Structure of potassium germanophosphate glasses by X-ray and neutron diffraction: 2. Medium-range order

Characteristics of the medium-range order (MRO) of K ₂O–GeO ₂–P ₂O ₅ (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% K ₂O/GeO ₂/P ₂O ₅. The pre-peaks are explained by a structural model in which K-rich and Ge-rich regions are separated by PO ₄ 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 GeO ₂–P ₂O ₅ system. The pre-peak is changed to a smooth scattering contribution and finally shifted to ∼9 nm ^–1 for glass compositions approaching the binary K ₂O–GeO ₂ system. The strong tendency of the PO ₄ units to coordinate K ⁺ and Ge neighbors at their four corners is the source for the special MRO of the KGP glasses.