Atomic and electronic structures of CaCO3 surfaces

The atomic structures and electronic states of CaCO3 surfaces in calcite and aragonite polymorph are theoretically investigated by performing electronic structure calculations within the density-functional theory. The calculated surface energies demonstrate that in both calcite and aragonite structures the surfaces, containing the same number of Ca atoms as CaCO3 groups around the top layer, are stabilized irrespective of the chemical potentials of constituting elements. The analysis of the electronic structures clarifies that the transfer of electrons between 4s orbitals of Ca atoms into 2p orbitals of O atoms, which are located near the surface, is crucial for the stabilization. Furthermore, we find the surface states caused by 2p orbitals of the O atoms in triangular CO32- located at the top or second layer appear above the valence band maximum of bulk CaCO3, leading to the reduction of C-O bonds of carbonate groups near the surfaces. The emergence of these surface states implies a possibility to identify the atomic structures by spectroscopic measurements.