Acoustic phonon behavior of PbWO4 and BaWO4 probed by low temperature Brillouin spectroscopy

Abstract

Temperature dependent acoustic phonon behavior of PbWO4 and BaWO4 using Brillouin spectroscopy has been explained for the first time. Low temperature Brillouin studies on PbWO4 and BaWO4 have been carried out from 320–20K. In PbWO4, we observe a change in acoustic phonon mode behavior around 180K. But in the case of BaWO4, we have observed two types of change in acoustic phonon mode behavior at 240K and 130K. The change in Brillouin shift ω and the slope dω/dT are the order parameter for all kinds of phase transitions. Since we do not see hysteresis on acoustic phonon mode behavior in the reverse temperature experiments, these second order phase transitions are not related to structural phase change and could be related to acoustic phonon coupled electronic transitions. In PbWO4, the temperature driven phase transition at 180K could be due to changes in the environment around the lead vacancy (VPb2−) changes the electronic states. In the case of BaWO4, the phase transition at 240K shows the decrease in penetration depth of WO3 impurity. So it becomes more metallic. The transition at 130K could be the same electronic transitions as that of PbWO4 as function of temperature. The sound velocity and elastic moduli of BaWO4 shows that it could be the prominent material for acousto-optic device applications.