ε-δ phase transition of nitrogen and the orientational behavior of the second-order transition within the δ phase:: A Monte Carlo study at 7.0 GPa

Monte Carlo simulations have been performed with an improved nitrogen-nitrogen potential at 7.0 GPa, to study the behavior near the epsilon-delta phase transition, to calculate the vibrational frequencies in the delta phase, and to examine the second-order phase transition within the delta phase. The epsilon-phase structure transformed to the delta-phase structure at 140 K, whereas the delta-phase structure is metastable below this temperature. The second-order transition could be detected in several ways. At high temperature the orientational behavior of all molecules is highly dynamical with weakly preferred orientations. At the second-order transition the molecules at the disk sites become localized with different orientations, whereas the orientational behavior of the molecules at the sphere sites does not change. The localization causes the frequency of the disk sites to split with an intensity of 1:2, and influences the frequency of the sphere sites. The frequency behavior calculated with this model system resembles the experimental behavior. Additional calculations revealed that the anisotropic energy term as well as the Coulomb term of the intermolecular potential influence not only the stability of the epsilon-phase structure, but also the orientational behavior, and thus the nature, of the second-order phase transition.