Aluminium oxide

Al₂O₃
Wikipedia: Aluminium oxide

Al₂O₃ (alumina) adopts two main structures, α-Al₂O₃ (corundum) and γ-Al₂O₃.

The crystal structure of corundum is rhombohedral (space group R3c, No. 167) and can be thought of as "an hcp array of oxide ions with Al ordered on two-thirds of the octahedral interstices" (G&E, based on Wells).

According to Greenwood & Earnshaw, "The structural relations between the many crystalline forms of aluminium oxide and hydroxide are exceedingly complex but they are of exceptional scientific interest and immense technological importance."

Good times!

α-Al₂O₃
Acta Cryst. (2001), A57, 290–303