Tetrahedral complexes extended bonding models

This approach has been successful in rationalizing the melting points, heats of formation and mixing, and various optical and electrical phenomena of tetrahedral semiconductor crystals (e.g., see Ref. 173). It is not clear how far it may be extended to other types of materials in the light of the complexities of bonding which many experiments demonstrate—also few experimental results may be related directly to a fractional ionicity for comparison. Levine, however, has generalized the dielectric model in terms of individual bond properties to several different structures. The critical ionicity ft = 0.785) between octahedral and tetrahedral coordination is not always appropriate for example PbS, PbSe, and PbTe (with rock-salt structures) have ft 0.6, and for LiH (also rock-salt)yi 0.1. This latter value would seem to be in... 

---