PHASE-DIAGRAM OF A HEXAGONAL MODEL WITH INCOMMENSURATE PHASES

A hexagonal two-dimensional model of particles with displacive degrees of freedom and interacting via a potential with harmonic and anharmonic third- and fourth-order terms has been considered. The calculated phase diagram has incommensurate and commensurate phases with both one-dimensional 1q and two-dimensional 3q modulations. The incommensurate 3q phase proves to be stable in the presence of the third-order anharmonic term and close to the phase boundary of the normal phase. The commensurate phases with 1q and 3q modulations, characterized by the wave vector 1/3, are found to be degenerate and stable in the same region of the phase diagram.