Alpha α-alumina (also called Sapphire) crystal is the most popular form of crystalline aluminium oxide and has a corundum structure. Sapphire is the hardest of all oxide crystals, has good thermal properties, excellent electrical and dielectric properties, and is resistant to chemical attack. The Al atoms are octahedrally coordinated by six oxygen atoms. The oxygen ions nearly form a hexagonal close-packed structure with aluminium ions (Al3+) filling two-thirds of the octahedral interstices. Each Al3+ center is octahedral.

Sapphire (α-Al2O3) as a hexagonal structure, belonging to the space group R3c, can be expressed both as a hexagonal as well as a  rhombohedral unit cell. The basic structure consists of hexagonal close-packed planes of oxygen intercalated with aluminum planes. As shown in Figure 2591, the aluminum planes have a similar hexagonal close-packed arrangement but with 1/3 of the sites vacant, resulting in an Al/O ratio of 2/3. Each Al atom is coordinated by 6 oxygen atoms, and each oxygen atom has 4 Al neighbours. The vacancies are aligned on the so-called R-planes, giving sapphire the ability to cleave along these rhombohedral planes. Due to these properties, the unit cell of sapphire is chosen to take into consideration of the position of the A1 vacancies.

Figure 2591.The crystal lattice of sapphire. (a) A-, M-, R-, and C-planes of the crystal; (b) Schematic illustration of the aluminum planes. Oxygen planes are intercalated with the aluminum planes (not shown).

Table 2591a shows the lattice properties of α-Al2O3.

Table 2591a. The lattice properties of α-Al2O3.

Table 2591b shows the lattice mismatch (%) between the substrates and epitaxial layers, and the resulting misfit dislocation separation (in Å) corresponding to complete misfit relaxation for the basal plane interfaces.

Table 2591b. The lattice mismatch (%) between the substrate and epitaxial layers, and the resulting misfit dislocation separation (in Å).

Table 2591c. Coordinates of ions of two molecules in the rhombohedral unit cell α-A1203, α = 5,124 Å. Origin of coordinates is in the rhombohedron vertex. 

Table 2591d. Interplanar distances in α-A1203. Cu-radiation.