Self-Consistent Calculations of the Lattice Modes of Solid Nitrogen

Abstract

Recently considerable interest has been shown in the lattice modes of solid nitrogen. The librational and translational modes at k = 0 have been measured by Raman and infrared scattering.^1,2 Solid nitrogen is observed in three phases: the α-phase, with an fcc molecular lattice, the hcp β-phase, and the body-centered tetragonal γ-phase. The space group for α-N₂ is P2₁3; however, the distortions from a Pa3 structure are very small.³ The α—β transition at zero pressure occurs at 35.6°K. This transition is accompanied by considerable anharmonicity in the librational modes.^3,4 In the α phase all lines broaden substantially with increasing temperature. The frequency of the most intense line at about 32 cm⁻¹ is found to decrease above about 20°K and to soften markedly near T _αβ. Similar behavior is observed for the translational modes.

Work supported by National Science Foundation Grant GP-22553.

Work performed under the auspices of the U. S. Atomic Energy Commission.