Preferential occupancy of strontium in the hydroxyapatite lattice in biphasic mixtures formed from non-stoichiometric calcium apatites

摘要

The present study reports the variations in phase content of biphasic mixtures and structural changes induced by different levels of strontium addition in calcium-deficient apatite (Ca/P = 1.60) powders during heat treatment. The synthesis was attempted by an in situ aqueous precipitation technique and X-ray diffraction, Raman spectroscopy and Rietveld refinement of the powder X-ray diffraction data were employed for comprehensive analysis. The results confirm the preferential occupancy of Sr2+ at two different Ca2+ sites of the hydroxyapatite [Ca-10(PO4)(6)(OH)(2), HAP] lattice, with the Ca2+ (2) site accommodating more Sr2+ than the Ca2+ (1) site. Increasing Sr2+ addition in calcium-deficient apatite has led to a decline in the phase content of beta-tricalcium phosphate [beta-Ca-3(PO4)(2), beta-TCP] in biphasic mixtures of HAP and beta-TCP. Sr2+ addition exceeding the critical limit of a (Ca + Sr)/P > 1.75 molar ratio has resulted in the formation of CaO as an additional phase, and this justifies the lack of enough PO43- ions to promote any kind of calcium phosphate precipitation. Sr2+ accommodation in the lattice sites of HAP has induced an increase in the lattice parameters and has also led to the significant distortion of the PO4 tetrahedron and OH groups, confirmed by Raman and FT-IR spectroscopic techniques.