THE EXTENT OF ZINC-OXIDE SOLUTION IN ZINC CHROMATE SPINEL

Atomistic simulation methods have been applied to an ionic model and u sed 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 (i i) show that the formation enthalpies of Schottky and Frenkel defects are sufficiently high that negligible intrinsic disorder of these kind s is expected. The possibility, suggested by experiment, that large am ounts of ZnO may dissolve in the ZnCr2O4 lattice has also been investi gated. However, the results of the calculations presented here rule ou t 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 nat ure of the second electron affinity of the oxygen atom. Nevertheless, the oxidation-driven mechanism is supported by the calculated variatio ns of lattice parameter with composition, which are not subject to the same uncertainty.