Spectral and quantum mechanical investigation of calcium apatites isomorphically substituted in the anionic sublattice

The evolution of the valence band, the charge states of atoms, and the optical and vibrational spectra in Ca₁₀(PO₄)_x(VO₄)_y(AsO₄)_z(OH)₂ (x + y + z = 6) compound with three different types of oxygen tetrahedra have been studied by XPS, IR, and optical spectroscopy with the use of quantum mechanical calculation methods in the DFT approximation. It is shown that the occupied part of the valence band of compounds with three different types of tetrahedra has a pronounced band character with varying lengths of individual subbands. Two structural regions separated by energy are observed: the valence band's upper part and the valence band's lower part (subvalent states). The structure of the valence band's middle part, the region of valence states with energies from ∼ 13.0–19.0 eV, is weakly expressed. The sublattice of oxygen tetrahedra is decisive in forming the shape and the main features of the total density of electronic states of the compounds under study. The vibrations anharmonicity in the crystal lattice of such compounds can vary depending on the concentration of tetrahedra of a specific type. Such changes are local, and with the help of directed substitutions, it is possible to create the required spatial distribution of the anharmonic component over the crystal.

The design of the calcium apatites structure by the method of isomorphic substitutions of (XO₄) tetrahedra with tetrahedra of various types and with different ratios makes it possible to control the energy band gap width.