Investigation of structural and optical properties in a new class Bi-Na-substituted fluorapatite: Bi3.13Ca2.74Na4.13(PO4)6F1.00

Crystals of bismuth sodium substituted fluorapatite with nominal formula Bi3Ca3Na4(PO4)6F were obtained via high-temperature synthesis. The compound was subjected to a complete structural characterization. Energy dispersive X-ray spectroscopy and inductively coupled plasma atomic emission spectroscopy confirmed the composition. Both powder and single-crystal X-ray diffraction analyses showed the typical hexagonal (space group P63/m) apatite structure, but characterized by the c unit-cell parameter tripled, 20.941(4) Å compared with conventional apatites. The structural study showed also that the compound is partially anionic deficient, and that stability is ensured by the presence of a fluorine atom in the Wyckoff (4e) position, on the senary axis of the tunnel. The study revealed five cationic sites, respectively three (4f) sites, one (6h) site and one (12i) site, occupied by the different cations, and two [PO4]3− environments. The refined crystal formula was Bi3.13Ca2.74Na4.13(PO4)6F1.00. The structural information was coupled by an optical characterization. UV-Vis spectroscopy revealed an absorption in the near ultraviolet (326.34, 336.33 and 352.31 nm), resulting from electronic transitions. The band gap energy values, determined by Tauc method at 4.35 eV and indirectly at 3.96 eV, confirmed the semiconducting nature of the sample. Besides, to evaluate the photoelectric potential, the compound was excited at 375 nm. Fluorescence measurements revealed, in the visible, an intense blue-green emission, with four peaks resulting from multisite distribution of active bismuth ion pairs, influenced by fluoride anion and phosphate groups. Overall, the semiconductor-like behavior and fluorescence properties of this compound suggest optical applications, e.g. photovoltaic systems, batteries, light-emitting diodes.