Phase diagrams in terms of pressure and temperature

Because pressure and temperature are two readily measured experimental quantities (e.g. Griesser etal. 1999), the relationships among the vapour, liquid, and polymorphs of a substance are often represented on diagrams of pressure vs temperature. These are also very useful in summarizing the polymorphic behaviour of a system. 

A review of some of the features of these diagrams in terms of the Phase Rule is enlightening (Findlay 1951). A system composed of two different solid forms of a substance will have one component and two solid phases. In the absence of a further definition of the system there will be one degree of freedom. In Fig. 2.7 this is either the temperature or the pressure along the I - II line. Choosing either variable fixes a point and defines the system. However, suppose that we are interested in the situation when the two phases are in equilibrium with the liquid or the vapour. Each one of those is an additional phase, making three in total and, by virtue of the Phase Rule, rendering the system invariant. Invariance results in a triple point for each case, defined by the intersection of the I - II curve with the I - v. and n - v. curves on the one hand and the intersection of the I - II curve with the I - 1. and II - 1. curves on the other hand. 

Recalling the classic definition of the triple point, say, for water as the intersection of the solid-vapour and the liquid-vapour curves, the analogy in Fig. 2.7 is the intersection of the II - v. and I - v. curves. Below the triple point only one of the solid phases (I) can exist in stable equilibrium with the vapour above the triple point only II 

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