Molecular-dynamics study of phase transitions between incommensurate and commensurate phases

Two models, one with an incommensurate phase, another with only commensurate phases have been simulated with the molecular-dynamics method, in order to study the mechanisms of the phase transitions in these systems. In the incommensurate model the mechanism relies on nucleation and growth of stripples. The commensurate-commensurate phase transition occurs in a system with a weak coupling along the modulation direction and is driven by the planar-nuclei mechanism. A planar nucleus nucleates and grows within a layer of a crystal. To transform one phase into another a large number of planar nuclei is needed. A kinetic run from one commensurate phase to another produces a spectrum of the diffuse scattering function which fills the gap between the characteristic wave vectors of the two commensurate phases.