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

Author(s)

Ren, Jinjun ^[1] ; Eckert, Hellmut ^{[2, 3]}

Affiliation

^[1] Chinese Acad Sci, Shanghai Inst Opt & Fine Mech, Key Lab Mat High Power Laser, Shanghai 201800 - Peoples R China

^[2] Univ Sao Paulo, Inst Fis Sao Carlos, CP 369, BR-13566590 Sao Carlos, SP - Brazil

^[3] Westfalische Wilhelms Univ Munster, Inst Phys Chem, Corrensstr 30, D-48149 Munster - Germany

• Document type: Journal article
• Source: Journal of Physical Chemistry C; v. 122, n. 48, p. 27620-27630, DEC 6 2018.
• Web of Science Citations: 1

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 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. (AU)

• FAPESP's process: 13/07793-6 - CEPIV - Center for Teaching, Research and Innovation in Glass
• Grantee: Edgar Dutra Zanotto
• Support type: Research Grants - Research, Innovation and Dissemination Centers - RIDC