LAUSR

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 AxB3–xO4−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 NixFe3–xO4−δ spinel can be predicted within a precision of 0.01 A. This model was used to monitor the value of x and δ of NixFe3–xO4−δ nanopowders (with initial x = 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 (x = 0.5, 1), and body-centered cubic for samples with low initial nickel conten...