Stoicheiometric defect

Anderson (13) lias used 5 for intrinsic disorder. However, we have used S for the stoicheiometric defect (see 1,2) together with other authors. 

We observe that the stoicheiometric defect is a quadratic function of the intrinsic disorder and that the equilibrium vapour pressure for the stoicheiometric crystal is Pn = WG(T)Y. The form of the 8-p isotherm is given in Fig. 7. Notice that there are no limitations on Sl and that the greater the ratio E +Eh lkT, the smaller is I and hence S and po in other words, the stoicheiometric compound is more closely approximated. 

Hg. 7.— Pressure dependence of stoicheiometric defect for small deviations from sloididiometric composition. 

Fig. 12.—lUustratioii of alternative model for large stoicheiometric defect. 

The extended defect structures that occur in certain non-stoicheiometric compounds have in recent years provided some of the most intriguing problems in solid-state chemistry. The most intensively studied phase showing this class of disorder is Ti02-x> which planar defects known as shear planes have been detected and characterized by electron microscopy. Examples of other simple inorganic compounds containing shear planes are provided by V02 x and The range of... 

Until recently very little was understood as to the factors which determine whether point or extended defects are formed in a non-stoicheiometric phase, although interesting empirical correlations between shear-plane formation and both dielectric and lattice dynamical properties of the defective solid had been noted. Theoretical techniques have, however, provided valuable insight into this problem and into the related one of the relative stabilities of extended and point defect structures. The role of these techniques is emphasized in this article. 

The predominance in non-stoicheiometric compounds of structures based on point defects or defect aggregates indicates that in most compounds > Ep the repulsion energy outweighs the defect elimination term. This suggests that in those materials where shear planes form we should look for some special factor which... 

A difficulty, however, arises in that detailed analysis of the data of Baumard et al. in the near-stoicheiometric regions suggests that the predominant point defects are metal interstitials rather than the oxygen vacancies assumed so far in... 

These results show that changes in the anion to cation ratios of the building components of the linked octahedra and tetrahedra which form the core of these silicates is accommodated structurally. There is no evidence to suggest that substantial stoicheiometric variation is accomplished by point-defect populations, and indeed, mineralogists have never had recourse to the point-defect model to account for such changes in stoicheiometry. The brief account above could be greatly expanded and many more examples will be found in the review article previously cited. ... 

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