Random Packing of Structural Units and the First Sharp Diffraction Peak in Glasses

Abstract

Diffraction studies of glasses often yield a structure factor S(Q) with a pronounced first sharp diffraction peak (FSDP) at low Q values in the vicinity of Q ≅ 1.0-1.5 A-1. This feature, while often quite prominent, has none-the-less eluded detailed explanation because it arises from correlations at distances of 4.5 – 6.0 A rather than from simple nearest- or next-nearest-neighbors. In fact, much of the local structure of the glass can be inferred without reference to this FSDP. We discuss here this diffraction feature and how it gives a direct clue to the connectivity of glasses and amorphous solids over intermediate ranges through the packing of basic structural or molecular units. These considerations are common to a large number of amorphous solids and lead to a better understanding of the atomic structure of glasses as nominally diverse, for example, as SiO2, GeSe2 A2S3 P2Se3 MoS3, and the elemental glasses for which dense random packing is a more familiar concept.