Cationic ordering and role of the A-site cation on the structure of the new double perovskites Ca2−xSrxRSbO6 (R = La, Sm) and (x = 0, 0.5, 1)

We have investigated the crystal structure and phase transitions of new series of ordered double perovskite oxides Ca2−xSrxLaSbO6 and Ca2−xSrxSmSbO6 (x = 0, 0.5, 1). Rietveld refinements of the X-ray powder diffraction data at room temperature shows that these compounds crystallize in monoclinic symmetry with P21/n space group, with cation distribution [Ca1−xSrxR]ACaSbO6 general formula. The La compounds have a total ordering between La and Ca in A- and B-sites, respectively; however, in Sm compounds, the degree of Sm–Ca ordering has been found decreasing with an increase in the Sr content. A linear increasing of the lattice parameters and the volume, at room temperature, has been found with increasing of Sr concentration. The temperature evolution of the structure has revealed that the compounds Ca1.5Sr0.5LaSbO6, CaSrLaSbO6 and CaSrSmSbO6 present two phase transitions, the first one being discontinuous and the second one continuous as a following phase-transition sequence: P21/n→R3¯→Fm3¯m. In Ca2LaSbO6, Ca2SmSbO6 and Ca1.5Sr0.5SmSbO6, no indication of any phase transitions from room temperature to 1770 K was observed.