PHONON RENORMALIZATION AT SMALL Q-VALUES IN THE HIGH-TEMPERATURE PHASE OF CSCUCL3

The cooperative Jahn-Teller system CsCuCl3 exhibits a structural phase transition al T = 423 K. Above that temperature the CuCl6 octahedra a n dynamically disordered. The c(44) dispersion branch shows an acousti c instability. The lattice dynamics of this high-temperature phase has been studied by inelastic neutron scattering measurements. The system could be modelled by a theory assuming pseudo-spin phonon coupling be tween reorienting Jahn-Teller-distorted CuCl6 octahedra and the harmon ic lattice vibrations. The theory predicts the occurrence of renormali zed softened phonons at very small q-values. The phonon peaks have bee n observed at various temperatures above T-c in the region q = 0.02 .. . 0.15 Angstrom(-1).