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Superstructural units involving six-coordinated silicon in sodium phosphosilicate glasses detected by solid-state NMR spectroscopy.
REN, Jinjun; ECKERT, Hellmut.
Abstract: 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 23Na, 29Si, and 31P on isotopically enriched glasses in the systems xSiO2-(1- x)(0.45Na2O-0.55P2O5) (x = 0.0, 0.1, and 0.2) and 0.2SiO2-0.2Na2O-0.6P2O5. 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. 29Si ? 23Na and 31P ? 23Na 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))6Na2 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.
Journal of Physical Chemistry C
v. 122, n. 48, p. 27620-27630 - Ano: 2018
Fator de Impacto: 4,484
    @article={002917525,author = {REN, Jinjun; ECKERT, Hellmut.},title={Superstructural units involving six-coordinated silicon in sodium phosphosilicate glasses detected by solid-state NMR spectroscopy},journal={Journal of Physical Chemistry C},note={v. 122, n. 48, p. 27620-27630},year={2018}}