The structure of strontium-doped hydroxyapatite: an experimental and theoretical studyElectronic supplementary information (ESI) available: Powder diffraction cif files for Sr1B2, Sr5B2, Sr10B2 and Sr15B2. See DOI: 10.1039/b802841a.

First-principles modeling combined with experimental methods were used to study hydroxyapatite in which Sr2 substituted for Ca2. Detailed analyses of cation–oxygen bond distributions, cation–cation distances, and site 1–oxygen polyhedron twist angles were made in order to provide an atomic-scale interpretation of the observed structural modifications. Density functional theory periodic band-structure calculations indicate that the Ca2 Sr2닦⭮똪 induces strong local distortion on the hydroxyapatite lattice: the nearest neighbor Sr–O bond structures in both cationic sites are comparable to pure SrHA, while Sr induces more distortion at site 2 than site 1. Infrared vibrational spectroscopy (FTIR) and extended X-ray absorption fine structure (EXAFS) analysis suggest increasing lattice disorder and loss of OH with increasing Sr content. Rietveld refinement of synchrotron X-ray diffraction patterns shows a preference for the Ca1 site at Sr concentrations below 1 at.%. The ideal statistical occupancy ratio Sr2/Sr1 = 1.5 is achieved for ∼5 at.%; for higher Sr concentrations occupation of the Ca2 site is progressively preferred. [ABSTRACT FROM AUTHOR]