Soft Mode Doublet in PbMg1/3Nb2/3O3 Relaxor Investigated with Hyper-Raman Scattering

Hyper-Raman scattering experiments suggest that a splitting of the lowest F-1u-symmetry mode of PbMg1/3Nb2/3O3 crystal occurs in a wide temperature range around its Burns temperature T-d approximate to 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 PbMg1/3Nb2/3O3, responsible for the Curie-Weiss behavior of its dielectric permittivity above T-d.