Zinc oxide crystals with additions of iron (III) oxide exhibited a characteristic inversion domain microstructure with domain boundaries on two different habit planes: parallel to {0001} basal planes and parallel to {2115} pyramidal planes of ZnO. The structural inversion of the domains was proved by electron diffraction experiments. In the present transmission electron microscopy study, emphasis was placed on the early stages of domain formation in sintered polycrystalline material and in diffusion couples with single crystals of ZnO. For solute iron content >0.5at% of the cations, defects nucleate at the surface and in the interior of ZnO grains at >900C. These primary defects propagate along the basal planes of ZnO and gradually widen in the positive c-axis direction of the ZnO host crystal. The widening along c was promoted by a second defect on {211l} planes that moves away from the basal plane defect. The c-axis orientation in the ZnO region swept by the second defect was inverted, finally resulting in the inversion domain microstructure. A low iron content ≈0.1at% was measured in the inverted domains. Energy-filtered imaging and quantitative electron energy-loss spectroscopy showed that the inversion domain boundaries (IDBs) parallel to the basal planes contain a full close-packed monolayer of iron whereas the pyramidal IDBs were occupied by iron with ≈2/3 of the content of the basal IDB. Based on experimental observations and arguments of structural chemistry, a mechanism was proposed explaining the nucleation and oriented growth of the inversion domains that were finally induced and driven by the trivalent iron ions at octahedral sites in the primary defects.

ZnO with Additions of Fe2O3: Microstructure, Defects, and Fe Solubility. Köster-Scherger, O., Schmid, H., Vanderschaeghe, N., Wolf, F., Mader, W.: Journal of the American Ceramic Society, 2007, 90[12], 3984-91