Structure of borate glasses from neutron-diffraction experiments.

Abstract

Systematic neutron-diffraction experiments have been performed on metal oxide modified borate glasses, ${\mathit{xM}}_{2}$O-${2\mathrm{B}}_{2}$${\mathrm{O}}_{3}$ with M=Ag,Li,Na, in order to investigate the short- and intermediate-range structure. The short-range order of the boron-oxygen network, involving the first B-O, B-B, and O-O interatomic distances, is found to be almost identical for the silver, lithium, and sodium borates of the same modifier concentration. A detailed analysis of the first peak in the atomic pair correlation function reveals two different B-O nearest-neighbor distances, 1.37 and 1.47 \AA{}, which are attributed to ${\mathrm{BO}}_{3}$ and ${\mathrm{BO}}_{4}$ groups, respectively. The relative abundance of three and four coordinated borons are determined for variations of the metal oxide content and found to be in accordance with reported nuclear magnetic resonance results. The low momentum transfer (Q) range of the structure factors of the various metal-oxide modified glasses are significantly different, which indicates differences in the structural organization on an intermediate length-scale 5--15 \AA{}. For ${\mathrm{Ag}}_{2}$O and ${\mathrm{Na}}_{2}$O modifications considerably longer range correlations are observed as compared with pure vitreous ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$. In contrast, in the case of ${\mathrm{Li}}_{2}$O modification no dramatic change of the intermediate-range order is observed. The different behavior is due to differences in the relative topological arrangements of the various borate groups formed, which in turn appears to be related to the size and degree of covalency of the cation and hence to its ability to form bridges between segments of the BO network.