Crystal Structures and Phase-Transitions Analysis of the Double Perovskites Sr2Co1−x Ni x TeO6 (x = 0.25, 0.5 and 0.75) Using X-ray Powder Diffraction, Raman and Infrared Spectroscopy

The reaction between the complexes of double perovskite formula Sr2Co1−xNixTeO6 in different stoichiometric proportions (x = 0.25, 0.5 and 0.75) have been processed in polycrystalline form by solid– state reaction mode in air. Based on the Rietveld refinements of x-ray powder diffraction data, the crystal structures and phase transitions, at room temperature of this double perovskite series are reported. The materials crystallize in a monoclinically distorted perovskite structure (the two compositions with x = 0.5 and 0.75 belong the to I2/m space group, while the composition with x = 0.25 crystallize in P21/n space group). We found a good agreement between the lattice parameters of this series and those of the two materials Sr2CoTeO6 and Sr2NiTeO6 with x = 0 and 1, respectively. The linear evolution of crystalline parameters proves the realization of the Vegard Law. The effect of the partial substitution of Co by Ni was also seen in the spectra of Raman and infrared, where a band shift was observed with increased nickel content.