Structural analysis of a series of strontium-substituted apatites

A series of Sr-substituted hydroxyapatites, (SrxCa1−x)5(PO4)3OH, where x = 0.00, 0.25, 0.50, 0.75 and 1.00, were made by a standard wet chemical route and investigated using X-ray diffraction (XRD), Rietveld refinement and Raman spectroscopy. We report apatites manufactured by two synthesis routes under 90 °C, and only the fully Sr-substituted sample had a small amount of an impurity phase, which is believed to be strontium pyrophosphate. Lattice parameters (a and c), unit cell volume and density were shown to increase linearly with strontium addition and were consistent with the addition of a slightly larger and heavier ion (Sr) in place of Ca. XRD Lorentzian peak widths increased to a maximum at x = 0.50, then decreased with increasing Sr content. This indicated an increase in crystallite size when moving away from the x = 0.50 composition (d ≈ 9.4 nm). There was a slight preference for strontium to enter the Ca(II) site in the mixed apatites (6 to 12% depending on composition). The position of the Raman band attributed to v1PO43- at around 963 cm−1 in hydroxyapatite decreased linearly to 949 cm−1 at full Sr-substitution. The full width at half maximum of this peak also correlated well and increased linearly with increasing crystallite size calculated from XRD.