The glass transition of BeF2: A Monte Carlo study

Abstract

A Monte Carlo study of the structure, equilibration, and glass transition of a model of BeF2 is described. The system consists of 54 ions in a box of side 8.949 Å. The Ewald sum is approximated by the spherically symmetric part and the repulsive potential is of exponential form. The resulting fluid is motionally hindered at low temperature and has structural features generally attributed to BeF2. The glass structure is found to be a three-dimensional continuous random network of BeF4 tetrahedra linked at the vertices with a broad distribution of Be–F–Be angles. The glass contains a small concentration of defects in the form of one- and threefold fluorine and threefold beryllium. The concentration of such defects in the fluid can be quantitatively related to the rate of structural rearrangements. At the glass transition temperature, diffusion ceases, the specific heat decreases suddenly and other averages show changes in slope as a function of temperature. A structural explanation is given for the change in specific heat. An argument is presented that, near the glass transition temperature, the same time-averaged ionic motions observed in this study will also be observed in the true time-dependent motion of the fluid (such as would be computed in a molecular dynamics simulation). The ionic motion both above and below the glass transition exhibits a long-term memory. Even after an ion moves a distance of 1 Å, there is a large probability of its returning to its starting place. The implications of this memory for studying diffusion and equilibration are pointed out. The glass shows the existence of localized structural rearrangements, in which a few ions undergo periodic hopping motions over 1–2 Å distances during times for which the majority of ions execute (many) simple vibrations of 0.1–0.2 Å displacement about their equilibrium positions. These latter motions do not lead to macroscopic diffusion.