Alloys high coordination number

In mercury rich alloys such as M Hg12 (M = K, Rb), a particular situation occurs due to high coordination number (CN) polyhedra (up to CN 22) in their structures. In these compounds the electropositive atoms are located in the centers of these polyhedra and are thus spatially separated from each other. The covalent Hg-Hg interaction, as discussed above, is of minor importance in these amalgams. 

In each phase an electropositive element with large size (alkaline earth) is alloyed with a partner with smaller size. As a consequence, the polyhedra around the alkaline earths present high coordination numbers ranging from 14 to 24. They have shapes generally different in each structure type and cannot be eeisily classified. On the contrary, the polyhedra around the partner elements have a lower number of vertices and a limited number of shapes, and they will be now considered. 

Narrow forbidden bands are observed whenever the bonding functions between atoms with similar effective nuclear charges extend to two or more sides of an atom and to two or more coordination partners. This situation is typical of metals and alloys in the crystalline and liquid states. However, it is encountered also in semiconductors such as Ge if the melt ias a high coordination number. 

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