The formation mechanism of the β-TCP phase in synthetic fluorohydroxyapatite with different fluorine contents

Synthetic hydroxyapatite (HAP) and fluorohydroxyapatite (F_xAP) products may form the β-tricalcium phosphate (β-TCP) phase in a calcination process. The β-TCP phase has a greater tendency for degradation in vivo than HAP and F_xAP. Hence, controlling the content of the β-TCP phase in the apatite is a pivotal factor to affect their lifetime and stability in vivo. It is particularly important to explore the formation mechanism of the β-TCP phase in synthetic apatite. In this work, F _xAP products with a chemical composition of Ca₁₀(PO ₄)₆(OH)₂-_xF_x are synthesized, with x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0, using a precipitation method and a calcination process. The effect of fluorine substitution for hydroxyl is investigated by using x-ray diffraction analysis, Fourier transform infrared spectroscopy, and thermogravimetry and differential thermal analysis. The results show that addition of fluorine forms F_xAP that exhibits high thermal stability. The β-TCP phase produced as a result of the structural refinement by heat treatment is gradually reduced and dramatically suppressed with the fluorine content.