WAXS and NMR studies of intermediate and short range order in K2O–SiO2 glasses

Wide-angle X-ray scattering and 29Si NMR have been employed to investigate the medium-range structure of xK2O–(1−x)SiO2 glasses, with x varying in the limits 5%<x<35%. The diffractograms show a first sharp diffraction peak (FSDP) in the 1.4 Å−1<q<2.2 Å−1 range. The peaks broaden below x=20%, and at the lowest K2O fraction, a bimodal line shape is found. This broadening is interpreted in terms of phase separation at low K2O fraction. The NMR spectra consist of several (usually three) Gaussian components assigned to the different Q species (SiO4 tetrahedra with different connectivity) present. All three components are uniformly deshielded as K2O is incorporated into the structure. The fraction of non-bridging oxygens (NBOs) derived from the distribution of Q species matches the value obtained from the overall composition, except for the x=17.41% sample, again indicating phase-separation at x<20%. The inhomogeneities found by WAXS and NMR in the xK2O–(1−x)SiO2 glasses are interpreted in terms of broken bond-bending constraints at the NBOs. Constraint theory assigns the critical concentration for glass forming at xc=20%, which may explain the tendency of the glasses to phase-separate at concentrations below xc.