Superstructural Units Involving Six-Coordinated Silicon in Sodium Phosphosilicate Glasses Detected by Solid-State NMR Spectroscopy

摘要

Sodium phosphosilicate glasses with high phosphate contents represent an unusual case in glass science as they are known to contain large amounts of six-coordinated silicon species (SiO6/2 units, Si-(6)). Although the network connectivity of these units has been previously investigated, the overall structural organization of this system at the medium-range order level is still incompletely understood. In the present study, this issue is addressed by using a comprehensive suite of homo- and heteronuclear dipolar recoupling studies involving the nuclear isotopes Na-23, Si-29, and P-31 on isotopically enriched glasses in the systems xSiO(2)-(1 - x)(0.45Na(2)O-0.55P(2)O(5)) (x = 0.0, 0.1, and 0.2) and 0.2SiO(2)-0.2Na(2)O-0.6P(2)O(5). Four- and six-fold coordinated silicon species (Si-(4)) and Si-(6)) coexist in these glasses. Invariably, Si-(6) selectively connects with phosphate species, which are exclusively of the P-(3) type, whereas the Si-(4) species are coordinated to both P-(2) and P-(3) units. Si-29 - Na-23 and P-31 - Na-23 rotational echo double resonance experiments indicate much closer sodium/phosphorus than sodium/silicon proximities. Evidently, the bond valence gradient of the Si-(6)-O-P-(3) linkages redistributes the anionic charge onto the phosphate nonbridging oxygen species, which attract Na+ cations. The Si-(6)(P-(3))(6)Na-2 superstructural units (stoichiometry Na2SiP6O18) thus formed represent an exceptionally high degree of medium-range order, accounting for high thermal and mechanical stability of these glasses. The Na+ ions neutralize the negative charge located at nonbridging oxygen atoms of P-(3) tetrahedra, whereas the bridging oxygen atoms in the Si-(6)-O-P bonds are electrically neutral. Finally, we show that the structural speciations of silicon and phosphorus in SiO2-Na2O-P2O5 glasses can be predicted straightforwardly from the elemental glass compositions in excellent agreement with experimental results.