Cubic calcite and its structural phase transitions

Calcite, CaCO₃, has been reported to exist in as many as seven different structural forms. The structure at room temperature and pressure (space group R3¯c, ‘Phase I’) was established by Bragg many years ago. A phase transition to a higher temperature phase (space group R3¯m, ‘Phase V’) was noted to occur at around 1240 K—this may proceed via an intermediate phase (space group again R3¯c, referred to as ‘Phase IV’). These phases differ primarily in the disposition of the CO₃ groups. Additional phases are found at higher pressures. We report a para-phase (parent phase, virtual prototype, aristotype) which assists in understanding the different phases, the phase transitions, and especially the domain structures and twin wall boundaries associated with these transitions. Molecular dynamics methods were used to study the temperature evolution of an isothermal-isobaric (NPT) ensemble of some 384,000 atoms. These computations reproduced the features of the known structures in R3¯c and R3¯m and then, at higher temperature, revealed a structure of the sodium chloride type (space group Fm3¯m) in which the entities were the Ca²⁺ cation and the CO₃²⁻ anion, this latter with effectively spherical symmetry. On this basis we have upon cooling a necessarily first order ferroelastic transition from cubic Fm3¯m to rhombohedral R3¯m, computed to occur at a simulated temperature of 1900 K, and a possibly continuous transition from the R3¯m to rhombohedral (on a doubled cell) R3¯c computed to occur at about 1525 K. The computations also allowed us to follow the domain structure and twin walls as a function of temperature, during both heating and cooling. The structure just below the R3¯m to R3¯c transition shows strong disorder in the orientation of the CO₃ groups, and this may be what is sometimes referred to as Phase IV. The domain structure just below the cubic to rhombohedral transition shows twinning of typical ferroelastic character. The doubling of the cell below the R3¯m to rhombohedral (on a doubled cell) R3¯c leads to a more complicated twin pattern. Indeed, the different structures can be identified from patterns of twinning. Differences between domain structures obtained on heating and cooling indicate extensive thermal metastabilities.