One of the most common oxide crystal structures is of corundum or a-alumina (Al2O3). The stoichiometry of the corundum-type structures is M2X3. These com­pounds are also known as “sesquioxides.” The basic structure contains hexagonal close-packed layers of the anion (i. e., O2~) sublattice. If the cations need to fill in all the octahedral sites, the stoichiometric ratio becomes MX as the number of the octahedral sites is equal to the number of the regular lattice sites in a HCP struc­ture. Because of the particular stoichiometry of Al2O3 (i. e., cation/anion = 2:3), the cations only fill in two-thirds of the available octahedral interstitial sites, however,

maintaining maximum cation separation in an orderly manner. A simpler way to understand the corundum structure is to see the ion positions from both the top and a vertical section. A basal plane of a corundum structure is shown in Figure 2.24a. The picture shows a layer of cations (indicated by filled circles) forming a hexagonal network with the same ion spacing between the two anion layers (only one layer is delineated by the larger open circles in this figure). Note that x positions indicate the unfilled octahedral sites. The next cation layer would have the same hexagonal configuration, but shifted to one atom spacing in the direction of “vector 1,” as shown in Figure 2.24a. When an another close- packed oxygen layer is added on top of this cation layer, an another cation layer will sit on it, although shifted by yet another atom spacing in the direction of “vector 2” while maintaining the similar hexagonal configuration. The structure becomes clearer if we take a vertical plane {1010} across the corundum struc­ture, as shown in Figure 2.24b. This shows that each octahedral column of cat­ions contains two cations in a row followed by one missing cation and so on. If one takes into account the periodic spacing of both the cations and anions at the same time, we find that the structure repeats itself after six such layers (shown as ABABAB), thus making the lattice constant in the vertical direction (c) as 1.299 nm. As the octahedral sites in the corundum share faces and two out of every three octahedral sites are occupied, the electrostatic repulsion between cations helps move them slightly into the unfilled octahedral site, whereas oxy­gen ions also shift slightly from their ideal close-packed configuration, thus making the corundum structure bit inherently distorted.

The corundum structure is exhibited by a multiple number of oxides, such as Fe2O3, Cr2O3, Ti2O3, V2O3, Ga2O3, Rh2O3, La2O3, Nd2O3, and so on. There are many ternary compounds such as ilmenite (FeTiO3) and lithium niobate (LiNbO3)

that assume “derivative” corundum structures. The Cr-containing steels in contact with water generally have corrosion layers composed of Fe2O3 and Cr2O3, both of which have a corundum-type crystal structure. Furthermore, the rare earth-based fission products, such as Nd2O3 and La2O3, have corundum-type crystal structure.

2.1.9