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 Pmmm. 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 2,000K. We 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 ~8GPa. 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, delta H between C2221 structure and post-aragonite (Pmmm) decresases with pressure, but the Pmmm structure remains energetically favorable to pressure 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.
J.P. Townsend, Y.-Y. Chang, X. Lou, M. Merino, S.J. Kirklin, J. W. Doak, A. Issa, C. Wolverton, S.N. Tkachev, P. Dera, S.D. Jaconsen, “Stability and equation of state of post-aragonite BaCO3“, Phys. Chem. Minerals, 2013, 40:447-453. DOI 10.1007/s00269-013-0582-8.
High-pressure polymorphs of BaCO3. Aragonite (Pmcn) (top) is stable under ambient conditions and features carbon in 3 coordination. Post-aragonite (Pmmm) (middle) is stable from ~9GPa to at least 150 GPa. Post-post-aragonite (C2221) (bottom) has not yet been observed experimentally, but features carbon in four coordination. In all figures, the large spheres represent barium, the small light spheres represent carbon, and the small dark spheres represent oxygen.