HIGH-PRESSURE RAMAN-STUDY OF THE N-2 STRETCHING VIBRATION IN ARGON-NITROGEN MIXTURES AT ROOM-TEMPERATURE

We present room-temperature Raman investigations of the nitrogen molec ule stretching vibration in Ar-1-x(N-2)(x) from x=1 to n=0.59 in the p ressure range 1 less than or equal to p less than or equal to 31 GPa. Ar substitution leads to an inhomogeneous broadening of the vibrationa l signal. The beta-delta phase transition of pure N-2 is shifted to hi gher pressures for x=0.75. The substituted Ar atoms preferentially occ upy the 2a sites of the cubic elementary cell of the orientationally d isordered delta phase. This interpretation of the experimental data is supported by our model calculations. For x=0.75 a superstructure form s in which all the a sites are occupied by Ar atoms. No direct experim ental evidence is found for a disorder-order delta-epsilon phase trans ition observable in pure N-2. However, calculated orientational potent ial energy barriers propose freezing of the orientational dynamics at reduced pressures.