| Abstract
| - Reduction (in H2) and re-oxidation (in air) behavior of Ca9Fe(PO4)7 was studied by X-raypowder diffraction, Mössbauer spectroscopy, thermogravimetry, hydrogen absorption, electrical conductivity, and magnetic-susceptibility measurements. The β-Ca3(PO4)2-like frameworkof Ca9Fe(PO4)7 was stable in 100% H2 up to 820 K. In the temperature range from 680 to820 K, reversible redox reactions occurred without changing the stoichiometry of oxygenand phosphorus atoms and destroying the structure. The reduction process in Ca9Fe(PO4)7included the change of the oxidation state of Fe atoms and incorporation of hydrogen atomsinto the structure to form Ca9FeHx(PO4)7 (0 < x ≤ 1). Above 820 K, treatment in 100% H2was accompanied by loss in mass, partial destruction of the structure, and appearance ofFeP and Fe2P phases. Re-oxidation studies in air showed that samples partially lostphosphorus atoms during reduction above 820 K. Behavior of Ca9Fe(PO4)7 under a reductionatmosphere depended on H2 concentration and temperature. At low H2 concentration, thesample destruction started from ca. 870 K and proceeded with maintaining the oxidationstate of Fe, releasing H2O, and forming a β-Ca3(PO4)2-like phase and β-Ca2P2O7. In thiscase, iron phosphides appeared above ca. 1200 K.
- Reduction (in H2) and re-oxidation (in air) behavior of Ca9Fe(PO4)7 was studied. 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. Above 820 K, treatment in 100% H2 was accompanied by a loss in mass, partial destruction of the structure, and appearance of FeP and Fe2P phases. At low H2 concentration, the sample destruction started from ca. 870 K.
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