Structure study of multi-component borosilicate glasses from high-Q neutron diffraction measurement and RMC modeling

A neutron diffraction structure study has been performed on multi-component borosilicate glasses with compositions (65 - x)SiO2 center dot xB(2)O(3) center dot 25Na(2)O center dot 5BaO center dot 5ZrO(2), x = 5-15 mol%. The structure factor has been measured up to a rather high momentum transfer value of 30 angstrom(-1), which made high r-space resolution available for real space analyses. Reverse Monte Carlo simulation was applied to calculate the partial atomic pair correlation functions, nearest neighbor atomic distances and coordination number distributions. The Si-O network consists of tetrahedral SiO4 units with characteristic first neighbor distances at r(Si-O) = 1.60 angstrom and r(Si-Si) = 3.0 angstrom. The boron environment contains two well-resolved B-O distances at 1.40 and 1.60 angstrom and both 3- and 4-fold coordinated B atoms are present. A chemically mixed network structure is proposed including B-[4]-O-Si-[4] and B-[3]-O-Si-[4] chain segments. The O-O and Na-O distributions suggest partial segregation of silicon and boron rich regions. The highly effective ability of Zr to stabilize glassy and hydrolytic properties of sodium-borosilicate materials is interpreted by the network-forming role of Zr ions. (C) 2008 Elsevier B.V. All rights reserved.