Thermodynamic relations in polymorphs

In terms of thermodynamics, one of the key questions regarding polymorphic systems is the relative stability of the various crystal modifications and the changes in thermodynamic relationships accompanying phase changes and different domains of temperature, pressure, and other conditions. Buerger s (1951) treatment of these questions provides the fundamentals upon which to base further discussion. 

At absolute zero TS vanishes and the Helmholz free energy equals the internal energy. As a consequence, at absolute zero the most stable polymorphic modification should have the lowest internal energy. Above absolute zero the entropy term will 

---