Synthesis, structure, magnetic properties and structural distortion under high pressure of a new osmate, Sr₂CuOsO₆

A new osmate Sr₂CuOsO₆ was synthesized and its structure refined using powder synchrotron X-ray diffraction. The results of the Reitveld refinements indicate complete B-cation order in a double perovskite crystal structure. Furthermore, the analysis of the B-cation bond lengths indicates a symmetric coordination around Os, as opposed to a significant distortion of Cu-O bond lengths. The octahedral distortion around Cu(II) is characteristic of a Jahn-Teller distortion. Within the crystal structure of Sr₂CuOsO₆, the long Cu-O bonds are aligned in the same direction along the c-axis in the tetragonal unit cell. This parallel arrangement of long Cu-O bonds produces a lattice parameter ratio, c/(2^1/2a), that is greater than unity. The magnetic susceptibility of Sr₂CuOsO₆ was measured using a SQUID magnetometer and was observed to be consistent with an assignment of Cu(II)-Os(VI) formal oxidation states, thus confirming the bond valences calculated on the basis of the crystal structure. In perovskites, octahedral tilting and bond shortening are two competing compression mechanisms. Compression mechanisms of this double perovskite were characterized using high-pressure synchrotron X-ray diffraction. Application of hydrostatic pressure up to 6 GPa significantly decreased the lattice parameter ratio, demonstrating the primary compression mechanism is a shortening of the long Cu-O bond.