Temperature dependence of the cation distribution in CoAl2O4 spinel
O’neill, Hugh St.C.
European Journal of Mineralogy Volume 6 Number 5 (1994), p. 603 - 610
published: Sep 28, 1994
manuscript accepted: Jun 1, 1994
manuscript received: Feb 16, 1994
ArtNo. ESP147050605008, Price: 29.00 €
Abstract 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 (O'Neill & Navrotsky, 1983, 1984) with αC-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 CoAI2O4 thus behaves similarly to the majority of other oxide spinels which show temperature-dependent cation distributions. Since Co2+ and Fe2+ usually show similar high temperature crystal-chemical behaviour, the present results emphasize the anomalous nature of the recent results for the temperature dependence of the cation distribution in FeAl2O4 (Larsson et al., 1994), which could not be well fitted by the usual thermodynamic model. The rate of cation re-ordering in CoAI2O4 at low temperatures (700°C) is qualitatively similar to other aluminate spinels, with the exception of FeAl2O4, in which the rates are much faster, due to intrinsic Fe3+/ vacancy defects. The apparently anomalous behaviour of FeAl2O4 may be an artefact of these rapid kinetics