Coordination Environments of Highly Charged Cations (Ti, Cr, and Light REE’s) in Borosilicate Glass/Melts to 1120°C

The local environments around Ti, Cr, and several light rare-earth elements (La, Ce, and Nd) were investigated by in-situ XANES spectroscopy in a number of complex borosilicate glasses and melts (to 1120 °C) that are used for nuclear waste storage. Examination of the high-resolution XANES spectra at the Ti K-edge shows that the average coordination of Ti changes from ~5 to ~4.5. Cr is dominantly trivalent in the melts studied. However, its average coordination is probably lower in the melt (tetrahedral ?) as revealed by the more intense Cr-K pre-edge feature. Ce also changes its average valence from dominantly +4 to +3.5 upon glass melting. These changes are reversible at Tg, the glass transition temperature (~500-550°C for these glasses). In contrast, the local environments of Nd, Pr, and La are unaffected by melting. Therefore, structural reorganization of these borosilicate glass/melts above Tg is variable, not only in terms of valence (as for Ce) but also speciation (Ti and Cr). Both the ability of B to adopt various coordination geometries (triangular and tetrahedral) and the chemical complexity of the glass/melts explain these changes.