Structure of fast-ion-conducting lithium and sodium borate glasses by neutron diffraction and reverse Monte Carlo simulations

The structures of fast-ion-conducting glasses $\left(\mathrm{N}\mathrm{a}{Z)}_x\right({\mathrm{Na}}_2{\mathrm{O}-2\mathrm{B}}_2{\mathrm{O}}_3{)}_{1-x}$ and $\left(\mathrm{Li}{Z)}_x\right({\mathrm{Li}}_2{\mathrm{O}-2\mathrm{B}}_2{\mathrm{O}}_3{)}_{1-x}$ $(Z=\mathrm{C}\mathrm{l},\mathrm{B}\mathrm{r})$ have been examined by neutron diffraction and reverse Monte Carlo (RMC) simulations. The short-range structure of the boron-oxygen network is almost unchanged for increasing dopant salt concentration and independent of the dopant salt, whereas the intermediate range order of the B-O network decreases significantly for increasing dopant salt concentration. The sodium borate glasses are generally slightly more ordered than the corresponding lithium borate glasses. The differences may be explained by the fact that the Li-borate glasses consist of a disordered random mixture of many different types of borate configurations, while the Na-borate glasses are built up by randomly distributed diborate groups, as suggested previously from NMR results. The RMC simulations of the highest LiCl- and NaCl-doped glasses show, however, that large density fluctuations are present within the boron-oxygen network. The voids are of widely different sizes and geometrical shapes. The present findings show that the intermediate-range order of the LiCl- and NaCl-doped glasses is significantly different from that recently reported for the corresponding AgI-doped glass. For the latter the boron-oxygen network forms a more ordered chainlike structure with the salt ions cross linking between the “chains.”