Temperature dependence of the cation distribution in CuAl2O4.

The temperature dependence of the cation distribution in CuAl2O4 spinel has been determined from 600 to 1100°C on\ud quenched specimens by powder X-ray diffraction, and from room temperature to 1000°C by in-situ high-temperature powder\ud neutron diffraction. The results, in agreement with earlier work, show that CuAl2O4 is a largely normal spinel, which is already\ud highly disordered at 600°C, the lowest temperature at which the cation distribution is inferred to be in equilibrium, with an inversion\ud parameter, x, at this temperature of 0.35 ± 0.005 from the neutron diffraction experiments. Increasing temperature to 1000°C\ud only causes a modest increase in x, to 0.40 ± 0.005. This somewhat unusual behaviour may be ascribed to local distortions from\ud the Jahn-Teller effect in both octahedrally and tetrahedrally coordinated Cu2+. Thermodynamically, the ordering may be described\ud either with a large entropy of disordering, or alternatively, with a large and positive quadratic term in the enthalpy of disordering.\ud The Jahn-Teller effect produces no long-range distortion of the structure, which has cubic symmetry (space group Fd–3m) under\ud all investigated condition