THE EXTENT OF ZINC-OXIDE SOLUTION IN ZINC CHROMATE SPINEL

Atomistic simulation methods have been applied to an ionic model and used to investigate the enthalpies of formation and reaction of defects in the spinel, ZnCr2O4. For the stoichiometric material our results (i) suggest that small amounts of exchange disorder are expected and (ii) show that the formation enthalpies of Schottky and Frenkel defects are sufficiently high that negligible intrinsic disorder of these kinds is expected. The possibility, suggested by experiment, that large amounts of ZnO may dissolve in the ZnCr2O4 lattice has also been investigated. However, the results of the calculations presented here rule out straightforward dissolution. On the other hand, they show that it is likely that the solubility of ZnO is substantially increased when the material is simultaneously oxidized. Unfortunately, the methods used to calculate the corresponding reaction energies in non-stoichiometric ZnCr2O4 are subject to some uncertainty arising from the variable nature of the second electron affinity of the oxygen atom. Nevertheless, the oxidation-driven mechanism is supported by the calculated variations of lattice parameter with composition, which are not subject to the same uncertainty.