Calcite V: a hundred-year-old mystery has been solved

Abstract

Since Boeke's finding of a reversible phase transition of calcite (calcium carbonate, CaCO 3 ) at elevated temperatures [Boeke, H. E. (1912). Neues Jahrb. Mineral. 1, 91-121], and following W. L. Bragg's determination of the structure of the room-temperature Phase I [Bragg, W. L. (1914). Proc. R. Soc. Lond. A 89,, the high-temperature Phase V of calcite has been an enduring mystery. Here, we summarize a paper on the structure of Phase V [Ishizawa, N., Setoguchi, H. and Yanagisawa, K. (2013). Sci. Rep. 3, 2832], as well as the intermediate Phase IV which exists between Phases I and V, and add new aspects. An in situ single-crystal X-ray diffraction study revealed that the I-IV and IV-V transitions occurred reversibly at approximately 985 and 1240 K, respectively, in a carbon dioxide atmosphere. Phase V was stable only over a narrow temperature range between 1240 and 1275 K. The crystal decomposed immediately at temperatures above 1275 K, leaving a nanoporous calcium oxide reaction product which retained the shape of the parent calcite crystal. The I-IV transition can be described as an orientational order/disorder transition of the carbonate group, occurring within the same space group R 3c. In Phase V, the oxygen sublattice is melted. The joint-probability density function obtained from the anharmonic atomic displacement parameters of the oxygen atoms revealed that the oxygen triangles of the carbonate group in Phase V do not sit still at specified Wyckoff positions in the space group R 3m, but are instead distributed with equal probability along the undulated circular orbital about the central carbon. The carbonate group in Phase V is no longer flat on the basal plane when the oxygen triangle comes to troughs or peaks in the undulated orbital, but is instead deformed like an umbrella. Assuming that the oxygen triangle migrates about carbon, the carbonate group should repeat the umbrella inversion in Phase V as a function of time. Finally, possible thermal decomposition mechanisms of calcite are briefly discussed.

Superlattice ordering transitions driven by short-range structure in barium calcium carbonates

“…Carbonate rotational disorder at high temperatures is common. Calcite (R 3c) undergoes a second-order phase transition to 'carbonate disordered' calcite IV approaching a critical temperature of T c ¼ 970 C. 33,34 This transition is characterized by the rotation of carbonates by 60 about their threefold axes, effectively inverting about the carbon center. Above 970 C calcite IV becomes further disordered into a 'carbonate-melted' calcite V with R 3m symmetry.…”

New High-Pressure and High-Temperature CaCO3 Polymorph

“…Although the mentioned “calcite-I → calcite-II → calcite-IIIb → calcite-III” phase transitions in CaCO 3 were thoroughly studied both structurally and spectroscopically and the aragonite equation of state (EoS) was constrained throughout its stability field up to 29 GPa and 1673 K, there is a wide gap between the high-temperature boundary of the aragonite stability field and CaCO 3 melting line (Figure ) where neither structures nor properties of thermodynamically stable phases have ever been reported. In a few experimental works, the high-temperature transition of aragonite into a previously unknown phase was studied; the latter was described as “disordered calcite” , “disordered phase” , or “new phase” , , or associated with a low-pressure calcite-V polymorph ( R 3̅ m space group) known to be stable above ∼1240 K at ambient pressure, , and no attempts to reveal any crystal structures, symmetries, or even unit cells have been reported.…”

Ba3(BO3)2: the first example of dynamic disorder in a borate crystal

“…Dynamic disorder is a well-known phenomenon observed upon heating of organic crystals. It is also quite widespread in inorganic compounds with rigid triangular anionic groups such as carbonates and nitrates, 1 for example, NaNO 3 , 2-5 KNO 3 , 6,7 AgNO 3 8,9 and alkaline earth metal carbonates such as CaCO 3 , [10][11][12][13][14] SrCO 3 14,15 and BaCO 3 . 14,15 For the listed compounds, the transition to the dynamically disordered state is realized at high temperatures, usually close to the temperature of melting or decomposition.…”

“…With a further increase in the temperature, CaCO 3 -IV transforms into a ''fully'' disordered state, CaCO 3 -V, in which [CO 3 ] groups rotate continuously and do not remain for a long time at any position. [11][12][13] Similarly, this is applicable to [BO 3 ] groups in cube cages in Ba 3 (BO 3 ) 2 . The transition of CaCO 3 to a disordered state begins when the angle of librations of the [CO 3 ] groups around the axis perpendicular to the threefold molecular axis exceeds the critical value of B451.…”