Discovery of phase transitions in Cs6In0.5W10.5O36

We report the first observation of structural phase transitions in Cs6In0.5 W10.5O36 crystallizing with a structure intermediary between the hexagonal tungsten bronze and pyrochlore structure. The recorded Raman spectra indica te the onset of three phase transitions at about 256, 505 and above 800 K. The crystal structure above the highest temperature transition point seems to be trigonal and the Raman study suggests that below the transition point the unit cell volume is multiplied. This transition is observed also as a marked dielectric anomaly at 867 K. The character of this anomaly suggests that this transition may be ferroelectric and diffuse or order-disorder typ e. The heat capacity, dielectric and Raman studies show that the transition at 505 K is second order or nearly so. It is of displacive type and is ind uced by a soft mode, which is most likely due to a tilt of WO6 octahedra. T he low-temperature transition at 256 K has been observed in Raman experimen t only. However, the observed changes in Raman spectra, which may indicate the onset of a phase transition at 256 K, are slight and therefore the conf irmation of this transition needs further studies.