A Monte Carlo study of metastable structures of the cyanoadamantane crystal

Abstract

We report results of the Monte Carlo simulations of stable and metastable phases of cyanoadamantane crystal, which is a model of an orientational glass forming system. The structures which have been found stable within the Monte Carlo runs include the plastic cubic phase, two different monoclinic phases, and an orthorhombic phase. The calculations have shown that all those structures remain metastable in the whole studied range of temperatures between 100 and 400 K. The orthorhombic arrangement is energetically the most favorable but the cubic phase is the most stable phase at high temperature because of the disorder and large entropy term in the free energy. The transient orthorhombic phase is playing an important role in the transformations from quenched cubic phase into stable low temperature structure. A possibility of a disorder in the orthorhombic phase is discussed and pointed out as a factor which may affect the mechanism and kinetics of the transformation between quenched cubic structure and the low temperature ordered phase. The present model is not able to stabilize thermodynamically the monoclinic structure at low temperature, and the simplified representation of the intermolecular interaction seems to be the cause.