Neutron scattering phase study of the normal-incommensurate transition in Rb2ZnBr4

The soft phonon modes, which are related to the normal-incommensurate phase transition of Rb2ZnBr4, have been measured by inelastic neutron scattering . The existence of both phase and amplitude modes has been confirmed, and t he phonon dispersion curves have been determined in the low-temperature com mensurate phase. On the other hand, the phonon dispersion curve of the tran sverse acoustic phonon along the (0 0 xi) direction does not vary appreciab ly in the normal phase. The soft mode is fully overdamped above the normal- incommensurate phase transition. Diffuse scattering develops around h0l +/- 5/17 as temperature approaches T-i = 347 K from above.