Phenomenological theory of charge-density-wave states in trigonal-prismatic, transition-metal dichalcogenides

A free-energy expression (differing in its symmetry from that of McMillan) is proposed for describing charge-density-wave (CDW) states in transition-metal dichalcogenides having trigonal-prismatically coordinated layers (e.g., 2H-Ta${\mathrm{Se}}_2$). An analysis of the single CDW state is given. In disagreement with previous work, we find that, close to the triple point where the normal, commensurate, and incommensurate phases coexist, the incommensurate-commensurate transition is first order; the transition becomes second order only beyond a multicritical point which is located relatively far from the triple point. We obtain analytical solutions for the order parameter in the single CDW state at the triple point.