Self-Consistent Calculations of the Lattice Modes of Solid Nitrogen
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.
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