Wide-angle X-ray scattering and Si-29 NMR have been employed to investigate
the medium-range structure of xK(2)O-(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 Angstrom (-1) < q < 2.2 Angstrom (-1) range. The pe
aks broaden below x = 20%, and at the lowest K2O fraction, a bimodal line s
hape is found. This broadening is interpreted in terms of phase separation
at low K2O fraction. The NMR spectra consist of several (usually three) Gau
ssian components assigned to the different Q species (SiO4 tetrahedra with
different connectivity) present. All three components are uniformly deshiel
ded as K2O is incorporated into the structure. The fraction of non-bridging
oxygens (NBOs) derived from the distribution of Q species matches the valu
e 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 xK(2)O-(1- x)SiO2 glasses are interpreted in terms of b
roken bond-bending constraints at the NBOs. Constraint theory assigns the c
ritical concentration for glass forming at x(c) = 20%, which may explain th
e tendency of the glasses to phase-separate at concentrations below x(c). (
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