Raman studies of sulfur in borosilicate waste glasses: sulfate environments

Abstract

Raman spectroscopy has been used to characterize sulfur environments in a variety of borosilicate glass formulations developed for long-term radioactive waste storage. The spectra of these glasses all have S–O symmetrical stretch modes ( ν 1) near 1000 cm −1 from tetrahedral SO 4 (sulfate) environments. The Raman data indicate that the sulfate environments are independent of the borosilicate network; in particular, isolated SO 4 tetrahedra in the glass are surrounded by network modifying cations, such as Na. By changing the type of network modifying cations in the borosilicate glass, the ν 1 peak shifts, in such a way that larger cation charge densities correlate to higher ν 1 frequencies. The ν 1 peak for Li, Ca, Na, K, and Cs borosilicate glasses is broadened and shifted to lower frequencies with respect to ν 1 for Li, Ca, Na, K, and Cs sulfate crystals, respectively; this indicates that sulfate tetrahedra in borosilicate glass are more disordered and more weakly bonded to their surrounding environments than sulfate tetrahedra in the corresponding sulfate crystals.