Ring compaction as a mechanism of densification in amorphous silica

Abstract

The structure of permanently densified silica glass is investigated by neutron and x-ray diffraction, and by the production of atomistic models that reproduce the diffraction results. The focus is on the nature of the ordering on an intermediate length scale that originates from the formation of $n$-rings, where $n$ is the number of Si or O atoms within a ring. The densification process has a large effect on this ordering, as identified by pronounced changes to the first sharp diffraction peak (FSDP) in the measured structure factors. A clear and systematic dependence of the distribution of ring sizes on the position and shape of the FSDP could not be found, i.e., the ring statistics are not directly encoded into the form of this peak. Instead, a significant contribution to the glass densification originates from compaction of the $n$-rings, a metric that is quantified by the radius of gyration and lifetime of the rings. We thereby uncover and quantify a key densification mechanism in amorphous materials.