The Structure of Real Crystals

The ideal crystal consists of a perfectly ordered array of atoms, ions, or molecules in three dimensions. This ideal is difficult to attain experimentally, although in one instance at least (NaCl)(42) it appears to have been closely approached. There catinot be any ionic conductivity or atomic diffusion a perfect lattice which conserves its ideal order in all circumstances, and the observation that conductivity and diffusion do occur is only one of a number of lines of evidence which lead to the conclusion that a real crystal is not perfectly regular under all conditions. 

As a result of a large number of researches it has been established that two types of feult system may exist in crystals which may be called reversible and irreversible fault systems. The former have reproducible properties in many respects, but properties of the latter depend upon the previous history of the specimen. In particular, irreversible fault systems give rise to the structure-sensitive diffusion and conductivity data described in the following chapter, while reversible fault systems give rise to reproducible conductivity and diffusion phenomena. Some of the reproducible and equilibrium types of fault systems may be briefiy discussed before the conductivity and diffusion data are given. 

Besides defects which have been artificially introduced in crystals, other types of imperfection exist of the greatest significance for understanding ionic mobility in crystal lattices. Lattice imperfections of this kind are  

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Experimental studies of crystallization indicate that in real systems there is no significant kinetic retardation present in the case when crystal growth takes place at low saturations. This is related to those defects that are present in the structure of real crystals, and in particular to the presence of a... 

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