Temperature dependence of the cation distribution in nickel aluminate (NiAl₂O₄) spinel: a powder XRD study

NiAl₂O₄ is a largely inverse spinel, which in detail shows increasing randomisation with temperature of Ni and Al between the octahedral and tetrahedral cation sites of the spinel structure. We have used powder XRD to determine this cation distribution in various samples of NiAl₂O₄ quenched after annealing between 700 and 1400° C. The inversion parameter (x) can be measured with a precision of ± 0.004 (one standard deviation), and a comparison of different methods of synthesis, X-ray diffraction and refinement techniques, suggests a probable accuracy of better than 0.01. The results are supported by some preliminary single crystal refinements on flux-grown samples. Below 800° C the rate of cation ordering becomes very slow, and, despite reaching an apparently steady state, it is doubtful if our samples attained complete internal equilibrium. Above 1250° C the cation redistribution becomes so fast that the quenching method becomes unreliable. Between 800 and 1250° C inclusive, the degree of inversion changes smoothly from 0.87 at 800° C to 0.79 at 1250° C, and is accompanied by linear changes in u, the oxygen parameter, from 0.2555 to 0.2563 (±0.0002), and a₀, the lattice parameter, from 8.0462 to 8.0522 Å (±0.0002 Å). If no non-configurational entropy of disordering is assumed, the change of x with temperature can be described by a non-linear enthalpy of disordering model (O'Neill and Navrotsky 1983, 1984) with α^Ni-Al=17.5 ±2.3, β=-18.4± 1.4 kJ/g-atom. The β term thus agrees with the -20 kJ/g-atom suggested for all 2-3 spinels by O'Neill and Navrotsky (1984). However, if the expected electronic entropy of Ni²⁺ in tetrahedral co-ordination is included (9.13 J/K g-atom), a very different best fit value of β=+8.3 kJ/mol is obtained. The present data, despite their precision, cannot be used to distinguish between these alternative models.