Atomistic Simulation Study of Spinel Oxides: Zinc Aluminate and Zinc Gallate

Stoichiometric zinc aluminate (ZnAl2O4) and zinc gallate (ZnGa2O4) are simulated in the framework of the shell model, for which a new set of two-body interatomic potential parameters has been developed. Using these parameters, a reasonable prediction is made for elastic and dielectric constants of ZnAl2O4 and ZnGa2O4. Both oxides are stable against decomposition to the component oxides. The fitting of the potential energy surface of these oxides to the equation of state yields the bulk modulus and its pressure derivative. The bulk modulus is predicted to be higher in ZnAl2O4 as compared with that in ZnGa2O4, whereas the pressure derivative remains the same in both oxides. On the other hand, the octahedral and tetrahedral volumes of ZnGa2O4 are greater than those of ZnAl2O4. These differences in compressibility behavior can be attributed to the size difference between Al3+ and Ga3+ in the spinel oxides considered here. The calculated formation energies of the native defects suggest the preference of disorder in the cation sublattice over the Schottky and Frenkel defects. Although the degree of disorder is expected to be small, it is likely to influence the vacancy population in the lattice. Finally, deviations from stoichiometry are considered in which a preference for the dissolution of Al2O3/Ga2O3 via the formation of zinc vacancies is predicted relative to that of ZnO in ZnAl2O4/ZnGa2O4.