Single-crystal X-ray diffraction of Ba <i>x</i> Ca1- <i>x</i> CO3 (0 ≤  <i>x</i>  ≤ 0.79) solid solutions from 100 to 500 K: Implications for thermal expansivity of disordered calcite

Abstract Disordered CaCO3 calcite is the high-temperature (high-T) polymorph of aragonite in the upper mantle. Its thermoelastic properties have not been determined since it cannot be quenched to the ambient conditions, although these properties are relevant to tracing carbonates in the Earth’s deep interior by seismological methods. In the prior synthetic experiments at 2.5 GPa and 1473 K [Hu et al. (2024) J. Phys. Chem. C., 128, 6138-6148], the BaxCa1-xCO3 solid solutions crystallized in the ordered (0 ≤ x ≤ 0.19) and disordered (0.27 ≤ x ≤ 0.79) calcite structures. In this study, in situ single-crystal X-ray diffraction (XRD) measurements were performed on these samples from 100 to 500 K. The volumetric thermal expansion coefficients for the disordered samples fall within a narrow range of 5.8 ∼ 6.3 × 10-5 K-1, closely resembling those of aragonite and witherite, and systematically exceeding the ones in the ordered calcite-type structure. Ba2+ exhibits minimal influence on the thermal expansivities in the disordered solid solutions, and the linearly extrapolated αV(T) for disordered CaCO3 calcite is quite similar to that for aragonite. The anisotropy of axial thermal expansivity follow the trend: ordered calcite &amp;gt; aragonite &amp;gt; disordered calcite. The MO9-polyhedra (M = Ca and Ba) in both aragonite and disordered calcite could mitigate the anisotropy of thermal expansion, and are more sensitive to temperature variation, as compared with the MO6-octahedron in ordered calcite. The longer M-O bonds exhibit larger thermal expansivity than the shorter ones in the disordered samples. The Kumar V(T) equation, established on Debye model, is adopted to extrapolate the αV(T) models to higher temperatures, and the results for the disordered solid solutions still match well with aragonite and witherite at geologically relevant temperatures. Low-temperature measurements are necessary and important for comprehensive investigation on pressure-volume-temperature (P-V-T) equations of state (EOSs), as the αV coefficient increases rapidly below 300 K, and progressively at higher temperatures. Conclusively, this study exemplifies the successful utilization of ambient-stable BaxCa1-xCO3 analogs to explore the thermoelastic properties of disordered calcite.