Molecular dynamics simulations of the structure of rare-earth-doped beryllium fluoride glasses

Abstract

We have simulated the atomic structure of a series of rare-earth-doped beryllium fluoride glasses using the molecular dynamics technique. Compositions include unitary BeF2 and CaF2 glasses and binary BeF2–NaF, BeF2–RbF, and BeF2–CaF2 glasses. Results are presented in the form of radial distribution functions and average coordination numbers about the rare-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.