Electronic, optical and bonding properties of CaCO3 calcite

The electronic, optical and bonding properties of CaCO3 (calcite) are investigated using first-principles calculations within density-functional theory. The indirect energy gap is estimated to be 5.07 eV and the direct allowed optical transitions along high symmetry directions in the Brillouin zone give the fundamental absorption edge (optical band gap) of ∼6.0 eV, which shows that the optical band gap is higher than the indirect band gap ( Γ → M ) and shows consistency with the experiment. The results also reveal a significant optical anisotropy of the calcite structure. Mulliken population analyses show the bonding properties, which reveal the presence of concurrent ionic and covalent bonding in calcite.