Soft acoustic mode in ferroelastic BiVO4

Brillouin scattering spectroscopy of bismuth vanadate has revealed a soft acoustic mode which is characteristic of a proper ferroelastic phase transition. Raman scattering has revealed an optic mode which is doubly degenerate $E_g$ in the paraelastic phase and splits into two $B_g$ modes in the ferroelastic phase. The temperature dependence of the splitting is also consistent with a proper ferroelastic transition.