Liquid−Liquid Phase Separation in Model Nuclear Waste Glasses: A Solid-State Double-Resonance NMR Study

Abstract

Double-resonance nuclear magnetic resonance (NMR) techniques are used in addition to single-resonance NMR experiments to probe the degree of mixing between network-forming cations Si and B, along with the modifier cations Cs+ and Na+ in two molybdenum-bearing model nuclear waste glasses. The double-resonance experiments involving 29Si in natural abundance are made possible by the implementation of a CPMG pulse-train during the acquisition period of the usual REDOR experiments. For the glass with lower Mo content, the NMR results show a high degree of Si−B mixing, as well as an homogeneous distribution of the cations within the borosilicate network, characteristic of a non-phase-separated glass. For the higher-Mo glass, a decrease of B−Si(Q4) mixing is observed, indicating phase separation. 23Na and 133Cs NMR results show that although the Cs+ cations, which do not seem to be influenced by the molybdenum content, are spread within the borate network, there is a clustering of the Na+ cations, very likely around the molybdate units. The segregation of a Mo-rich region with Na+ cations appears to shift the bulk borosilicate glass composition toward the metastable liquid−liquid immiscibility region and induce additional phase separation. Although no crystallization is observed in the present case, this liquid−liquid phase separation is likely to be the first stage of crystallization that can occur at higher Mo loadings or be driven by heat−treatment. From this study emerges a consistent picture of the nature and extent of such phase separation phenomena in Mo-bearing glasses, and demonstrates the potential of double−resonance NMR methods for the investigation of phase separation in amorphous materials.