Non-convergent disordering in spinels

The molar configurational entropy of a spinel can be expressed in terms of the composition parameter x as 

Both ordered and normal spinels should disorder at high temperature, since the entropy of the random spinel is larger. The disordering reaction for a normal spinel can be described by a quasi-chemical reaction 

The Gibbs energy of this defect reaction defines the degree of disorder through the equilibrium constant. 

Similar methods have been used in other cases as well, and a recent example is an analysis of the cation distribution in the complex oxide, LaSr2 xCaxCu2Ga07. Here site preference enthalpies for La, Sr and Ca have been derived [21]. 

---