R3m: A New High-Pressure and/or High-Temperature Phase of PbCO3, SrCO3, BaCO3, and Possibly of CaCO3

In the present study, we use the crystal structure prediction technique to reveal the new high-pressure structures of PbCO 3, SrCO 3, and BaCO 3 . As a result, we found the new calcite-like R 3 m structure thermodynamically stable for PbCO 3, SrCO 3, and BaCO 3 . Without consideration of the temperature effect, PbCO 3 - R 3 m is stable in the pressure range of 15–30 GPa. With increasing temperature, the stability field of PbCO 3 - R 3 m in P–T coordinates reduces, and above 2000 K, the aragonite phase directly transforms into the post-aragonite phase without the formation of R 3 m . SrCO 3 - R 3 m is thermodynamically stable at pressures 10–20 GPa and temperatures above 750 K. BaCO 3 - R 3 m is dynamically stable at ambient pressure and the available experimental data indicate its appearance at temperatures above 1086 K. By analogy with KNO 3, we assume the possibility of the appearance of BaCO 3 - R 3 m during rapid quenching of high-temperature dynamically disordered phases of BaCO 3, as well as metastable crystallization of the R 3 m phase at ambient pressure.