Evidence for the orientationally disordered cubic phase of ${\mathrm{Ar}}_{0.15}$(${\mathrm{N}}_2$${)}_{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 ${\mathrm{Ar}}_{0.15}$(${\mathrm{N}}_2$ ${)}_{0.85}$ 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 ${\mathit{A}}_{\mathit{g}}$ and the ${\mathit{T}}_{\mathit{g}}$ 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 α-β phase transition in solid ${\mathrm{N}}_2$ and in ${\mathrm{Ar}}_{\mathit{x}}$(${\mathrm{N}}_2$ ${)}_{1\mathrm{-}\mathit{x}}$ 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.