Self-consistent phonon calculation of the β phase and the α-β transition in solid nitrogen

The self-consistent phonon approximation of anharmonic lattice dynamics is applied to solid β nitrogen. Completely disordered molecular orientations and a Lennard-Jones 6–12 intermolecular potential are assumed. The quantities calculated are the phonon spectrum and the thermal expansion as a function of temperature at zero pressure. The α-β fcc-hcp transition temperature is estimated on the basis of two models for the molecular ordering in the β phase. Although the computations are intended primarily to explore the application of the self-consistent phonon method to a treatment of structural phase transitions in simple molecular crystals, comparison is made with existing experimental data.