High‐coordinated alumina and oxygen triclusters in modified aluminosilicate glasses

Abstract

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.

Modeling Oxygen Tricluster Formation in Calcium Aluminosilicate Supercooled Liquids and Glasses

“…Prior studies investigating the temperature dependence of triclusters and higher coordinated Al units noted several trends. Thompson and Stebbins 12 used NMR spectroscopy to show that five-coordinated Al structures in 30 and 60 mol % silica glasses formed more frequently as the fictive temperature increases, with a maximum of 8% [5] Al in a glass when 12 Similarly, NMR results by Dubinsky and Stebbins 13 showed that in CAS glass systems of R = 2, the percentage of [5] Al increased from 7.6 to 9.4% with a 200 K increase in fictive temperature. 13 This is in contrast with the reasoning that MD's high fictive temperature causes artificially high concentrations of triclusters.…”

“…5 The dominant mechanism of charge compensation within peraluminous compositions is debated within the literature, and an in-depth review of this structural debate can be found elsewhere. 5 In general, spectroscopic studies on calcium and sodium aluminosilicate glasses using techniques such as nuclear magnetic resonance (NMR) show support for the formation of higher coordinated Al units. 6,7 In contrast, atomistic simulations of the same systems with techniques such as molecular dynamics (MD) show support for oxygen triclusters and a minimal percentage of higher coordinated aluminum.…”

“…If MD triclusters only existed due to high fictive temperatures, the number of highly coordinated aluminum should increase (corresponding to less triclusters) as the fictive temperature increases. However, a 2004 study by Sen and Youngman 14 disputed these studies, claiming that NMR spectra of binary Al 2 O 3 −SiO 2 systems show no correlation between the fictive temperature and [5] Al formation. 14 A 2007 study by Vo Van Hoang 15 reported that for binary Al 2 O 3 −SiO 2 systems, there is only a weak temperature dependence for the creation of oxygen triclusters; instead, composition showed a stronger correlation.…”

“…15 Their study concluded that more NBO structures form with the increasing fictive temperature, when normalized for the composition. Dubinsky and Stebbins 13 argued that because there is no stoichiometric relationship between NBOs and [5] Al structures, there must be an increased amount of higher coordinated oxygen species or triclusters. 13 However, an extensive investigation into the temperature dependence of triclusters and other charge-compensating species has not been thoroughly studied for a wide compositional range.…”

“…O 3 ]/[CaO] being 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0. These parameters were chosen to explore the impact of low-and high-silica content on tricluster and [5] Al formation within both the percalcic and peraluminous regimes. MD simulations were performed using the LAMMPS 23 modeling package with each simulation consisting of ∼3000 atoms.…”