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

Abstract

The local structural environment and the spatial distribution of the lithium ions in lithium silicate glasses with composition ${({\mathrm{Li}}_{2}\mathrm{O})}_{x}{({\mathrm{SiO}}_{2})}_{1\ensuremath{-}x}(0<x\ensuremath{\leqslant}0.40)$ is studied by nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulation experiments. Site resolved $^{29}\mathrm{Si}{^{7}\mathrm{Li}}$ rotational echo double resonance (REDOR) studies reveal that the $^{7}\mathrm{Li}$ 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 ${M}_{2}(^{29}\mathrm{Si}\text{\ensuremath{-}}^{7}\mathrm{Li})$ extracted from REDOR and partial pair correlation functions ${g}_{\mathrm{SiLi}}(r)$ 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 ${({\mathrm{Li}}_{2}\mathrm{O})}_{0.33}{({\mathrm{SiO}}_{2})}_{0.67}$ 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.