Reduction and Oxidation Behavior of Ni<i>_x</i>Fe_3–<i>x</i>O_4−δ Spinels Probed by Reactive in Situ XRD

A semiempirical crystal model based on the hard sphere model is proposed to determine the oxygen deviation from stoichiometry (δ) of a mixed metal spinel of general formula A<i>_x</i>B_3–<i>x</i>O_4−δ from its lattice parameter. The model was calibrated with data for Ni- and Mn-ferrites taken from the literature. We demonstrate that the lattice parameter of a Ni<i>_x</i>Fe_3–<i>x</i>O_4−δ spinel can be predicted within a precision of 0.01 Å. This model was used to monitor the value of <i>x</i> and δ of Ni<i>_x</i>Fe_3–<i>x</i>O_4−δ nanopowders (with initial <i>x</i> = 0, 0.25, 0.5, and 1) during reactive in situ X-ray diffraction H₂ reduction and CO₂ oxidation at 400 °C. Results show that H₂ reduction occurs in two steps: (i) transition from a γ-type (δ < 0) to a regular (δ ≈ 0) spinel and (ii) preferential reduction of nickel from the spinel lattice to form a (Ni,Fe) solid solution. The face-centered cubic configuration for this alloy is favored in cases of high initial contents of nickel (<i>x</i> = 0.5, 1), and body-centered cubic for samples with low initial nickel content (<i>x</i> = 0, 0.25, 0.5). A subsequent CO₂ reoxidation of the samples shows that the process is partly reversible: iron will first be preferentially reintegrated into the lattice, and the initial excess of oxygen will be partially replenished. In addition to providing a thorough description of the phases and their evolution during reaction, these results describe the thermochemical behavior of nonstoichiometric nickel ferrites for the first time