Orthocarbonates are a newly discovered class of compounds that are stable at high pressures. The presence of sp3-hybridized carbon, having structural similarity to orthosilicates, and their potential participation in the global planetary carbon cycle have triggered intensive theoretical and experimental investigations into these compounds. Here, based on the density functional theory and crystal structure prediction calculations, we predict new stable crystal structures of the orthocarbonates Sr3CO5-Cmcm, Sr3CO5-I4/mcm, Ba2CO4-Pnma, and Ba3CO5-I4/mcm. Summarizing the obtained data, we show that orthocarbonates of alkaline-earth metals are isotypic to ambient-pressure orthosilicates with only rare exceptions. The lower-pressure stability limit for Ba-orthocarbonates is around 5 GPa. However, the stability limit increases with decreasing cation radius and reaches 13 GPa for Ca-orthocarbonates. Based on the calculations of Gibbs free energies with the quasi-harmonic approximation, the reaction 2M2CO4 = M3CO5 + MCO3 (M = Sr and Ba) is established. At 20 GPa, this reaction is realized at temperatures above 1080 K for Sr2CO4 and above 740 K for Ba2CO4, and the Clapeyron slope is positive in both cases. The obtained P-T diagrams for SrCO3 and BaCO3 show that equilibrium between the structures of aragonite and postaragonite is observed at 15-17 GPa for SrCO3 and 5-7 GPa for BaCO3. The transition pressure is almost independent of temperature. No other more favorable structures than postaragonite have been found for these compounds in the considered pressure range, up to 200 GPa. Thus, in contrast to CaCO3 and MgCO3, the transition from sp2 to sp3 hybridization is not realized for these compounds. Two of the found structures, Sr2CO4-Pnma and Sr3CO5-Cmcm, are dynamically stable at ambient pressure. This indicates the possibility of recovering the crystals from a high-pressure environment and conducting further laboratory investigation.
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