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Disordering of MgAl₂O₄ spinel from first principles

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK

^* E-mail: m.warren{at}esc.cam.ac.uk

At high temperature, MgAl₂O₄ spinel is stabilized by disorder of Mg and Al between octahedral and tetrahedral sites. This behaviour has been measured up to 1700 K in recent neutron experiments, but the extrapolation of subsequently fitted thermodynamic models is not reliable. First principles simulation of the electronic structure of such minerals can in principle accurately predict disorder, but would require unfeasibly large computing resources. We have instead parameterized on-site and short-ranged cluster potentials using a small number of electronic structure simulations at zero temperature. These potentials were then used in large-scale statistical simulations at finite temperatures to predict disordering thermodynamics beyond the range of experimental measurements. Within the temperature range of the experiment, good agreement is obtained for the degree of order. The entropy and free energy are calculated and compared to those from macroscopic models.

KEYWORDS: disorder, spinel, neutron experiment, electronic structure calculation, monte carlo

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