Exotic Fe^II/Fe^III 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).