Two-mode behavior of the PbMg1/3Nb2/3O3 relaxor

The low-frequency lattice dynamics of lead magnoniobate has been studied using inelastic neutron scattering in the temperature range 650 K < T < 1050 K. The measurements have been performed in two Brillouin zones, (200) and (300). The data corresponding to these two zones and different values of the reduced wave vector have been treated in a common frame. It has been shown that the model assuming one optical phonon mode fails in an adequate description of scattering in both zones. The model involving a two-mode behavior of soft optical phonons provides a good agreement with experimental data. It has been demonstrated that the frequency of the low-energy soft-mode component follows the Curie-Weiss law with the Curie temperature T (C) = 400 K, which fits well dielectric spectroscopy measurements.