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

Raman spectra of Pb{[}(Mg1/3Nb2/3)(1-x),Ti-x]O-3 and Pb{[}(Zn1/3Nb2/3)(1-x),Ti-x]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.