Inelastic neutron scattering measurements of the relaxor ferroelectric PbMg$_{ 1 / 3}$Nb$_{ 2 / 3}$O$_{ 3}$ (PMN) in the temperature range 490~K$<$T$<$880~K directly observe the soft mode (SM) associated with the Curie-Weiss behavior of the dielectric constant $\varepsilon $(T). The results are treated within the framework of the coupled SM and transverse optic (TO1) mode and the temperature dependence of the SM frequency at q=0.075 a* is determined. The parameters of the SM are consistent with the earlier estimates and the frequency exhibits a minimum near the Burns temperature ($\approx $ 650K)
Categories: cond-mat.mtrl-sci