., Raja, A. n., Guo, Y. n., DiChiara, A. n., Mao, W. n., Cao, L. n., Tongay, S. n., Sun, J. n., Singh, D. J., Heinz, T. F., Xu, X. n., MacDonald, A. H., Reed, E. n., Wen, H. n., Lindenberg, A. M. 2017; 17 (12): 7761–66
Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. This work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.
High-pressure compressibility and vibrational properties of (Ca,Mn)CO3 AMERICAN MINERALOGIST Liu, J., Caracas, R., Fan, D., Bobocioiu, E., Zhang, D., Mao, W. L. 2016; 101 (12): 2723-2730
View details for DOI 10.2138/am-2016-5742
Effects of Molecular Geometry on the Properties of Compressed Diamondoid Crystals JOURNAL OF PHYSICAL CHEMISTRY LETTERS Yang, F., Lin, Y., Baldini, M., Dahl, J. E., Carlson, R. M., Mao, W. L. 2016; 7 (22): 4641-4647
Diamondoids are an intriguing group of carbon-based nanomaterials, which combine desired properties of inorganic nanomaterials and small hydrocarbon molecules with atomic-level uniformity. In this Letter, we report the first comparative study on the effect of pressure on a series of diamondoid crystals with systematically varying molecular geometries and shapes, including zero-dimensional (0D) adamantane; one-dimensional (1D) diamantane, [121]tetramantane, [123]tetramantane, and [1212]pentamantane; two-dimensional (2D) [12312]hexamantane; and three-dimensional (3D) triamantane and [1(2,3)4]pentamantane. We find the bulk moduli of these diamondoid crystals are strongly dependent on the diamondoids' molecular geometry with 3D [1(2,3)4]pentamantane being the least compressible and 0D adamantane being the most compressible. These diamondoid crystals possess excellent structural rigidity and are able to sustain large volume deformation without structural failure even after repetitive pressure loading cycles. These properties are desirable for constructing cushioning devices. We also demonstrate that lower diamondoids outperform the conventional cushioning materials in both the working pressure range and energy absorption density.
Substantial tensile ductility in sputtered Zr-Ni-Al nano-sized metallic glass ACTA MATERIALIA Liontas, R., Jafary-Zadeh, M., Zeng, Q., Zhang, Y., Mao, W. L., Greer, J. R. 2016; 118: 270-285
View details for DOI 10.1016/j.actamat.2016.07.050
Pressure tuning the lattice and optical response of silver sulfide APPLIED PHYSICS LETTERS Zhao, Z., Wei, H., Mao, W. L. 2016; 108 (26)
High-pressure behavior of the polymorphs of FeOOH AMERICAN MINERALOGIST Reagan, M. M., Gleason, A. E., Daemen, L., Xiao, Y., Mao, W. L. 2016; 101 (5-6): 1483-1488
Pressure-dependent isotopic composition of iron alloys SCIENCE Shahar, A., Schauble, E. A., Caracas, R., Gleason, A. E., Reagan, M. M., Xiao, Y., Shu, J., Mao, W. 2016; 352 (6285): 580-582
Our current understanding of Earth's core formation is limited by the fact that this profound event is far removed from us physically and temporally. The composition of the iron metal in the core was a result of the conditions of its formation, which has important implications for our planet's geochemical evolution and physical history. We present experimental and theoretical evidence for the effect of pressure on iron isotopic composition, which we found to vary according to the alloy tested (FeO, FeH(x), or Fe3C versus pure Fe). These results suggest that hydrogen or carbon is not the major light-element component in the core. The pressure dependence of iron isotopic composition provides an independent constraint on Earth's core composition.
View details for DOI 10.1126/science.aad9945
High-Pressure Single-Crystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties. ACS central science Jaffe, A., Lin, Y., Beavers, C. M., Voss, J., Mao, W. L., Karunadasa, H. I. 2016; 2 (4): 201-209
We report the first high-pressure single-crystal structures of hybrid perovskites. The crystalline semiconductors (MA)PbX3 (MA = CH3NH3 (+), X = Br(-) or I(-)) afford us the rare opportunity of understanding how compression modulates their structures and thereby their optoelectronic properties. Using atomic coordinates obtained from high-pressure single-crystal X-ray diffraction we track the perovskites' precise structural evolution upon compression. These structural changes correlate well with pressure-dependent single-crystal photoluminescence (PL) spectra and high-pressure bandgaps derived from density functional theory. We further observe dramatic piezochromism where the solids become lighter in color and then transition to opaque black with compression. Indeed, electronic conductivity measurements of (MA)PbI3 obtained within a diamond-anvil cell show that the material's resistivity decreases by 3 orders of magnitude between 0 and 51 GPa. The activation energy for conduction at 51 GPa is only 13.2(3) meV, suggesting that the perovskite is approaching a metallic state. Furthermore, the pressure response of mixed-halide perovskites shows new luminescent states that emerge at elevated pressures. We recently reported that the perovskites (MA)Pb(Br x I1-x )3 (0.2 < x < 1) reversibly form light-induced trap states, which pin their PL to a low energy. This may explain the low voltages obtained from solar cells employing these absorbers. Our high-pressure PL data indicate that compression can mitigate this PL redshift and may afford higher steady-state voltages from these absorbers. These studies show that pressure can significantly alter the transport and thermodynamic properties of these technologically important semiconductors.
View details for DOI 10.1021/acscentsci.6b00055
In situ measurement of lithiation-induced stress in silicon nanoparticles using micro-Raman spectroscopy NANO ENERGY Zeng, Z., Liu, N., Zeng, Q., Lee, S. W., Mao, W. L., Cui, Y. 2016; 22: 105-110
View details for DOI 10.1016/j.nanoen.2016.02.005
General 2.5 power law of metallic glasses PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Zeng, Q., Lin, Y., Liu, Y., Zeng, Z., Shi, C. Y., Zhang, B., Lou, H., Sinogeikin, S. V., Kono, Y., Kenney-Benson, C., Park, C., Yang, W., Wang, W., Sheng, H., Mao, H., Mao, W. L. 2016; 113 (7): 1714-1718
Metallic glass (MG) is an important new category of materials, but very few rigorous laws are currently known for defining its "disordered" structure. Recently we found that under compression, the volume (V) of an MG changes precisely to the 2.5 power of its principal diffraction peak position (1/q1). In the present study, we find that this 2.5 power law holds even through the first-order polyamorphic transition of a Ce68Al10Cu20Co2 MG. This transition is, in effect, the equivalent of a continuous "composition" change of 4f-localized "big Ce" to 4f-itinerant "small Ce," indicating the 2.5 power law is general for tuning with composition. The exactness and universality imply that the 2.5 power law may be a general rule defining the structure of MGs.
High-pressure single-crystal structures of 3D lead-halide hybrid perovskites and pressure effects on their electronic and optical properties ACS Cent. Sci Jaffe, A., Lin, Y., Beavers, C. M., Voss, J., Mao, W. L., Karunadasa, H. I. 2016; 2: 201
Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Bell, E. A., Boehnke, P., Harrison, T. M., Mao, W. L. 2015; 112 (47): 14518-14521
Evidence of life on Earth is manifestly preserved in the rock record. However, the microfossil record only extends to ∼ 3.5 billion years (Ga), the chemofossil record arguably to ∼ 3.8 Ga, and the rock record to 4.0 Ga. Detrital zircons from Jack Hills, Western Australia range in age up to nearly 4.4 Ga. From a population of over 10,000 Jack Hills zircons, we identified one >3.8-Ga zircon that contains primary graphite inclusions. Here, we report carbon isotopic measurements on these inclusions in a concordant, 4.10 ± 0.01-Ga zircon. We interpret these inclusions as primary due to their enclosure in a crack-free host as shown by transmission X-ray microscopy and their crystal habit. Their δ(13)CPDB of -24 ± 5‰ is consistent with a biogenic origin and may be evidence that a terrestrial biosphere had emerged by 4.1 Ga, or ∼ 300 My earlier than has been previously proposed.
Fractal atomic-level percolation in metallic glasses SCIENCE Chen, D. Z., Shi, C. Y., An, Q., Zeng, Q., Mao, W. L., Goddard, W. A., Greer, J. R. 2015; 349 (6254): 1306-1310
Metallic glasses are metallic alloys that exhibit exotic material properties. They may have fractal structures at the atomic level, but a physical mechanism for their organization without ordering has not been identified. We demonstrated a crossover between fractal short-range (<2 atomic diameters) and homogeneous long-range structures using in situ x-ray diffraction, tomography, and molecular dynamics simulations. A specific class of fractal, the percolation cluster, explains the structural details for several metallic-glass compositions. We postulate that atoms percolate in the liquid phase and that the percolating cluster becomes rigid at the glass transition temperature.
View details for DOI 10.1126/science.aab1233
A Novel Phase of Li15Si4 Synthesized under Pressure ADVANCED ENERGY MATERIALS Zeng, Z., Zeng, Q., Liu, N., Oganov, A. R., Zeng, Q., Cui, Y., Mao, W. L. 2015; 5 (12)
View details for DOI 10.1002/aenm.201500214
Pressure-induced phase transition in MnCO3 and its implications on the deep carbon cycle JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH Boulard, E., Goncharov, A. F., Blanchard, M., Mao, W. L. 2015; 120 (6): 4069-4079
View details for DOI 10.1002/2015JB011901
Structural transition and amorphization in compressed alpha-Sb2O3 PHYSICAL REVIEW B Zhao, Z., Zeng, Q., Zhang, H., Wang, S., Hirai, S., Zeng, Z., Mao, W. L. 2015; 91 (18)
High pressure: Compressed hydrogen heats up. Nature materials Mao, W. L. 2015; 14 (5): 466-8
View details for DOI 10.1038/nmat4245
Strain-induced modification of optical selection rules in lanthanide-based upconverting nanoparticles. Nano letters Wisser, M. D., Chea, M., Lin, Y., Wu, D. M., Mao, W. L., Salleo, A., Dionne, J. A. 2015; 15 (3): 1891-1897
NaYF4:Yb(3+),Er(3+) nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the nonoptimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. Although hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly 2-fold increase in efficiency. In-situ X-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter.
View details for DOI 10.1021/nl504738k
Pressure-Induced Conductivity and Yellow-to-Black Piezochromism in a Layered Cu-Cl Hybrid Perovskite. Journal of the American Chemical Society Jaffe, A., Lin, Y., Mao, W. L., Karunadasa, H. I. 2015; 137 (4): 1673-1678
Pressure-induced changes in the electronic structure of two-dimensional Cu-based materials have been a subject of intense study. In particular, the possibility of suppressing the Jahn-Teller distortion of d(9) Cu centers with applied pressure has been debated over a number of decades. We studied the structural and electronic changes resulting from the application of pressures up to ca. 60 GPa on a two-dimensional copper(II)-chloride perovskite using diamond anvil cells (DACs), through a combination of in situ powder X-ray diffraction, electronic absorption and vibrational spectroscopy, dc resistivity measurements, and optical observations. Our measurements show that compression of this charge-transfer insulator initially yields a first-order structural phase transition at ca. 4 GPa similar to previous reports on other Cu(II)-Cl perovskites, during which the originally translucent yellow solid turns red. Further compression induces a previously unreported phase transition at ca. 8 GPa and dramatic piezochromism from translucent red-orange to opaque black. Two-probe dc resistivity measurements conducted within the DAC show the first instance of appreciable conductivity in Cu(II)-Cl perovskites. The conductivity increases by 5 orders of magnitude between 7 and 50 GPa, with a maximum measured conductivity of 2.9 × 10(-4) S·cm(-1) at 51.4 GPa. Electronic absorption spectroscopy and variable-temperature conductivity measurements indicate that the perovskite behaves as a 1.0 eV band-gap semiconductor at 39.7 GPa and has an activation energy for electronic conduction of 0.232(1) eV at 40.2 GPa. Remarkably, all these changes are reversible: the material reverts to a translucent yellow solid upon decompression, and ambient pressure powder X-ray diffraction data taken before and after compression up to 60 GPa show that the original structure is maintained with minimal hysteresis.
View details for DOI 10.1021/ja512396m
Strain-induced modification of optical selection rules in lanthanide-based upconverting nanoparticles Nano Letters Wisser, M., Chea, M., Lin, Y., Wu, D., Mao, W. L., Salleo, A., Dionne, J. 2015: 1891–97
Tetrahedrally coordinated carbonates in Earth's lower mantle. Nature communications Boulard, E., Pan, D., Galli, G., Liu, Z., Mao, W. L. 2015; 6: 6311-?
Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in the Earth's crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO3. Using ab-initio calculations, we assign the new infrared signature to C-O bands associated with tetrahedrally coordinated carbon with asymmetric C-O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low-pressure threefold coordinated carbonates, as well as different chemical properties in the liquid state. Hence, this may have significant implications for carbon reservoirs and fluxes, and the global geodynamic carbon cycle.
View details for DOI 10.1038/ncomms7311
Compressed hydrogen heats up Nature Materials Mao, W. L. 2015
Ultrafast visualization of crystallization and grain growth in shock-compressed SiO2. Nature communications Gleason, A. E., Bolme, C. A., Lee, H. J., Nagler, B., Galtier, E., Milathianaki, D., Hawreliak, J., Kraus, R. G., Eggert, J. H., Fratanduono, D. E., Collins, G. W., Sandberg, R., Yang, W., Mao, W. L. 2015; 6: 8191-?
Pressure- and temperature-induced phase transitions have been studied for more than a century but very little is known about the non-equilibrium processes by which the atoms rearrange. Shock compression generates a nearly instantaneous propagating high-pressure/temperature condition while in situ X-ray diffraction (XRD) probes the time-dependent atomic arrangement. Here we present in situ pump-probe XRD measurements on shock-compressed fused silica, revealing an amorphous to crystalline high-pressure stishovite phase transition. Using the size broadening of the diffraction peaks, the growth of nanocrystalline stishovite grains is resolved on the nanosecond timescale just after shock compression. At applied pressures above 18 GPa the nuclueation of stishovite appears to be kinetically limited to 1.4±0.4 ns. The functional form of this grain growth suggests homogeneous nucleation and attachment as the growth mechanism. These are the first observations of crystalline grain growth in the shock front between low- and high-pressure states via XRD.
Pressure induced metallization with absence of structural transition in layered molybdenum diselenide. Nature communications Zhao, Z., Zhang, H., Yuan, H., Wang, S., Lin, Y., Zeng, Q., Xu, G., Liu, Z., Solanki, G. K., Patel, K. D., Cui, Y., Hwang, H. Y., Mao, W. L. 2015; 6: 7312-?
Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.
High-pressure storage of hydrogen fuel: ammonia borane and its related compounds CHINESE SCIENCE BULLETIN Lin, Y., Mao, W. L. 2014; 59 (36): 5235-5240
Deviatoric stress-induced phase transitions in diamantane JOURNAL OF CHEMICAL PHYSICS Yang, F., Lin, Y., Dahl, J. E., Carlson, R. M., Mao, W. L. 2014; 141 (15)
The high-pressure behavior of diamantane was investigated using angle-dispersive synchrotron x-ray diffraction (XRD) and Raman spectroscopy in diamond anvil cells. Our experiments revealed that the structural transitions in diamantane were extremely sensitive to deviatoric stress. Under non-hydrostatic conditions, diamantane underwent a cubic (space group Pa3) to a monoclinic phase transition at below 0.15 GPa, the lowest pressure we were able to measure. Upon further compression to 3.5 GPa, this monoclinic phase transformed into another high-pressure monoclinic phase which persisted to 32 GPa, the highest pressure studied in our experiments. However, under more hydrostatic conditions using silicone oil as a pressure medium, the transition pressure to the first high-pressure monoclinic phase was elevated to 7-10 GPa, which coincided with the hydrostatic limit of silicone oil. In another experiment using helium as a pressure medium, no phase transitions were observed to the highest pressure we reached (13 GPa). In addition, large hysteresis and sluggish transition kinetics were observed upon decompression. Over the pressure range where phase transitions were confirmed by XRD, only continuous changes in the Raman spectra were observed. This suggests that these phase transitions are associated with unit cell distortions and modifications in molecular packing rather than the formation of new carbon-carbon bonds under pressure.
Deviatoric stress-induced phase transitions in diamantane. journal of chemical physics Yang, F., Lin, Y., Dahl, J. E., Carlson, R. M., Mao, W. L. 2014; 141 (15): 154305-?
Bandgap closure and reopening in CsAuI3 at high pressure PHYSICAL REVIEW B Wang, S., Kemper, A. F., Baldini, M., SHAPIRO, M. C., Riggs, S. C., Zhao, Z., Liu, Z., Devereaux, T. P., Geballe, T. H., Fisher, I. R., Mao, W. L. 2014; 89 (24)
Pressure induced second-order structural transition in Sr3Ir2O7 JOURNAL OF PHYSICS-CONDENSED MATTER Zhao, Z., Wang, S., Qi, T. F., Zeng, Q., Hirai, S., Kong, P. P., Li, L., PARK, C., Yuan, S. J., Jin, C. Q., Cao, G., Mao, W. L. 2014; 26 (21)
View details for DOI 10.1088/0953-8984/26/21/215402
Disproportionation of (Mg,Fe)SiO3 perovskite in Earth's deep lower mantle. Science Zhang, L., Meng, Y., Yang, W., Wang, L., Mao, W. L., Zeng, Q., Jeong, J. S., Wagner, A. J., Mkhoyan, K. A., Liu, W., Xu, R., Mao, H. 2014; 344 (6186): 877-882
The mineralogical constitution of the Earth's mantle dictates the geophysical and geochemical properties of this region. Previous models of a perovskite-dominant lower mantle have been built on the assumption that the entire lower mantle down to the top of the D″ layer contains ferromagnesian silicate [(Mg,Fe)SiO3] with nominally 10 mole percent Fe. On the basis of experiments in laser-heated diamond anvil cells, at pressures of 95 to 101 gigapascals and temperatures of 2200 to 2400 kelvin, we found that such perovskite is unstable; it loses its Fe and disproportionates to a nearly Fe-free MgSiO3 perovskite phase and an Fe-rich phase with a hexagonal structure. This observation has implications for enigmatic seismic features beyond ~2000 kilometers depth and suggests that the lower mantle may contain previously unidentified major phases.
View details for DOI 10.1126/science.1250274
Tuning the crystal structure and electronic states of Ag2Se: Structural transitions and metallization under pressure PHYSICAL REVIEW B Zhao, Z., Wang, S., Oganov, A. R., Chen, P., Liu, Z., Mao, W. L. 2014; 89 (18)
Universal fractional noncubic power law for density of metallic glasses. Physical review letters Zeng, Q., Kono, Y., Lin, Y., Zeng, Z., Wang, J., Sinogeikin, S. V., Park, C., Meng, Y., Yang, W., Mao, H., Mao, W. L. 2014; 112 (18): 185502-?
As a fundamental property of a material, density is controlled by the interatomic distances and the packing of microscopic constituents. The most prominent atomistic feature in a metallic glass (MG) that can be measured is its principal diffraction peak position (q_{1}) observable by x-ray, electron, or neutron diffraction, which is closely associated with the average interatomic distance in the first shell. Density (and volume) would naturally be expected to vary under compression in proportion to the cube of the one-dimensional interatomic distance. However, by using high pressure as a clean tuning parameter and high-resolution in situ techniques developed specifically for probing the density of amorphous materials, we surprisingly found that the density of a MG varies with the 5/2 power of q_{1}, instead of the expected cubic relationship. Further studies of MGs of different compositions repeatedly produced the same fractional power law of 5/2 in all three MGs we investigated, suggesting a universal feature in MG.
High Pressure Raman and X-ray Diffraction Study of [121] Tetramantane JOURNAL OF PHYSICAL CHEMISTRY C Yang, F., Lin, Y., Dahl, J. E., Carlson, R. M., Mao, W. L. 2014; 118 (14): 7683-7689
View details for DOI 10.1021/jp500431k
Phase transitions in metastable phases of silicon JOURNAL OF APPLIED PHYSICS Zeng, Z., Zeng, Q., Mao, W. L., Qu, S. 2014; 115 (10)
Strain derivatives of T-c in HgBa2CuO4+delta: The CuO2 plane alone is not enough PHYSICAL REVIEW B Wang, S., Zhang, J., Yan, J., Chen, X., Struzhkin, V., Tabis, W., Barisic, N., Chan, M. K., Dorow, C., Zhao, X., Greven, M., Mao, W. L., Geballe, T. 2014; 89 (2)
Five-dimensional visualization of phase transition in BiNiO3 under high pressure APPLIED PHYSICS LETTERS Liu, Y., Wang, J., Azuma, M., Mao, W. L., Yang, W. 2014; 104 (4)
Five-dimensional visualization of phase transition in BiNiO3 under high pressure. Applied physics letters Liu, Y., Wang, J., Azuma, M., Mao, W. L., Yang, W. 2014; 104 (4): 043108
Colossal negative thermal expansion was recently discovered in BiNiO3 associated with a low density to high density phase transition under high pressure. The varying proportion of co-existing phases plays a key role in the macroscopic behavior of this material. Here, we utilize a recently developed X-ray Absorption Near Edge Spectroscopy Tomography method and resolve the mixture of high/low pressure phases as a function of pressure at tens of nanometer resolution taking advantage of the charge transfer during the transition. This five-dimensional (X, Y, Z, energy, and pressure) visualization of the phase boundary provides a high resolution method to study the interface dynamics of high/low pressure phase.
Evidence for photo-induced monoclinic metallic VO2 under high pressure APPLIED PHYSICS LETTERS Hsieh, W., Trigo, M., Reis, D. A., Artioli, G. A., Malavasi, L., Mao, W. L. 2014; 104 (2)
Pressure-induced densification in GeO2 glass: A transmission x-ray microscopy study APPLIED PHYSICS LETTERS Lin, Y., Zeng, Q., Yang, W., Mao, W. L. 2013; 103 (26)
Elastic moduli of polycrystalline Li15Si4 produced in lithium ion batteries JOURNAL OF POWER SOURCES Zeng, Z., Liu, N., Zeng, Q., Ding, Y., Qu, S., Cui, Y., Mao, W. L. 2013; 242: 732-735
View details for DOI 10.1016/j.jpowsour.2013.05.121
High-pressure Raman spectroscopy of phase change materials APPLIED PHYSICS LETTERS Hsieh, W., Zalden, P., Wuttig, M., Lindenberg, A. M., Mao, W. L. 2013; 103 (19)
Formation of an interconnected network of iron melt at Earth's lower mantle conditions NATURE GEOSCIENCE Shi, C. Y., Zhang, L., Yang, W., Liu, Y., Wang, J., Meng, Y., Andrews, J. C., Mao, W. L. 2013; 6 (11): 971-975
View details for DOI 10.1038/NGEO1956
Nanoscale Elemental Sensitivity Study of Nd2Fe14B Using Absorption Correlation Tomography MICROSCOPY RESEARCH AND TECHNIQUE Kao, T. L., Shi, C. Y., Wang, J., Mao, W. L., Liu, Y., Yang, W. 2013; 76 (11): 1112-1117
Transmission X-ray microscopy (TXM) is a rapidly developing technique with the capability of nanoscale three dimensional (3D) real-space imaging. Combined with the wide range in energy tunability from synchrotron sources, TXM enables the retrieval of 3D microstructural information with elemental/chemical sensitivity that would otherwise be inaccessible. The differential absorption contrast above and below absorption edges has been used to reconstruct the distributions of different elements, assuming the absorption edges of the interested elements are fairly well separated. Here we present an "Absorption Correlation Tomography" (ACT) method based on the correlation of the material absorption across multiple edges. ACT overcomes the significant limitation caused by overlapping absorption edges, significantly expands the capabilities of TXM, and makes it possible for fully quantitative nano-scale 3D structural investigation with chemical/elemental sensitivity. The capability and robustness of this new methodology is demonstrated in a case study of an important type of rare earth magnet (Nd₂Fe₁₄B).
View details for DOI 10.1002/jemt.22273
Bonding and electronic changes in rhodochrosite at high pressure AMERICAN MINERALOGIST Farfan, G. A., Boulard, E., Wang, S., Mao, W. L. 2013; 98 (10): 1817-1823
View details for DOI 10.2138/am.2013.4497
Symmetrization driven spin transition in epsilon-FeOOH at high pressure EARTH AND PLANETARY SCIENCE LETTERS Gleason, A. E., Quiroga, C. E., Suzuki, A., Pentcheva, R., Mao, W. L. 2013; 379: 49-55
View details for DOI 10.1016/j.epsl.2013.08.012
Pressure-induced structural transitions and metallization in Ag2Te PHYSICAL REVIEW B Zhao, Z., Wang, S., Zhang, H., Mao, W. L. 2013; 88 (2)
Strength of iron at core pressures and evidence for a weak Earth's inner core NATURE GEOSCIENCE Gleason, A. E., Mao, W. L. 2013; 6 (7): 571-574
Sound velocities for hexagonally close-packed iron compressed hydrostatically to 136GPa from phonon density of states GEOPHYSICAL RESEARCH LETTERS Gleason, A. E., Mao, W. L., Zhao, J. Y. 2013; 40 (12): 2983-2987
View details for DOI 10.1002/grl.50588
Elastic and inelastic behavior of graphitic C3N4 under high pressure CHEMICAL PHYSICS LETTERS Jia, R., Amulele, G., Zinin, P. V., Odake, S., Eng, P., Khabashesku, V., Mao, W. L., Ming, L. C. 2013; 575: 67-70
View details for DOI 10.1016/j.cplett.2013.04.065
Three-dimensional coherent x-ray diffraction imaging of molten iron in mantle olivine at nanoscale resolution. Physical review letters Jiang, H., Xu, R., Chen, C. C., Yang, W., Fan, J., Tao, X., Song, C., Kohmura, Y., Xiao, T., Wang, Y., Fei, Y., Ishikawa, T., Mao, W. L., Miao, J. 2013; 110 (20): 205501
We report quantitative 3D coherent x-ray diffraction imaging of a molten Fe-rich alloy and crystalline olivine sample, synthesized at 6 GPa and 1800 °C, with nanoscale resolution. The 3D mass density map is determined and the 3D distribution of the Fe-rich and Fe-S phases in the olivine-Fe-S sample is observed. Our results indicate that the Fe-rich melt exhibits varied 3D shapes and sizes in the olivine matrix. This work has potential for not only improving our understanding of the complex interactions between Fe-rich core-forming melts and mantle silicate phases but also paves the way for quantitative 3D imaging of materials at nanoscale resolution under extreme pressures and temperatures.
Three-Dimensional Coherent X-Ray Diffraction Imaging of Molten Iron in Mantle Olivine at Nanoscale Resolution PHYSICAL REVIEW LETTERS Jiang, H., Xu, R., Chen, C., Yang, W., Fan, J., Tao, X., Song, C., Kohmura, Y., Xiao, T., Wang, Y., Fei, Y., Ishikawa, T., Mao, W. L., Miao, J. 2013; 110 (20)
The effect of composition on pressure-induced devitrification in metallic glasses APPLIED PHYSICS LETTERS Zeng, Q., Mao, W. L., Sheng, H., Zeng, Z., Hu, Q., Meng, Y., Lou, H., Peng, F., Yang, W., Sinogeikin, S. V., Jiang, J. 2013; 102 (17)
Single-crystal structure determination of (Mg,Fe)SiO3 postperovskite PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Zhang, L., Meng, Y., Dera, P., Yang, W., Mao, W. L., Mao, H. 2013; 110 (16): 6292-6295
Knowledge of the structural properties of mantle phases is critical for understanding the enigmatic seismic features observed in the Earth's lower mantle down to the core-mantle boundary. However, our knowledge of lower mantle phase equilibria at high pressure (P) and temperature (T) conditions has been based on limited information provided by powder X-ray diffraction technique and theoretical calculations. Here, we report the in situ single-crystal structure determination of (Mg,Fe)SiO3 postperovskite (ppv) at high P and after temperature quenching in a diamond anvil cell. Using a newly developed multigrain single-crystal X-ray diffraction analysis technique in a diamond anvil cell, crystallographic orientations of over 100 crystallites were simultaneously determined at high P in a coarse-grained polycrystalline sample containing submicron ppv grains. Conventional single-crystal structural analysis and refinement methods were applied for a few selected ppv crystallites, which demonstrate the feasibility of the in situ study of crystal structures of submicron crystallites in a multiphase polycrystalline sample contained within a high P device. The similarity of structural models for single-crystal Fe-bearing ppv (~10 mol% Fe) and Fe-free ppv from previous theoretical calculations suggests that the Fe content in the mantle has a negligible effect on the crystal structure of the ppv phase.
Pressure-induced symmetry breaking in tetragonal CsAuI3 PHYSICAL REVIEW B Wang, S., Hirai, S., Shapiro, M. C., Riggs, S. C., Geballe, T. H., Mao, W. L., Fisher, I. R. 2013; 87 (5)
Pressure-induced tetragonal-orthorhombic phase transitions in CeRuPO APPLIED PHYSICS LETTERS Hirai, S., Kamihara, Y., Wakatsuki, A., Matoba, M., Mao, W. L. 2013; 102 (5)
Novel pressure-induced phase transitions in Co3O4 APPLIED PHYSICS LETTERS Hirai, S., Mao, W. L. 2013; 102 (4)
Giant atomic displacement at a magnetic phase transition in metastable Mn3O4 PHYSICAL REVIEW B Hirai, S., dos Santos, A. M., SHAPIRO, M. C., Molaison, J. J., Pradhan, N., Guthrie, M., Tulk, C. A., Fisher, I. R., Mao, W. L. 2013; 87 (1)
Nanoprobes for Deep Carbon CARBON IN EARTH Mao, W. L., Boulard, E. 2013; 75: 423-448
View details for DOI 10.2138/rmg.2013.75.13
Lithographically fabricated gratings for the interferometric measurement of material shear moduli under extreme conditions JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B Gleason, A. E., Tiberio, R. C., Mao, W. L., Ali, S., Bolme, C. A., Lazicki, A., Bordonaro, G., Treichler, J., Genova, V., Eggert, J. H. 2012; 30 (6)
Pressure-induced tuning of a magnetic phase separation in Nd0.53Sr0.47MnO3 PHYSICAL REVIEW B Baldini, M., Ding, Y., Wang, S., Lin, Y., Tulk, C. A., dos Santos, A. M., Mitchell, J. F., Haskel, D., Mao, W. L. 2012; 86 (9)
Long-Range Ordered Carbon Clusters: A Crystalline Material with Amorphous Building Blocks SCIENCE Wang, L., Liu, B., Li, H., Yang, W., Ding, Y., Sinogeikin, S. V., Meng, Y., Liu, Z., Zeng, X. C., Mao, W. L. 2012; 337 (6096): 825-828
Solid-state materials can be categorized by their structures into crystalline (having periodic translation symmetry), amorphous (no periodic and orientational symmetry), and quasi-crystalline (having orientational but not periodic translation symmetry) phases. Hybridization of crystalline and amorphous structures at the atomic level has not been experimentally observed. We report the discovery of a long-range ordered material constructed from units of amorphous carbon clusters that was synthesized by compressing solvated fullerenes. Using x-ray diffraction, Raman spectroscopy, and quantum molecular dynamics simulation, we observed that, although carbon-60 cages were crushed and became amorphous, the solvent molecules remained intact, playing a crucial role in maintaining the long-range periodicity. Once formed, the high-pressure phase is quenchable back to ambient conditions and is ultra-incompressible, with the ability to indent diamond.
Bonding and structural changes in siderite at high pressure AMERICAN MINERALOGIST Farfan, G., Wang, S., Ma, H., Caracas, R., Mao, W. L. 2012; 97 (8-9): 1421-1426
High pressure nano-tomography using an iterative method 6th International Conference on the Study of Matter at Extreme Conditions (SMEC) Wang, J., Yang, W., Wang, S., Xiao, X., De Carlo, F., Liu, Y., Mao, W. L. AMER INST PHYSICS. 2012
Families of Superhard Crystalline Carbon Allotropes Constructed via Cold Compression of Graphite and Nanotubes PHYSICAL REVIEW LETTERS Niu, H., Chen, X., Wang, S., Li, D., Mao, W. L., Li, Y. 2012; 108 (13)
We report a general scheme to systematically construct two classes of structural families of superhard sp(3) carbon allotropes of cold-compressed graphite through the topological analysis of odd 5+7 or even 4+8 membered carbon rings stemmed from the stacking of zigzag and armchair chains. Our results show that the previously proposed M, bct-C(4), W and Z allotropes belong to our currently proposed families and that depending on the topological arrangement of the native carbon rings numerous other members are found that can help us understand the structural phase transformation of cold-compressed graphite and carbon nanotubes (CNTs). In particular, we predict the existence of two simple allotropes, R and P carbon, which match well the experimental x-ray diffraction patterns of cold-compressed graphite and CNTs, respectively, display a transparent wide-gap insulator ground state and possess a large Vickers hardness comparable to diamond.
Ultrafast pump-probe measurements of short small-polaron lifetimes in the mixed-valence perovskite Cs2Au2I6 under high pressures PHYSICAL REVIEW B Trigo, M., Chen, J., Jiang, M. P., Mao, W. L., Riggs, S. C., SHAPIRO, M. C., Fisher, I. R., Reis, D. A. 2012; 85 (8)
Effect of pressure and composition on lattice parameters and unit-cell volume of (Fe,Mg)SiO3 post-perovskite EARTH AND PLANETARY SCIENCE LETTERS Zhang, L., Meng, Y., Mao, W. L. 2012; 317: 120-125
Experimental and Theoretical Studies on a High Pressure Monoclinic Phase of Ammonia Borane JOURNAL OF PHYSICAL CHEMISTRY C Lin, Y., Ma, H., Matthews, C. W., Kolb, B., Sinogeikin, S., Thonhauser, T., Mao, W. L. 2012; 116 (3): 2172-2178
View details for DOI 10.1021/jp206726t
Crystal structures of (Mg1-x,Fe-x)SiO3 postperovskite at high pressures PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Yamanaka, T., Hirose, K., Mao, W. L., Meng, Y., Ganesh, P., Shulenburger, L., Shen, G., Hemley, R. J. 2012; 109 (4): 1035-1040
X-ray diffraction experiments on postperovskite (ppv) with compositions (Mg(0.9)Fe(0.1))SiO(3) and (Mg(0.6)Fe(0.4))SiO(3) at Earth core-mantle boundary pressures reveal different crystal structures. The former adopts the CaIrO(3)-type structure with space group Cmcm, whereas the latter crystallizes in a structure with the Pmcm (Pmma) space group. The latter has a significantly higher density (ρ = 6.119(1) g/cm(3)) than the former (ρ = 5.694(8) g/cm(3)) due to both the larger amount of iron and the smaller ionic radius of Fe(2+) as a result of an electronic spin transition observed by X-ray emission spectroscopy (XES). The smaller ionic radius for low-spin compared to high-spin Fe(2+) also leads to an ordered cation distribution in the M1 and M2 crystallographic sites of the higher density ppv structure. Rietveld structure refinement indicates that approximately 70% of the total Fe(2+) in that phase occupies the M2 site. XES results indicate a loss of 70% of the unpaired electronic spins consistent with a low spin M2 site and high spin M1 site. First-principles calculations of the magnetic ordering confirm that Pmcm with a two-site model is energetically more favorable at high pressure, and predict that the ordered structure is anisotropic in its electrical and elastic properties. These results suggest that interpretations of seismic structure in the deep mantle need to treat a broader range of mineral structures than previously considered.
Nanoscale diffraction imaging of the high-pressure transition in Fe1-xO APPLIED PHYSICS LETTERS Ding, Y., Cai, Z., Hu, Q., Sheng, H., Chang, J., Hemley, R. J., Mao, W. L. 2012; 100 (4)
Pressure induced stabilization of antiferromagnetic phase in Nd0.53Sr0.47MnO3 Physical Review B Baldini, M., Ding, Y., Wang, S., Lin, Y., Tulk, C. A., Santos, A. M. Dos, Haskel, D., Mao, W. L. 2012; 86
Effect of Compressive Strain on the Raman Modes of the Dry and Hydrated BaCe0.8Y0.2O3 Proton Conductor JOURNAL OF PHYSICAL CHEMISTRY C Chen, Q., Huang, T., Baldini, M., Hushur, A., Pomjakushin, V., Clark, S., Mao, W. L., Manghnani, M. H., Braun, A., Graule, T. 2011; 115 (48): 24021-24027
View details for DOI 10.1021/jp208525j
Amorphous Diamond: A High-Pressure Superhard Carbon Allotrope PHYSICAL REVIEW LETTERS Lin, Y., Zhang, L., Mao, H., Chow, P., Xiao, Y., Baldini, M., Shu, J., Mao, W. L. 2011; 107 (17)
Compressing glassy carbon above 40 GPa, we have observed a new carbon allotrope with a fully sp(3)-bonded amorphous structure and diamondlike strength. Synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced sp(2)-to-sp(3) bonding change, while x-ray diffraction confirmed the perseverance of noncrystallinity. The transition was reversible upon releasing pressure. Used as an indenter, the glassy carbon ball demonstrated exceptional strength by reaching 130 GPa with a confining pressure of 60 GPa. Such an extremely large stress difference of >70 GPa has never been observed in any material besides diamond, indicating the high hardness of this high-pressure carbon allotrope.
High-pressure EXAFS measurements of crystalline Ge using nanocrystalline diamond anvils PHYSICAL REVIEW B Baldini, M., Yang, W., Aquilanti, G., Zhang, L., Ding, Y., Pascarelli, S., Mao, W. L. 2011; 84 (1)
Application of a new composite cubic-boron nitride gasket assembly for high pressure inelastic x-ray scattering studies of carbon related materials REVIEW OF SCIENTIFIC INSTRUMENTS Wang, L., Yang, W., Xiao, Y., Liu, B., Chow, P., Shen, G., Mao, W. L., Mao, H. 2011; 82 (7)
We have developed a new composite cubic-boron nitride (c-BN) gasket assembly for high pressure diamond anvil cell studies, and applied it to inelastic x-ray scattering (IXS) studies of carbon related materials in order to maintain a larger sample thickness and avoid the interference from the diamond anvils. The gap size between the two diamond anvils remained ~80 μm at 48.0 GPa with this new composite c-BN gasket assembly. The sample can be located at the center of the gap, ~20 μm away from the surface of both diamond anvils, which provides ample distance to separate the sample signal from the diamond anvils. The high pressure IXS of a solvated C(60) sample was studied up to 48 GPa, and a pressure induced bonding transition from sp(2) to sp(3) was observed at 27 GPa.
Compressional, temporal, and compositional behavior of H-2-O-2 compound formed by high pressure x-ray irradiation JOURNAL OF CHEMICAL PHYSICS Kung, A., Goncharov, A. F., Zha, C. S., Eng, P., Mao, W. L. 2011; 134 (23)
X-ray irradiation was found to convert H(2)O at pressures above 2 GPa into a novel molecular H(2)-O(2) compound. We used optical Raman spectroscopy to explore the behavior of x-ray irradiated H(2)O samples as a function of pressure, time, and composition. The compound was found to be stable over a period of two years, as long as high pressure conditions (>2 GPa) were maintained. The Raman shifts for the H(2) and O(2) vibrons behaved differently from pure H(2) and O(2) as pressure was increased on the compound up to 70 GPa, indicating that it remains a distinct, molecular compound. Based on spectra taken from different locations in a single sample, it appears that multiple forms of the H(2)-O(2) compound exist. The structure and composition of the starting material plays an important role in compound formation, as we found that hydrogen-filled ice clathrate C(2) (H(2))H(2)O did not undergo the same dissociation as observed in ice VII upon x-ray irradiation until pressure was increased to above 10 GPa.
Long-Range Topological Order in Metallic Glass SCIENCE Zeng, Q., Sheng, H., Ding, Y., Wang, L., Yang, W., Jiang, J., Mao, W. L., Mao, H. 2011; 332 (6036): 1404-1406
Glass lacks the long-range periodic order that characterizes a crystal. In the Ce(75)Al(25) metallic glass (MG), however, we discovered a long-range topological order corresponding to a single crystal of indefinite length. Structural examinations confirm that the MG is truly amorphous, isotropic, and unstrained, yet under 25 gigapascals hydrostatic pressures, every segment of a centimeter-length MG ribbon devitrifies independently into a face-centered cubic (fcc) crystal with the identical orientation. By using molecular dynamics simulations and synchrotron x-ray techniques, we elucidate that the mismatch between the large Ce and small Al atoms frustrates the crystallization and causes amorphization, but a long-range fcc topological order still exists. Pressure induces electronic transition in Ce, which eliminates the mismatch and manifests the topological order by the formation of a single crystal.
Low temperature transport properties of Ce-Al metallic glasses JOURNAL OF APPLIED PHYSICS Zeng, Q. S., Rotundu, C. R., Mao, W. L., Dai, J. H., Xiao, Y. M., Chow, P., Chen, X. J., Qin, C. L., Mao, H., Jiang, J. Z. 2011; 109 (11)
Compressional Behavior of Bulk and Nanorod LiMn2O4 under Nonhydrostatic Stress JOURNAL OF PHYSICAL CHEMISTRY C Lin, Y., Yang, Y., Ma, H., Cui, Y., Mao, W. L. 2011; 115 (20): 9844-9849
View details for DOI 10.1021/jp112289h
Studying single nanocrystals under high pressure using an x-ray nanoprobe REVIEW OF SCIENTIFIC INSTRUMENTS Wang, L., Ding, Y., Patel, U., Yang, W., Xiao, Z., Cai, Z., Mao, W. L., Mao, H. 2011; 82 (4)
In this report, we demonstrate the feasibility of applying a 250-nm focused x-ray beam to study a single crystalline NbSe(3) nanobelt under high-pressure conditions in a diamond anvil cell. With such a small probe, we not only resolved the distribution and morphology of each individual nanobelt in the x-ray fluorescence maps but also obtained the diffraction patterns from individual crystalline nanobelts with thicknesses of less than 50 nm. Single crystalline diffraction measurements on NbSe(3) nanobelts were performed at pressures up to 20 GPa.
Persistence of Jahn-Teller Distortion up to the Insulator to Metal Transition in LaMnO3 PHYSICAL REVIEW LETTERS Baldini, M., Struzhkin, V. V., Goncharov, A. F., Postorino, P., Mao, W. L. 2011; 106 (6)
High pressure, low temperature Raman measurements performed on LaMnO3 up to 34 GPa provide the first experimental evidence for the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase. This result resolves the ongoing debate about the nature of the pressure driven insulator to metal transition (IMT), demonstrating that LaMnO3 is not a classical Mott insulator. The formation of domains of distorted and regular octahedra, observed from 3 to 34 GPa, sheds new light on the mechanism behind the IMT suggesting that LaMnO3 becomes metallic when the fraction of undistorted octahedra domains increases beyond a critical threshold.
Electronic Structure of Crystalline He-4 at High Pressures PHYSICAL REVIEW LETTERS Mao, H. K., Shirley, E. L., Ding, Y., Eng, P., Cai, Y. Q., Chow, P., Xiao, Y., Shu, J., Hemley, R. J., Kao, C., Mao, W. L. 2010; 105 (18)
Using inelastic x-ray scattering techniques, we have succeeded in probing the high-pressure electronic structure of helium at 300 K. Helium has the widest known valence-conduction band gap of all materials a property whose high-pressure response has been inaccessible to direct measurements. We observed a rich electron excitation spectrum, including a cutoff edge above 23 eV, a sharp exciton peak showing linear volume dependence, and a series of excitations and continuum at 26 to 45 eV. We determined the electronic dispersion along the Γ-M direction over two Brillouin zones, and provided a quantitative picture of the helium exciton beyond the simplified Wannier-Frenkel description.
High-pressure evolution of Fe2O3 electronic structure revealed by x-ray absorption PHYSICAL REVIEW B Wang, S., Mao, W. L., Sorini, A. P., Chen, C., Devereaux, T. P., Ding, Y., Xiao, Y., Chow, P., Hiraoka, N., Ishii, H., Cai, Y. Q., Kao, C. 2010; 82 (14)
Pressure-induced behavior of the hydrogen-dominant compound SiH4(H-2)(2) from first-principles calculations PHYSICAL REVIEW B Chen, X., Wang, S., Mao, W. L., Fu, C. L. 2010; 82 (10)
Size-Dependent Amorphization of Nanoscale Y2O3 at High Pressure PHYSICAL REVIEW LETTERS Wang, L., Yang, W., Ding, Y., Ren, Y., Xiao, S., Liu, B., Sinogeikin, S. V., Meng, Y., Gosztola, D. J., Shen, G., Hemley, R. J., Mao, W. L., Mao, H. 2010; 105 (9)
Y2O3 with particle sizes ranging from 5 nm to 1 μm were studied at high pressure using x-ray diffraction and Raman spectroscopy techniques. Nanometer-sized Y2O3 particles are shown to be more stable than their bulk counterparts, and a grain size-dependent crystalline-amorphous transition was discovered in these materials. High-energy atomic pair distribution function measurements reveal that the amorphization is associated with the breakdown of the long-rang order of the YO6 octahedra, while the nearest-neighbor edge-shared octahedral linkages are preserved.
Properties of polyamorphous Ce75Al25 metallic glasses PHYSICAL REVIEW B Zeng, Q. S., Struzhkin, V. V., Fang, Y. Z., Gao, C. X., Luo, H. B., Wang, X. D., Lathe, C., Mao, W. L., Wu, F. M., Mao, H., Jiang, J. Z. 2010; 82 (5)
Distortions and stabilization of simple-cubic calcium at high pressure and low temperature PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Mao, W. L., Wang, L., Ding, Y., Yang, W., Liu, W., Kim, D. Y., Luo, W., Ahuja, R., Meng, Y., Sinogeikin, S., Shu, J., Mao, H. 2010; 107 (22): 9965-9968
Ca-III, the first superconducting calcium phase under pressure, was identified as simple-cubic (sc) by previous X-ray diffraction (XRD) experiments. In contrast, all previous theoretical calculations showed that sc had a higher enthalpy than many proposed structures and had an imaginary (unstable) phonon branch. By using our newly developed submicrometer high-pressure single-crystal XRD, cryogenic high-pressure XRD, and theoretical calculations, we demonstrate that Ca-III is neither exactly sc nor any of the lower-enthalpy phases, but sustains the sc-like, primitive unit by a rhombohedral distortion at 300 K and a monoclinic distortion below 30 K. This surprising discovery reveals a scenario that the high-pressure structure of calcium does not go to the zero-temperature global enthalpy minimum but is dictated by high-temperature anharmonicity and low-temperature metastability fine-tuned with phonon stability at the local minimum.
Elastic anisotropy of ferromagnesian post-perovskite in Earth's D '' layer PHYSICS OF THE EARTH AND PLANETARY INTERIORS Mao, W. L., Meng, Y., Mao, H. 2010; 180 (3-4): 203-208
Effect of composition, structure, and spin state on the thermal conductivity of the Earth's lower mantle PHYSICS OF THE EARTH AND PLANETARY INTERIORS Goncharov, A. F., Struzhkin, V. V., Montoya, J. A., Kharlamova, S., KUNDARGI, R., Siebert, J., Badro, J., Antonangeli, D., Ryerson, F. J., Mao, W. 2010; 180 (3-4): 148-153
Nanoprobe measurements of materials at megabar pressures PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Wang, L., Ding, Y., Yang, W., Liu, W., Cai, Z., Kung, J., Shu, J., Hemley, R. J., Mao, W. L., Mao, H. 2010; 107 (14): 6140-6145
The use of nanoscale x-ray probes overcomes several key limitations in the study of materials up to multimegabar (> 200) pressures, namely, the spatial resolution of measurements of multiple samples, stress gradients, and crystal domains in micron to submicron size samples in diamond-anvil cells. Mixtures of Fe, Pt, and W were studied up to 282 GPa with 250-600 nm size synchrotron x-ray absorption and diffraction probes. The probes readily resolve signals from individual materials, between sample and gasket, and peak pressures, in contrast to the 5-microm-sized x-ray beams that are now becoming routine. The use of nanoscale x-ray beams also enables single-crystal x-ray diffraction studies in nominally polycrystalline samples at ultrahigh pressures, as demonstrated in measurements of (Mg,Fe)SiO(3) postperovskite. These capabilities have potential for driving a push toward higher maximum pressures and further miniaturization of high-pressure devices, in the process advancing studies at extreme conditions.
Origin of Pressure-Induced Polyamorphism in Ce75Al25 Metallic Glass PHYSICAL REVIEW LETTERS Zeng, Q., Ding, Y., Mao, W. L., Yang, W., Sinogeikin, S. V., Shu, J., Mao, H., Jiang, J. Z. 2010; 104 (10)
Using high-pressure synchrotron x-ray absorption spectroscopy, we observed the Ce 4f electron in Ce(75)Al(25) metallic glass transform from its ambient localized state to an itinerant state above 5 GPa. A parallel x-ray diffraction study revealed a volume collapse of about 8.6%, coinciding with 4f delocalization. The transition started from a low-density state below 1.5 GPa, went through continuous densification ending with a high-density state above 5 GPa. This new type of electronic polyamorphism in densely packed metallic glass is dictated by the Ce constituent, and is fundamentally distinct from the well-established structural polyamorphism in which densification is caused by coordination change and atomic rearrangement.
High-pressure EXAFS study of vitreous GeO2 up to 44 GPa PHYSICAL REVIEW B Baldini, M., Aquilanti, G., Mao, H., Yang, W., Shen, G., Pascarelli, S., Mao, W. L. 2010; 81 (2)
New Structure and Spin State of Iron-Rich (Mg,Fe)SiO3 Post-Perovskite Joint AIRAPT-22 and HPCJ-50 Conference/International Conference on High Pressure Science and Technology Yamanaka, T., Mao, W. L., Mao, H., Hemley, R. J., Shen, G. IOP PUBLISHING LTD. 2010
View details for DOI 10.1088/1742-6596/215/1/012100
Bonding in boranes and their interaction with molecular hydrogen at extreme conditions JOURNAL OF CHEMICAL PHYSICS Wang, S., Mao, W. L., Autrey, T. 2009; 131 (14)
The effects of high pressure and temperature on the bonding in ammonia borane (AB), NH(3)BH(3) and decaborane (DB), B(10)H(14) and their interactions with molecular hydrogen (H(2)) were investigated using Raman spectroscopy in a diamond anvil cell. At 0.7 GPa, AB becomes amorphous between 120 and 127 degrees C, indicating a positive Clapeyron slope. Heated to 140 degrees C, AB begins to undergo decomposition to polyaminoborane. The amorphous and decomposed AB does not recrystallize back to AB during slow cooling to room temperature or upon application of high pressure up to 3 GPa, underscoring the challenge of rehydrogenation of decomposed AB. The molecular Raman modes broaden in the reacted phase, and the NH(3) modes show no pressure dependence. DB was studied at room temperature up to 11 GPa. The observed frequency dependence with pressure (dnu/dP) and mode Gruneisen parameters varied for different spectral groups, and a new transition was identified at approximately 3 GPa. In both DB and heated AB, we found that they could store additional H(2) with the application of pressure. We estimate that we can store approximately 3 wt % H(2) in heated AB at 3 GPa and 1 wt % H(2) in DB at 4.5 GPa.
High pressure chemistry in the H-2-SiH4 system PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Wang, S., Mao, H., Chen, X., Mao, W. L. 2009; 106 (35): 14763-14767
Understanding the behavior of hydrogen-rich systems at extreme conditions has significance to both condensed matter physics, where it may provide insight into the metallization and superconductivity of element one, and also to applied research areas, where it can provide guidance for designing improved hydrogen storage materials for transportation applications. Here we report the high-pressure study of the SiH4-H2 binary system up to 6.5 GPa at 300 K in a diamond anvil cell. Raman measurements indicate significant intermolecular interactions between H2 and SiH4. We found that the H2 vibron frequency is softened by the presence of SiH4 by as much as 40 cm(-1) for the fluid with 50 mol% H2 compared with pure H2 fluid at the same pressures. In contrast, the Si-H stretching modes of SiH4 shift to higher frequency in the mixed fluid compared with pure SiH4. Pressure-induced solidification of the H2-SiH4 fluid shows a binary eutectic point at 72(+/-2) mol% H2 and 6.1(+/-0.1) GPa, above which the fluid crystallizes into a mixture of two nearly end-member solids. Neither solid has a pure end-member composition, with the silane-rich solid containing 0.5-1.5 mol% H2 and the hydrogen-rich solid containing 0.5-1 mol% SiH4. These two crystalline phases can be regarded as doped hydrogen-dominant compounds. We were able to superpressurize the sample by 0.2-0.4 GPa above the eutectic before complete crystallization, indicating extended metastability.
Storage of molecular hydrogen in an ammonia borane compound at high pressure PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Lin, Y., Mao, W. L., Mao, H. 2009; 106 (20): 8113-8116
We studied ammonia borane (AB), NH(3)BH(3), in the presence of excess hydrogen (H(2)) pressure and discovered a solid phase, AB(H(2))(x), where x approximately 1.3-2. The new AB-H(2) compound can store an estimated 8-12 wt % molecular H(2) in addition to the chemically bonded H(2) in AB. This phase formed slowly at 6.2 GPa, but the reaction rate could be enhanced by crushing the AB sample to increase its contact area with H(2). The compound has 2 Raman H(2) vibron peaks from the absorbed H(2) in this phase: one (nu(1)) at frequency 70 cm(-1) below the free H(2) vibron, and the other (nu(2)) at higher frequency overlapping with the free H(2) vibron at 6 GPa. The peaks shift linearly over the pressure interval of 6-16 GPa with average pressure coefficients of dnu(1)/dP = 4 cm(-1)/GPa and dnu(2)/dP = 6 cm(-1)/GPa. The formation of the compound is accompanied by changes in the N-H and B-H stretching Raman peaks resulting from the AB interactions with H(2) which indicate the structural complexity and low symmetry of this phase. Storage of significant amounts of additional molecular H(2) in AB increases the already high hydrogen content of AB, and may provide guidance for developing improved hydrogen storage materials.
X-ray diffraction studies and equation of state of methane at 202 GPa CHEMICAL PHYSICS LETTERS Sun, L., Yi, W., Wang, L., Shu, J., Sinogeikin, S., Meng, Y., Shen, G., Bai, L., Li, Y., Liu, J., Mao, H., Mao, W. L. 2009; 473 (1-3): 72-74
Substitutional alloy of Ce and Al PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Zeng, Q., Ding, Y., Mao, W. L., Luo, W., Blomqvist, A., Ahuja, R., Yang, W., Shu, J., Sinogeikin, S. V., Meng, Y., Brewe, D. L., Jiang, J., Mao, H. 2009; 106 (8): 2515-2518
The formation of substitutional alloys has been restricted to elements with similar atomic radii and electronegativity. Using high-pressure at 298 K, we synthesized a face-centered cubic disordered alloy of highly dissimilar elements (large Ce and small Al atoms) by compressing the Ce(3)Al intermetallic compound >15 GPa or the Ce(3)Al metallic glass >25 GPa. Synchrotron X-ray diffraction, Ce L(3)-edge absorption spectroscopy, and ab initio calculations revealed that the pressure-induced Kondo volume collapse and 4f electron delocalization of Ce reduced the differences between Ce and Al and brought them within the Hume-Rothery (HR) limit for substitutional alloying. The alloy remained after complete release of pressure, which was also accompanied by the transformation of Ce back to its ambient 4f electron localized state and reversal of the Kondo volume collapse, resulting in a non-HR alloy at ambient conditions.
High-pressure induced phase transitions of Y2O3 and Y2O3:Eu3+ APPLIED PHYSICS LETTERS Wang, L., Pan, Y., Ding, Y., Yang, W., Mao, W. L., Sinogeikin, S. V., Meng, Y., Shen, G., Mao, H. 2009; 94 (6)
Raman spectroscopy study of ammonia borane at high pressure JOURNAL OF CHEMICAL PHYSICS Lin, Y., Mao, W. L., Drozd, V., Chen, J., Daemen, L. L. 2008; 129 (23)
Ammonia borane, NH(3)BH(3), has attracted significant interest as a promising candidate material for hydrogen storage. The effect of pressure on the bonding in NH(3)BH(3) was investigated using Raman spectroscopy to over 20 GPa in a diamond anvil cell, and two new transitions were observed at approximately 5 and 12 GPa. Vibrational frequencies for the modes of the NH(3) proton donor group exhibited negative pressure dependence, which is consistent with the behavior of conventional hydrogen bonds, while the vibrational frequencies of the BH(3) proton acceptor group showed positive pressure dependence. The observed behavior of these stretching modes supports the presence of dihydrogen bonding at high pressure. In addition, the BH(3) and NH(3) bending modes showed an increase in spectral complexity with increasing pressure together with a discontinuity in d nu/d P which suggests rotational disorder in this molecule. These results may provide guidance for understanding and developing improved hydrogen storage materials.
Experimental determination of the elasticity of iron at high pressure JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH Mao, W. L., Struzhkin, V. V., Baron, A. Q., Tsutsui, S., Tommaseo, C. E., Wenk, H., Hu, M. Y., Chow, P., Sturhahn, W., Shu, J., Hemley, R. J., Heinz, D. L., Mao, H. 2008; 113 (B9)
Cubic to tetragonal phase transformation in cold-compressed Pd nanocubes NANO LETTERS Guo, Q., Zhao, Y., Mao, W. L., Wang, Z., Xiong, Y., Xia, Y. 2008; 8 (3): 972-975
Pd nanocubes with an average side length of approximately 10 nm were compressed up to 24.8 GPa in a diamond-anvil cell (DAC). In situ synchrotron X-ray diffraction was used to monitor structural changes, and a face-centered cubic (fcc) to face-centered tetragonal (fct) distortion was observed for the first time. This novel discovery not only provides new insights into the pressure-induced behavior of faceted nanocrystals of palladium and other noble metals but also gives guidance for finding new phases in close-packed metals.
View details for DOI 10.1021/nl0731217
Phase transformation in Sm2O3 at high pressure: In situ synchrotron X-ray diffraction study and ab initio DFT calculation SOLID STATE COMMUNICATIONS Guo, Q., Zhao, Y., Jiang, C., Mao, W. L., Wang, Z. 2008; 145 (5-6): 250-254
View details for DOI 10.1016/j.ssc.2007.11.019
Hydrogen storage in molecular clathrates CHEMICAL REVIEWS Struzhkin, V. V., Militzer, B., Mao, W. L., Mao, H., Hemley, R. J. 2007; 107 (10): 4133-4151
View details for DOI 10.1021/cr050183d
Pressure-induced cubic to monoclinic phase transformation in erbium sesquioxide Er2O3 INORGANIC CHEMISTRY Guo, Q., Zhao, Y., Jiang, C., Mao, W. L., Wang, Z., Zhang, J., Wang, Y. 2007; 46 (15): 6164-6169
Cubic Er(2)O(3) was compressed in a symmetric diamond anvil cell at room temperature and studied in situ using energy-dispersive X-ray diffraction. A transition to a monoclinic phase began at 9.9 GPa and was complete at 16.3 GPa and was accompanied by a approximately 9% volume decrease. The monoclinic phase was stable up to at least 30 GPa and could be quenched to ambient conditions. The normalized lattice parameter compression data for both phases were fit to linear equations, and the volume compression data were fit to third-order Birch-Murnaghan equations of state. The zero-pressure isothermal bulk moduli (B(0)) and the first-pressure derivatives (B(0)') for the cubic and monoclinic phases were 200(6) GPa and 8.4 and also 202(2) GPa and 1.0, respectively. Ab initio density functional theory calculations were performed to determine optimized lattice parameters and atom positions for the cubic, monoclinic, and hexagonal phases of Er(2)O(3). The calculated X-ray spectra and predicted transition pressure are in good qualitative agreement with the experimental results.
View details for DOI 10.1021/ic070154g
High-pressure/low-temperature neutron scattering of gas inclusion compounds: Progress and prospects PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Zhao, Y., Xu, H., Daemen, L. L., Lokshin, K., Tait, K. T., Mao, W. L., Luo, J., Currier, R. P., Hickmott, D. D. 2007; 104 (14): 5727-5731
Alternative energy resources such as hydrogen and methane gases are becoming increasingly important for the future economy. A major challenge for using hydrogen is to develop suitable materials to store it under a variety of conditions, which requires systematic studies of the structures, stability, and kinetics of various hydrogen-storing compounds. Neutron scattering is particularly useful for these studies. We have developed high-pressure/low-temperature gas/fluid cells in conjunction with neutron diffraction and inelastic neutron scattering instruments allowing in situ and real-time examination of gas uptake/release processes. We studied the formation of methane and hydrogen clathrates, a group of inclusion compounds consisting of frameworks of hydrogen-bonded H(2)O molecules with gas molecules trapped inside the cages. Our results reveal that clathrate can store up to four hydrogen molecules in each of its large cages with an intermolecular H(2)-H(2) distance of only 2.93 A. This distance is much shorter than that in the solid/metallic hydrogen (3.78 A), suggesting a strong densification effect of the clathrate framework on the enclosed hydrogen molecules. The framework-pressurizing effect is striking and may exist in other inclusion compounds such as metal-organic frameworks (MOFs). Owing to the enormous variety and flexibility of their frameworks, inclusion compounds may offer superior properties for storage of hydrogen and/or hydrogen-rich molecules, relative to other types of compounds. We have investigated the hydrogen storage properties of two MOFs, Cu(3)[Co(CN)(6)](2) and Cu(3)(BTC)(2) (BTC = benzenetricarboxylate), and our preliminary results demonstrate that the developed neutron-scattering techniques are equally well suited for studying MOFs and other inclusion compounds.
Pressure-induced distortive phase transition in chromite-spinel at 29 GPa Symposium on Materials Research at High Pressure held at the 2006 MRS Fall Meeting Shu, J., Mao, W. L., Hemley, R. J., Mao, H. MATERIALS RESEARCH SOCIETY. 2007: 179–184
Effect of iron on the properties of post-perovskite silicate, The Last Mantle Phase Transition Mao, W. L., Campbell , A. J., Prakapenka, V. B., Hemley , R. J., H-k. Mao , H. edited by Hirose, K., Brodholt, J., Lay, T., Yuen, D. American Geophysical Union. 2007: 37–46
Diamond Anvil Cells and Ultra-High P/T Experimental Methods Treatise on Geophysics Mao, H., Mao, W. L. edited by Price, G. D. Elsevier, Amsterdam. 2007: 231–267
High-P/Low-T Neutron Scattering of Hydrogen Inclusion Compounds-Progress and Prospects Proceedings of the National Academy of Sciences Zhao, Y., Xu, H., Daemen, L. L., Lokshin, K., Mao, W. L., Luo, J., Currier, R. P., Hickmott, D. D. 2007; 104: 5727-5731
Clathrate hydrates under pressure Physics Today Mao, W. L., Koh, C. A., Sloan, E. D. 2007; 60: 42-47
X-ray-induced dissociation of H2O and formation of an O-2-H-2 alloy at high pressure SCIENCE Mao, W. L., Mao, H., Meng, Y., Eng, P. J., Hu, M. Y., Chow, P., Cai, Y. Q., Shu, J., Hemley, R. J. 2006; 314 (5799): 636-638
When subjected to high pressure and extensive x-radiation, water (H2O) molecules cleaved, forming O-O and H-H bonds. The oxygen (O) and hydrogen (H) framework in ice VII was converted into a molecular alloy of O2 and H2. X-ray diffraction, x-ray Raman scattering, and optical Raman spectroscopy demonstrated that this crystalline solid differs from previously known phases. It remained stable with respect to variations in pressure, temperature, and further x-ray and laser exposure, thus opening new possibilities for studying molecular interactions in the hydrogen-oxygen binary system.
Ultrahigh-pressure experiment with a motor-driven diamond anvil cell JOURNAL OF PHYSICS-CONDENSED MATTER Mao, W. L., Mao, H. 2006; 18 (25): S1069-S1073
A Pt sample was compressed to ultrahigh pressures in a diamond anvil cell (DAC) using a motorized gearbox to change pressure remotely from outside the synchrotron x-ray hutch. In situ angle-dispersive x-ray diffraction (XRD) was used to determine pressure from known equations of state (EOS). The sample position was unperturbed during motor-driven pressure changes. By eliminating the need to realign the sample to the x-ray position after each pressure increment, 142 XRD patterns could be collected continuously over the course of three hours, and the maximum pressure of 230 GPa was reached before diamond failure ended the experiment. We demonstrate the advantages of this motor-driven assembly for smooth and efficient pressure change, and the possibility for fine pressure and temporal resolution.
View details for DOI 10.1088/0953-8984/18/25/S13
The effect of pressure on the structure and volume of ferromagnesian post-perovskite GEOPHYSICAL RESEARCH LETTERS Mao, W. L., Mao, H. K., Prakapenka, V. B., Shu, J. F., Hemley, R. J. 2006; 33 (12)
View details for DOI 10.1029/2006GL025770
Iron-rich post-perovskite and the origin of ultralow-velocity zones SCIENCE Mao, W. L., Mao, H. K., Sturhahn, W., Zhao, J. Y., Prakapenka, V. B., Meng, Y., Shu, J. F., Fei, Y. W., Hemley, R. J. 2006; 312 (5773): 564-565
The boundary layer between the crystalline silicate lower mantle and the liquid iron core contains regions with ultralow seismic velocities. Such low compressional and shear wave velocities and high Poisson's ratio are also observed experimentally in post-perovskite silicate phase containing up to 40 mol% FeSiO3 endmember. The iron-rich post-perovskite silicate is stable at the pressure-temperature and chemical environment of the core-mantle boundary and can be formed by core-mantle reaction. Mantle dynamics may lead to further accumulation of this material into the ultralow-velocity patches that are observable by seismology.
Phase relations of Fe-Ni alloys at high pressure and temperature PHYSICS OF THE EARTH AND PLANETARY INTERIORS Mao, W. L., Campbell, A. J., Heinz, D. L., Shen, G. Y. 2006; 155 (1-2): 146-151
Ultrahigh pressure experiment with a motor-driven diamond anvil cell Journal of Physics: Condensed Matter Mao, W., Mao, H. 2006; 18: S1069-S1073
Phase relations in Fe-Ni alloys at high pressure and temperature Physics of the Earth and Planetary Interiors Mao, W. L., Campbell, A. J., Heinz, D. L., Shen, G. 2006; 155: 146-151
Iron-rich silicates in the Earth's D '' layer PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Mao, W. L., Meng, Y., Shen, G. Y., Prakapenka, V. B., Campbell, A. J., Heinz, D. L., Shu, J. F., Caracas, R., Cohen, R. E., Fei, Y. W., Hemley, R. J., Mao, H. K. 2005; 102 (28): 9751-9753
High-pressure experiments and theoretical calculations demonstrate that an iron-rich ferromagnesian silicate phase can be synthesized at the pressure-temperature conditions near the core-mantle boundary. The iron-rich phase is up to 20% denser than any known silicate at the core-mantle boundary. The high mean atomic number of the silicate greatly reduces the seismic velocity and provides an explanation to the low-velocity and ultra-low-velocity zones. Formation of this previously undescribed phase from reaction between the silicate mantle and the iron core may be responsible for the unusual geophysical and geochemical signatures observed at the base of the lower mantle.
Pressure-temperature stability of the van der Waals compound (H-2)(4)CH4 CHEMICAL PHYSICS LETTERS Mao, W. L., Struzhkin, V. V., Mao, H. K., Hemley, R. J. 2005; 402 (1-3): 66-70
The stability and Raman spectra of ikaite, CaCO3•6H2O, at high pressure and temperature American Mineralogist Shahar, A., Bassett, W., Mao, H., Chou, I., Mao, W. 2005; 90: 1835-1839
Ferromagnesian postperovskite silicates in the D '' layer of the Earth PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Mao, W. L., Shen, G. Y., Prakapenka, V. B., Meng, Y., Campbell, A. J., Heinz, D. L., Shu, J. F., Hemley, R. J., Mao, H. K. 2004; 101 (45): 15867-15869
Natural olivine with 12 mol % Fe(2)SiO(4) and synthetic orthopyroxenes with 20% and 40% FeSiO(3) were studied beyond the pressure-temperature conditions of the core-mantle boundary. All samples were found to convert entirely or partially into the CaIrO(3) postperovskite structure, which was recently reported for pure MgSiO(3). The incorporation of Fe greatly reduces the pressure needed for the transition and establishes the new phase as the major component of the D'' layer. With the liquid core as an unlimited reservoir of iron, core-mantle reactions could further enrich the iron content in this phase and explain the intriguing seismic signatures observed in the D'' layer.
Structure and dynamics of hydrogen molecules in the novel clathrate hydrate by high pressure neutron diffraction PHYSICAL REVIEW LETTERS Lokshin, K. A., Zhao, Y. S., He, D. W., Mao, W. L., Mao, H. K., Hemley, R. J., Lobanov, M. V., Greenblatt, M. 2004; 93 (12)
The D2 clathrate hydrate crystal structure was determined as a function of temperature and pressure by neutron diffraction for the first time. The hydrogen occupancy in the (32+X)H2.136H(2)O, x=0-16 clathrate can be reversibly varied by changing the large (hexakaidecahedral) cage occupancy between two and four molecules, while remaining single occupancy of the small (dodecahedral) cage. Above 130-160 K, the guest D2 molecules were found in the delocalized state, rotating around the centers of the cages. Decrease of temperature results in rotation freezing followed by a complete localization below 50 K.
View details for DOI 10.1103/PhysRevLett.93.125503
Nuclear resonant x-ray scattering of iron hydride at high pressure GEOPHYSICAL RESEARCH LETTERS Mao, W. L., Sturhahn, W., Heinz, D. L., Mao, H. K., Shu, J. F., Hemley, R. J. 2004; 31 (15)
Hydrogen storage in molecular compounds PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Mao, W. L., Mao, H. K. 2004; 101 (3): 708-710
At low temperature (T) and high pressure (P), gas molecules can be held in ice cages to form crystalline molecular compounds that may have application for energy storage. We synthesized a hydrogen clathrate hydrate, H(2)(H(2)O)(2), that holds 50 g/liter hydrogen by volume or 5.3 wt %. The clathrate, synthesized at 200-300 MPa and 240-249 K, can be preserved to ambient P at 77 K. The stored hydrogen is released when the clathrate is warmed to 140 K at ambient P. Low T also stabilizes other molecular compounds containing large amounts of molecular hydrogen, although not to ambient P, e.g., the stability field for H(2)(H(2)O) filled ice (11.2 wt % molecular hydrogen) is extended from 2,300 MPa at 300 K to 600 MPa at 190 K, and that for (H(2))(4)CH(4) (33.4 wt % molecular hydrogen) is extended from 5,000 MPa at 300 K to 200 MPa at 77 K. These unique characteristics show the potential of developing low-T molecular crystalline compounds as a new means for hydrogen storage.
Phonon density of states and elastic properties of Fe-based materials under compression 3rd Nassau Mossbauer Conference on Development and Novel Application of the Technique to Science Struzhkin, V. V., Mao, H. K., Mao, W. L., Hemley, R. J., Sturhahn, W., Alp, E. E., L'abbe, C., Hu, M. Y., Errandonea, D. SPRINGER. 2004: 3–15
Generation of ultrahigh pressure using single-crystal chemical-vapor-deposition diamond anvils APPLIED PHYSICS LETTERS Mao, W. L., Mao, H. K., Yan, C. S., Shu, J. F., Hu, J. Z., Hemley, R. J. 2003; 83 (25): 5190-5192
Bonding changes in compressed superhard graphite SCIENCE Mao, W. L., Mao, H. K., Eng, P. J., Trainor, T. P., Newville, M., Kao, C. C., Heinz, D. L., Shu, J. F., Meng, Y., Hemley, R. J. 2003; 302 (5644): 425-427
Compressed under ambient temperature, graphite undergoes a transition at approximately 17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the pi-bonds between graphite layers convert to sigma-bonds, whereas the other half remain as pi-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form sigma-bonds, whereas the nonbridging carbon atoms remain unpaired with pi-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.
Displacive transition in magnesiowustite JOURNAL OF PHYSICS-CONDENSED MATTER Mao, W. L., Shu, J. F., Hu, J. Z., Hemley, R., Mao, H. 2002; 14 (44): 11349-11354
Hydrogen clusters in clathrate hydrate SCIENCE Mao, W. L., Mao, H. K., Goncharov, A. F., Struzhkin, V. V., Guo, Q. Z., Hu, J. Z., Shu, J. F., Hemley, R. J., Somayazulu, M., Zhao, Y. S. 2002; 297 (5590): 2247-2249
High-pressure Raman, infrared, x-ray, and neutron studies show that H2 and H2O mixtures crystallize into the sII clathrate structure with an approximate H2/H2O molar ratio of 1:2. The clathrate cages are multiply occupied, with a cluster of two H2 molecules in the small cage and four in the large cage. Substantial softening and splitting of hydrogen vibrons indicate increased intermolecular interactions. The quenched clathrate is stable up to 145 kelvin at ambient pressure. Retention of hydrogen at such high temperatures could help its condensation in planetary nebulae and may play a key role in the evolution of icy bodies.
Iron-nickel alloy in the Earth's Core Geophysical Research Letters Lin, J., Heinz, D., Campbell, A., Devine, J., Mao, W., Shen, G. 2002; 29