The glass transition temperature of lithium-alkali borates

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Glass transition temperatures, T g, were measured for mixed lithium-alkali borate glasses over wide ranges of composition in samples prepared from both oxide and carbonate starting materials. Raman spectroscopy was used to compare samples prepared from different sets of starting materials. Differences in T g were observed as a function of alkali. At low alkali content LiNa borates had the greatest T g and LiCs borates the least, while at high alkali contents the trends were reversed. Differences were also seen between samples prepared from carbonates when compared with oxides for the LiRb and LiCs systems with the carbonate-based glasses having higher T g. No such differences were found in the LiNa case. The trends of the data were explained in terms of models for the atomic arrangements. At low alkali contents, the dependence of T g on the fraction of four-coordinated borons was invoked while at high alkali contents differences between orthoborate units — tetrahedral in the cases of Rb and Cs and trigonal in the sodium and lithium systems — were used. The T g trends were also compared with density and glass formability observations.