Molecular dynamics simulations of the structure of rarehyphen;earthhyphen;doped beryllium fluoride glasses

摘要

We have simulated the atomic structure of a series of rarehyphen;earthhyphen;doped beryllium fluoride glasses using the molecular dynamics technique. Compositions include unitary BeF2and CaF2glasses and binary BeF2ndash;NaF, BeF2ndash;RbF, and BeF2ndash;CaF2glasses. Results are presented in the form of radial distribution functions and average coordination numbers about the rarehyphen;earth and other cations. In addition, by examining conditional probability distributions, we predict unusual structural features of the glasses such as a significant number of Be ions coordinated by five fluorine nearest neighbors, and the presence of F ions with no Be nearest neighbors. It is also found that the average coordination number of all cations, including Be, increases with an increasing amount of modifier. This result has been observed experimentally for oxides. We show that our runs are sufficiently long that the structural results are not due to insufficient equilibration. The relevance of our results to problems of solubility and crystallization are also discussed. The presence of fivefold coordinated Be is expected to have a dominant effect on glass kinetic properties.