Dense Hydrated\nMagnesium Carbonate MgCO<sub>3</sub>·3H<sub>2</sub>O Phases

The study of the\nstructural stability of carbonates under\ndifferent\npressure and temperature conditions is important for modeling the\ncarbon budget in the Earth’s interior and the stability of\ncarbonation products of carbon capture and storage (CCS) solutions.\nIn this work, we confirm the existence of the two dense polymorphs\nof the hydrated magnesium carbonate MgCO<sub>3</sub>·3H<sub>2</sub>O nesquehonite mineral previously reported, and we characterize their\nstructures using synchrotron single-crystal X-ray diffraction at 3.1\nand 11.6 GPa. Phase transitions entail the distortion and atomic rearrangement\nof the Mg-centered polyhedra and the tilting of the [CO<sub>3</sub>] carbonate units. In particular, the Mg coordination number increases\nfrom 6 in nesquehonite to 7 in the second high-pressure phase, while\nmaintaining a topology based on complex MgCO<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub> chains. We also studied their vibrational behavior\nupon compression using Raman spectroscopy and complemented the experimental\nresults with density-functional theory (DFT) calculations. The role\nplayed by hydrogen bonds in the compressibility and the polymorphism\nof this hydrated carbonate is also discussed.