Structural fluctuations and the order-disorder phase transition in calcite

Molecular-dynamics calculations have been carried out on the ordered (low-temperature) and orientationally disordered (high-temperature) phases of calcite. The potential model used is shown to reproduce the main features of the experimentally observed order-disorder transition. The significance of the results for the interpretation of recent inelastic neutron-scattering experiments is discussed with particular reference to a possible competition between different ordering processes. Comparison is made with previously reported simulations of the corresponding phase transition in sodium nitrate and differences in behaviour between the two systems are noted.