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 < 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 ∼25/50/25 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 (∼1 nm) is responsible for the pre-peak at ∼7.5 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 ∼9 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.