Stability and equation of state of post-aragonite BaCO3

• Published in: Physics and chemistry of minerals 2013-05, Vol.40 (5), p.447-453
• Main Authors: Townsend, Joshua P., Chang, Yun-Yuan, Lou, Xiaoting, Merino, Miguel, Kirklin, Scott J., Doak, Jeff W., Issa, Ahmed, Wolverton, Chris, Tkachev, Sergey N., Dera, Przemyslaw, Jacobsen, Steven D.
• Format: Article
• Language: English
• Subjects: Aragonite; Bulk modulus; Carbonates; Cations; Crystallography and Scattering Methods; Density functional theory; Earth and Environmental Science; Earth Sciences; Enthalpy; Equations of state; Geochemistry; High temperature; Mathematical analysis; Mineral Resources; Mineralogy; Original Paper; Phase transitions; Stability; Synchrotron radiation; X-ray diffraction
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-013-0582-8

At ambient conditions, witherite is the stable form of BaCO 3 and has the aragonite structure with space group Pmcn . Above ~10 GPa, BaCO 3 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 BaCO 3 , which remained stable to the highest experimental P – T conditions of 150 GPa and 2,000 K. We obtained a bulk modulus K 0  = 88(2) GPa with  = 4.8(3) and V 0  = 128.1(5) Å 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 BaCO 3 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 C222 1 ) featuring four-fold coordination of oxygen around carbon. In agreement with previous DFT studies, Δ H between the C222 1 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.