Density fluctuations, phase separation and microsegregation in silicate glasses

Microstructure in silicate glasses is reviewed from the standpoint of density fluctuations, phase separation and microsegregation. Recent experiments in Brillouin Scattering and Nuclear Magnetic Resonance (NMR) spin relaxation are used to characterise the length scale, magnitude and dimensionality of density fluctuations in monophasic silicate glasses and of phase separation in biphasic glasses. New Small Angle X-ray Scattering (SAXS) experiments are introduced which reveal the presence of much filler microstructure whose geometry is linked with the position and width of the First Sharp Diffraction Peak. For silica this structure resembles the quasiperiodicity recently identified in large tetrahedral model networks. For silicate glasses quasiperiodic order with a shorter spacing occurs in the microsegregation of glass modifiers from glass formers evident from Molecular Dynamic structures as well as from the empirically determined local structure.