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 treatments at 435 °C, i.e., 20 K below the laboratory glass transition temperature (T_g). 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 homonuclear 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 transition 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 investigating the structural relaxation process accompanying the internal crystallization of ion-conducting frozen supercooled melts containing suitable NMR active nuclear probes (⁷Li, ²³Na, ¹³³Cs, etc.).

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Recommended citation: Henrik Bradtmüller, Anuraag Gaddam, Hellmut Eckert, Edgar Dutra Zanotto, “Structural rearrangements during sub-Tg relaxation and nucleation in lithium disilicate glass revealed by a solid-state NMR and MD strategy”, Acta Materialia, 2022, 240, 118318.