A calculation of the entropy and free energy differences between the alpha and beta phases of solid nitrogen

The transition between fee and hcp solid phases of N-2 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 alpha and beta 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. (C) 1997 American Institute of Physics.