Molecular dynamics simulation of the rotational order–disorder phase transition in calcite

Molecular dynamics (MD) simulation of calcite was carried out with the interatomic potential model based on ab initio calculations to elucidate the phase relations for calcite polymorphs and the mechanism of the rotational order–disorder transition of calcite at high temperature at the atomic scale. From runs of MD calculations with increasing temperature within a pressure range of 1 atm and 2 GPa, the transition of calcite with symmetry into a high-temperature phase with symmetry was reproduced. In the high-temperature phase, CO₃ groups vibrate with large amplitudes either around the original positions in the structure or around other positions rotated ± 60°, and their positions change continuously with time. Moreover, contrary to the suggestion of previous investigators, the motion of CO₃ groups is not two-dimensional. At 1 atm, the transition between and is first order in character. Upon increasing temperature at high pressure, however, first a first-order isosymmetric phase transition between the phases occurs, which corresponds to the start of ± 120° flipping of CO₃ groups. Then, at higher temperatures, the transition of to phases happens, which can be considered second order. This set of two types of transitions at elevated pressure can be characterized by the appearance of an 'intermediate' phase between the stable region of calcite and the high-temperature phase, which may correspond to the CaCO₃-IV phase.