SEMINAR: Structural evolution of calcite (CaCO3) at high temperatures: A hundred-year old mystery has been resolved

Preceding W. L. Bragg’s deduction in 1914 [1] about the room-temperature atomic configuration of the calcite form of calcium carbonate CaCO3 (Phase I), H. E. Boeke in 1912 [2] found a reversible phase transition at around 1248 K. Since then, many experiments have been undertaken to determine the high-temperature structure of calcite, but none has succeeded. Here we first show the structure of the high temperature modification of calcite, named as Phase V by P. W. Mirwald [3], in addition to the intermediate state IV bridging Phases I and V [4], using the in-situ single-crystal X-ray diffraction technique. Phase V was stable only in a narrow temperature range between 1240 and 1275 K in a ~0.4 MPa CO2 atmosphere. The crystal was immediately decomposed upon further heating. The oxygen sublattice is melted in Phase V. The joint-probability density function calculated from the inverse Fourier transform of the anharmonic atomic displacement parameters using the 3rd order expansion of the Gram–Charlier series, revealed that the oxygen atoms of the carbonate group in Phase V do not sit still at specified Wyckoff positions in the space group R–3m, but are distributed with equal probability along the undulated circular orbital about the central carbon, like “Tigers turned into butter” [5]. The amplitude of undulation and the magnitude of atomic displacement parameters suggested that the carbonate group in Phase V is no longer flat on the basal plane when the oxygen triangle comes to troughs or peaks of the undulation, but deformed like an umbrella. It appears that the oxygen atoms in Phase V migrate about carbon with repeating the umbrella inversion, as the last hurrah before decomposition.