The Fluoridation of Nonstoichiometric Calcium Hydroxyapatite. An Infrared Study

Calcium deficient hydroxyapatites were impregnated to varying degrees with fluoride in very dilute solutions of NaF at room temperature. Ca/P ratios were determined before and after fluoridation. Phosphate loss upon fluoridation was consistent with an initial mechanism of OH − vacancy filling by F − , with equivalent amounts of PO 4 3− ions escaping to restore charge balance. Infrared measurements confirmed previous evidence of OH/F exchange and the formation of hydrogen bonds, which increased with progressive thermal activation, considerable exchange having occurred already at 120 °C and possibly below. The first signs of lattice collapse occurred near 600 °C, with the fluorided sample exhibiting somewhat better stability.