MEDIUM-RANGE ORDER IN SILICA, THE CANONICAL NETWORK GLASS

Most of the diffraction data for silica, the prototypical network glas s, 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 corre spondence between FSDPs in glasses and crystals and examine anisotropi c scattering from atomic models for a-SiO2. The FSDP is related to qua si-Bragg planes, specified by features to similar to 1 nm in the corre lation function, thus defining the medium-range structure. Arguments a re general, applicable to most amorphous materials.