X-ray diffraction and Raman spectroscopy studies of temperature and composition induced phase transitions in Ba2−xSrxMWO6 (M = Ni, Co and 0 ⩽ x ⩽ 2) double perovskite oxides

X-ray diffraction and Raman spectroscopy studies of Sr doped double perovskites compound Ba2−xSrxMWO6 with (M = Ni, Co and 0 ⩽ x ⩽ 2) were investigated. The samples show a transition from cubic to tetragonal phase as a function of composition while increasing strontium amount; both Rietveld refinements and Raman studies showed that this transition occurs between x = 1.2 and 1.4. Furthermore, increasing temperature for the tetragonal compositions (1.4 ⩽ x ⩽ 2), manifest the tetragonal to cubic phase transition. For this series, the transition from tetragonal (I4/m)/to cubic (Fm-3m) shows considerable changes in the composition and temperature dependence of the modes: all the Raman modes show a linear behavior when temperature is increased, then the slope change dramatically indicating the symmetry change from tetragonal to cubic. To better view this phase transition, the full width at half maximum (FWHM) temperature dependence of the most intense modes were studied; by this study, we have obtained a sensitive guide to the onset of the phase transition. A large increase of the FWHM with increasing temperature is observed for all modes in the spectrum, clear transitional effects were observed: the width behaves in a linear way as a function of temperature; when the temperature reached the transition temperature, a drop in the width of this mode was observed.