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 320–20 K. In PbWO₄, we observe a change in acoustic phonon mode behavior around 180 K. But in the case of BaWO₄, we have observed two types of change in acoustic phonon mode behavior at 240 K and 130 K. 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 180 K could be due to changes in the environment around the lead vacancy (V_Pb²⁻) changes the electronic states. In the case of BaWO₄, the phase transition at 240 K shows the decrease in penetration depth of WO₃ impurity. So it becomes more metallic. The transition at 130 K 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.