Cation clustering in lithium silicate glasses: Quantitative description by solid-state NMR and molecular dynamics simulations

The local structural environment and the spatial distribution of the lithium ions in lithium silicate glasses with composition (Li2O)(x)(SiO2)(1-x)(0 < x <= 0.40) is studied by nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulation experiments. Site resolved Si-29{Li-7} rotational echo double resonance (REDOR) studies reveal that the Li-7 dipolar fields measured at the Q((3)) sites are significantly stronger than those at the Q((4)) sites, and are almost independent of composition, implying a significant amount of cation clustering. For glasses with low lithia contents (x=0.10 and 0.17) these conclusions are qualitatively confirmed by molecular dynamics simulations and are consistent with the well-known tendency of such glasses to phase separate. Based on the combined interpretation of dipolar second moments M-2(Si-29-Li-7) extracted from REDOR and partial pair correlation functions g(SiLi)(r) determined by MD simulation, a structural model for the lithia-enriched domains is proposed: each Q((3)) unit is surrounded by approximately three lithium ions at an average distance of 320 pm, whereas the Q((4)) units are much more remote from lithium. Detailed quantitative comparisons indicate that the clustering tendency suggested by MD is generally less pronounced than that indicated by the NMR results, and a significant structural difference is observed for (Li2O)(0.33)(SiO2)(0.67) glass. Most likely, these discrepancies are consequences of the large difference in implicit cooling rates in the laboratory and the computer experiments, resulting in significantly different glass transition temperatures.