Al doped silica glass: investigation of structural response and defect interactions based on crystalline models

High purity quartz glass is an important material in high-tech industries like semiconductors and photovoltaics due to, among other properties, its good mechanical performance at high temperatures. Small amounts of Al in silica glass (in the range between 20 ppm and 100 ppm) have previously been shown to increase the viscosity of the SiO₂ glass. The underlying mechanism for this increase is, however, not well understood. In this paper we report on the local structural and electronic effects of the presence of Al in the SiO₂ structure by density functional theory (DFT). Comparing the quartz and cristobalite polymorphs, we found that the driving force for Al substitution is larger in the denser quartz structure compared to cristobalite, and that oxygen vacancy (Vo) formation is most stabilized in the nearest neighbour position relative to Al in both polymorphs. Al was not found to inherently strengthen the SiO₂ network in the two crystalline polymorphs considered. However, our results suggest that Al preferentially substitutes Si in denser ring configurations, which combined with local Vo formation could lead to locally favourable SiO₂ network reconstructions in SiO₂ glasses (likely towards 6-membered rings), which could propagate causing an increase in the viscosity. Furthermore, we show that the presence of Al can lower the stability of OH groups due to increased electrostatic interactions between the substitutional Al and H₂O which may also be a contributing factor in the increased viscosity of Al doped SiO₂ glass. The modelling results are in line with the experimental fluorescence and FT-IR spectroscopy data confirming that the presence of Al in the glass causes formation of oxygen vacancies and correlates with a lower fictive temperature which typically corresponds to a larger average Si-O-Si angle in the glass structure. Our results suggest that Al's contribution to high glass viscosity is not solely due to the substitution of Si atoms by Al atoms in the glass structure but rather due to structural changes of the silica network the substitution causes.