Monte Carlo simulations of the distribution of Ar and other noble-gas atoms in high-pressure solid N-2

The distribution of argon and other noble-gas atoms over the two types of l attice sites in the nitrogen crystal at high pressure, and its consequences for the phase behavior, has been investigated with Monte Carlo simulations as a function of p, T, and X-Ar. We have used (N,p,T) simulations, with an additional unphysical move, consisting of a simultaneous identity change o f two unlike molecules (a "swap" move). To speed up the calculation, orient ational biased sampling was performed. The preference of the guest atoms fo r the a sites turns out to be stronger in the delta* phase than in the epsi lon* phase. Within the delta* phase, three temperature regions exist. At lo w temperatures, the preference for the a sites is nearly complete and indep endent of T. At somewhat higher temperatures, the preference for the a site s rapidly decreases as a function of T. Above this region, only a small dec rease towards a random distribution is observed. For 5% argon, this general behavior is independent of pressure. A larger pressure results in a strong er preference for the a sites. In contrast, increasing the Ar content withi n the same phase results in a less profound preference. For smaller noble-g as atoms this preference is stronger. The results are in qualitative agreem ent with experiments.