The orientational and structural properties of N-2 and N-2-Ar solids at high pressure

The high pressure phases of N-2 and N-2-Ar mixed solids at 7 GPa have been investigated using Monte Carlo simulations. Orientational order parameters and displacements of the molecules with respect to the positions of the spa ce group were calculated. These tools provide an appropriate way to study t hese solids. In the epsilon* phase, the displacement a of the e sites was f ound to be 0.05 Angstrom. In the epsilon* phase, the orientational confinem ent to a disk of the N-2 molecules at the c sites increases with Ar concent ration. Within this disk, the distribution is nonuniform, even at high temp eratures. The orientationally ordered disk molecules show a displacement al ong the line of closest intermolecular contact. For the tetragonal modifica tion of the delta(loc)* phase, the c axis is perpendicular to these ordered and displaced disks. At high Ar concentrations, the delta(oo)* phase shows a large degree of alignment of the disk molecules. In this phase, the equi librium positions show a complicated structure. Therefore, there are small but significant structural changes between the delta(loc)*, delta(rot)*, an d delta(oo)*, phases.