DFT Calculations with van der Waals Interactions of Hydrated Calcium Carbonate Crystals CaCO₃·(H₂O, 6H₂O): Structural, Electronic, Optical, and Vibrational Properties

The role of hydration on the structural, electronic, optical, and vibrational properties of monohydrated (CaCO₃·H₂O, hexagonal, P3₁, Z = 9) and hexahydrated (CaCO₃·6H₂O, monoclinic, C2/c, Z = 4) calcite crystals is assessed with the help of published experimental and theoretical data applying density functional theory within the generalized gradient approximation and a dispersion correction scheme. We show that the presence of water increases the main band gap of monohydrocalcite by 0.4 eV relative to the anhydrous structure, although practically not changing the hexahydrocalcite band gap. The gap type, however, is modified from indirect to direct as one switches from the monohydrated to the hexahydrated crystal. A good agreement was obtained between the simulated vibrational infrared and Raman spectra and the experimental data, with an infrared signature of hexahydrocalcite relative to monohydrocalcite being observed at 837 cm^–1. Other important vibrational signatures of the lattice, water molecules, and CO₃^2– were identified as well. Analysis of the phonon dispersion curves shows that, as the hydration level of calcite increases, the longitudinal optical–transverse optical phonon splitting becomes smaller. The thermodynamics properties of hexahydrocalcite as a function of temperature resemble closely those of calcite, while monohydrocalcite exhibits a very distinct behavior.