Acoustic soft modes and the ferroelastic phase transition in BiVO4 and LaNbO4

Abstract
Brillouin scattering studies of BiVO4 and LaNbO4 have revealed acoustic soft modes in both crystals with similar temperature dependences, propagating normal to ferro-elastic domain walls. Raman scattering measurements show that BiVO4 has an optic soft mode while LaNbO4 does not, in agreement with previous results of other investigators. Attempts to fit the BiVO4 data to a two-coupled-oscillator model could not account for the observed temperature dependent intensities. We suggest that the frequencies of both the optic and acoustic modes are temperature dependent in both crystals.