Structural and spectroscopic properties of calcium-iron Ca9Fe(PO4)7 phosphate

Fresh (f-) and reduced (r-) Ca9Fe(PO4)7 whitlockite-like compounds were prepared and characterized by X-ray diffraction (XRD), IR, temperature programmed reduction and oxidation (TPR/TPO), UV-visible, electron paramagnetic resonance (EPR), and 57Fe Mِssbauer spectroscopy. TPR/TPO, XRD, and IR investigations showed that cycles of reduction and oxidation reversibly affected the iron cations without noticeable modification of the structure. The reduction by H2 was accompanied by a protonation of the sample and a slight increase of the unit-cell parameters. It was observed that the reduction of Fe3+ to Fe2+ never exceeded 91%. The simulation of f-Ca9Fe(PO4)7 EPR spectra and the Mِssbauer investigations confirmed that the Fe3+ cations are located in highly symmetrical sites. These techniques also revealed that in r-Ca9Fe(PO4)7 the residual ferric ions are in a rhombic symmetry and the Fe2+ ions are in octahedral sites slightly distorted by the Jahn-Teller effect.