THE EFFECT OF WATER ON THE STRUCTURE OF SILICATE-GLASSES - A NEUTRON-DIFFRACTION STUDY

The first neutron diffraction study of water in silicate glasses is re ported. Deuterated sodium tetrasilicate glasses, Na2Si4O9 . xD(2)O (x = 5, 10 wt%), as well as a protonated glass, Na2Si4O9 . 10 wt%H2O, wer e prepared in rapid-quench high pressure autoclaves. Infrared (IR) mea surements show, in accordance with results for other proton-containing glasses, the presence of both molecular H2O and OH species. Line shap e analysis of the first sharp diffraction peak (FSDP) indicates a grad ual decrease of the medium-range order with increasing total water (H2 O + OH) content, A structural mechanism explaining the observed change s in the FSDP is proposed. Combined least-squares fits, using Gaussian functions, of the first Si-O, Na-O and O-O peaks in the corresponding total pair correlation functions, T(r), indicate that the short-range order of the silicate network is nor affected by the presence of wate r. However, the root mean square displacements of the Na atoms decreas e, suggesting that the Na-O coordination becomes more ordered in hydro us silicate glasses. The first-difference of the correlation functions , D(r), for the glasses with 10 wt% total water content is obtained. T he OH contributions are identified and indicate, in accordance with th e analysis of the FSDP, that the OH groups lead to significant changes in the silicate network on the 3-5 Angstrom scale. The average O-H di stance determined from the total pair correlation functions is 0.99 An gstrom. Three types of hydrous species (H2O and/or OH), participating in hydrogen-bonding, are observed in the IR spectra with average O-H.. .O bond lengths, equal to 3.0, 2.65 and 2.55 Angstrom, respectively.