The Effect of Pressure on the Structure and Electronic Properties of Hydrated Calcium Carbonates

Abstract

This paper studies the effect of pressure on the structure and electronic properties of CaCO3-H2O and CaCO3-6H2O crystalline hydrates. The study is based on the density functional theory (DFT) and the linear combination of atomic orbitals (LCAO) method. Calculations are performed using the CRYSTAL17 software package and the PBE gradient functional. The calculated lattice parameters of hydrated calcium carbonates and their dependence on external hydrostatic pressure are shown to be in good agreement with the available experimental measurements. Dependencies of linear compressibility on the direction are obtained using the calculated pressure dependencies of the structural parameters. It is demonstrated that the linear compressibility of calcium carbonate hexahydrate, in contrast to calcium carbonate monohydrate, is highly anisotropic (the smallest and largest compressibility values are correlated as K max /K min ~ 4). In this case, the maximum compressibility is located between crystallographic axes (between the axes a and c) and not along the crystallographic axes. The bulk modulus for the monohydrate (CaCO3-6H2O) is greater than for the hexahydrate (CaCO3-6H2O). Total and partial densities of electronic states for CaCO3-H2O and CaCO3-6H2O are calculated. Also, dependencies of the band gap width on pressure for hydrated calcium carbonates are established. It is shown that, with increasing pressure, the increase of the band gap is greater for CaCO3-6H2O than for CaCO3-H2O.