Cationic ordering and role of the A-site cation on the structure of the new double perovskites Ca 2− xSr xRSbO 6 (R = La, Sm) and ( x = 0, 0.5, 1)

Abstract

We have investigated the crystal structure and phase transitions of new series of ordered double perovskite oxides Ca 2− x Sr x LaSbO 6 and Ca 2− x Sr x SmSbO 6 ( 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 P2 1/ n space group, with cation distribution [Ca 1− x Sr x R] A CaSbO 6 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 Ca 1.5Sr 0.5LaSbO 6, CaSrLaSbO 6 and CaSrSmSbO 6 present two phase transitions, the first one being discontinuous and the second one continuous as a following phase-transition sequence: P 2 1 / n → R 3 ¯ → Fm 3 ¯ m . In Ca 2LaSbO 6, Ca 2SmSbO 6 and Ca 1.5Sr 0.5SmSbO 6, no indication of any phase transitions from room temperature to 1770 K was observed.