Structure of potassium germanophosphate glasses by X-ray and neutron diffraction: 3. Reverse Monte Carlo simulations

Reverse Monte Carlo (RMC) is used to investigate the origin of the first sharp diffraction peaks (FSDP) found for K2O–GeO2–P2O5 glasses at very small scattering vector Q = ∼7.5 nm−1. Structures of the ternary glass with the greatest intensity of FSDP (KGeP5 – 25/50/25 mol% K2O/GeO2/P2O5), of the binary combinations of the three oxides and of vitreous GeO2 are modeled. Results are deduced from comparisons of the partial structure factors and inspections of model sections. The P sites are uniformly distributed in the structure of KGeP5. The K+ ions interact more with the PO4 units (via OT-corners) than with Ge-centered units. Main component of the FSDP comes from the SGeGe(Q) factor. The FSDP is due to separations of ∼1 nm between the longish Ge-rich clusters which are visible in the corresponding models. Different to our tentative structural models reported before, the PO4 tetrahedra possess a broad distribution of numbers of OT corners. The FSDP’s of the binary K2O–GeO2 and K2O–P2O5 glasses (∼10 nm−1) are due to a chemical order between network former and network modifier regions. The MRO of a mixed GeO2–P2O5 glass of small P2O5 content (FSDP at ∼16 nm−1) shows great similarity to the MRO of vitreous GeO2.