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 $1T$-Ti${\mathrm{Se}}_2$. 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 $1T$ phase fold into the zone center for the CDW phase. The calculated phonons at the symmetry points $\Gamma$, $M$, $A$, and $L$ for $1T$ 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.