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 PbCO3, SrCO3, and BaCO3. As a result, we found the new calcite-like R3m structure thermodynamically stable for PbCO3, SrCO3, and BaCO3. Without consideration of the temperature effect, PbCO3-R3m is stable in the pressure range of 15-30 GPa. With increasing temperature, the stability field of PbCO3-R3m in P-T coordinates reduces, and above 2000 K, the aragonite phase directly transforms into the post-aragonite phase without the formation of R3m. SrCO3-R3m is thermodynamically stable at pressures 10-20 GPa and temperatures above 750 K. BaCO3-R3m is dynamically stable at ambient pressure and the available experimental data indicate its appearance at temperatures above 1086 K. By analogy with KNO3, we assume the possibility of the appearance of BaCO3-R3m during rapid quenching of high-temperature dynamically disordered phases of BaCO3, as well as metastable crystallization of the R3m phase at ambient pressure.