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

The crystal and magnetic structures of brownmillerite-like Sr2Co1.2Ga0.8O5 with a stable Co3+ 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) angstrom, b = 15.702(2) angstrom, c = 5.4543(6) angstrom; R-wp = 0.0339, R-p = 0.0443, chi(2) = 0.775). The very large tetragonal distortion of Co-O-6 octahedra (1.9591(4) angstrom for Co-O-eq and 2.257(6) angstrom for Co-O-ax) could be beneficial for the stabilization of the long-sought intermediate-spin state of Co3+ in perovskite-type oxides. However, the large magnetic moment of octahedral Co3+ (3.82(7)mu(B)) indicates the conventional high-spin state of Co3+ 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,L-3 edges on Sr2Co1.2Ga0.8O5. A high-spin ground state of Co3+ in Sr2Co1.2Ga0.8O5 resulted in much lower in comparison with a LaCoO3 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.