Ferroelastic phase transition and phonons in a diatomic-molecular monolayer

Using a constant-pressure molecular-dynamics simulation, we have investigated the thermodynamics and the dynamics of a two-dimensional diatomic-molecular monolayer undergoing a ferroelastic phase transition. This system closely resembles the phase of oxygen molecules adsorbed on a graphite surface. For Lennard-Jones parameters appropriate for the oxygen molecules, we find a first-order transition from an orientationally ordered distorted triangular structure (ferro- elastic phase) to an orientationally disordered equilateral triangular structure (paraelastic phase). The transition temperature is 20.1 K compared with 26 K for oxygen on graphite [coverage 8 molecules (100 AI2)] and the entropy associated with this transition is 0.88kB. The orientational diffusion constant increases by a factor of 30 at the transition. In addition, there is a strong softening of the elastic constants near the transition, particularly in the paraelastic phase; this can be understood in terms of translation-rotation coupling. Comparison between phonon frequencies for certain symmetry directions obtained by using quasiharmonic approximation and molecular-dynamics simulation clearly shows the presence of large anharmonicity effects in the paraelastic phase. A rapid quench from the high-temperature phase to very low temperatures indicates the presence of small clusters (consisting of 612 molecules) with both ferroelastic and herringbone ordering. In addition, we find a large density of equilateral triangular plaquettes. These give rise to a three-peak structure in the center-of-mass radial distribution function. © 1989 The American Physical Society.