Low-Q Features In Diffraction Data For Borate Glasses And Crystals - An Examination Of Similaritities In Medium-Range Structures

Features in X-ray and neutron scattering data for amorphous solids at low values of the scattering vector, Q, are related to the medium-range structure of the glass. In many glasses (and some melts and simple liquids) the positions of low-Q peaks are related to features in equivalent crystalline phases. In silica, recent work has shown that low-Q peaks re associated with "quasi-Bragg" planes in the glass with a distinct similarity to those found in cristobalite and tridymite. X-ray and neutron scattering data for borate glasses containing cations like Ag+, Cs+ and Li+ are compared with simulated diffraction patterns for equivalent crystals. Comparison is close and supports the conclusion that large polyanionic "super-structural" units are intrinsic features of the vitreous phase. The packaging of such groups and the interstitial cations is likely to reflect that of the crystals too.