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

Abstract Reverse Monte Carlo (RMC) is used to investigate the origin of the first sharp diffraction peaks (FSDP) found for K 2 O–GeO 2 –P 2 O 5 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% K 2 O/GeO 2 /P 2 O 5 ), of the binary combinations of the three oxides and of vitreous GeO 2 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 PO 4 units (via O T -corners) than with Ge-centered units. Main component of the FSDP comes from the S GeGe ( 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 PO 4 tetrahedra possess a broad distribution of numbers of O T corners. The FSDP’s of the binary K 2 O–GeO 2 and K 2 O–P 2 O 5 glasses (∼10 nm −1 ) are due to a chemical order between network former and network modifier regions. The MRO of a mixed GeO 2 –P 2 O 5 glass of small P 2 O 5 content (FSDP at ∼16 nm −1 ) shows great similarity to the MRO of vitreous GeO 2 .