Abstract
The crystallographic phase transition in the compound VS is explained in terms of the formation of a charge-density-wave (CDW) state. In an attempt to neutralize the charge density of the electron gas, the positive ions shift in position and produce a superlattice structure. In VS it is found to be energetically favorable for the superlattice to have a periodicity which is commensurate with the undistorted lattice, as found experimentally. The elementary excitations in the CDW state contain an acoustic-plasmon branch which is mixed with the transverse-optical phonons.