Theoretical Study of Lattice Instability and Lattice Dynamics of 2H-TaSe2 and 2H-NbSe2 II. Lattice Dynamics and Phonon Anomalies

Phonon anomalies of the 2 H -polytype TaSe 2 and NbSe 2 , which reveal a structural phase transition at T 0 =122.3 K and 33.5 K, respectively, have beenstudied theoretically. First, the phonon dispersion curves are calculated using the rigid ion model, and are in good agreement with the observation except for the Σ 1 mode along the \( M\) line. Then by taking into account the effective ion-ion interaction caused by the electron-lattice interaction, the observed anomalous behavior of the acoustic Σ 1 phonon branch is explained. It is shown that the temperature dependence of the effective interaction leads to a softening of the Σ 1 mode of the wave vector \( q=(2/3) M\) which characterizes the superlattice below T 0 .