Nonstoichiometry and P–T–x diagrams of binary systems

A P–T–x three-dimensional space diagram forms the complete representation between pressure (P), temperature (T) and composition (x) of coexisting phases—solid (S), liquid (L) and vapor (V). It gives the number of nonstoichiometric compounds that may be formed by the components and the stability limits of phases which are in equilibrium. Definitions of stoichiometry and nonstoichiometry are given. Some features of P–T–x diagrams with nonstoichiometric compounds are considered: maximum (Tmmax) and congruent (Tmc) melting points, difference between the compositions of solid (xS), liquid (xL) and vapor (xV): xL≠xS≠xV at maximum melting point T=Tmmax, the width and position of the homogeneity range, and the nonstoichiometry caused by defects are also discussed. A new technique for the investigation of P–T–x diagrams is presented.