Analysis of normal-state phonon properties of2H-TaSe2and -NbSe2

We have introduced a phenomenological lattice dynamical model for $2H$-Ta${\mathrm{Se}}_2$ and -Nb${\mathrm{Se}}_2$. The parameters of the model were fitted to much of the Raman and inelastic neutron diffraction data for $2H$-Ta${\mathrm{Se}}_2$ and the model accurately predicted mode frequencies not used in the fitting procedure and, with the appropriate mass change but the same force parameters, all the available Raman and neutron data for $2H$-Nb${\mathrm{Se}}_2$. The model takes into account the anomalous ${\Sigma }_1$ acoustic dispersion through an explicit wave-vector-dependent metal-atom—metal-atom interaction in the dynamical matrix. The remaining portion of the dynamical matrix is given by valence-force-field parameters, two of which are found to be much smaller in magnitude than for $2H$-Mo${\mathrm{S}}_2$.