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Stability and equation of state of post-aragonite BaCO3 |
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| Authors: | Joshua P Townsend Yun-Yuan Chang Xiaoting Lou Miguel Merino Scott J Kirklin Jeff W Doak Ahmed Issa Chris Wolverton Sergey N Tkachev Przemyslaw Dera Steven D Jacobsen |
| Institution: | 1. Department of Earth and Planetary Sciences, Northwestern University, 2145 Sheridan Rd., Evanston, IL, 60208, USA 2. Department of Materials Science and Engineering, Northwestern University, Evanston, IL, 60208, USA 3. Argonne National Laboratory, Center for Advanced Radiation Sources, The University of Chicago, Argonne, IL, 60439, USA |
| 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 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 K 0 = 88(2) GPa with $K'$ = 4.8(3) and V 0 = 128.1(5) Å3 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 C222 1 ) featuring four-fold coordination of oxygen around carbon. In agreement with previous DFT studies, ΔH 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. |
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