Lattice Dynamics of 1<i>T</i>-TiSe₂

Abstract

Phonon dispersion curves for the high temperature phase of a layer compound 1 T -TiSe 2 , which reveals a second order structural phase transition at T c =200 K, are calculated using the rigid ion model and the shell model, The calculated phonon dispersion curves, especially with the shell model, are in good agreement with the observation with regard to their gross features. In particular, the calculated dispersion curve for acoustic phonons propagating in the layer and polarized perpendicular to the layer shows an upward bend as this is observed. However,the observed dip at the L point cannot be reproduced. with our models in which only central two-body forces are assumed; the L point corresponds to the wave vector of the observed lattice distortion. The results are discussed in connection with other possible effects.