Ionicity in solids

The authors review the use of the ideas of 'ionicity' and 'covalency' in quantitative studies of the solid state. Such ideas have taken several distinct forms: a description of charge distributions of bonding, a spectroscopic classification of perfect solids, and a framework for quantitative modelling of defects in polar solids. The limits on the usefulness of ionic pictures clearly depend on which form of idea and which application is involved. Nevertheless, many misunderstandings persist through failing to recognise that unique, universal, charges cannot be assigned except in trivial cases. Further confusion arises from the temptation to ascribe almost any unexpected features of ionic crystals to covalence. They discuss in particular the use of ionicity in the quantitative modelling of polar solids. Here, it appears, the ionic picture provides a framework for defect models and theories which, properly used, covers both the traditional 'ionic' crystals and ones, like silicates, where appreciable covalency is acknowledged.