Anharmonic effects in librational motion of N₂-type crystals

The dynamics of librational motion in N₂-type crystals (α-N₂, α-CO, N₂O, CO₂) is treated by taking into account both anharmonic and correlation effects. The method used is similar to Tyablikov's method in the theory of magnetism. The main thermodynamic characteristics of the librational subsystem are calculated: the order parameter, rms librational angle, librational mode frequencies and corresponding Grüneisen parameters, librational heat capacity, and internal and free energies. The librational isotope effects for α-¹⁴N₂ and α-¹⁵N₂ are considered. An explanation of the anomalous isotope effects in the heat capacity is proposed. A theory of the phase transition into the orientationally disordered state is developed.