Effect of lithium concentration on the network connectivity of nuclear waste glasses

Structure of borosilicate glasses with varying Li₂O contents were investigated using Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR), employing ⁶Li, ¹¹B, ²³Na, ²⁷Al and ²⁹Si nuclei. ¹¹B MAS NMR revealed that increasing Li₂O contents result in formation of [BO₄]⁻ sites at the expense of BO₃. ¹¹B{⁶Li} and ²⁷Al{⁶Li} dipolar heteronuclear multiple quantum correlation (D-HMQC) NMR revealed Li as a charge compensator for anionic tetrahedral sites with increased Li. ¹¹B{⁶Li} J-coupling mediated HMQC NMR suggested the possible association of Li with non-bridging oxygens on Q² and Q³ sites, indicating its dual role in the glass network. ²⁹Si and ²³Na MAS NMR spectra showed depolymerisation of the silicate network and shortening of Na-O bond lengths with increased Li. NMR results are supported by Raman spectroscopy and thermal analysis that indicate depolymerisation of the silicate network and a reduction in glass transition temperature at higher Li content.