Structural studies of a mixed-valence state in the incommensurate composite crystal Sr_1.261CoO₃

The incommensurate modulated crystal structure of the hexagonal cobalt oxide Sr_1.261CoO₃ has been studied using a four-dimensional (4D) superspace profile analysis of neutron powder diffraction data. Sr_1.261CoO₃ is a composite crystal that consists of the [CoO₃] and [2Sr] subsystems. The [CoO₃] subsystem forms 1D chains that run parallel to the c-axis and consist of face-sharing CoO₆ polyhedra with octahedral (Oh) and trigonal prismatic (TP) coordinations. The structure analysis reveals that the [CoO₃] chains contain 73.9% Oh and 26.1% TP sites, and that the TP sites have longer Co-O bonds than the Oh sites: d_av. = 2.039(4)Å (TP) and 1.895(3)Å (Oh). The averaged Co bond valences are Co3.56(3)+ in the Oh sites and Co^2.45(3)+ in the TP sites, suggesting that a considerable amount of Co³⁺ ions mixed with Co⁴⁺ions in the Oh sites and with Co²⁺ ions in the TP sites. The observed magnetic susceptibility can be well explained assuming that the compound has the Co mixed-valence state with the spin configurations of S = 0 low-spin state for Co³⁺(dε⁶), S = 1/2 low-spin state for Co ⁴⁺(dε⁵) and S = 3/2 high-spin state for Co ²⁺(dε⁵). The Weiss temperature, approximately 0.8 K, implies that Sr_1.261CoO₃ naturally assumes a Curie paramagnetic state, probably owing to the obstruction of the intrachain magnetic interaction by the nonmagnetic Co³⁺ ions. These results suggest that the nonmagnetic Co³⁺ ions play an essential role in the magnetism of Sr_2γ CoO₃ systems.