Nuclear Magnetic Resonance Investigation of the Structures of Phosphate and Phosphate-containing Glasses: A Review

This paper presents a review of the nuclear magnetic resonance (NMR) data for phosphate and phosphate-containing glasses obtained primarily within the past 10 years and of the structural interpretations based on those data. Compositions discussed include P2O5, alkali and alkaline earth phosphates, aluminophosphates, borophosphates, fluorophosphates, and phosphate-containing silicate and aluminosilicate glasses. 31P NMR data, in conjunction with 27Al, 29Si, 11B, 7Li, and 23Na data if appropriate, have proven very powerful in providing direct evidence about the local structural environments present in the these materials and in many cases have allowed interpretation of the physical and chemical behavior of these glasses in terms of polyhedral structures. © 1995

Structure of Li,Na Metaphosphate Glasses by ³¹P and ²³Na MAS-NMR Correlated with the Mixed Alkali Effect

31P and 23Na NMR spectra for glasses along the Li,Na metaphosphate join have been obtained by MAS-NMR spectroscopy. The 31P results are interpreted in terms of phosphate Q2 (two bridging oxygens) tetrahedra. The 31P chemical shifts become less shielded (less negative) with increasing [Na]/[Na + Li] ratio. This observation is rationalized in terms of increased paramagnetic deshielding due to increased average electron density in the Psingle bondO bonds with increasing [Na]/[Na + Li] ratio. The 23Na peak maxima also become deshielded with increased [Na]/[Na + Li], suggesting the effects of Lisingle bond(non-bridging oxygen)single bondNa linkages. Both 31P and 23Na peaks have greater full-widths at half-height (FWHH) for the intermediate (mixed Li,Na) compositions compared with the Li or Na end-member, reflecting a greater distribution of P and Na sites due to the presence of both Li and Na. The glass transition temperatures have a minimum at [Na]/[Na + Li] = 0.6, thus showing a mixed alkali effect, previously explained by mixed alkali pairs (Lisingle bondNBOsingle bondNa) detected by the NMR experiments. © 1992 Elsevier Science Publishers B.V. All rights reserved

Local Structure of XAl₂O₃·(1 - X)NaPO3 Glasses. an NMR and XPS Study

Phosphate glasses are of interest because of their potential applicability in high-expansion, low-temperature glass-to-metal seals. However, their generally poor chemical durabilities limit their usefulness. Addition of Al2O3 to sodium phosphate glass has long been known to improve its aqueous durability. A series of xAl2O3·(1 - x)NaPO3 glasses were prepared and characterized by magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and by X-ray photoelectron spectroscopy (XPS)

Multinuclear MAS NMR Study of the Short-Range Structure of Fluorophosphate Glass

We have examined the bonding arrangements in Na-P-O-F and Na-Al-P-O-F glasses using 19F, 27Al, and 31P solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. For the Al-free series of glasses, the 19F NMR spectra are dominated by peaks near +90 ppm, representative of F terminating P-chains. The formation of these bonds has little effect on the 31P chemical shifts, indicating that F preferentially replaces bridging oxygen on the phosphate tetrahedra, consistent with previous NMR studies of crystalline fluorophosphates and other spectroscopic studies of fluorophosphate glass. For the Na-Al-P-O-F glasses, 27Al NMR detects only octahedral Al-sites, the 19F NMR spectra include a second peak near -12 ppm due to F bonded to Al, and the 31P NMR spectra contain signals due to Q1-sites with one or more Al next-nearest neighbors. The relative intensity of the two 19F peaks correlates well with previous spectroscopic studies and shows that a greater fraction of F-P bonds forms when the base glass is remelted in NH4HF2

The Short Range Structure of Sodium Phosphate Glasses I. MAS NMR studies

A series of x(Na2O + H2O)·(1 - x)P2O5 glasses have been characterized by 31P and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. High-resolution 31P NMR spectra reveal the presence of Q2-(2 bridging oxygen/tetrahedron) and Q3-(3 bridging oxygen/tetrahedron) tetrahedral sites in glasses with x \u3c 0.5, and Q2- and Q1-tetrahedral sites in glasses with x \u3e 0.5. Quantitative measurements of the respective NMR site populations are in excellent agreement with Van Wazer\u27s predictions for the structures of ionic phosphates and illustrate the depolymerizing effects of residual H2O on the glass structure. the systematic change in the Q2_31P chemical shift is consistent with an increase in the average π-bond character of the phosphorus-nonbridging oxygen bond as x decreases

Surface Structure and Chemistry of High Surface Area Silica Gels

Combined Raman and 29Si NMR investigations of high surface area silica gels indicate that dehydroxylation of the a-SiO2 surface results preferentially in the formation of cyclic trisiloxanes (3-membered rings). Estimates of the maximum experimentally observed concentration of 3-membered rings correspond to a surface coverage of about 28-58%. Two consequences of 3-membered rings on the a-SiO2 surface are enhanced hydrolysis rates and increased skeletal densities. © 1990

Raman Spectroscopy Study of the Structure of Lithium and Sodium Ultraphosphate Glasses

Anhydrous binary phosphate glasses containing from 0 to 50 mol% Li 2O or Na 2O have been prepared and examined by Raman scattering spectroscopy. The unpolarized Raman spectrum of vitreous P 2O 5 has intense bands near 640 cm -1, attributed to the symmetric stretching mode of P-O-P bridging oxygens, (POP) sym, between Q 3 phosphate tetrahedra, and at 1390 cm -1 due to the symmetric stretch of the P=O terminal oxygens, (P=O) sym. With the addition of alkali oxide to P 2O 5, a new feature appears in the Raman spectra near 1160 cm -1 indicating the formation of Q 2 phosphate tetrahedra with two bridging and two non-bridging oxygens. The increase in relative amplitude of this new (PO 2) sym band with increasing modifier content is consistent with a simple depolymerization of the phosphate network. From 20 to 50 mol% alkali oxide, the position of the (P=O) sym Raman band decreases by ∼ 130 cm -1 whereas the frequency of the (POP) sym band increases by ∼ 60 cm -1. These frequency shifts are the result of π-bond delocalization on Q 3 species that effectively lengthens the P=O terminal oxygen bond and strengthens the P-O-P linkages with increasing alkali oxide content. The compositional dependence of the π-bond delocalization on Q 3 tetrahedra is described by considering the interconnections between neighboring Q 3 and Q 2 tetrahedra. The onset of π-bond delocalization on Q 3 species corresponds with the anomalous T g minimum at 20 mol% alkali oxide in alkali ultraphosphate glasses. The increase in T g between 20 and 50 mol% alkali oxide is attributed to the increased ionic interconnection of what becomes a chain-like phosphate network at higher alkali contents. Finally, the Raman spectra of several alkali ultraphosphate glasses show high frequency shoulders on the Raman bands attributed to the (PO 2) sym and (PO 2) asym vibrational modes. These shoulders represent the presence of strained structural units, possibly three-or four-membered rings. © 1998 Elsevier Science B.V

The Short-range Structure of Sodium Ultraphosphate Glasses

Anhydrous sodium ultraphosphate glasses were prepared with Na 2O contents between 0 and 50 mol% and were characterized by several structurally sensitive spectroscopic probes to determine the nature of the phosphate tetrahedra that constitute the short-range glass structure. Solid state 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy reveals that Na 2O depolymerizes the branched (Q 3) P-O network of P 2O 5 to form metaphosphate (Q 2) sites, in quantitative agreement with Van Wazer\u27s “chemically simple” model. X-ray photoelectron spectroscopy reveals that the concomitant increase in non-bridging oxygen with increasing Na 2O content is also in quantitative agreement with this structural model. Raman spectroscopic analyses of glasses with approximately 40 mol% Na 2O suggest that some intermediate-range order, perhaps associated with strained rings, also exists within the glass network. Strained sites are eliminated when the solid glass is heated to melt temperatures. © 1994