Structural Characterization of Tunnel-Structured MgCO₃·H₂O and β‑MgCO₃: Two-Step Topotactic Dehydration of Nesquehonite Using High Pressure

This study achieved the structural characterization of two novel phases: monohydrated magnesium carbonate (MHMC; MgCO3·H2O) and a dehydrated phase of MHMC, a previously unknown crystalline polymorph of MgCO3 (β-MgCO3). MHMC was synthesized from nesquehonite (Nq; MgCO3·3H2O) at 500 MPa and 100 °C, while β-MgCO3 was synthesized by dehydrating MHMC. Due to the fine rod-shaped MHMC and β-MgCO3 crystals, microelectron diffraction was used to characterize the MHMC structure (a = 5.8102(6) Å, b = 5.3759(3) Å, c = 9.259(1) Å, space group: P212121, calculated density: 2.350 g/cm3). The MgCO3 rod modules of Nq were polymerized in the MHMC structure, accompanied by dehydration, and MHMC had a tunnel-type MgCO3 framework structure. The structure of β-MgCO3 was theoretically predicted based on the dehydrated MHMC structure and verified by the Rietveld refinement of the powder X-ray diffraction of β-MgCO3 (a = 5.1730(4) Å, b = 5.1597(5) Å, c = 8.8066(5) Å, space group: P212121, calculated density: 2.383 g/cm3). The calculated bulk moduli of MHMC and β-MgCO3 were 57.1(1) and 20.0(2) GPa, respectively.