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Yttrium aluminosilicate glasses with 25–78 mol% silica were studied using molecular dynamics simulations to understand their structural and property changes. The results show that Al3+ ions primarily exist as four-fold coordinated, with <5% in higher-coordinated states that increase with decreasing silica content. The formation of significant concentrations (4–9%) of oxygen tri-clusters and small amounts of free oxygen were also observed, suggesting a perturbed glass network structure. An average Y-O bond distance of 2.26 Å and Y coordination number of 6.3 were found. The glass transition temperatures are relatively insensitive to composition, agreeing with experiments. A 16% and 30% increase in Young's and bulk moduli, respectively, was observed with decreasing silica contents which was explained by the strong Y-O bond and formation of oxygen tri-clusters that aggregate higher coordinated Al species. These results were discussed in the context of optical and acoustic properties of YAS optical fibers that exhibit reduced nonlinearities.