Librational Motion in Solid alpha-N-2

A description of the librational molecular motions in solid alpha-N-2 in terms of spherical harmonics is applied to give a temperature-dependent molecular field theory. Electrostatic quadrupole-quadrupole interactions between molecules on a fcc lattice with space group Pa3 are assumed. A first-order transition from a phase where the molecules are librating to one where they are rotating freely is found. In addition, the librational frequencies at k = 0 and zero temperature are calculated, using the single-particle states of the molecular fleld theory, and compared with Raman scattering experiments.