Nepheline crystallization in boron-rich alumino-silicate glasses as investigated by multi-nuclear NMR, Raman, & Mössbauer spectroscopies

• Alumino-boro-silicate simulant nuclear waste glasses crystallize nepheline.
• 23Na, 27Al, 29Si, 11B NMR, Fe Mössbauer, and Raman used to study glass structure
• Two different sites each of Na, Al, and Si shown by NMR in crystallized glass
• Raman shows predominance of metaborate chains and rings in glass.

A spectroscopic study was conducted on six simulant nuclear waste glasses using multi-nuclear NMR, Raman, and Mössbauer spectroscopies exploring the role of Si, Al, B, Na, and Fe in the glass network with the goal of understanding melt structure precursors to deleterious nepheline crystal formation. NMR showed two sites each for Al, Si, and Na in the samples which crystallized significant amounts of nepheline, and B speciation changed, typically resulting in more B(IV) after crystallization. Raman spectroscopy suggested that some of the glass structure is composed of metaborate chains or rings, thus significant numbers of non-bridging oxygen and a separation of the borate from the alumino-silicate network. Mössbauer, combined with Fe redox chemical measurements, showed Fe playing a minor role in these glasses, mostly as Fe3 +, but iron oxide spinel forms with nepheline in all cases. A model of the glass network and allocation of non-bridging oxygens (NBOs) was computed using experimental B(IV) fractions which predicted a large amount of NBO consistent with Raman spectra of metaborate features.