Low frequency excitation modes in the (1-x)[PbZn_1/3Nb_2/3O_3]-x[PbTiO_3] relaxor ferroelectric

There is great current interest in the solid solution (1-x)[Pb Zn_1/3Nb_2/3O_3]-x[PbTiO_3], known as PZNT. This relaxor ferroelectric system displays significant enhancement in permitivity and piezoelectric properties near a morphotropic phase boundary (MPB) between rhombohedral and tetragonal structures (x\cong 0.1). We report the results of a Raman scattering study of excitation modes of PZNT. The measurements were carried out over a large segment of the composition range. In the results reported here we cover the frequency range between 3cm^-1 to 120cm^-1. Strong Raman bands at energies above 40cm^-1 are assigned to optical vibration modes. However, these spectra display relatively small changes when crossing the MPB. We observe very strong inelastic light scattering at frequencies smaller than 30cm^-1 down to frequencies reaching to 4cm^-1 and possibly even lower. The intensity of light scattering by such low frequency excitation modes has a marked enhancement on the tetragonal side of the MPB. This response is observed in ceramic samples and in crystals, indicating it is not due to ceramic grains. The possible existence of a sharp (FWHM < 4cm^-1) central mode peak is being explored.