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
$K'$ = 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.
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