NA+ COORDINATION IN SODIUM DIBORATE AND TRIBORATE GLASSES BY X-RAY-DIFFRACTION

Na+ coordination in diborate (Na2O-2B2O3) and triborate (Na2O-3B2O3) glasses was investigated by X-ray diffraction through a difference method in which radial distribution functions of lithium borate samples are subtracted from those of sodium borate glasses of the same molar composition. The difference curves exhibited peaks arising from the distribution of nearest neighbour O atoms around Na+ ions. The quantitative analysis of these peaks and a careful evaluation of the reliability of structural parameters were performed. Average coordination numbers were 4.0+/-0.4 for diborate and 4.8+/-0.5 for triborate. Finally, a comparison was made of Na+ environments in glasses and crystals of equal composition, and the similarities and the differences were discussed.