Structure and intermolecular excitations of β-N2 at 400 MPa and 55 K

Abstract

Elastic and inelastic neutron scattering measurements have been made on a single crystal of β‐N₂ at 55 K and a pressure of 400 MPa. The integrated intensities of 34 Bragg reflections were measured and analyzed in terms of symmetry‐adapted harmonics. None of the coefficients of the higher‐order harmonics could be determined and, within experimental error, the N₂ molecule is randomly oriented. No well‐defined librational modes were observed and even translational acoustic phonons are broad. The line shapes of translational acoustic phonons propagating along the hexagonal axis are analyzed in terms of both Gaussian and Lorentzian line shapes and in terms of the Michel–Naudts theory of rotation–translation interactions. In terms of the latter theory, β‐N₂ is in the slow relaxation regime and has a strong rotation–translation interaction. The temperature dependence observed for four of these line shapes suggests that the librational excitations are no longer overdamped at 46 K. Values for three of the elastic constants were determined and are (in units of 10⁹ N m⁻²): C₁₁=4.43±0.14, C₃₃=5.92±0.14, and C₄₄=0.61±0.02.