Anionic vacancy: definition

Since the ratio of number of anions to cations in a unit cell for the Fluorite structure is 1 to 2, the compound Zro.gsCao isOj 5 can be said to be non-stoichiometric. The possible defect types are anion vacancies or interstitial cations. X-ray diffraction studies have definitely confirmed that the former type of defect structure is dominant therefore, there exist oxygen vacancies up to 7.5 per cent. The concentration of oxygen vacancies must depend on Po., as is usual for the metal oxides. 

On the basis of a definite analogy between e tx and F-centers we may expect the appearance under certain conditions of (e tr)2- particles of the type of F -centers. From the polaron model (57) it follows that the bipolaron (two electrons localized in a common polarization well) can not exist. In accordance with the work of Vinetskii and Giterman (63), in some cases the formation of the bipolarons becomes energetically possible in the result of interaction of the polarization wells of two separate polarons. However, the saving in energy for such bipolaron states is not large and hence they will not be stable in liquids under room temperature. Actually, up to the present time a series of attempts have been made to detect (e aq)2 in the irradiated liquid water but these attempts were not successful. The polaron theory (57) predicts that F -centers (two electrons in the anionic vacancy) may be stable. For this it is necessary that the ratio e/n2 (e and n2 are the static and optical dielectric constants, n—refraction index) should be more than 1.5. Evidently, in the glassy systems under consideration this requirement is fulfilled. 

Dowden (27) considers the active centers for carbonium ion formation to be associated with surface cation vacancies. A proton, derived from water contained in the catalyst, is attracted to the anions surrounding the vacancy. A hydrocarbon molecule is assumed to be held by polarization forces above this lattice defect and the proton will be distributed between the hydrocarbon and the anions, forming a carbonium ion of a definite lifetime. 

In compound crystals, balanced-defect reactions must conserve mass, charge neutrality, and the ratio of the regular lattice sites. In pure compounds, the point defects that form can be classified as either stoichiometric or nonstoichiometric. By definition, stoichiometric defects do not result in a change in chemistry of the crystal. Examples are Schottky (simultaneous formation of vacancies on the cation and anion sublattices) and Frenkel (vacancy-interstitial pair). 

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