Electronic structure of carbonate-containing lead and calcium apatites

The atomic and electronic structure of carbonate-containing calcium and lead apatites were studied by experimental and computational methods. It was found, that under all types of doping of stoichiometric calcium (lead) apatite matrix with carbonate ions, a increase in electron density on calcium and phosphorus atoms observed suggests a rise in the ionic character within the overall chemical bonding balance. The density of states analysis shows that the isostructural doping of calcium and lead apatite matrix by carbonate ions does not violate the general trends in the formation of the valence band. The structure of the occupied part of the valence band of the studied compounds, as in the case of stoichiometric samples, has a pronounced band character with different width of individual subbands. In both Ca- and Pb- apatites, A-type substitution causes the unit cell to expand, mostly along a, b axis due to necessity of fitting the larger CO^3- into the central channel, which also causes C–O bond lengths contraction.