Nonstoichiometric compounds vaporization

In nonstoichiometric compounds, the partial vapor pressures of various chemical species in the gaseous phase depend on the composition in the solid phase, and the composition in the gaseous phase is not necessarily the same as that in the solid phase. Hence, in such a case, the vaporization process proceeds, the composition in the solid phase gradually varies, and this is called incongruent vaporization. If the composition in the gaseous phase happens to be equal to that in the solid phase, the latter remains unchanged with time, which is called congruent vaporization. 

The binary systems actually and potentially important as nuclear fuel include oxides, carbides, nitrides, phosphides, and sulfides of uranium, plutonium, and thorium. An increasing amount of detailed information is becoming available on the phase equilibria of these compounds, but the relations existing between the composition (especially nonstoichiometric) and the vapor pressure (or activity) of each component are known only for a limited number of systems. 

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