[Encyclios]

Ionic compensation

There are four possibilities for cation substitution, as with non-stoichiometric compounds:

1. substitution with higher-charge cations > creation of cation vacations: if the substituent cation has a higher charge, cation vacations can be created to preserve electroneutrality. For example NaCl can dissolve a small amount of CaCl2 by substituting two Na+ ions with one Ca2+ ion leaving one Na site vacant.
2. Substitution with higher charged cations > creation of interstitial anions: the other mechanism, if the substituting cation has a higher charge, is to create interstitial anions. This is not very common because in many structures there are not large enough interstitial sites to accommodate extra anions. Only in certain cases can fluorite type structures function in this way.
3. substitution with less charged cations > creation of anionic vacancies: if the cation to be substituted has a higher charge than the one being replaced, the charge balance can be maintained by creating anionic vacancies or interstitial cations. The best known example is still with the structure of fluorite, in oxides such as zirconia, ZrO2. These materials are very important in modern technology, both as ceramic materials and as solid electrolytes (with oxide ions as conductors).
4. substitution with lower charged cations > creation of interstitial cations: this is a common mechanism of solid solution formation, although the most important structural examples (some aluminosilicates) are quite complex.

A similar pattern is possible for anionic substitution, but this is much less common in solid solutions.