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A computational investigation of cation ordering phenomena in the binary spinel system MgAl₂O₄-FeAl₂O₄

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

^* E-mail: rjh40{at}esc.cam.ac.uk

The systematics of cation ordering in binary spinel solid solutions have been investigated using an interatomic potential model combined with Monte Carlo simulations. The formalism to describe a system containing three cation species ordering over two non-equivalent sub-lattices is developed and the method applied to the MgAl₂O₄-FeAl₂O₄ binary solid solution. Our results compare favourably with experimental measurements of site-occupancy data, although the experiments display a slightly larger degree of non-ideality than the simulations. A possible kinetic origin of the non-ideal behaviour was examined by performing simulations in which only exchange of Mg and Fe²⁺ between tetrahedral and octahedral sites was permitted below the Al-blocking temperature of 1160 K. This approach improves the agreement with the experimental site occupancies, and suggests that the blocking temperature for moving Mg and Fe²⁺ between tetrahedral and octahedral sites is significantly lower than for moving Al.

KEYWORDS: spinel, hercynite, thermodynamics, Monte Carlo simulations, cation ordering