Preparation and characterization of metastable trigonal layered MSb2O6 phases (M = Co, Ni, Cu, Zn, and Mg) and considerations on FeSb2O6.

MSb2O6 compounds (M = Mg, Co, Ni, Cu, Zn) are known in the tetragonal trirutile forms, slightly distorted monoclinically with M = Cu due to the Jahn-Teller effect. In this study, using a low-temperature exchange reaction between ilmenite-type NaSbO3 and molten MSO4-KCl (or MgCl2-KCl) mixtures, these five compositions were prepared for the first time as trigonal layered rosiaite (PbSb2O6)-type phases. Upon heating, they irreversibly transform to the known phases via amorphous intermediates, in contrast to previously studied isostructural MnSb2O6, where the stable phase is structurally related to the metastable phase. The same method was found to be applicable for preparing stable rosiaite-type CdSb2O6. The formula volumes of the new phases show an excellent correlation with the ionic radii (except for M = Cu, for which a Jahn-Teller distortion is suspected) and are 2-3% larger than those for the known forms although all coordination numbers are the same. The crystal structure of CoSb2O6 was refined via the Rietveld method: P3[combining macron]1m, a = 5.1318(3) Å, and c = 4.5520(3) Å. Compounds with M = Co and Ni antiferromagnetically order at 11 and 15 K, respectively, whereas the copper compound does not show long-range magnetic order down to 1.5 K. A comparison between the magnetic behavior of the metastable and stable polymorphs was carried out. FeSb2O6 could not be prepared because of the 2Fe2+ + Sb5+ = 2Fe3+ + Sb3+ redox reaction. This electron transfer produces an additional 5s2 shell for Sb and results in a volume increase. A comparison of the formula volume for the stable mixture FeSbO4 + 0.5Sb2O4 with that extrapolated for FeSb2O6 predicted that the trirutile-type FeSb2O6 can be stabilized at high pressures.

An unusual high-spin ground state of Co3+ in octahedral coordination in brownmillerite-type cobalt oxide.

The crystal and magnetic structures of brownmillerite-like Sr(2)Co(1.2)Ga(0.8)O(5) with a stable Co(3+) oxidation state at both octahedral and tetrahedral sites are refined using neutron powder diffraction data collected at 2 K (S.G. Icmm, a = 5.6148(6) Å, b = 15.702(2) Å, c = 5.4543(6) Å; R(wp) = 0.0339, R(p) = 0.0443, χ(2) = 0.775). The very large tetragonal distortion of CoO(6) octahedra (1.9591(4) Å for Co-O(eq) and 2.257(6) Å for Co-O(ax)) could be beneficial for the stabilization of the long-sought intermediate-spin state of Co(3+) in perovskite-type oxides. However, the large magnetic moment of octahedral Co(3+) (3.82(7)µ(B)) indicates the conventional high-spin state of Co(3+) ions, which is further supported by the results of a combined theoretical and experimental soft X-ray absorption spectroscopy study at the Co-L(2,3) edges on Sr(2)Co(1.2)Ga(0.8)O(5). A high-spin ground state of Co(3+) in Sr(2)Co(1.2)Ga(0.8)O(5) resulted in much lower in comparison with a LaCoO(3) linear thermal expansion coefficient of 13.1 ppm K(-1) (298-1073 K) determined from high-temperature X-ray powder diffraction data collected in air.

Temperature evolution of sodium nitrite structure in a restricted geometry.

The NaNO2 nanocomposite ferroelectric material in porous glass was studied by neutron diffraction. For the first time, the details of the crystal structure including positions and anisotropic thermal parameters were determined for the solid material, embedded in a porous matrix, in ferro- and paraelectric phases. It is demonstrated that in the ferroelectric phase the structure is consistent with bulk data, but above transition temperature the giant growth of amplitudes of thermal vibrations is observed, resulting in the formation of a "premelted state." Such a conclusion is in good agreement with the results of dielectric measurements published earlier.