First-order displacive structural phase transitions studied by computer simulation.

Abstract

We have constructed a lattice-dynamical model which possesses many of the features occurring at first-order structural phase transitions in solids. The model includes an asymmetric nonlinear on-site potential and anharmonic interparticle interactions. The anharmonicity in the interaction is introduced in a way which lowers the phonon frequencies in the high-temperature, metastable phase. The interaction provides a mechanism for a vibrational-entropy-driven first-order phase transition. We present results from molecular-dynamics calculations which show (i) clear evidence in the thermodynamic functions for the existence of a first-order phase transition produced by heating from low temperature, and (ii) unusual properties for the position probability distribution and the dynamic structure factor. These spectral functions have significant intensity in the quasielastic region, and this contribution is strongly maximized near the transition temperature. The wave-vector dependence of this ``central peak'' clearly points to the existence of propagating nonlinear modes.