Inelastic Light Scattering Near the Ferroelectric Phase-Transition Point in Bismuth Vanadate Crystals

The Raman and Brillouin spectra in bismuth vanadate crystals near the ferroelectric phase-transition point are studied. The intensity maximum corresponding to the soft mode in light scattering spectra is found. This mode is responsible for the instability of the bismuth vanadate crystal lattice in the vicinity of the ferroelectric phase-transition point. Strong interaction of the soft optical mode with an acoustic mode of the corresponding symmetry is revealed by an analysis of the Raman and Brillouin spectra observed. The theory of light scattering by coupled lattice modes based on the model of two strongly coupled oscillators is developed. The introduction of an additional oscillator strongly interacting with the fundamental soft mode is shown to result in a temperature shift of the structural phase transition to higher temperatures. The results obtained confirm the possibility of changing the phase-transition temperature by modification of the vibrational spectrum through introduction of additional degrees of freedom or comminution of the macroscopic sample with formation of the ordered superdispersed structure (globular crystal). These results are general and can be subsequently used to increase the phase-transition temperature in ferroelastics, ferroelectrics, and superconductors.