Brillouin scattering study of the ferroelastic transition in some crystals with the scheelite structure

The structural phase transitions in BiVO4 and LaNbO4 have been studied by means of the Brillouin scattering. It has turned out that the velocity of a transverse acoustic wave propagating along a special direction [cosΘ0, sinΘ0, 0] decreases, showing a typical soft-mode behavior, as the transition temperature is approached both from above and below (the direction specified by Θ0 coincides with that of intersection of the tetragonal ab-plane and the ferro-elastic twin boundary observed in the monoclinc phase).