Substitution mechanism of alkali metals for strontium in strontium hydroxyapatite

Strontium hydroxyapatites substituted by alkali metals are synthesized by double decomposition method in basic medium. Structures of Sr9.50Na0.30(PO4)6(OH)1.30 (SrNaHAp) and Sr9.81K0.12(PO4)6(OH)1.74 (SrKHAp) are determined by X-ray powder diffraction. Both compounds are isotypic and crystallize in hexagonal system (space group P63/m) with the following cells: a=9.751(3) إ and c=7.279(3) إ for SrNaHAp and a=9.755(4) إ and c=7.284(3) إ for SrKHAp. Results are compared to those of Sr10(PO4)6(OH)2. According to the site occupancy factors, in SrNaHAp sodium is localized in site (I) and in SrKHAp potassium in site (II). Both structures contain vacancies in hydroxyl and metal sites. The mechanism of alkali metals substitution for strontium proposed explains the vacancies formation.