Lattice vibrations in the quasi-one-dimensional charge-density-wave (CDW) system (MSe4)nI are investigated by Raman scattering in the (TaSe4)2I and (NbSe4)2I 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 MSe4 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.
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