Nanocubes or nanorhombohedra? Unusual crystal shapes of corundum-type indium oxide

Sol-gel processing via the modified nonalkoxide route resulted in the formation of nanocrystals of the corundum-type In2O3 polymorph, which revealed characteristic rhombohedra with a morphology very close to cubes. Because the cubic bixbyite-type In2O3 polymorph is more abundant under ambient conditions, the seemingly cube-shaped morphology might confuse the experimentalist. As shown by transmission electron microscopy, this morphology does not reflect any relation to a cubic structure but appears accidentally with form {012} at a specific axis ratio of corundum-type structures. The pseudocubic particles have also been reported for alpha-Fe2O3, also in this case the pseudocubic nanorhombohedra are terminated by {012} facets.