Geometric configuration of five-coordinated Al and Si in tectosilicate calcium aluminosilicate glasses and its effect on plastic flow

Abstract Using molecular dynamics simulations of calcium aluminosilicate glasses, AlV, SiV, and their surrounding O environments were analyzed. Unlike a trigonal bipyramidal structure, which is common for five-coordinated species, the bond angle distributions suggest that AlV and SiV adopt configurations consistent with the range of geometries found in a Berry pseudorotation. This allows AlV and SiV to adopt any configuration between two extremes, which are trigonal bipyramids, with minimal energy cost. It is shown that this range of geometries, in combination with the ease with which Al changes coordination, facilitates mechanisms that lead to plastic deformation or viscous flow. This understanding of the geometry of these five-coordinated network formers, along with knowledge of plastic deformation mechanisms, can be used to explain the effects of pressure, temperature, and aluminum content on the plastic flow behavior of aluminosilicates.