Crystal structure and phase transitions of Sr2CdWO6Sr2CdWO6

The crystal structure of Sr2CdWO6Sr2CdWO6, prepared by solid state reaction, was determined by high-resolution X-ray diffraction at different temperatures. At room temperature, this compound has a monoclinic structure (space group P21/nP21/n) with a=5.7463(1)a=5.7463(1), b=5.8189(1)b=5.8189(1), c=8.1465(1)c=8.1465(1), β=90.071(1)β=90.071(1). At 1105 K the structure is converted to tetragonal (space group I4/mI4/m). Diffraction data also suggest that a cubic phase exists above 1220 K. Comparing the phase transition temperatures of Sr2CdWO6Sr2CdWO6 with those of other compounds of the Sr2MWO6Sr2MWO6 family reported previously, it was observed that the transition temperatures are higher in compounds with low-tolerance factors. At the same time, the temperature range in which the intermediate tetragonal phase exists is reduced.

The phase transition temperatures as a function of the tolerance factor t, calculated using bond lengths distances that give the nominal oxidation states of the cations in the bond–valence method. The phase transition temperatures increase at lower values of t. Also, the temperature range in which the tetragonal phase exists is narrower at low t  . These results are very similar to those observed in Rb2KBF2(B=Sc,In,Lu,Er,Ho) elpasolites, and confirm that the most important factor governing the appearance of octahedral tilts in the double perovskite structure and the temperature range in which they exist is the mismatch between the size of the A cation and interstitial space between the BO6BO6 and B′O6B′O6 octahedra.Figure optionsDownload full-size imageDownload as PowerPoint slide