Behavior of Iron Ions in Iron Spinel (Iron Aluminum Oxide) Fired under Various Oxygen Partial Pressure

Abstract

Iron Oxide (III) and aluminum oxide (α) were mixed at the molar ratio of 1 to 2, fired for 3 hours under various oxygen partial pressure (P02); 3.9×10-7Pa≤P02≤2.1×104Pa at 1300°C, , or 4.0×10-6Pa≤P02≤2.1×104Pa at 1400°C. The lattice constant of the iron spinel and the valence of iron ions were measured. From the X-ray powder diffraction pattern, the distribution of iron ions was obtained by the PFSR method. The lattice constant was almost constant at low P02, increased gradually with the increase of P02, then increased steeply at P02≈10-1.5Pa (1300°C), or P02≈100Pa (1400°C), and finally remained unchanged at high P02. The valence of iron ions showed the similar change. At low P02, the iron ions were all bivalent, and FeAl2O4 was produced. About 80% of the tetrahedral sites and 10% of the octahedral sites were occupied by iron ions, because of the tetrahedral site preference of Fe2+ ions to Al3+ ions. With the increase of P02, a part of Fe2+ ions was oxidized to Fe3+ ions, and substituted for Al3+ ions with a smaller radius, resulting in an increase in the lattice constant. In iron spinel containing Fe3+ ions, the ratio of the tetrahedral sites occupied by iron ions increased because of their tetrahedral site preference over Fe2+ ions. In the low P02 region, the oxidation of the iron ions and the substitution of Al3+ ions in the tetrahedral sites proceed stepwise with increasing P02. The steep increase of the lattice constant and the amount of Fe3+ ions observed at a certain P02 has been explained as due to the remarkable change in the cation distribution accompanying the promotion of the oxidation of Fe2+ ions.