Crystal to Glass Transition and Melting in Two Dimensions
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Molecular & Materials Simulation Center, Beckman Institute, 139-74, California Institute of Technology, Pasadena, California 91125 ABSTRACT Thermodynamic properties, structures, defects and their configurations of a twodimensional Lennard-Jones (LJ) system are investigated close to crystal to glass transition (CGT) via molecular dynamics simulations. The CGT is achieved by saturating the LJ binary arrays below glass transition temperature with one type of the atoms which has different atomic size from that of the host atoms. It was found that for a given atomic size difference larger than a critical value, the CGT proceeds with increasing solute concentrations in three stages, each of which is characterized by distinct behaviors of translational and bond-orientational order correlation functions. An intermediate phase which has a quasi-long range orientational order but short range translational order has been found to exist prior to the formation of the amorphous phase. The destabilization of crystallinity is observed to be directly related to defects. We examine these results in the context of two dimensional (2D) melting theory. Finite size effects on these results, in particular on the intermediate phase formation, are discussed. INTRODUCTION Both crystal to glass transition and melting are topological order to disorder transitions. The similarities of the starting and end phase and the disappearance of long range translational symmetries at the transitions suggest that they are the same [1]. In general the thermodynamics and in particular, the kinetics of these transitions resemble each other remarkably [1]. Although the mechanism of melting is still in debate [2J, the theories and models developed for melting have been extensively used in the understanding of the CCT [1, 3-10]. However, these analogous criteria suffer from the same problem as they
do in melting, namely that they are not able to provide a detailed microscopic picture of how these transitions proceed. In this paper, 2D LJ binary arrays under the polymorphic constraint are studied. By keeping the array at a sufficiently low temperature and thus eliminating chemical inhomogeneity, we expect to see certain intrinsic mechanisms of the CGT and to establish relationships between microscopic properties (such as atomic interactions and atomic size differences) with the thermodynamic and kinetic properties during the CGT. The crystal to glass transitions can be obtained by a number of methods [1,11]. There are rarely cases where a pure system can be amorphized unless some extreme kinetic constraints are imposed. As in rapid quenching of melts, the majority of systems that can be amorphized usually contain several components. These multicomponent amorphizable systems invariably become metastable prior to the transition with much increased free energy. The composition-induced crystal to glass transition seems to be present in most
of these systems and thus can be regarded as a general characteristic of solid state amorphization. Even mechanical d
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