EVIDENCE FOR THE ORIENTATIONALLY DISORDERED CUBIC PHASE OF AR0.15(N2)0.85 FROM LIBRATIONAL AND VIBRATIONAL RAMAN-SCATTERING

Vibrational and librational Raman scattering yields spectroscopic evidence for the existence of an orientationally disordered fcc structure of the Ar0.15(N2)0.85 mixed crystal, and directly probes its dynamical properties. At the orientational phase transition, the librational motion suddenly becomes overdamped and the factor group splitting between the A(g) and the T(g) vibrons disappears, which is accompanied by a jump in vibrational frequency. The purely structural fcc-hcp transition is observed through a different temperature dependence of the stretching frequency for the two phases. The anomalous decrease of the linewidth of the vibrational transition when passing the alpha-beta phase transition in solid N2 and in Ar(x)(N2)1-x mixed crystals with low Ar content can now be ascribed to the difference in orientational status in the two phases and not to structural changes.