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.

• Received 3 May 2002

DOI:https://doi.org/10.1103/PhysRevB.66.224107