The use of crystallographic data in interpreting the correlation function for complex glasses

The interpretation of the total correlation function, T(r), for a complex glass is extremely difficult without complementary information, even in the case of the peaks at low r arising from the basic structural units. For example, in alkali borosilicate glasses, there are no less than 10 possible structural units, viz. SiØ4, SiØ3O−, Si Ø 2 O 2 2 - , Si Ø O 3 3 - , SiO 4 4 - , B Ø 4 - , BØ3, BØ2O−, B Ø O 2 2 - and BO 3 3 - , where Ø represents a bridging and O− a (negatively charged) non-bridging oxygen atom. All of these structural units have slightly different bond lengths and, in addition, the borate structural unit parameters change when they are incorporated into the various superstructural units arising from the borate component. The present paper discusses the role of crystallographic information in the analysis of the correlation functions for complex glasses and a summary is presented of accurate crystallographic data for alkali borates, silicates and borosilicates, which are employed to establish optimum bond lengths and angles for the various structural units within the glasses, and the detailed geometry of the borate superstructural units. These parameters form the basis for a more comprehensive interpretation of the neutron correlation functions for a number of glasses from the sodium borosilicate system.