Transitioning beyond a trial-and-error based approach for the compositional design of next-generation borosilicate-based bioactive glasses requires a fundamental understanding of the underlying compositional and structural drivers controlling their degradation and ion release in vitro and in vivo. Accordingly, the present work combines magic-angle spinning (MAS) NMR techniques, MD simulations, and DFT calculations based on GIPAW and PAW algorithms, to build a comprehensive model describing the short-to-medium-range structure of potentially bioactive glasses in the Na 2 O−P 2 O 5 −B 2 O 3 −SiO 2 system over a broad compositional space. P 2 O 5 preferentially tends to attract network modifier species, thus resulting in a repolymerization of the silicate network and a restructuring of the borate component. 11 B{ 31 P} and 31 P{ 11 B} dipolar recoupling experiments suggest that the ability of glasses to incorporate P 2 O 5 without phase separation is related to the formation of P−O−B(IV) linkages integrated into the borosilicate glass network. An analogous approach is used for elucidating the local environments of the Na + network modifiers. This work, along with future studies aimed at elucidating composition−structure−solubility/bioactivity relationships, will lay the foundation for the development of quantitative structure−property relationship (QSPR) models, thus representing a leap forward in the design of functional borosilicate bioactive glasses with controlled ionic release behavior.

NMR determinations of fluorine environments in transparent oxyfluoride glass-ceramics were made to learn about the crystallization of LaF 3 , as well as to ascertain the structural role of fluorine in the surrounding glassy matrix. The fraction of fluorine in LaF 3 was measured as a function of heat treatments, demonstrating significant differences between glasses modified with barium and those containing sodium. The results of these measurements showed that not all of the fluorine formed LaF 3 in these glass-ceramics, with resolution of additional fluoride sites at ؊135 and ؊185 ppm, due primarily to Si-F and Al-F bonding, respectively. Not only is the evidence of Si-F bonding unexpected, given the presence of aluminum, but the amount of Si-F bonding is sensitive to the type of modifier in the glass. Samples containing barium oxide as the modifier showed a higher fraction of Si-F bonding than those modified with sodium oxide.

Aluminosilicate glasses are ubiquitous in high-performance displays due to their favorable thermal, mechanical, and optical properties. They also exhibit interesting structural features depending on the ratio of alumina to modifiers in the glass system. Excess modifiers exist in the metaluminous region, while the peraluminous region contains more negatively charged alumina structures than modifiers. As the composition switches from metaluminous to peraluminous, anomalous changes in properties such as the glass transition temperature, viscosity, and refractive index occur. This has been explained with two contrasting structural transformations to accommodate the lack of charge-balancing modifiers: either aluminum increases in coordination (forming five-coordinated or six-coordinated Al) and/or oxygens become three-coordinated (known as triclusters). The precise charge-balancing mechanism remains a subject of much debate in the community. This review highlights this structural debate by providing a chronological understanding of how these two theories evolved. We also summarize the state-of-the-art understanding of the aluminosilicate glass structure. By gaining a more comprehensive view of the two opposing structural views within the aluminosilicate glass system, we can gain insights on valuable future research from both experimental and modeling perspectives.

The structure and properties of glass can be modified through compression near the glass transition temperature (T), and such modified structure and properties can be maintained at ambient temperature and pressure. However, once the compressed glass undergoes annealing near T at ambient pressure, the modified structure and properties will relax. The challenging question is how the property relaxation is correlated with both the local and the medium-range structural relaxation. In this paper, we answer this question by studying the volume (density) and structural relaxation of a sodium borate glass that has first been pressure-quenched from its T at 1 GPa, and then annealed at ambient pressure under different temperature-time conditions. Using B MAS NMR and Raman spectroscopy, we find that the pressure-induced densification of the glass is accompanied by a conversion of six-membered rings into non-ring trigonal boron (B) units, i.e. a structural change in medium-range order, and an increase in the fraction of tetrahedral boron (B), i.e. a structural change in short-range order. These pressure-induced structural conversions are reversible during ambient pressure annealing near T, but exhibit a dependence on the annealing temperature, e.g. the ring/non-ring B ratio stabilizes at different values depending on the applied annealing temperature. We find that conversions between structural units cannot account for the pressure-induced densification, and instead we suggest the packing of structural units as the main densification mechanism.