Zincblende, limiting structure

Yet a third simple cubic structure, not found among the alkali halides, occurs nevertheless in a limited number of ionic compounds of composition AX. This is the zincblende arrangement, shown in fig. 3.04, in which the co-ordination is fourfold or tetrahedral, each ion being... 

Among the covalent AX compounds which crystallize with either the zincblende or wurtzite structure are ZnS itself, ZnO, AIN, A1P, HgS, CuCl and many more. At first sight it would seem that such an arrangement is inconsistent with the known covalencies of the atoms involved. In ZnS, for example, the zinc and sulphur atoms, with two and six electrons, respectively, in the outermost shell, would be expected to be limited to a covalency of 2 and therefore to be capable of covalent... 

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