A MONTE-CARLO STUDY OF METASTABLE STRUCTURES OF THE CYANOADAMANTANE CRYSTAL

We report results of the Monte Carlo simulations of stable and metasta ble phases of cyanoadamantane crystal, which is a model of an orientat ional glass forming system. The structures which have been found stabl e within the Monte Carlo runs include the plastic cubic phase, two dif ferent monoclinic phases, and an orthorhombic phase. The calculations have shown that all those structures remain metastable in the whole st udied range of temperatures between 100 and 400 K. The orthorhombic ar rangement is energetically the most favorable but the cubic phase is t he most stable phase at high temperature because of the disorder and l arge entropy term in the free energy. The transient orthorhombic phase is playing an important role in the transformations from quenched cub ic phase into stable low temperature structure. A possibility of a dis order in the orthorhombic phase is discussed and pointed out as a fact or which may affect the mechanism and kinetics of the transformation b etween quenched cubic structure and the low temperature ordered phase. The present model is not able to stabilize thermodynamically the mono clinic structure at low temperature, and the simplified representation of the intermolecular interaction seems to be the cause. (C) 1998 Ame rican Institute of Physics. [S0021-9606(98)50240-1].