Low-frequency spectroscopic study of ferroelectric and ferroelastic phase transitions

A review of low-frequency spectroscopic study of ferroelectrics and ferroelastics is given with emphasis on their on their phase transition mechanism: In common with many ferroelectrics, anomalous broadening of Brillouin scattering spectra observed just below Tc shows a strong dependence on the direction of the wave vector of the acoustic mode with respect to the spontaneous polarization. The anisotropy indicates, via the coupling between the acoustic mode and fluctuating polarization or soft polar mode, the characteristic fluctuation of the electric polarization caused by electrostatic long-range forces. In the ferroelastic phase transition, the acoustic mode softens only along a direction related closely to the ferroelastic domain boundary. Low-frequency Raman scattering was applied for KDP under hydrostatic pressure using a diamond anvil cell in order to remove overdamping of the soft modes in the ferroelectric spectra. The spectra observed for two different scattering angles suggest breakdown of the soft-mode model of KDP due to the measured wave vector dispersion being inconsistent with coupling to the ordinary soft ferroelectric mode.

Keywords:

• Brillouin scattering
• ferroelectric
• ferroelastic
• phase transition
• KDP
• Raman scattering
• central peak
• high pressure