Hydrothermal synthesis of multi-cationic high-entropy layered double hydroxides

High-entropy materials are compositionally complex materials which often contain five or more elements. The most commonly studied materials in this field are alloys and oxides, where their composition allows for tunable materials properties. High-entropy layered double hydroxides have been recently touted as the next focus for the field of high-entropy materials to expand into. However, most previous work on multi-cationic layered double hydroxides has focused on syntheses with 5 or less cations in the structure. To bridge this gap into high-entropy materials, this work explores the range and extent of different compositional combinations for high-entropy double layered hydroxides. Specifically, pure layered double hydroxides were synthesized with different combinations of 7 cations (Mg, Co, Cu, Zn, Ni, Al, Fe, Cr) as well as one combination of 8 cations by utilizing a hydrothermal synthesis method. Furthermore, magnetic properties of the 8-cation LDH were investigated.


Introduction
In the last two decades, the concept of high-entropy materials has become a subject that has garnered increasing attention in the material science community. What has drawn particular interest is their tunable material properties across many different applications. Initially, alloys based on combinations of ve or more elements in near-equimolar compositions were designed. 1,2 The main idea behind these materials is to maximize congurational entropy in order to minimize the Gibbs free energy (DG ¼ DH À TDS), thus stabilizing a system towards robustness and tunability. Furthermore, the large number of elements incorporated in the structure may lead to cocktail effects resulting in unexpected materials properties that would not be otherwise exhibited by each element individually. 3 The eld of research on HEMs (HEMs -High Entropy Materials) has expanded signicantly and today includes not merely HEAs (HEAs -High Entropy Alloys), but also oxides, 4 diborides, 5 carbides, 6 nitrides, 7 polymers, 8 uorides 9 or suldes. 10 With the expanding portfolio of HEMs, there is conjecture in the HEM community 11,12 of other potential materials that could benet from high-entropy congurations. One such material class are hydroxides, specically layered double hydroxides (LDH). 13,14 These materials have also been referred to by several other names or abbreviations, including highentropy hydroxides (HEH), high-entropy layered hydroxides (HELH), or high-entropy layered double hydroxides (HE-LDH).
Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds (when obtained synthetically), are a large family of anionic clay materials, which can be represented by the formula [M 2 1Àx + M 3 x + (OH) 2 ]A n x/n $mH 2 O, where M is a metal and A is a n-valent anion. The structure of LDHs includes brucite-like sheets separated by anionic species and water. 14,15 LDHs with various cations substituted into the structure are widely used in many elds, such as catalysis, 16,17 biomedical science, 18,19 environmental remediation, 20,21 polymerization, 15 and electrochemistry. 22 Additionally, LDHs can be used as precursors for such oxides structures like spinel 23 where the incorporation of a few cations in the LDH can be used to tailor the spinel's nal chemistry and shape/size. [24][25][26] The expansion of high-entropy materials to LDHs is a logical one, as different transition metal cations can readily be substituted into their crystal structure. 13,14 However, highentropy layered double hydroxides remains largely unexplored since most attention has been on incorporating two or three cations rather than the number of incorporated cations relevant to high-entropy materials.
Recently a universal congurational entropy metric (labelled EM) has been proposed that accounts for complex crystal structures as well as multiple sub lattices. 27 The number of cations needed to be incorporated for high-entropy classication varies depending on the structure and the cation's sub lattice positions. With this metric, for a spinel structure, the high-entropy distinction only starts with 7 cation or morethus, were an LDH to be used as a precursor for a high-entropy spinel structure, it would require at least 7 cations. In order to be considered a high-entropy material directly, an LDH would need at least 8 cations.
Currently there have only been a few attempts to synthesize HE-LDHs. [28][29][30] However, the focus was on only 5 or 6 cations in the LDH structure 28,29 or on identifying the maximum number of cations incorporated. 30 Though these studies are limited, they show that LDHs with a large number of cations can act as electrocatalysts themselves 29 and also conrm that HE-LDHs can be used precursors to high-entropy oxides. 30 Since this eld of high-entropy layered double hydroxides is only just emerging and a promising one, strategies to synthesize them with relevant ranges of cation congurations are now needed. Therefore, this work focuses on the synthesis of multicationic layered double hydroxides with 7 or 8 cations by the hydrothermal method. This synthesis study is especially timely as this eld of high-entropy materials is expanded to layered double hydroxides.

Synthesis procedure
The synthesis procedure was adapted for multiple cations from a hydrothermal synthesis method originally for two cations (magnesium and aluminum). 26 The cations considered were Mg, Zn, Cu, Ni, Co (as divalent M(II) cations) and Al, Fe, Cr (as trivalent M(III) cations). The molar ratio of M(II) to M(III) cations was kept at 2. In all synthesis batches, a total of 0.005 moles of M(II) cations and a total of 0.0025 moles of M(III) cations were used. This means the molar contribution of each individual metal cation was scaled according to the number of cations of the same valence used in each synthesis formulation.
For each formulation, metal nitrates (weighed according to Table 1) and 2.25 grams of urea were dissolved into 31.25 ml of distilled water. The solution was stirred by magnetic stirrer for 30 minutes. Aerwards, the solutions were poured into stainless steel autoclaves with PTFE-lined vessels with a capacity of 50 ml. The autoclaves containing the solutions were placed in oven under static conditions at 150 C and 2 h for all syntheses unless otherwise noted. Crystallization times ranging from 1-4 h and temperatures ranging from 120-180 C were also used for a selected number of syntheses with 8 cations, to study these parameters' effect on the resulting crystalline phase. Aer the completion of the crystallization time, the autoclaves were cooled to room temperature. The solid fraction was collected by centrifuge and washed three times with deionized water. The obtained powder was le to dry in air in an oven at 120 C for 16 hours.

Materials and reagents
Eight different nitrates (Sigma-Aldrich and Fluka, all at least 97% purity) were used as raw materials, without additional

Solution with 8 cations: synthesis condition variation and resulting crystalline phases
The crystallization time and temperature of 2 h at 150 C specied in Section 2.2 were arrived at through a synthesis optimization procedure, where crystallization times of 1-4 h and crystallization temperatures of 120, 150, 180 C were tested with the 8-cation formulation listed in Table 1, to monitor the inuence of these conditions on the resulting crystal structure. In order to monitor the formation of layered double hydroxides as well as secondary crystalline phases, an X-ray powder diffraction was performed on all synthesized samples. Experimental and equipment details can be found in the ESI. † Fig. 1 (le) shows the inuence of crystallization temperature on the formation of LDHs when 8 cations are present in the starting solution. All patterns exhibit the (003), (006), (009), (012), (015) (018), (110), and (113) characteristic reections of a layered double hydroxide phase; 31,32 however, at a crystallization temperature of 180 C and crystallization time of 2 hours, a secondary carbonate phase starts to form. Conversely at lower crystallization temperatures, reections for the LDH structure appear broad with low intensity, likely due to low crystallinity. In Fig. 1 (right), the crystallization time and temperature were both varied in order to nd synthesis conditions which produced pure LDH for each crystallization temperature. Overall, LDHs from 8-cation solutions can be synthesized within a considerable range of crystallization temperatures and times common in hydrothermal synthesis. Pure and crystalline LDH could be observed for temperature range of 150-180 C, though the time needed was longer at 150 C, needing 2 h, compared to 1 h at 180 C.

Solutions with 7 cations: synthesis and resulting crystalline phases
The crystallization temperature of 150 C and time of 2 hours were selected to perform the syntheses of the series of 7-cation solutions. The '-' in Table 1 represent which cation from the 8 cations was not included in each 7-cation solution, and Fig. 2 shows the X-ray powder diffraction patterns for all 7-cation syntheses. All of these led to LDH as the major crystalline phase.

Characterization of 8-cation LDH and 7-cation LDH
3.3.1 Morphology. Platy morphology is oen observed for the hydrothermal synthesis of hydrotalcite-like structure with only two cation (Al and Mg), 32 and it appears that even at 7 or more cations, this platy morphology also develops. Fig. 3 shows the morphology of the 8-cation LDH sample as observed by scanning electron microscope. The plates appear to be heavily agglomerated. To estimate the crystallite size, the Scherrer equation was used for [003] reection, and the crystallites were estimated to be about 14 nm.
Similar morphology of agglomerated platelets were observed for all the 7-cation formulations. It appears these LDHs heavily favor the formation of platelets despite the varying number of cations, chemistries, or synthesis conditions.

Chemical analysis.
To further understand the chemical nature of this multi-cationic LDH structure, SEM/EDS and X-ray absorption were conducted on the 8-cation LDH.
One criterion for high entropy materials is that elements are homogeneously distributed throughout the structure. To conrm homogeneous distribution, EDS elemental mapping was conducted. Fig. 4 shows SEM EDS maps of the 8-cation LDH. No tendency of segregation of cations was observed for the 8-cation LDH.
Atomic concentrations for each element were also estimated by EDS. Ideally elements of similar oxidation states should be equimolar, in order to increase the congurational entropy. Table 2 shows atomic concentrations of elements for the 8cation LDH and all 7-cation LDHs. Measurements were taken on ve different particles and averaged. Across all formulations, zinc and copper were found to exhibit the poorest incorporation, while aluminum and magnesium have slightly higher incorporation than expected. Of the M(II) cations, nickel and cobalt mostly exhibit the expected amounts and are equimolar to each other; similarly, for the M(III) cations, iron and chromium are as expected in concentration and are equimolar to each other. Of all the samples, 7cat-H (Mg, Cu, Zn, Ni, Co, Al, Fe) is the closest of the samples to achieving an equimolar distribution, but even though the copper amount is higher than in the other 7-cation formulations, it still below the amount needed for equimolarity. The nal M(II) and M(III) ratio is slightly lower than the starting M(II) and M(III) ratio (1.3-1.6 rather than 2). This discrepancy is likely due to the poor incorporation of copper and zinc. The remaining liquid from the synthesis was a brown/bluish color and likely contained cations that were not fully incorporated in the LDH. There appears to be preference for incorporating certain cations over others.
The transition metals substituted into the LDHs in this study can exist in many different oxidation states. To conrm the oxidation state of the transition metals (Fe, Cr, Co, Ni, Zn and Cu), and evaluate the effect of the removal of specic cations from the structure on oxidation states, X-ray absorption spectroscopy (XAS) was performed on the 8-cation LDH and one 7cation LDH (7cat-H with Mg, Zn, Cu, Co, Ni, Al, Fe). The edge position in the XANES region of the XAS spectra can indicate the oxidation state. Since the oxidation state is known for the nitrate precursors, these were used as a reference for the oxidation state in the LDHs. Analysis of XAS data was conducted on the demeter soware. 33 The edge positions for the LDH samples were not signicantly shied from the nitrate references as shown in Fig. S8. † Therefore, there appears to be no oxidation-reduction chemistry occurring during the synthesis to accommodate the multitude of cations into the structure. Fig. 5 shows the EXAFS spectra k 3 c(k) and the real space c(R) for Cu, Zn, Ni, Co, Cr, and Fe. For all probed cations, the c(R) spectrum is consistent with a layered double hydroxide system. The rst peak (1-2Å) corresponds to the scattering between the cation absorber and the nearest oxygen neighbour, and the second peak (2.5-3.5Å) corresponds to the scattering between the cation absorber and the second shell that is populated by other cations. The EXAFS spectra suggest that the nickel, cobalt, and zinc have similar scattering environments. The similar local geometry of nickel, cobalt, and zinc in the LDH structure is further corroborated by tting of the rst shell where obtained coordination numbers are 6.1 AE 0.7, 6.0 AE 0.8, and 6.0 AE 0.7 respectively, and the distances between each probed cation and nearest oxygen was 2.05Å AE 0.01, 2.06Å AE 0.01, and 2.03Å AE 0.02, respectively. Of the M(II) cations, copper is the outlier with a distinctly different EXAFS  spectrum and initial ttings showing a lower than expected coordination number (3.9 AE 0.4), and indicate possible Jahn-Teller distortion. [34][35][36] The EXAFS spectra for chromium and iron show that these M(III) cations have a local geometry different from the M(II) cations. The rst shell scattering is shorter for M(III) than for M(II) cations, with the nearest oxygen neighbour for iron and chromium being 1.98Å AE 0.02 and 1.99Å AE 0.02, respectively. This different in distances between M(II) and M(III) cations has also been observed from LHDs with two or three cations incorporated as well. 37 However, the coordination of the iron and chromium is lower than expected with 5.5 AE 0.9 and 5.0 AE 0.9, respectively.
The 7-cation sample (7cat-H) shows generally similar coordination numbers and distances for rst shell scattering events as the 8-cation LDH. High resolution X-ray powder diffraction was also conducted on both the 8-cation and 7cat-H samples. The calculated lattice parameters are summarized in Table 3 where the unit cell parameter c (¼3d 003 ) corresponds to three times the interlayer distance and a (¼2d 110 ) is the average distance between the two cations. Generally little geometry and structural differences were observed between the 7-cation (7cat-  H) and 8-cation systems by XRD and XAS. The multitude of incorporated cations could dampen any changes in the local structure when specic cations are omitted or included.

Magnetic properties
In order to investigate the magnetic properties of the 8-cation sample, we performed DC magnetization measurements by collecting temperature-dependent zero eld cooling (ZFC) and eld cooling (FC) magnetization M(T) data under various external magnetic elds (Fig. 6a) and by measuring magnetization M(H) loops at different temperatures (Fig. 6b). From Fig. 6a and b, it can be observed that the 8-cation LDH shows a rather complex magnetic response. For M(T) measured between 5 and 90 K, a separation between ZFC and FC can be observed below T z 45 K when exposed to magnetic elds m 0 H # 0.1 T, indicative of a history-dependent behavior. In high magnetic elds (i.e., m 0 H z 1 T), however, the M(T) curve appears to fully overlap in the measured temperature range and within the resolution limit of the measurement. In Fig. 6b, we show corresponding magnetization M(H) loops taken at 5, 15, 30, 60, and 90 K, respectively. Above z30 K, the M(H) curves display almost a linear magnetic eld dependence without any hysteresis signs, suggesting a paramagnetic behavior. A hysteretic behavior appears at temperatures #30 K, with coercivity ranging from z0.019 T at 5 K to z0.006 T at 30 K (inset of Fig. 6b), respectively, reminiscent of a ferromagnetic-like behavior.
Because of the observed paramagnetic to ferromagnetic-like behavior transition, additional time-dependent magnetization experiments were performed on the same 8-cation LDH. These included AC magnetic susceptibility measurements with AC frequencies between 1 and 300 Hz in absence of a static external magnetic eld, as well as aging and memory experiments for the DC magnetization, as shown in Fig. 6c. A frequency-dependent behavior is observed in the temperature-dependent real part of the AC susceptibility c 0 (T) at temperatures ranging from z10 to z60 K (Fig. 6c). Moreover, a plateau can be observed, the shoulder of which shis with increasing frequency from z33 to z40 K, while its amplitude decrease gradually, which may suggest a possible spin-glass freezing behavior.
The frequency dependence of the c 0 (T) plateaus in the AC magnetization measurements were further analyzed and tted to a critical dynamical scaling model 38 as well as the Vogel-Fulcher model. 39 The ts can be found in the inset of Fig. 6c, and the tting parameters can be found in the ESI. † The tting results allowed to further corroborate the suggested spin-glass of the 8-cation LDH with a freezing temperature T s of z30 K.
Memory and aging effects are additional experimental signatures of a spin-glass behavior in magnetic materials. 40 Therefore, FC memory and aging effects were studied by intermittent-stop cooling (ISC) and continuous warming (CW) as shown in Fig. 6d. In the spin-glass state, a magnetization shi can be observed when the cooling is resumed (Fig. 6d). In the CW process, the sample is continuously heated up from 5 to  90 K under a constant magnetic eld (5 mT) following an ISC run. The CW magnetization M(T) exhibits kink-like features at the previous intermittent-stopping temperatures (below the spin-glass freezing temperature) suggesting a memory effect characteristic for many spin-glasses. [40][41][42] In spin-glass states, spin dynamics are known to slow down whilst approaching the freezing temperature T s , impeding the magnetization recovery and provoking measurable magnetization steps in ISC curves. 43 Following this discussion, it is worth noting that superconductivity has been previously observed in hydroxide materials, albeit only under high mechanical pressure. 44 Since mechanical and chemical pressures similarly allow changing the Fermi surface of the sample, it may be suggested that a high chemical pressure may be reached via compositional changes and/or high-entropy structures which may allow reaching superconductivity. Note also that in some superconductive systems, spinglass states and superconductor states are not independent, meaning that via elemental and/or structural tuning a shi from a spin-glass to a superconductor state may be achievable. More research is therefore needed in order to corroborate whether HE-LDH may be able to become superconductive.

HE-LDHs in the context of highentropy materials
Recently different nomenclature has emerged to categorize high-entropy materials, specically in reference to oxides, where materials have been labeled as entropy-stabilized oxides (ESO) and/or high-entropy oxides (HEO). Although these terms had oen been used interchangeably, there has been recent discussion of clarifying these denitions. 27,[45][46][47] Since multicationic LDHs are only just emerging, it is pertinent to ask how they t generally into the high-entropy materials discussion.
Entropy-stabilized materials possess congurational entropy that drives the stabilization of a single phase and oen have equivalent concentration of all cations. The most elegant examples of these materials are entropy-stabilized rock salt 4 and a uorite oxide, 45 where reversibility of multiple phases to a single phase can be observed during different annealing temperatures. In the case of HE-LDHs, it would be difficult to conrm entropy stabilization using this technique, as LDHs are not stable at elevated temperatures due to its very nature. No loops with mag magnetic fields ranging from À7 to 7 T in temperatures between 5 and 90 K, inset of (b) an expanded scale to show the hysteretic behavior in low magnetic fields. (c) Real part (c 0 ) of the AC susceptibility as a function of temperature and frequency from 1 Hz to 300 Hz, inset of (c) measurement period (s ¼ 1/2pf where f is the frequency of applied magnetic field) vs. peak temperatures of the real part of the AC magnetic susceptibility c 0 ; (d) aging and memory measurements according to intermittent-stop cooling (ISC) and continuous warming-up (CW) processes, respectively, in an external field of 5 mT, with a waiting time in the ISC process of 5000 s, inset of (b) the time dependence of the magnetic moment (triangle) at 25 K after switching off the magnetic field. thermal stability advantages were observed in the 8-cation LDH in comparison with hydrotalcite (2 cations incorporated) during thermogravimetric analysis (ESI, Fig. S7 †).
Multicationic LDHs will likely not fall into the entropystabilized category of high-entropy materials from the high temperature criterion, which however may not rightfully be applicable here. The eld of HE-LDHs, however, should in any case fall in the broader denition of high-entropy materials, where the number of different cations and their concentrations (equivalent or non-equivalent to each other) contribute to the increased congurational entropy leading to interesting properties from cocktail effects and/or lattice distortions. Additionally, the role of high entropy in precursors to high-entropy oxides is yet to be fully explored, and may prove to have advantages in reducing calcination and sintering temperatures as well as expand the range of elements incorporated into an oxide structure.

Conclusions
In this work, we present a hydrothermal synthesis method for obtaining high-entropy layered double hydroxides, which represent an emerging branch of high-entropy materials.
A total of eight formulations using 7 or 8 cations (Mg, Cu, Zn, Co, Ni, Al, Fe, Cr) were successfully synthesized, with the incorporated elements showing a homogeneous distribution in the structure with oxidation state inherited from the metal salts. The 7-cation LDHs incorporated a wide range of different chemistries, highlighting the tailorability of this material class and opening the door for synthesis of further materials. Considering the number of possible combinations as well as the number of cations and their nature, many types of high-entropy hydroxides can be synthesized with properties still waiting to be investigated. The present study provides an important foundational synthesis of these materials, a required rst step on the way to future investigations and applications.