Structure of Potassium Germanophosphate Glasses by X-ray and Neutron Diffraction: 2. Medium-range Order

Characteristics of the medium-range order (MRO) of K2O-GeO2-P2O5 (KGP) glasses are obtained from X-ray and neutron scattering data. The behavior of the MRO is expressed in changes of pre-peaks and smooth contributions in the structure factors, S(Q), for Q \u3c 12 nm-1. Peaks at Q = (7.5 ± 0.5) nm-1 (Q - magnitude of the scattering vector) are the outstanding features and reach maximum intensity for a glass of not, vert, similar25/50/25 mol% K2O/GeO2/P2O5. The pre-peaks are explained by a structural model in which K-rich and Ge-rich regions are separated by PO4 units. The distance of repetition of similar regions (not, vert, similar1 nm) is responsible for the pre-peak at not, vert, similar7.5 nm-1. The intensity of this pre-peak is reduced and finally eliminated for glass compositions approaching the binary GeO2-P2O5 system. The pre-peak is changed to a smooth scattering contribution and finally shifted to not, vert, similar9 nm-1 for glass compositions approaching the binary K2O-GeO2 system. The strong tendency of the PO4 units to coordinate K+ and Ge neighbors at their four corners is the source for the special MRO of the KGP glasses