  We report a thorough characterization of the glassy phases of mixtures of
succinonitrile and glutaronitrile via dielectric spectroscopy and differential
scanning calorimetry. This system is revealed to be one of the rare examples
where both glassy states of matter, a structurally disordered supercooled
liquid and an orientationally disordered plastic crystal, can be prepared in
the same material. Both disordered states can be easily supercooled, finally
arriving at a structural-glass or a glassy-crystal state. Detailed
investigations using broadband dielectric spectroscopy enable a comparison of
the glassy dynamics in both phases. Just as previously demonstrated for
supercooled-liquid and plastic-crystalline ethanol, our experiments reveal very
similar relaxational behavior and glass temperatures of both disordered states.
Thus the prominent role of orientational degrees of freedom in the glass
transition, suggested on the basis of the findings for ethanol, is fully
corroborated by the present work. Moreover, the fragilities of both phases are
determined and compared for different mixtures. The findings can be well
understood within an energy-landscape based explanation of fragility.
