A b i n i t i o description of large amplitude motions in solid N2. II. Librons in the β-phase and the α–β phase transition

Abstract

Using an ab initio pair potential, we have performed mean field and time-dependent Hartree calculations for the reorientational motions in disordered β nitrogen, in a basis of tesseral harmonics. The results show that orientationally localized librational solutions with neighboring molecules rotated over 180° around the crystal c axis are energetically more favorable than (nearly) free precession of the molecules. The experimental symmetry can be obtained by allowing rapid jumps between six equivalent localized states; such a jump model predicts the α–β phase transition at the correct temperature.