An unusual high-spin ground state of Co super(3+) in octahedral coordination in brownmillerite-type cobalt oxide

The crystal and magnetic structures of brownmillerite-like Sr sub(2)Co sub(1.2)Ga sub( 0.8)O sub(5) with a stable Co super(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) Aa, b= 15.702(2) Aa, c= 5.4543(6) Aa; R sub(wp) = 0.0339, R sub(p) = 0.0443, chi super(2) = 0.775). The very large tetragonal distortion of CoO sub(6) octahedra (1.9591(4) Aa for Co-O sub(eq) and 2.257(6) Aa for Co-O sub(ax)) could be beneficial for the stabilization of the long-sought intermediate-spin state of Co super(3+) in perovskite-type oxides. However, the large magnetic moment of octahedral Co super(3+) (3.82(7) mu sub(B)) indicates the conventional high-spin state of Co super(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 sub(2,3) edges on Sr sub(2)Co sub(1.2)Ga sub( 0.8)O sub(5). A high-spin ground state of Co super(3+) in Sr sub(2)Co sub(1.2)Ga sub( 0.8)O sub(5) resulted in much lower in comparison with a LaCoO sub(3) linear thermal expansion coefficient of 13.1 ppm K super(-1) (298-1073 K) determined from high-temperature X-ray powder diffraction data collected in air.