Interphonon interactions at the charge-density-wave phase transitions in (${\mathrm{TaSe}}_4$${)}_2$I and (${\mathrm{NbSe}}_4$${)}_2$I

Lattice vibrations in the quasi-one-dimensional charge-density-wave (CDW) system (M${\mathrm{Se}}_4$ ${)}_{\mathrm{n}}$I are investigated by Raman scattering in the (${\mathrm{TaSe}}_4$ ${)}_2$I and (${\mathrm{NbSe}}_4$ ${)}_2$I compounds. The existence of three CDW-related TO modes is found in this experiment, and the following model for the CDW transition is proposed. The compounds of this system have strong interaction between the Kohn-anomaly TO mode and the TA mode, both of which are on the same branch in the extended Brillouin zone of the M${\mathrm{Se}}_4$ unit. As a consequence of the interaction, the TA mode condenses prior to the Kohn-anomaly TO mode. The theory of the CDW phase transition in the coupled system is presented.