The change in phase behaviour and orientational order of solid N-2 at highpressure under the influence of Ar: a simulation study

We have studied the influence of Ar atoms on the high-pressure phase behavi our of N-2 by a Monte Carlo study of Ar-x(N-2)(1-x) mixed solids. The resul ts were obtained on the N2 rich Side (0 less than or equal to x less than o r equal to 0.25), in the region where the epsilon and delta phases exist in pure N-2. The epsilon*-delta* transition shifts rapidly to lower temperatu res with x: it has vanished for x greater than or similar to 0.05. For x gr eater than or similar to 0.12, a cubic orientationally ordered phase appear s. The transition from this phase to the delta* phases shifts to higher tem peratures with increasing x. The temperature of the onset as well as the co mpletion of the cascade process, responsible for the delta(loc)*-delta(rot) * transition, is independent of composition. This transition disappears in the x-region where the ordered phase is present for all tempera tures below the transition to the delta(rot)* phase. Therefore, the influence of the i sotropic diluents on the orientational behaviour of the N-2 molecules is th reefold. In the delta(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.