Crystal Structure and Spectroscopic Studies of Ba_xCa_1–xCO₃: The Order–Disorder Phase Transition of Calcite

Calcium carbonate (CaCO₃) is an important carbon host on Earth, playing a key role in the global carbon cycle. Barium-bearing carbonate has unique properties that demand a systematic study. Nine Ba_xCa_1–xCO₃ (0.10 ≤ x ≤ 0.89) solid solutions were synthesized at 2.5 GPa (gigapascal) and 1200 °C. Single-crystal X-ray diffraction analyses indicate that the phases with x ≤ 0.19 and x ≥ 0.27 form in the symmetry of R3̅c and R3̅m, respectively. The a and c axes and average Ca(Ba)–O bond length increase gradually with the increasing Ba²⁺ concentration. The replacement of Ca²⁺ by Ba²⁺ helps stabilize the aplanarity and disordered orientations of CO₃ groups down to ambient condition. X-ray photoelectron spectroscopy was measured on these synthetic samples, and their structural environments of atoms are similar to those in pure calcite. The Raman- and IR-active modes generally show a red-shift with the incorporation of Ba²⁺, without any significant difference between the ordered and disordered structures. The R3̅m phases transform to the aragonite-type structure around 1000 K. This study provides useful structural information for both bioinspired materials and carbonate minerals, with implications to the phase transitions in carbonates at high P–T conditions in Earth’s interior.