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

Abstract

The crystal and magnetic structures of brownmillerite-like Sr₂Co_1.2Ga_0.8O₅ 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) Å, b = 15.702(2) Å, c = 5.4543(6) Å; R_wp = 0.0339, R_p = 0.0443, χ² = 0.775). The very large tetragonal distortion of CoO₆ 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 Co³⁺ in perovskite-type oxides. However, the large magnetic moment of octahedral Co³⁺ (3.82(7)μ_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_2,3 edges on Sr₂Co_1.2Ga_0.8O₅. A high-spin ground state of Co³⁺ in Sr₂Co_1.2Ga_0.8O₅ 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.