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

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.

Non-linear dissolution mechanisms of sodium calcium phosphate glasses as a function of pH in various aqueous media

“…Formation of six-fold coordinated Si ions, which can reduce the dissolution rate, has been previously observed in phosphate glasses. However, this requires a higher degree of network connectivity within the phosphate glass network (mostly Q 3 phosphates) than is present here (only Q 2 or Q 1 ), as well as a greater concentration of Si, so their presence is not expected in these glass compositions [38,39]. Formation of six-fold coordinated Si ions would also result in the appearance of a unique peak at 670 cm -1 in the IR spectrum [38], which was not observed in our experiments (Fig.…”

A modified random network model for P2O5–Na2O–Al2O3–SiO2 glass studied by molecular dynamics simulations

“…Besides, there is a strong affinity of phosphorus to bond with aluminum as tetrahedral coordinated phosphorus have a bond valence of 1.25, the bond valence of [AlO 4 ] species is 0.75, thus a P-O-Al connectivity leaves no charge on the bridging oxygen and local electrostatic neutrality would be satised. 18,52 Therefore, Si-O-Al connection has been consumed to form Si-O-Si and Al-O-P with P 2 O 5 addition. 16 Based on the propensity of connection between different network former cations, the addition of P 2 O 5 has induced separated aluminum-phosphorus-rich and silicon-rich region, exacerbating the heterogeneity of glass structure, which might not be distinguished in emission scanning microscope (SEM).…”

Modification of silicophosphate glass composition, structure, and properties via crucible material and melting conditions

“…However, it is not possible to identify contributions to 31 P signals from P–O–Si or P–O–Al bonds based on the chemical shift alone. MAS NMR cannot distinguish between Q n groups, that is, phosphate tetrahedra connected to n neighboring phosphate tetrahedra, and or groups, that is, phosphate tetrahedra connected to n −1 phosphate tetrahedra and one aluminum or one silicon atom, respectively . P–O–Si bonds detected by vibrational spectroscopy for the aluminum‐free glasses, 900Pt1 and 1200Si0.5, are therefore likely to be included in these two signals as groups.…”