The Al array in Al2O3. A comparison with the [Al2]3− and [Al2Si2]2− Zintl polyanions in Li3Al2 and CaAl2Si2

Abstract The Al subnet in corundum is related to the diamond-like structure of Si. Each Al atom is four-fold coordinated, forming an inverted tetrahedron (umbrella-like). Three Al–Al distances (2.79 Å) are close to those of the parent fcc -Al (2.85 Å), and one has a shorter value of 2.66 Å. Unexpectedly, this cation subnet is similar, in topology and distances, to the 2D diamond-like layers formed by the [Al 2 ] 3− polyanions in the Zintl phase Li 3 Al 2 and is also topologically related to the [Al 2 Si 2 ] 2− subnet in CaAl 2 Si 2 .