Crystal structures and high-temperature phase transitions of the new ordered double perovskites Sr{sub 2}SmSbO{sub 6} and Sr{sub 2}LaSbO{sub 6}

In the present work we report X-ray powder diffraction measurements of Sr{sub 2}SmSbO{sub 6} and Sr{sub 2}LaSbO{sub 6}, at different temperatures. The crystal structures at room temperature of both compounds are determined; and results showing the existence of high-temperature phase transitions in them are presented. Both compounds have double perovskite structure with 1:1 ordered arrangement of the B site cations. At room temperature their symmetries are described with the P2{sub 1}/n space group, that correspond to the (a{sup +}b{sup -}b{sup -}) tilt system. The evolution with temperature of the structure of both compounds shows the presence of two phase transitions: a discontinuous one, at 885 and 945 K, for Sr{sub 2}SmSbO{sub 6} and Sr{sub 2}LaSbO{sub 6}, respectively; and a continuous one, at 1170 and 118 K, for Sr{sub 2}SmSbO{sub 6} and Sr{sub 2}LaSbO{sub 6}, respectively, with the following phase transition sequence: P2{sub 1}/n->R3-bar->Fm3-barm. - Clinographic projections of the structures of the new ordered double perovskites (a) Sr{sub 2}SmSbO{sub 6} and (b) Sr{sub 2}LaSbO{sub 6}, along the [001]{sub p} and [010]{sub p} directions, as indicated, in the upper and in the lower panels, respectively. SmO{sub 6} octahedra are shown green, LaO{sub 6} in cyan, SbO{sub 6} in blue and Sr cations more » in gray. Sr{sub 2}LaSbO{sub 6} is more distorted than Sr{sub 2}SmSbO{sub 6}, as can be appreciated in the upper panel in the projection of the cuboctahedral site occupied by the Sr. The angle of rotation of the octahedra around the simple perovskite cubic axis is greater in the more distorted Sr{sub 2}LaSbO{sub 6} phase. « less