Phonon Dispersion and Mode Coupling in High-Quartz Near the Incommensurate Phase Transition

In order to test the possible dynamical origin of the incommensurate phase of quartz, the dispersion of low-energy phonon branches in high quartz has been determined by inelastic neutron scattering. The optical sof tmode associated with the α-β transition and the transverse acoustic mode corresponding to u xy strains exhibit a large anisotropy within the hexagonal plane. The relations of these results to the phenomenological gradient coupling model and to microscopic lattice dynamical models are discussed.