Crystal Structure and Spectroscopic Studies of Ba_<i>x</i>Ca_1–<i>x</i>CO₃: 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_<i>x</i>Ca_1–<i>x</i>CO₃ (0.10 ≤ <i>x</i> ≤ 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 <i>x</i> ≤ 0.19 and <i>x</i> ≥ 0.27 form in the symmetry of <i>R</i>3̅<i>c</i> and <i>R</i>3̅<i>m</i>, respectively. The <i>a</i> and <i>c</i> 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 <i>R</i>3̅<i>m</i> 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 <i>P</i>–<i>T</i> conditions in Earth’s interior.