An extension of the McMillan (1975) and Nakanishi and Shiba (1977) models of charge-density-wave formation which fully reflects the space-group symmetry of the $1T-\mathrm{Ta}{\mathrm{S}}_2$ structure is presented. The model allows an incommensurate $c$-axis wave vector immediately below the normal to incommensurate transition temperature, and a $c$-axis wave-vector lock-in transition at a lower temperature. The model also allows a number of different commensurate phases characterized by different charge-density-wave stacking configurations. A triple-honeycomb domain structure is described.

• Received 19 January 1983

DOI:https://doi.org/10.1103/PhysRevB.28.2766