Structural, electronic and thermodynamic properties of britholites Ca10−xLax(PO4)6−x(SiO4)xF2 (0 ≤ x ≤ 6): Experiment and theory

• The structural and electronic properties of apatites with the general formula Ca10−xLax(PO4)6−x(SiO4)xF2 (0 ≤ x ≤ 6) have been investigated by means of the density functional theory.
• The calculated enthalpy of formation of the compounds increased with the substitution degree.
• The decrease in stability is probably due to the disorder induced in the Ca/LaO6F polyhedron, following the substitution.

The apatite-type compounds Ca10−xLax(PO4)6−x(SiO4)xF2 with 0 ≤ x ≤ 6 were prepared using a high temperature solid state reaction and were characterized by X-ray diffraction. The crystal structure, chemical bonding, electronic structure and formation energy of all relaxed structures were analyzed by density functional theory (DFT). The calculated results show that the predicted geometry can well reproduce the structural parameters. The incorporation of La3+ into the fluorapatite (FA) structure induced especially at the level of the S(2) sites a certain disorder which is responsible for the weakening in the stability with x. Excellent agreement were obtained between the calculated and experimental results. Moreover, the band structure indicates that despite the reduction of the band gap with x content all materials remain insulating.

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