Structural aspects of lead borate glasses prepared in platinum and alumina crucibles

Lead borate glasses in the system xPbO-(100-x)B₂O₃ have been prepared in platinum and alumina crucibles. Glasses were investigated qualitatively by Raman and infrared spectroscopies, and quantitatively by NMR spectroscopy, to determine the effect of Al₂O₃ contamination on the glass network. Glasses melted in a platinum crucible had a formation range of 30 ≤ x ≤ 80 mol% PbO, while glasses prepared in alumina crucibles experienced an expansion in the formation range, i.e. 20 ≤ x ≤ 85 mol%, as a result of Al₂O₃ contamination from the alumina crucible. Both the mid-frequency Raman and infrared data, as well as NMR spectroscopy, suggest that the introduction of Al₂O₃ within the glass results in the under-modification of the borate network due to the formation of anionic AlO_n type units charge balanced by Pb²⁺ ions. This is also reflected in the low-frequency region of the Raman and infrared spectra, which relates to the Pb-O vibrations, where the Pb-O bonding remains predominately ionic for a larger composition range in the Al₂O₃ contaminated glasses (x ≤ 60), while the Pb-O bond also acquires covalent character for x ≥ 70 mol%.