Solid-State NMR and Raman Spectroscopic Investigation of Fluoride-Substituted Apatites Obtained in Various Thermal Conditions

Fluoride-substituted apatites were synthesized by the standard sol-gel method and then calcined at three different temperatures: 800 °C, 1000 °C, and 1200 °C. Using a similar method, hydroxyapatite was synthesized and used as a reference material. The obtained powders were characterized by physicochemical methods: powder X-ray diffractometry, Raman spectroscopy, transmission electron microscopy, and solid-state nuclear magnetic resonance. All these methods allowed to identify additional α-TCP phase (tricalcium phosphate) in the HAP samples heated at 1000 °C and 1200 °C while fluoridated apatites turned out to be thermally stable. Moreover, Raman spectroscopy and NMR allowed to establish that the powders substituted with fluoride ions are not pure fluorapatite and contain OH- groups in the crystal structure. All the obtained materials had crystals with a shape similar to that of biological apatite.