Fowler Guggenheims quasi-chemical solution

Model of Fowler Guggenheim s quasi-chemical solution 

In this expression, the first and second term in the denominator, respectively, are relative to the numbers of pairs A-A and B-B, and the third and fourth term respectively relate to the numbers of pairs A-B and B-A. 

Suppose we use g i ) to denote the possible number of arrangements of pairs when they are distributed at random . According to equation [3.70], we have  

However, we know that, for a random distribution, this number is  

By multipl57ing and dividing relation [3.70] by the values of g(X) given respectively by relations [3.72] and [3.71], which are equal, we obtain  

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In order to evaluate the functions g(5) and E s), we need to know the distribution of the atoms on the lattice for the given value of s. Two models have been developed the Gorsky, Bragg and Williams model and the quasi-chemical model. The hypotheses upon which these models are based are similar, respectively, to those used for the model of a strictly-regular solution (see section 2.3.3) and those used for Fowler and Guggenheim s quasi-chemical solution model (see section 2.3.5). 

Fowler and Guggenheim s quasi-chemical model of the solution... 

Figure 3.3. Variation of the degree of order as a function of the composition ofa binary solution in Fowler and Guggenheim s quasi-chemical model (reproducedfrom [DBS 10], p.87)...

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