Structural rearrangements during sub- T g relaxation and nucleation in lithium disilicate glass revealed by a solid-state NMR and MD strategy

Structural rearrangements taking place during relaxation and crystal nucleation in lithium-disilicate (LS2) glass have been investigated by a comprehensive set of solid-state nuclear magnetic resonance (NMR) experiments, supported by molecular dynamics (MD) simulations. Samples were subjected to heat treat-ments at 435 degrees C, i.e., 20 K below the laboratory glass transition temperature (Tg). Raman and NMR data indicate that under these conditions both relaxation and nucleation occur without detectable changes in the network former unit distribution of the glassy silicon-oxide network. Instead, relaxation of the frozen supercooled melt and nucleation of LS2 crystals occur principally in terms of a changing lithium local environment: 7Li spin-echo decay NMR indicates average Li-Li distances, characterized by homonu-clear dipolar second-moment measurements, are reduced after very short heat treatments and approach those found in the isochemical crystal. This finding is supported by molecular dynamics (MD) simulations predicting a dependence of the Li + ion distribution on the melt-cooling rate. In addition, the structural reorganization also impacts the distribution of electric field gradients as detected by 7Li satellite tran-sition NMR spectroscopy. Finally, crystal nucleation becomes most evident by the appearance of minor amounts of sharp 29Si MAS resonances and a significant change in the 7Li NMR satellite transitions, as visualized by difference spectroscopy. This study defines a new NMR strategy, generally applicable for in-vestigating the structural relaxation process accompanying the internal crystallization of ion-conducting frozen supercooled melts containing suitable NMR active nuclear probes (7Li, 23Na,133Cs, etc.).(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The structural rearrangements during relaxation and crystal nucleation in lithium-disilicate glass were investigated using solid-state nuclear magnetic resonance (NMR) experiments and molecular dynamics (MD) simulations. It was found that the change in the local environment of lithium, rather than the network former unit distribution, played a key role in the relaxation and nucleation processes. The crystal nucleation was accompanied by changes in the electric field gradients.