Reduction and Re-oxidation Behavior of Calcium Iron Phosphate, Ca9Fe(PO4)7

Reduction (in H2) and re-oxidation (in air) behavior of Ca9Fe(PO4)7 was studied by X-ray powder diffraction, Mössbauer spectroscopy, thermogravimetry, hydrogen absorption, electrical conductivity, and magnetic-susceptibility measurements. The β-Ca3(PO4)2-like framework of Ca9Fe(PO4)7 was stable in 100% H2 up to 820 K. In the temperature range from 680 to 820 K, reversible redox reactions occurred without changing the stoichiometry of oxygen and phosphorus atoms and destroying the structure. The reduction process in Ca9Fe(PO4)7 included the change of the oxidation state of Fe atoms and incorporation of hydrogen atoms into the structure to form Ca9FeH x (PO4)7 (0 < x ≤ 1). Above 820 K, treatment in 100% H2 was accompanied by loss in mass, partial destruction of the structure, and appearance of FeP and Fe2P phases. Re-oxidation studies in air showed that samples partially lost phosphorus atoms during reduction above 820 K. Behavior of Ca9Fe(PO4)7 under a reduction atmosphere depended on H2 concentration and temperature. At low H2 concentration, the sample destruction started from ca. 870 K and proceeded with maintaining the oxidation state of Fe, releasing H2O, and forming a β-Ca3(PO4)2-like phase and β-Ca2P2O7. In this case, iron phosphides appeared above ca. 1200 K.