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

[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 (Fm (3) over barm) to tetragonal (I4/m) to monoclinic structural phases (I2/m, P2(1)/n). These transitions have been confirmed by Raman studies Fm (3) over barm(x=0) -> I4/m(0.1 <= x <= 0.2) -> I2/m(0.3 <= x <= 0.5) -> P2(1)/n(0.6 <= x <= 1) Furthermore, increasing the temperature for the compositions [Ba2CaWO6](1-x)[Sr2CaWO6](x) (0.1 <= x < 1), manifests the P2(1)/n to I2/m, the I2/m to I4/m and the I4/m to Fm<(3)over bar>m 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 Fm (3) over barm structures. Finally, for the room temperature P2(1)/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. (C) 2015 Elsevier Inc. All rights reserved.