Orthocarbonates of Ca, Sr, and BaThe Appearance of sp³‑Hybridized Carbon at a Low Pressure of 5 GPa and Dynamic Stability at Ambient Pressure

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.