Medium-range order in silica, the canonical network glass

Most of the diffraction data for silica, the prototypical network glass, can be understood through local structural parameters. Failures to fit the prominent first (sharp) diffraction peak (FSDP), giving vital clues to medium-range order, are spectacular, however. We show a correspondence between FSDPs in glasses and crystals and examine anisotropic scattering from atomic models for a-SiO2. The FSDP is related to quasi-Bragg planes, specified by features to similar to 1 nm in the correlation function, thus defining the medium-range structure. Arguments are general, applicable to most amorphous materials.