Structural evolution of calcite at high temperatures: Phase V unveiled

The calcite form of calcium carbonate CaCO₃ undergoes a reversible phase transition between Rc and Rm at ~1240 K under a CO₂ atmosphere of ~0.4 MPa. The joint probability density function obtained from the single-crystal X-ray diffraction data revealed that the oxygen triangles of the CO₃ group in the high temperature form (Phase V) do not sit still at specified positions in the space group Rm, but migrate along the undulated circular orbital about carbon. The present study also shows how the room temperature form (Phase I) develops into Phase V through an intermediate form (Phase IV) in the temperature range between ~985 K and ~1240 K.