LATTICE-DYNAMICS OF TISE2

Wakabayashi et al. fitted a force-constant model to the purely transverse phonon dispersion curves from their neutron scattering measurements on 1 T-TiSe2. Their model is extended here to fit the remaining phonon branches observed by them and the LA branch in the c direction studied by Stirling et al. As the crystal is cooled below 200°K, it develops charge-density waves (CDW) and the c and a axes of the new unit cell are twice those of the old one. Due to this the symmetry points A, M, and L at the zone boundary for the 1 T phase fold into the zone center for the CDW phase. The calculated phonons at the symmetry points Γ, M, A, and L for 1 T phase are compared with the observed infrared and Raman modes in the CDW phase. Except for some very-low-frequency observed infrared modes, the calculated results explain the origin of the observed data reasonably well. © 1979 The American Physical Society.