Dielectric properties of fluorine substituted hydroxyapatite: the effect of the substitution on configuration of hydroxide ion chains

Abstract

Hydroxyapatite (HAp) [Ca₅(PO₄)₃OH] is an extremely popular biomaterial. Moreover, HAp durability is well-known to be enhanced by fluoridation. We prepared fluoridated HAp (F-HAp; Ca₅(PO₄)₃(OH)_1−xF_x) ceramics by substituting F⁻ ions for OH⁻ ions of HAp. F-substitution dependence of dielectric properties in F-HAp was measured to detect the configuration change of OH⁻ ion chains. Dielectric relaxation of two kinds was observed. Each kind had a different relaxation time. Faster relaxation, which is associated with the reorientation motion of OH⁻ ions, was only observed at the low F-substitution range (0 ≤ x < 0.35). The relaxation strength decreased as the F-substitution increased. It became zero at x = 0.35, suggesting that F-substitution induces hydrogen bonds. One F⁻ ion substituted for an OH⁻ ion forms two hydrogen bonds with the two neighboring OH⁻ ions and inhibits the motion of OH⁻ ions, which results in some kind of ordered configuration in F-HAp. The configuration might be an origin that enhances the durability of the F-HAp.