Medium-range structure in glasses and low-Q structure in neutron and X-ray scattering data

Attempts to understand the medium-range structure of glasses and other amorphous materials from X-ray and neutron scattering data necessarily focus on the low-Q region, where Q is the modulus of the scattering vector. Data for Q in the range 8-20 nm−1 reveal real-space detail on a scale 2π/Q. This corresponds to the region often denoted as 'medium-range structure', 0.5-1.5 nm. An attempt is made to describe the origin of low-Q structure in terms of spatial correlation of atomic density fluctuations and to delineate the inter-atomic distances involved. Amorphous silicon and silica are considered in detail showing that real-space data extending to about 1.1 and 1.6 nm, respectively, are required to specify accurately the detail in the first peaks of the structure factor. The degree of order associated with spatial atomic density fluctuations then becomes a key question. Completely random fluctuations seem unlikely to offer an explanation. Aperiodic, non-crystallographic packing offers some promise, but remains inadequately tested. The most satisfactory results for simple oxide glasses, at least, come from medium-range organization based on the principles underlying compositionally and thermodynamically equivalent crystalline structures. Low-Q structures in neutron scattering data for lithium, sodium and lithium/potassium disilicate glasses are treated as examples.