Mechanisms of phase transitions between commensurate and incommensurate phases

A three-dimensional model with one-dimensional incommensurate and commensurate phases has been simulated using the molecular-dynamics method. The results of simulation show that the commensurate{→}incommensurate phase transition starts from nucleation of a stripple whose existence, in turn, promotes nucleation of the next stripples in its vicinity. That process repeats and leads to cascade nucleation of stripples, effectively regarded as a frontal motion of the commensurate-incommensurate interface. The mechanism of the phase transition between two incommensurate phases is different and relies, not on random nucleation of stripples, but on correlated nucleation of a series of stripples that all grow and propagate in one thin layer of the crystal, which is perpendicular to the modulation direction. Easy nucleation of stripples at some surfaces of the crystallite suppresses the serial nucleation effect. Stripple nucleations and growths are accompanied by heat release.