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) Å, b = 15.702(2) Å, c = 5.4543(6) Å; Rwp = 0.0339, Rp = 0.0443, χ2 = 0.775). The very large tetragonal distortion of CoO6 octahedra (1.9591(4) Å for Co–Oeq and 2.257(6) Å for Co–Oax) 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)μB) 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-L2,3 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.
- This article is part of the themed collection: Perovskites