The change in phase behaviour and orientational order of solid N₂ at high pressure under the influence of Ar: a simulation study

Abstract

We have studied the influence of Ar atoms on the high-pressure phase behaviour of N₂ by a Monte Carlo study of Ar _x (N₂)_1-x mixed solids. The results were obtained on the N₂ rich side (0 ≤ x ≤ 0.25), in the region where the ∊ and δ phases exist in pure N₂. The ∊∗—δ∗ transition shifts rapidly to lower temperatures with x: it has vanished for x ≲ 0.05. For x ≳ 0.12, a cubic orientationally ordered phase appears. The transition from this phase to the δ∗ phases shifts to higher temperatures with increasing x. The temperature of the onset as well as the completion of the cascade process, responsible for the δ∗_loc—δ∗_rot transition, is independent of composition. This transition disappears in the x-region where the ordered phase is present for all temperatures below the transition to the δ∗_rot phase. Therefore, the influence of the isotropic diluents on the orientational behaviour of the N₂ molecules is threefold. In the δ∗_loc phase, the vibron frequencies show a splitting that shifts as a function of x, which might be related to the occurrence of a tetragonal modification of this phase.