Structural and Thermodynamic Variation in Nickel Aluminate Spinel

New structural and calorimetric data for samples of NiAl₂O₄ quenched from 600° to 1560°C are presented The spinel remains stoichiometric for all heat treatments. Based on the refinement of X-ray powder patterns, it is shown that the degree of disorder, defined as the mole fraction of tetrahedral sites occupied by Al³⁺, changes from x = 0.82 at 600°C to 0.78 at 1560°C. Simultaneously, the lattice parameter and enthalpy vary in a complex manner with quench temperature. The largest lattice parameter (0.80500 ± 0.00004 nm) and most exothermic enthalpy of annealing (heat released when sample is equilibrated at 780°C; -10.1 kJ/mol) occur for the sample quenched from 1100°C. A linear correlation exists between the heat of annealing and the lattice parameter. The results have been interpreted as a superposition of at least two effects: (1) the disordering of Ni²⁺ and Al³⁺ ions between octahedral (16d) and tetrahedral (8a) sites and (2) a second process, which may be a small amount of the disordering of ions into the usually empty (16c) sites.