Effect of the M3+ cation size on the structural and high temperature phase transitions in Sr2 MSbO6 (M = Ln, Y) double perovskites

Abstract

In the present work, synchrotron X-ray powder-diffraction measurements at room temperature and laboratory X-ray powder-diffraction measurements at high temperatures of nine members of the antimony family Sr2MSbO6 M = Ln (Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb) and Y ; are reported. Six of them are synthesized for the first time in this work. The crystal structures at room temperature of these compounds are determined and their symmetries are described in the P21/n (a-a-b+) space group (No. 14, non-standard setting). The materials have double perovskite structure with 1:1 ordered arrangement of the B sites. The high-temperature measurements show the existence of two temperature driven phase-transitions: a discontinuous one, from the RT symmetry to R3¯ (a-a-a-) and a continuous one from the latter symmetry to the highest one, the cubic Fm3¯m (a0a0a0). The phase transition temperatures are correlated to the M3+ cation size. The analysis of the phase transitions and the structural refinements have done using the symmetry-adapted modes. A step forward in these analysis is performed: parametrization of the refinements by fixing a selected common direction in the sub-space spanned by X5+ , tri-linearly coupled to the order parameters of the cubic to monoclinic first order phase transition.