Ionic formula variable charge

To overcome this problem an extension of the sublattice model was proposed by Hillert et al. (1985) which is now known as the ionic two-sublattice model for liquids. As in the previous case it uses constituent fractions as composition variables, but it also considers that vacancies, with a charge corresponding to the charge of the cations, can be introduced on the anion sublattice so that the composition can move away from the ideal stoichiometry and approach an element with an electropositive character. The necessary neutral species of an electronegative element are added to the anion sublattice in order to allow the composition to approach a pure element. The sublattice formula for the model can then be written as... 

Nonstoichiometry may occur for some ceramic materials in which two valence (or ionic) states exist for one of the ion types. Iron oxide (wustite, FeO) is one such material because the iron can be present in both Fe " and Fe states the number of each of these ion types depends on temperature and the ambient oxygen pressure. The formation of an Fe " ion disrupts the electroneutrality of the crystal by introducing an excess -1-1 charge, which must be offset by some type of defect. This may be accomplished by the formation of one Fe " vacancy (or the removal of two positive charges) for every two Fe " ions that are formed (Figure 12.20). The crystal is no longer stoichiometric because there is one more O ion than Fe ion however, the crystal remains electrically neutral. This phenomenon is fairly common in iron oxide, and, in fact, its chemical formula is often written as Fei 0 (where x is some small and variable fraction substantially less than unity) to indicate a condition of nonstoichiometry with a deficiency of Fe. 

---