1. Field of the Invention
This invention relates to oxynitride perovskites, and in particular to partially or fully ordered oxynitride perovskites of the general formula ABO2N that have a high polarization.
2. Description of Related Art
The first perovskite discovered was calcium titanium oxide (CaTiO3). The term “perovskite” is now used to describe a group of oxides that have similar structures to calcium titanium oxide and the general formula of ABO3. The parent, or high-temperature, crystal structure of an ABO3 perovskite is cubic, with an A-cation in the middle of the cube, a B-cation in the corner and the anion, commonly oxygen, in the center of the edges. The structure is stabilized by the six coordination of the B-cation (octahedron) and twelve coordination of the A cation. Thus, packing of the ions is such that the A and O ions together form a cubic close packed array, where the B ions occupy a quarter of the octahedral holes. Differences in ratio between the A and B cations can cause a number of different distortions in the structure, usually consisting of tilting of the octahedra, which leads to centrosymmetric or non-polar structures, or off-centering of the cations, leading to polar structures. Polar perovskite structures have the curious property that the central atom does not “touch” its coordination neighbors, in violation of Pauling's rules.
Perovskites have many uses such as e.g., in ferroelectrics, catalysts, sensors, and superconductors. In particular, perovskite oxides have been of great interest, as they appear to be an alternative to complementary-metal-oxide-semiconductor (C-MOS) gate dielectrics and dynamic random access memory (DRAM) storage capacitors. Many superconductors are based on the perovskite structure.
Given the interesting properties of perovskites, researchers have attempted to synthesize complex perovskites, which are variants of the general structural formula of ABO3. Such complex perovskites may contain two or more different B-site cations. This results in ordered and disordered variants. Most work has centered on variations in the cations, but the anions can also be varied. One of the latter variants gives an oxynitride perovskite, such as e.g., CaTaO2N or Na3WO3N.
U.S. Pat. No. 4,734,390 to Marchand et al. discloses non-ordered oxynitride perovskites of the general structure ABO3-nNn. A is a metal from Group IA, Group IIA, yttrium or the lanthanides; B is metal from Groups IVA to IB. In particular, A is selected from the group consisting of Li+, Na+, K+, Rb+, Cs+, Ag+, Tl+, Ca2+, Ba2+, Sr2+, Pb2+, Cd2+, Ln3+, Bi3+, Y3+, Th4+, U4+ and trans-U4+. B is selected from the group consisting of W6+, Re6+, Mo6+, Ta5+, Nb5+, Mo5+, W5+, Ti4+, Zr4+, Sn4+, Ge4+, Nb4+, Ta4+, Al3+, Ga3+, In3+, Tl3+, Fe3+, and Cr3+. Furthermore, n is equal to 1, 2, or 3. The cationic charge a of metal A and the cationic charge b of metal B have to satisfy the equations (i) a+b=6+n and (ii) a≧n. These oxynitride perovskites are synthesized via ammoniazation and sintering, which must be carried out at high temperature. This process, however, does not yield ordered, or even partially ordered, oxynitride perovskites.
In addition, Marchand et al. reportedly also prepared ATaO2N (A=Ca, Sr, Ba), AbNbO2N (A=Sr, Ba), AMON2 (A=La, Dy; M=Nb, Ta), ATiO2N (A=La, Yb) and LnWOxN3-x (Ln=La, Nd) (Grins et al. Material Research Bulletin 29(7): 801-809 (1994)).
U.S. Pat. No. 6,383,416 (the '416 patent) to Hamada et al. is directed to an electron-emitting material containing perovskite oxynitrides of the general formula MIMIIO2N and other components. U.S. Pat. No. 6,432,325 (the '325 patent) also to Hamada et al. (and filed on the same day as the '416 patent) discloses electrodes comprising electron-emitting materials, which contain MIMIIO2N type oxynitride perovskites and have restrained evaporation during electric discharge as well as a high resistance to ion sputtering.
The '416 patent and the '325 patent disclose identical electron emitting materials containing MIMIIO2N type oxynitride perovskites with identical components. As disclosed in both the '416 and the '325 patent, the electron-emitting material may also contain tantalum, zirconium, niobium, titanium, hafnium, and mixtures thereof. In addition, the material may also contain compounds of general formula MI4MIIO3, MI5MII4O15, MI7MII6O22, and MI6MIIMII4O18. MI is selected from barium (Ba), strontium (Sr) and calcium (Ca). MII is selected from tantalum (Ta), zirconium (Zr), niobium (Nb), titanium (Ti), and hafnium (Hf). In addition, the electron emitting material may contain magnesium (Mg), scandium (Sc), yttrium (Y), lanthanum (La), vanadium (V), chromium (Cr), molybdenum (Mo), tungsten (W), iron (Fe), nickel (Ni), or aluminum (Al). Preferably, this electron emitting material satisfies the equation 0.8≦X/Y≦1.5, wherein X and Y are the molar ratios of the first and second component (MI and MII), respectively. In the case of '416 patent, a range of 0.9≦X/Y≦1.2 is even more preferred. These compounds are produced by sintering and they are not ordered.
Clarke et al. (Chem. Mater. 14: 288-294 (2002)) and Kim et al. (Chem. Mater. 16: 1267-1276 (2004)) synthesized other oxynitride perovskites of the general formula MIIIMIVO2N.
None of these oxynitride perovskites has an ordered or a partially ordered structure; they all have disordered structures, and centrosymmetric space groups. Since these compounds are non-polar, they cannot be used as ferroelectrics, piezoelectrics, non-linear optics, or other polar applications.
There is a need for partially ordered and ordered oxynitride perovskites that advantageously exploit the perovskite structure or a variant thereof for practical application.