STRUCTURE AND BONDING OF CALCITE - A THEORETICAL-STUDY

The ground-state structural and electronic properties of bulk calcite (CaCO3) are calculated from first principles by working within the ful l-potential linearized augmented plane wave implementation of density functional theory. Calculated charge density plots for bulk calcite sh ow clearly the mixed ionic and covalent bonding in this mineral. From these plots the relative size of the constituent atoms are determined by calculating bonded radii. In calcite, the 0 atoms are shown to be s maller than expected from ionic measures of the atomic size and of a s imilar size to the Ca atoms. Examination of the one-electron eigenfunc tions and the computed density of states reveals the character of loca l bonding. Comparison of the computed density of states with recent X- ray photoemission data allows the orbital character of the experimenta l valence band peaks to be assigned. The highest occupied states are o f O 2p character, with Ca 3p states mixing slightly with carbonate gro up states in the valence region. The calculated value of the band gap in calcite is consistent with the experimental value, as measured by r eflection electron energy-loss spectroscopy. Lattice parameters are re produced to within 5% of their experimental equilibrium values. Under pressure the carbonate group is shown to act as a rigid structural uni t, with the C-0 bond length decreasing by approximately 3 mA, for a 5% compression in volume.