Crystal Growth and Structure Determination of Oxygen-Deficient Sr₆Co₅O₁₅

Large single crystals of oxygen-deficient Sr₆Co₅O_15-δ compounds, i.e., Sr₆Co₅O_14.70 and Sr₆Co_4.9Ni_0.1O_14.36, were obtained by using K₂CO₃ flux in the presence of additives of transition metal oxides. The single-crystal structure determination shows that the structures of Sr₆Co₅O_14.70 and Sr₆Co_4.9Ni_0.1O_14.36 crystallize in the space group R3̄ and can be described as one-dimensional face-sharing CoO₃ polyhedral chains and Sr cation chains. Unlike the other known 2H-perovskite-related oxides in which the polyhedral chains consist of octahedra (Oh) and trigonal prism (TP), the structure of Sr₆Co₅O_14.70 and Sr₆Co_4.9Ni_0.1O_14.36 contain Oh and intermediate polyhedra (IP) and can be attributed to a general structure formula A₆A^‘₂B₃O_15-δ, which is closely related to the known A₆A^‘B₄O₁₅ phases by shifting of a B atom and the O₃ triangle along the c axis. Further study on O3 reveals that this oxygen position splits into two independent positions, corresponding to polyhedral geometry of IP and TP, respectively. Therefore, the polyhedral chain in the structure should be more precisely described as a random composite of the 4Oh + TP and 3Oh + 2IP. This model is used to interpret the magnetic properties, although not quantitatively. The 4-D structure analysis was also conducted for both Sr₆Co₅O_14.70 and Sr₆Co_4.9Ni_0.1O_14.36 with a commensurate modulated structure in a 4-D superspace group, R3̄m(00γ)0s, γ = p/k = 3/5. By considering the same 4-D superspace group R3̄m(00γ)0s but different t-phases, one can understand the structure relationship between Sr₆Co₅O_14.70 and Sr₆Rh₅O₁₅.