Structural characterization of noncrystalline solids and glasses using solid state NMR

This Primer critically reviews the possibilities and limitations of probing the short-range structure of an oxide-based glass by routine magic-angle spinning (MAS) or triple-quantum MAS (3QMAS) nuclear magnetic resonance (NMR) experiments. We briefly outline the structural features of oxide-based glasses and the basics of solid-state NMR. Besides suggesting guidelines for selecting favorable experimental conditions and important considerations for recording high-quality MAS NMR spectra amenable for subsequent analysis, we review options for extracting NMR observables and their distributions from spins-1/2 as well as half-integer spin quadrupolar nuclei. Considering that the isotropic chemical shift remains the primary probe of local structure by MAS NMR, we thoroughly review its dependence on the short-range oxide-based glass parameters from the viewpoint of a very simple and intuitive but qualitative model. The utility of deconvolutions of notably ²⁹Si and ³¹P MAS NMR spectra are discussed critically. We also suggest alternative yet qualitative analysis options that are available whenever MAS NMR spectral deconvolutions are not warranted without additional information, which incidentally applies to a majority of modern multicomponent glasses.

In this paper we studied the effect of NaBO₂ addition to a phase-separated alkali-free bioactive glass with a composition of 38.49 SiO₂ • 36.07 CaO • 19.24 MgO • 5.61 P₂O₅ • 0.59 CaF₂. Microscopy reveals binodal phase separation involving two Si-containing microphases with a droplet size of ∼200 μm, driven by the thermodynamic LLPS mechanism. The local environments and spatial distribution of silicate, phosphate, and fluoride ions in this phase-separated system were studied, using ²⁹Si, ³¹P, ¹¹B, ¹⁹F, ²⁵Mg, and ²³Na nuclear magnetic resonance (NMR) and infrared spectroscopy. The silicate units are dominantly of the metasilicate (Si²) type. The phosphate units exist mostly as orthophosphate (P⁰) while the borate is present in the form of pyroborate (B¹). Multinuclear dipolar re-coupling experiments indicate that the minority components F, P, B and Na all occur within a common phase. Thus, atomic distribution scenarios involving the separation of these components into separate phases can be excluded. The ³¹P spin echo decay (SED) method was used along with Monte Carlo simulations to characterize the spatial distribution of the phosphate component. Based on the analysis, the phosphate component forms clusters of sizes 1-4 nm, which are embedded in an environment more dilute in phosphate, having a random distribution. While ¹⁹F SED results indicate that the fluoride ions do not form clusters and are close to randomly distributed, dipolar recoupling of ³¹P suggests a local environment resembling that of fluorapatite.

Calcium aluminosilicate (CAS) glasses are ubiquitous in nature and play an important role in diverse technological applications ranging from structural glasses to sustainable cementitious materials. Understanding the relationship between the chemical composition and the structure of CAS glasses is an essential step towards optimizing their properties for different uses in the future. Here, we use extensive molecular dynamics (MD) simulations to characterize the multiscale structure of CAS glasses over the full compositional range. Analysis of the short and medium range order of the glasses reveals that Lowenstein's rule is widely violated, that silica is more susceptible than alumina to the depolymerizing effects of calcium, and that high-silica glasses favor calcium in lower oxygen coordination states, while the opposite is true for high-alumina ones. We also find the presence of highly coordinated aluminum and tricluster oxygens in high-alumina glasses, which form as a charge compensation mechanism. We find that current theoretical models used to predict oxygen species, oxygen bridge types, or tetrahedral coordination, while overall qualitatively reasonable, simplify the complex interplay between the different oxides which results in inaccurate predictions, particularly for glasses in intermediate compositional regions. Our analysis of the cluster, chain, and ring topological structures in the aluminosilicate network reveals a sharp transition from a connected to a disconnected graph which depends not only on the calcium content of the glass, but also on the ratio of silica to alumina. Glasses in the compositional region corresponding to such transition display the largest ring and longest chain structures of any glass studied.

Due to a lack of well-established industrial utilization technology, the large storage of coal gasification fine slag (CGFS) has caused environmental pollution and resource waste. The high-temperature melting (HTM) system can separate carbon from CGFS and make ash melt to form slag. In this study, the capacity of HTM with two different cooling structures in disposing massive CGFS was explored through bench-scale experiments, the resulting slag was characterized and the feasibility of slag as the starting material to prepare porous glass-ceramic through inorganic gel casting was studied. The rate of capturing slag and decarburization of the system with a molten bath are 88.60% and 91.00%, respectively, both better than the system with a radiation heat exchanger. As for the indicators of alkali activation potential for product slag, such as the content of unburned materials and vitreous phase, the system with the molten bath had a more optimistic performance. But the problems of low vitreous phase content and uneven composition are faced with it. In order to improve the ability of the slag to be alkali-activated, it is necessary to take measures to promote the full mixing of the melt and increase the cooling rate of the slag.

Alkali-borosilicate glasses 16M₂O–10B₂O₃–74SiO₂ were studied for M = Li to Cs. ¹¹B NMR showed 57% four-fold coordinated boron for Li, but 88 to 97% for M = Na to Cs. The fraction of neutral borate triangles decreases from Na to Cs. Vibrational spectroscopy shows significant non-bridging oxygen on silicate species for Li, decreasing to a low level from Na to Cs. The glass transition temperature is highest for M = Cs and correlates with increasing network connectivity. Sub-liquidus phase separation and homonuclear bonding was observed for M = Li, while M = Na to Cs show good linking of silicate with borate tetrahedra. Raman spectroscopy reveals a maximum of danburite rings for M = K, the glass with the highest packing density. Glasses doped with Ni₂O₃ changed color from yellow (Li) to purple (Na), to blue in samples of higher optical basicity (K, Rb, Cs). Tetrahedral Ni²⁺ likely coexists with trivalent nickel, Ni³⁺, in blue glasses, while higher coordinated Ni²⁺ prevails in the yellow Li-sample.

This article provides an overview of a variety of applications of ²⁹Si NMR to materials, with an emphasis on solid-state ²⁹Si NMR of siloxane polymers, silicates, functionalized silica, porous materials, zeolites, and glasses. The unique features of the ²⁹Si isotope with respect to NMR spectroscopy and an overview of the ²⁹Si chemical shift ranges of various structural environments in silicon-containing compounds and materials (in both solution and in solids) are presented. The main techniques of solid-state ²⁹Si NMR spectroscopy are briefly described, including a number of important advanced two-dimensional correlation techniques. A range of applications of ²⁹Si NMR to siloxanes, silicates, functionalized silica, zeolites, and glasses are described. Finally, recent developments in dynamic nuclear polarization techniques that have led to remarkable increases in sensitivity and have enabled the application of ²⁹Si NMR to new materials and new aspects of materials are outlined.