Soft vibrations and acoustic-optic mode couplings in ferroelectrics with large piezoelectric responses

Abstract

Raman spectra of $\mathrm{Pb}\left[\right({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{)}_{1-x},{\mathrm{Ti}}_x]{\mathrm{O}}_3$ and $\mathrm{Pb}\left[\right({\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}{)}_{1-x},{\mathrm{Ti}}_x]{\mathrm{O}}_3$ solid solutions display transverse optic vibration modes that soften with composition and reveal interactions between acoustic and optic modes with enhanced low frequency responses at compositions close to the tetragonal-rhombohedral phase boundaries. The results suggest that intrinsic microscopic interactions may coexist with effects characteristic of disordered ferroelectrics in creating the giant piezoelectric responses of these materials.