LIGHT-SCATTERING IN ORIENTATIONAL GLASSES

Brillouin scattering experiments on Ar:N-2 mixtures at ambient pressur e and low temperatures clearly exhibit the hcp-Pa3 transition for low Ar concentrations. At 30% Ar admixture, indications are found for a lo w temperature orthorhombic modification of the orientational glass. Ra man spectra were obtained from Ar:N-2 mixtures at high pressures (5 GP a < p < 27 GPa) and room temperature. Similar phase transitions as in pure N-2 are observed. In addition, partial site-ordering of the Ar at oms is found on the 2a-sites of the cubic delta-N-2-phase above 10 GPa , leading to a superstructure of Ar-0.25(N-2)(0.75). Above 16 GPa, the N-2 molecules undergo orientational ordering yielding a trigonal epsi lon-N-2 phase. This system and those with higher Ar concentrations can be used as model systems for orientational glasses similar to the cya nides.