Evidence for a transient metastable state during the isothermal transformation of the quenched plastic phase of cyanoadamantane from Raman scattering investigations

Abstract

We report Raman investigations on stable and metastable phases of cyanoadamantane crystal, which is a model of an orientational glass forming system. The isothermal transformation of the deeply quenched plastic phase of cyanoadamantane at 190 K toward the ultimately stable phase was analyzed from Raman investigations both in lattice- and internal-mode region. The evidence for the change in the local order during the ordering process is given through a frequency shift of the bending vibration of the CC–N protuberance. The careful analysis of this Raman band in the different states of cyanoadamantane suggests the existence of a local order of molecular dipoles in the plastic phase, which isothermally transforms before its spatial expansion into the stable phase. The development of the original local order on a mesoscopic scale was observed through a slight structuration of the low-frequency Raman spectrum. It is a direct evidence for a transient metastable state during the isothermal ordering process of CNa. However, no trace of the long-range order corresponding to this state has been detected in the early stages of transformation.