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

Abstract

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 [Ca₁₀(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 β-tricalcium phosphate [β-Ca₃(PO₄)₂, β-TCP] in biphasic mixtures of HAP and β-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.