Crystal dynamics of nitrogen: The cubicα-phase

The technique of inelastic neutron scattering has been used to determine translational and librational lattice modes in a single crystal of solid nitrogen in the cubic $\alpha$-phase at 15 K. The measurements were concentrated at the high-symmetry points $\Gamma$, $R$, and $M$, where the degeneracies ease the task of symmetry assignments to the observed modes. The assignment was made on the correlation of the observed structure-factor variation with force-model predictions. The calculations were based on a general potential function which includes Lennard-Jones or six-exponential-type interactions as well as electric quadrupole forces. Good agreement was found between both the observed frequencies and the observed intensities and their calculated counterparts. The temperature dependence of selected modes has been established and it is compared to a recent self-consistent phonon calculation.