Diffuse scattering in Pb(Zn_1/3Nb_2/3)O₃ with 8% PbTiO₃ by quasi-elastic neutron scattering

Diffuse scattering (DS) due to the polar nanoregions in perovskite relaxor ferroelectric Pb(Zn_1/3Nb_2/3)O₃ with 8% PbTiO₃ was investigated by cold neutron scattering with high energy resolution (8 GHz). The anisotropy of DS near the (100) Bragg reflection cannot be explained merely by the polar character of the inhomogeneous displacement field. The occurrence of 'soft' [110]-type directions suggests a close relation to the [110]-type ridges seen in x-ray scattering on Pb(Mg_1/3Nb_2/3)O₃. Transverse (quasishear) acoustic phonons with wavevectors near the half maximum of DS density in the 'soft' [110] direction have anomalously large damping but their integrated intensity is small in comparison with the intensity of DS. Since no quasielastic width of the DS was found in this experiment, it is concluded that the characteristic relaxation frequency associated with this DS is less than 1-2 GHz even at 500 K. Results are compared with predictions based on recently proposed models for DS in perovskite relaxors.