Symmetrical disorders in stoichiometric binary solids

A disorder is symmetrical if one of the two defects which make it up involves the lattice of the element A and the other the lattice of the element B. In practice, we find Schottky disorder and the anti-stmcture disorder. 

Schottky disorder is the simultaneous presence of vacancies of each of the two chemical species, with these vacancies being in stoichiometric proportions to maintain the ratio between the numbers of sites. 

This disorder is formed under the influence of the temperature, by transformation of the normal sites of A and B within the solid at the normal sites of A and B on the surface, which results in an increase in the dimensions of the crystal. In the case of a compound with the theoretical formula BmA , the reaction of formation of the disorder can be written as  

The equilibrium constant depends on the temperature, through the van t Hoff law, and the standard enthalpy of the reaction.  

The anti-structure disorder is the simultaneous presence of two types of exchanged atoms Ab and Ba. As these exchanges do not alter the stoichiometry, the equilibrium of formation of the disorder is written as follows, regardless of the values of the munbers m and n in the theoretical formula  

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