Structural properties of lithium and sodium tetrasilicate glasses: Molecular dynamics simulations versus NMR experimental and first-principles data

In this work we present the structural properties of lithium tetrasilicate glass (LS4) obtained by combined classical and Car–Parrinello molecular dynamics simulations. The computed features are compared with corresponding experimental results as well as to those obtained for sodium tetrasilicate glass (NS4) models generated using the same approach, and containing the some percentage of alkali oxide. The ab initio description improves the comparison of the structural characteristics with experimental data, in particular concerning the Li–O, Si–Li and Si–Si bond lengths. The influence of the network modifier nature on the relevant structural parameters is further examined in terms of their correlations to the first-principles calculated NMR parameters of our glassy models. For both glasses, 29Si MAS NMR measurements are reported and the obtained experimental spectra are compared to the theoretical ones.

We have studied the structural properties of two (Li/Na) single-alkali silicate glasses with the same cation content (i.e. 20%) by combined classical and Car–Parrinello molecular dynamics simulations. The influence of the network modifier nature on the relevant structural parameters have been further examined in terms of their correlations to the first-principles NMR spectra and parameters computed for our glassy models using the GIPAW method. Hence, we found that the 29Si isotropic chemical shift of Q(4) units upon the mean Si–O–T bond angle (where T denotes the Q(n) connected tetrahedron, n = 4, 3, 2) could be well described by means of a single (i.e. the same for both glasses) simple linear dependence. This comparative study may be helpful to obtain a precise microscopic insight into one of the unusual features of oxide glasses, namely the so-called mixed alkali effect.Figure optionsDownload as PowerPoint slide