PbSe, ionicity

We should also mention another class of ionic solids, the ten-electron solids, such as PbS, PbSe, and PbTe (in which lead contributes four and S, Se, or Te contributes six electrons per molecular unit). These form in the same structure as NaCl and can be thought of, by analogy, as consisting of closed-shell S , Se" ", or Te ions and Pb ions that have fully-occupied s states and completely empty p states the Pb is closed-shell in almost the sense that He is hence, the same approaches that are developed here for other ionic crystals are appropriate to the ten-electron solids. Another such set of compounds are TlCl, TlBr, and Til, which form in the same structure as CsCl. Still another ionic compound without an inert-... 

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... 

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