Effect of alumina on the structure and properties of Li2O–B2O3–P2O5 glasses

The structure of glasses within the system Li2O–Al2O3–B2O3–P2O5 has been studied through 31P, 11B and 27Al Nuclear Magnetic Resonance, and the effect of Al2O3 substitution by B2O3 and P2O5 network formers on the structure and properties investigated for a constant Li2O content. Multinuclear NMR results reveal that substitution of Al2O3 for B2O3 and P2O5 network formers in a glass with composition 50Li2O·15B2O3·35P2O5 produces a change in boron environment from four-fold to three-fold coordination. Meanwhile aluminum can be present in four-, five- and six-fold coordinations a higher amount of Al(IV) groups is found for increasing alumina contents. The behavior of the glass transition temperature and electrical conductivity of the glasses has been interpreted as a function of the structural changes induced in the glass network when alumina is substituted for B2O3, P2O5 or both. Small additions of alumina produce a drastic increase in glass transition temperature, while it does not change for [Al2O3] greater than 3 mol.%. However, the electrical conductivity shows very different behavior depending on the type of substitution; it can remain constant when B2O3 content decreases or sharply decrease when P2O5 is substituted by Al2O3, which is attributed to a higher amount of BO3 and phase separation.

Research highlights► Al2O3 induces the conversion of BO4 groups to BO3 ones and increase its former character through a bigger amount of AlO4. ► Small alumina addition produces a sharp increase in Tg of the glasses, while further additions only produce a small decrease. ► The electrical conductivity of the glasses either remains constant or decreases sharply when Al2O3 is substituted for P2O5. ► The higher the amount of BO3 units the higher the tendency for phase separation.