Neutron Scattering Study of the Normal-Incommensurate
Phase Transition in Rb₂ZnBr₄

Abstract

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 the phonon dispersion curves have been determined in the low-temperature commensurate phase. On the other hand, the phonon dispersion curve of the transverse acoustic phonon along the (0 0 ξ) direction does not vary appreciably 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.