Symbolism of defects

All statements which have been made up to now also apply for ionic crystals. At first sight it would seem that we do not have the prerequisites necessary for a definition of the chemical potentials of the defects in ionic crystals relative to convenient standard states, since firstly the electroneutrality of the crystal must be preserved, and secondly the proper relationships between the number of crystallographic different lattice sites must be preserved. How- 

In formulating reactions in which structural elements take part, one must observe three balances 1. the material balance, 2. the charge balance, and 3. the site balance. It is understood that the ratios to one another of crystallographically possible sites are not permitted to change for the formulated reaction. 

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