Calculation of high-pressure phase transitions in solid N₂ and the pressure dependence of intramolecular mode frequencies

Abstract

A calculation that minimizes the energy of solid N₂ with respect to a rhombohedral distortion of the Pm3n structure shows that a low-temperature phase transition occurs into the R3c calcite structure at P≈19.2 kbar with a volume change of 0.125 cm³/mole. This transition agrees with recent Raman scattering measurements. Another transition from R3c into R3^- m is predicted at P≈67.5 kbar, with a volume change of 0.1 cm³/mole. The pressure dependence of the intramolecular mode frequencies for the R3c structure are in reasonably good agreement with the two main branches observed experimentally.