Polar modes in relaxor PbMg1/3Nb2/3O3 by hyper-Raman scattering

Phonon modes in PbMg1/3Nb2/3O3 relaxor are studied by backward and near-forward hyper-Raman scattering technique. Polar mode frequencies obtained from hyper-Raman experiment fully agree with results of infrared spectroscopy. An additional mode observed in cross-polarized spectra near 250 cm(-1) is assigned as F-2u mode. Most surprisingly, experiments reveal (i) a strong enhancement of hyper-Raman scattering by longitudinal optic modes and (ii) remarkable correspondence between near-forward hyper-Raman scattering and imaginary part of the inverse dielectric function.