PHASE-TRANSITION MECHANISMS BETWEEN HEXAGONAL COMMENSURATE AND INCOMMENSURATE STRUCTURES

A hexagonal two-dimensional model of particles with displacive degrees of freedom and interacting via potential energy with harmonic and anh armonic third- and fourth-order terms has been studied by the molecula r-dynamics technique. The modulated phases characterized by the wave v ectors lying in the interval from 1/4 to 1/3 have been specifically co nsidered. The simulations have shown that: (i) the 1q commensurate 1/4 --> 1/3 1q incommensurate phase transition is driven by the stripple mechanism. The stripples are built from four discommensuration planes and the serial mechanism of stripple correlated nucleation has been ob served. (ii) The reverse phase transition from 1q incommensurate to 1q commensurate 1/4 phase was obtained only under extreme metastable con ditions. No antistripples with four discommensurations were observed. Instead, a rearrangement of same domains gave a possibility to remove discommensuration planes by simple antistripples built up from two dis commensurations only. (iii) At the 3q --> 1q phase transition, the col umns of 3q phase merged together to form a stripe of a 1q incommensura te phase. (iv) The commensurate phase 1/3 is degenerate with respect t o the 1q and 3q modulation patterns.