Raman studies of sulfur in borosilicate waste glasses: sulfate environments

Raman spectroscopy has been used to characterize sulfur environments in a v ariety of borosilicate glass formulations developed for long-term radioacti ve waste storage, The spectra of these glasses all have S-O symmetrical str etch modes (v(1)) near 1000 cm(-1) from tetrahedral SO4 (sulfate) environme nts. The Raman data indicate that the sulfate environments are independent of the borosilicate network; in particular, isolated SO4 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 v(1) p eak shifts, in such a way that larger cation charge densities correlate to higher v(1) frequencies. The v(1) peak for Li, Ca, Na, K, and Cs borosilica te glasses is broadened and shifted to lower frequencies with respect to v( 1), for Li, Ca, Na, K, and Cs sulfate crystals, respectively; this indicate s that sulfate tetrahedra in borosilicate glass are more disordered and mor e weakly bonded to their surrounding environments than sulfate tetrahedra i n the corresponding sulfate crystals. (C) 2001 Elsevier Science B.V. All ri ghts reserved.