The local structural environment and the spatial distribution of the lithium ions in lithium silicate glasses with composition ${\left({\mathrm{Li}}_2\mathrm{O}\right)}_x{\left({\mathrm{SiO}}_2\right)}_{1-x}(0<x⩽0.40)$ 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 $Q^{\left(3\right)}$ sites are significantly stronger than those at the $Q^{\left(4\right)}$ 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 $M_2\left({}^{29}\mathrm{Si}\text{−}{}^7\mathrm{Li}\right)$ extracted from REDOR and partial pair correlation functions $g_{\mathrm{SiLi}}\left(r\right)$ determined by MD simulation, a structural model for the lithia-enriched domains is proposed: each $Q^{\left(3\right)}$ unit is surrounded by approximately three lithium ions at an average distance of 320 pm, whereas the $Q^{\left(4\right)}$ 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 ${\left({\mathrm{Li}}_2\mathrm{O}\right)}_{0.33}{\left({\mathrm{SiO}}_2\right)}_{0.67}$ 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.

• Received 18 March 2005

DOI:https://doi.org/10.1103/PhysRevB.72.064207