a b s t r a c tCharacteristics of the medium-range order (MRO) of K 2 O-GeO 2 -P 2 O 5 (KGP) glasses are obtained from Xray 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 (Qmagnitude of the scattering vector) are the outstanding features and reach maximum intensity for a glass of $25/50/25 mol% K 2 O/GeO 2 /P 2 O 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 2 O 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 2 O-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.
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