Stability and equation of state of post-aragonite BaCO₃

At ambient conditions, witherite is the stable form of BaCO₃ and has the aragonite structure with space group Pmcn. Above ~10 GPa, BaCO₃ adopts a post-aragonite structure with space group Pmmn. High-pressure and high-temperature synchrotron X-ray diffraction experiments were used to study the stability and equation of state of post-aragonite BaCO₃, which remained stable to the highest experimental P-T conditions of 150 GPa and 2,000 K. We obtained a bulk modulus K₀ = 88(2) GPa with K′ = 4.8(3) and V₀ = 128.1(5) ų using a third-order Birch-Murnaghan fit to the 300 K experimental data. We also carried out density functional theory (DFT) calculations of enthalpy (H) of two structures of BaCO₃ relative to the enthalpy of the post-aragonite phase. In agreement with previous studies and the current experiments, the calculations show aragonite to post-aragonite phase transitions at ~8 GPa. We also tested a potential high-pressure post-post-aragonite structure (space group C222₁) featuring four-fold coordination of oxygen around carbon. In agreement with previous DFT studies, ΔH between the C222₁ structure and post-aragonite (Pmmn) decreases with pressure, but the Pmmn structure remains energetically favorable to pressures greater than 200 GPa. We conclude that post-post-aragonite phase transformations of carbonates do not follow systematic trends observed for post-aragonite transitions governed solely by the ionic radii of their metal cations.