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

Neutron scattering results on the lattice dynamics and Peierls phase t ransition of (TaSe4)2I are presented. The room temperature phonon disp ersion curves reflect the stiffness of the (TaSe4)infinity chains and the comparatively weak transverse interchain interactions. The respons e of the [110] polarised TA mode, connected with the Peierls instabili ty, is found to be weakly temperature-dependent. Correlation lengths a ssociated with the diverging ''central peak'' are determined and are f ound to be nearly isotropic. We discuss the significance of these resu lts from the point of view of electron-phonon theory and stress the im portance of allowing for the interaction between optic and elastic deg rees of freedom.