Design of the electronic structure and properties of calcium apatites via isomorphic modification of the cation sublattice, and prospects of their application

Abstract

The evolution of the valence band, charge states of atoms, and optical and vibrational spectra in compounds Ca10−xMx(PO4)xY2, M = Fe, Ni, Cu, Mg; Y = OH, Cl, F was studied by using XPS, infrared, and optical spectroscopy, with the addition of quantum mechanics calculations. The changes in the bandgap in these compounds were analyzed. Isomorphic substitution of calcium ions in the cationic sublattice of calcium hydroxyapatite by metal ions changes the shape of the curve that represents the occupied part of the valence band only slightly. It retains a pronounced gapped character with different lengths of individual subbands—the upper and lower parts of the valence band. It is shown that the predominant position of rare earth and uranium atoms in the apatite structure is the Ca(2)-position. Isomorphic substitution of calcium atoms by metal atoms (Fe, Ni, Cu, Mg) in the apatite structure in the range of 1%–2% of atoms leads to the narrowing of the energy gap. The most significant narrowing is observed when calcium is substituted by nickel and copper. The theoretically calculated bandgap width in calcium apatites can be well described in terms of the generalized gradient approximation. The design of the structure of calcium apatites via the method of isomorphic substitutions in the cation sublattice makes it possible to control the bandgap width, thus expanding the field of practical application of these compounds.