The structure of sodium tetrasilicate glass from neutron diffraction, reverse Monte Carlo simulations and Raman spectroscopy

Abstract

The reverse Monte Carlo (RMC) method was used for modelling the three-dimensional structure of sodium tetrasilicate glass (Na2Si4O9, NS4). Neutron diffraction data over a large momentum transfer range and nuclear magnetic resonance results (Q-species distribution) as well as chemical bonding considerations have been used to constrain the RMC simulations. Very good agreement with the experimental structure factor was achieved. The three-dimensional model is analysed in terms of partial pair distribution functions, bond-angle distributions, short-range order (SRO) clusters and ring statistics. The average Si–O–Si bridging angle is different for the different Qi–Qj units. The effect of different Q species on the network connectivity is analysed by comparison with statistical SRO distributions. While the Q2 species are randomly distributed, a high preference for the formation of Q4–Q4 units in three-membered rings is found. This explains why during the initial stages of hydration of NS4 glass, Si–OH groups are preferentially formed by breaking Q4–Q4 linkages. The RMC model also provides an independent test for the assignment of bands in the Si–O stretching region of the Raman spectrum. It is shown that the bands at about 1050 and 1100 cm−1 are probably due to Q3 units surrounded by different numbers of Q4 units.