Soft mode doublet in PbMg{1/3}Nb{2/3}O{3} relaxor investigated with hyper-Raman scattering.

Hyper-Raman scattering experiments suggest that a splitting of the lowest F{1u}-symmetry mode of PbMg{1/3}Nb{2/3}O{3} crystal occurs in a wide temperature range around its Burns temperature T{d}≈630  K. The upper-frequency component, earlier investigated by inelastic neutron scattering experiments above T{d}, appears to be underdamped even hundred of degrees below T{d}. The lower-frequency component, known below T{d} from far-IR spectroscopy, actually becomes underdamped above T{d}. This suggests that the lower-frequency mode is the "primary" polar soft mode of PbMg{1/3}Nb{2/3}O{3}, responsible for the Curie-Weiss behavior of its dielectric permittivity above T{d}.