NEUTRON STUDY OF THE PEIERLS TRANSITION AND LOW-FREQUENCY EXCITATIONS IN (TASE4)2I

Neutron scattering results on the lattice dynamics and Peierls phase transition of (TaSe4)2I are presented. The room temperature phonon dispersion curves reflect the stiffness of the (TaSe4)infinity chains and the comparatively weak transverse interchain interactions. The response of the [110] polarised TA mode, connected with the Peierls instability, is found to be weakly temperature-dependent. Correlation lengths associated with the diverging ''central peak'' are determined and are found to be nearly isotropic. We discuss the significance of these results from the point of view of electron-phonon theory and stress the importance of allowing for the interaction between optic and elastic degrees of freedom.