Exotic FeII/FeIII Local Environments in the Hexagonal Channels of Hydroxyapatite

In this fundamental solid-state chemistry study, two sample series were investigated in depth: iron­(III)-doped hydroxyapatite (HA) compounds obtained from a co-sintering process of hematite and pure HA under air and iron­(III)-doped HA compounds obtained from a co-sintering process from iron­(II) acetate and pure HA under an argon atmosphere. X-ray diffraction, UV–visible, Fourier transform infrared, 1H and 31P NMR, electron paramagnetic resonance (EPR,) and Mössbauer spectroscopy methods were coupled to unravel the Fe valence states, the interactions with other anionic species (OH– and PO4 3–), and finally the complex local environments in hexagonal channels in both the series. In particular, we highlighted the associated mechanism to ensure electroneutrality with a focus on deprotonation versus calcium substitution. By diverging mechanisms, Fe3+ and Fe2+ ions were found to be located in different coordinated sites: 4­(+1) coordinated site for Fe3+ and 2­(+3) coordinated site for Fe2+ and clearly associated with very different Mössbauer and EPR signatures as various absorption bands (leading to different sample colors).