Preparation and characterization of fluoride-substituted apatites.

Apatites were prepared with three different fluoride concentrations: 0.0 mM (pure hydroxyapatite) 2.5 mM and 5 mM. Reactions were performed in aqueous medium using a reaction between diammonium orthophosphate and calcium nitrate 4-hydrate and ammonium fluoride at temperatures of 3 degrees, 25 degrees, 60 degrees and 90 degrees C. The effects of reaction temperature and fluoride concentration on the crystal morphology, phase purity and crystallinity of the precipitates were observed, using transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and ion chromatography. Transmission electron micrographs revealed that the crystallites precipitated at 3 degrees C were spheroidal, but became increasingly acicular with increasing precipitation temperature. X-ray diffraction results indicated that all the materials produced were phase pure and that the crystallinity of apatites prepared at higher precipitation temperatures was higher than those prepared at lower precipitation temperatures. A significant difference in the a-axis dimension of fluoride-substituted apatites was observed, as compared to hydroxyapatite. FTIR spectroscopy revealed a hydroxyl band at 3568 cm-1, along with a broad peak of adsorbed water in the region of 3568 cm-1 to 2670 cm-1 in the hydroxyapatite and fluoride-substituted apatite powders. Hence by careful selection of the precipitation conditions and fluoride contents, the composition and morphology of fluoride-substituted apatite may be controlled and this has interesting implications for the development of these materials for biomedical implantation.