Molar axes

If one starts from a potential phase diagram, one may decide to replace one of the potentials by its conjugate variable. However, the potential phase diagram has no information on the size of the system and one should thus accept introducing a molar quantity rather than its extensive variable. By replacing all the potentials with their conjugate molar variables, one gets a molar diagram. One would like to retain the diagram's character of a true phase diagram, which means that there should be a unique answer as to which phase or phases are stable at each location. In this chapter we shall examine the properties of molar diagrams and we shall find under what conditions they are true phase diagrams. Only then may they be called molar phase diagrams. However, we shall start with a simple demonstration of how a diagram changes when molar axes are introduced.

Figure 9.1(a)–(d) demonstrates what happens to a part of the T, P potential phase diagram for Fe when Sm and Vm axes are introduced. Initially the P axis is plotted in the negative direction because V is conjugate to −P. It can be seen that the one-phase fields separate and leave room for a two-phase field. It can be filled with tie-lines connecting the points representing the individual phases in the two-phase equilibrium. It is self-evident how to draw them when one axis is still a potential but they yield additional information when all axes are molar (Fig. 9.1(d)).