Eyrings semi-microscopic formulation of the vacancy model

Eyring s semi-microscopic formulation of the vacancy model 

Of course, the movement of a vacancy is, in fact, the movement of a molecule neighboring that vacancy. 

Solids, just below dieir melting point, are assumed to contain no vacancies. If there are any, they are few in number in relation to the molecules. In a liquid, on the other hand, the number of vacancies is of the same order of magnitude as the number of molecules. If N denotes the number of molecules behaving like a solid, the number of vacancies would be  

the fiaction of sites with molecules would be  

The mean number of vacancies neighboring a molecule (z is the coordination index of the lattice) would be  

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