Fe²⁺/Fe³⁺ substitution in hydroxyapatite: Theory and experiment

Electron paramagnetic resonance, Mössbauer spectroscopy, and electronic structure calculations were combined in order to study the local geometry of Fe²⁺/Fe³⁺ 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 Fe²⁺/Fe³⁺. 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 Fe²⁺ and Fe³⁺ and a surface hematitelike Fe³⁺ species.