Temperature dependence of the cation distribution in CuAl2O4 spinel

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