Structural characterization of high-zirconia borosilicate glasses using Raman spectroscopy

Polarized Raman spectra were obtained for a collection of borosilicate glasses that have been developed as candidate compositions for the immobilization of wastes generated from the reprocessing of Zircaloy-clad spent nuclear fuel. Raman spectra were obtained for borosilicate glasses with zirconia compositions as high as 21 wt%, as well as for crystalline ZrO2 (baddeleyite) and crystalline ZrSiO4 (zircon). As zirconia content in the glass is increased, two trends in the spectra indicate that the partially polymerized silicate tetrahedral network becomes more depolymerized: one, the polarized mid frequency envelope near 450 cm−1, assigned to Si–O–Si symmetrical bend modes, decreases in area; and two, the higher frequency band assigned to Si–O stretch in Q2 units (silicate chains) increases in area, while band areas decrease for modes assigned to Si–O stretch in more polymerized Q3 and Q4 units (silicate sheets and cages). These trends take place whether the glass composition is relatively simple or considerably more complex. As zirconia concentrations in the glass increase beyond 15 wt%, a series of sharp lines are observed in the spectra from baddeleyite crystals, and to a minor extent Zn–Cr spinel phases, superimposed on broad features from the glass matrix. A low intensity, broad band near 1400 cm−1 in the glass spectra is probably due to B–O stretch modes within BO3 units.