Crystal structures and phase transitions of the double-perovskite oxides Sr2CaWO6 and Sr2MgWO6

In the present work we report x-ray powder diffraction measurements of Sr2CaWO6 and Sr2MgWO6, at different temperatures. The crystal structures at room temperature of both compounds are determined; and results showing the existence of phase transitions in them are presented. For the structure at room temperature of the Ca containing compound, the space group P2(1)/n is obtained, which is different from that previously suggested. The evolution with temperature of the structure of this compound shows the presence of two phase transitions, a discontinuous one at 1130 K and a continuous one at 1250 K, with the following phase transition sequence: P2(1)/n --> I4/m --> Fm (3) over barm. The crystal structure of the Mg containing compound at room temperature has the I4/m space group. The temperature evolution of the structure has revealed a continuous phase transition at 570 K, changing the symmetry from tetragonal (I4/m) to cubic (Fm (3) over barm).