The structural role of tellurium dioxide in lead borophosphate glasses

• The structure of lead borophosphate glasses modified with 0–70 mol% TeO2 was studied.
• From 11B MAS NMR spectra, the relative numbers of BO3 and B(OP)4 − n(OTe)n units were obtained.
• Raman spectroscopy showed the preference of TeO3 units at a low TeO2 content and TeO4 units at a high TeO2 content.
• The effect of different B2O3/P2O5 ratio on the glass structure was evaluated.

Lead borophosphate glasses formulated with TeO2 were prepared and studied in the compositional series (1 − x)[50PbO-10B2O3-40P2O5]-xTeO2 with x = 0–0.7 and glasses with 20 and 60 mol% TeO2 with also the B2O3/P2O5 ratio of 5/45 and 15/35. Structural studies by 31P and 11B MAS NMR and Raman spectroscopies revealed the formation of P–O–Te bonds. The addition of TeO2 resulted in the depolymerisation of phosphate chains as revealed from the 31P MAS NMR spectra. The addition of TeO2 to the parent lead borophosphate glass resulted in the formation of B–O–Te bonds and B(OP)4 − n(OTe)n mixed structural units, the number of which increases with increasing TeO2 content and also with the increasing B2O3/P2O5 ratio in the glasses. Only a small number of BO3 units (less than 4%) are formed in glasses with a higher TeO2 content and at a high B2O3/P2O5 ratio. The Raman spectra also indicated that at low TeO2 concentrations, tellurium atoms form preferentially TeO3 structural units, whereas at higher TeO2 content, TeO4 units are predominant in the glass network. With the replacement of P2O5 by B2O3 in the studied glasses, TeO4 units are converted into TeO3 units as the number of oxygen atoms in the glass decreases when the B2O3/ P2O5 ratio increases.