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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2015-06, Vol.44 (23), p.178-1713
• Main Authors: Istomin, S. Ya, Tyablikov, O. A, Kazakov, S. M, Antipov, E. V, Kurbakov, A. I, Tsirlin, A. A, Hollmann, N, Chin, Y. Y, Lin, H.-J, Chen, C. T, Tanaka, A, Tjeng, L. H, Hu, Z
• Format: Article
• Language: English
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c4dt03670k

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. High-spin ground state of Co 3+ was found in brownmillerite-like Sr 2 Co 1.2 Ga 0.8 O 5 by low-temperature NPD and XAS study.