Stability and equation of state of post-aragonite BaCO3

Abstract

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 exper- iments 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 0 = 4.8(3) and V0 = 128.1(5) A u 3 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 calcula- tions 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, DH 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 system- atic trends observed for post-aragonite transitions governed solely by the ionic radii of their metal cations.