Temperature dependence of the cation distribution in CoAl₂O₄ spinel

The cation distribution in synthetic CoAl₂O₄ spinel has been determined in samples quenched from 750 to 1200°C, by Rietveld structure refinement of powder X-ray diffraction data. The inversion parameter, x, increases smoothly from 0.11 at 750°C to 0.25 (± 0.005) at 1200°C, and can be described by a conventional thermodynamic model with α^Co-Al = 45.3 ± 0.5 and β = -18.3 ± 1.2 kJ/mol. The β term agrees well with the -20 kJ/mol suggested for all 2-3 spinels by O'Neill & Navrotsky (1984), and CoAl₂O₄ thus behaves similarly to the majority of other oxide spinels which show temperature-dependent cation distributions. -from Author