Most oxides can be considered close packings of oxygen ions with the cations occupying the tetrahedral and/or octahedral sites in the structure. As an example, a-alumina (a- Al2O3) consists of an hep packing of O2~ with two thirds of the octahedral sites occupied by Al3+ in an orderly fashion.

Since for each O2" there exist one octahedral and two tetrahedral sites, in Al2O3 there would be three octahedral sites in which two Al3+ are placed thus two-thirds of the octahedral and none of the tetrahedral sites are filled.
The compound is electrically neutral, since 2 X (3+) (Al) = 3 X (2-) (O). If the Al is shared by six O's, then 3/6 = l/2 of its charge is contributed to each O. For the charge on each O to be satisfied, four Al's need to be coordinated to each O, since 4(1A) = 2.

A notation to indicate the coordination scheme for a-Al2O3 is 6:4—each Al is coordinated to six O's and each oxygen is coordinated to four Al's.

The structure of silicates is complicated, but the basic unit is the SiO4 tetrahedron. The three polymorphs of SiO2-quartz, tridymite and cristobalite—have different arrangements for the linking of all four vertices of the tetrahedron.

Each Si is bonded to four O's and each O is bonded to two Si's. In the layer silicates such as micas, clays, and talc, only three of the vertices are linked. The result is a laminar structure in which the bonding between layers is a weaker ionic bonding, hydrogen bonding, or van der Waals bonding, respectively, for mica, clay, and talc.

Of particular importance in semiconductors is the diamond structure. In this structure, each atom is tetrahedrally coordinated to four other atoms. The predominant covalent bonding of the structure is manifested by the high degree of directionality in the bonding.

In addition to diamond, Si and Ge have this structure, as do other semiconductors that have been doped with other elements.

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