EXAFS study of a "mixed-alkali" type effect in sodium-calcium borate glass

Replacement of one alkali in a glass by another is known to have a profound effect on many physical properties in that glass. In the case of electrical conductivity, there is a nonlinear variation between the values of the two relevant single-alkali glasses, with a minimum at some mixed composition. This is referred to as the mixed-alkali effect (MAE), and has often been investigated using EXAFS, which shows a distinct change in the first-shell Debye-Waller factor at a composition usually corresponding to the minimum conductivity. However, investigations of the MAE have normally been restricted to alkali metal oxides, R2O, and other monovalent cation oxides. The authors have undertaken an EXAFS study of a series of glasses of general formula xNa(2)O . (1 - x)CaO . 2B(2)O(3) in order to ascertain whether a "mixed-alkali" type effect occurs between cations of differing charge, and whether any significant change in Debye-Waller factor occurs at the same percentage replacement as in glasses containing cations of similar charge.