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.
