The formation of strontium apatites through alkaline hydrolysis of strontium hydrogen phosphate and their crystallographic characterization

Abstract

Pure and carbonate- or silicate-substituted strontium apatites (i.e., SrHAP, SrCAP and SrSiP) were synthesized via alkaline hydrolysis of strontium hydrogen phosphate using sodium hydroxide, carbonate, and metasilicate, respectively. Zinc substitution in SrSiP was performed by an ion exchange reaction to yield SrZnSiP. Characterization of these strontium apatites was carried out by powder XRD, Fourier-transform infrared (FTIR) spectroscopy, Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), SEM- Energy dispersive X-ray spectroscopy (EDS), FE-SEM and TEM- Selected Area Electron Diffraction (SAED). From elemental analyses, the Sr/P ratios of all samples were found to be around 1.5, suggesting that these are strontium-deficient apatites. From FTIR analysis, presence of both the structural OH group and the HPO4 group could not be confirmed. Crystal morphology strongly depends on the pH during sample preparation. At high pH, bundles of spindle-like nanocrystalline SrHAP, SrSiP and SrZnSiP were produced, whereas at lower pH, cubical crystallite of SrCAP with increased crystallinity was formed. Powder XRD analysis revealed the effects of these substituents on the hexagonal lattice parameters which agreed well with the known dependence for the corresponding substituted hydroxyapatite. The relationship between the lattice parameters a and c for calcium to barium hydroxyapatite groups collected from various studies together with our data, showed a universal linear relationship represented by the following equation, c = 1.1348a −3.8043 with a coefficient of determination (R2) of 0.9982. Dissolution behavior of SrZnSiP in pure water was investigated by the surface coating on β-TCP disc. For over 24 days, Sr, Si and Zn ions were released constantly within moderate concentration ranges.