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Temperature dependence of the cation distribution in CuAl₂O₄ spinel

¹ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
² Bragg Institute, Building 58, Australian Nuclear Science and Technology Organisation (ANSTO), PMB 1, Menai NSW 2234, Australia
³ Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA
⁴ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

^* E-mail: hugh.oneill{at}anu.edu.au

The temperature dependence of the cation distribution in CuAl₂O₄ spinel has been determined from 600 to 1100°C on quenched specimens by powder X-ray diffraction, and from room temperature to 1000°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°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°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 Fdm) under all investigated conditions.

Key-words: spinel, order-disorder.

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