A calculation of the entropy and free energy differences between the α and β phases of solid nitrogen

The transition between fcc and hep solid phases of N₂ is examined by calculating their Helmholtz free energies using the Monte Carlo method. This shows that the transition occurs at about 41 K, with an entropy change very close to the experimental value. No plastic phase intermediate between the α and β structures is indicated. However, within a few degrees of the transition, orientational exchange of the molecules from one body diagonal to another commence. Another precursor to the transition are considerably increased orientational fluctuations.