Temperature dependence of the cation distribution in CoAl (sub 2) O (sub 4) spinel

The cation distribution in synthetic CoAl (sub 2) O (sub 4) was determined, in samples quenched from 750-1200 degrees C, by Rietveld structure refinement of XRD data. The inversion parameter, x, increases smoothly from 0.11 at 750 to 0.25 (+ or - 0.005) at 1200 degrees C, and can be described by a conventional thermodynamic model, with alpha (super Co-Al) = 45.3 + or - 0.5 and beta = -18.3 + or - 1.2 kJ/mol. Since Co (super 2+) and Fe (super 2+) usually show similar high-T crystal-chemical behaviour, the present results emphasize the anomalous nature of recent results for the T-dependence of cation distribution in FeAl (sub 2) O (sub 4) , which could not be reasonably fitted to the usual thermodynamic model. This apparently anomalous behaviour of FeAl (sub 2) O (sub 4) may be an artefact of unusually fast rate of cation re-ordering in FeAl (sub 2) O (sub 4) due to intrinsic Fe (super 3+) /vacancy defects.