First-principles modeling combined with experimental methods were used to study hydroxyapatite in which Sr²⁺ is 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⁺ to Sr²⁺ substitution 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.

Electron paramagnetic resonance, Mössbauer spectroscopy, and electronic structure calculations were combined in order to study the local geometry of ${\mathrm{Fe}}^{2+}/{\mathrm{Fe}}^{3+}$ in Fe-doped hydroxyapatite. Atomistic simulations were carried out to obtain estimates of local geometry and lattice strain associated with fourfold, fivefold, and sixfold Fe sites. First-principles embedded cluster density functional calculations were performed to investigate the electronic structure associated with the substitution of calcium by ${\mathrm{Fe}}^{2+}/{\mathrm{Fe}}^{3+}.$ Mössbauer isomer shift, quadrupole splitting, and the hyperfine magnetic field were calculated for each site and local coordination, for comparison to an experimental fit to a five-line model consisting of two bulk sites each for ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ and a surface hematitelike ${\mathrm{Fe}}^{3+}$ species.