Sequence of phase transitions induced by chemical composition and high temperature in [Ba2CaWO6](1−x)[Sr2CaWO6]x double perovskite tungsten oxides

Abstract

[Ba2CaWO6]1−x[Sr2CaWO6]x (0≤x≤1) materials were synthesized by the high temperature solid state reaction and firing methods, and characterized using techniques of X-ray diffraction and Raman spectroscopy. The crystal structures were determined by Rietveld refinements on the laboratory X-ray powder diffraction data. As a function of composition, upon increasing the strontium content, the samples exhibit a sequence of three phase transitions: from cubic (Fm3̅m) to tetragonal (I4/m) to monoclinic structural phases (I2/m, P21/n). These transitions have been confirmed by Raman studiesFm3¯mx=0→I4/m0.1≤x≤0.2→I2/m0.3≤x≤0.5→P21/n0.6≤x≤1Furthermore, increasing the temperature for the compositions [Ba2CaWO6]1−x[Sr2CaWO6]x (0.1≤x<1), manifests the P21/n to I2/m, the I2/m to I4/m and the I4/m to Fm3̅m phase transitions. For the compositions (0.1≤x≤0.2) the tetragonal to cubic phase transition is well illustrated.For the room temperature I2/m monoclinic compositions, two phase transitions were observed for all the compositions with x ranging from 0.3 to 0.5: from the monoclinic (I2/m) to tetragonal (I4/m), and from I4/m to Fm3̅m structures. Finally, for the room temperature P21/n monoclinic compositions, only two phase transitions are observed in the temperature range probed by Raman spectroscopy, the temperature was not high enough to reach the tetragonal-to-cubic phase transition.