A NEW CONCEPT ABOUT THE STATIC STRUCTURE AND ALKALI NON-BRIDGING OXYGEN DEPENDENCE OF THE MIXED ALKALI EFFECT IN BISMUTHATE GLASSES

Abstract. The X-ray measurements for the glass system of composition (Bi2O3)50(Fe2O3)10(Li2O)x(K2O)40−x, with x between 0 and 40 in steps of 5 have been carried out by means of X-ray powder diffraction using Mo– Kα radiation over [scattering vector, q between 1 and 16 A−1] range. X-ray diffraction has been used to calculate the pair distribution function g(r) ap-plying the pair potential analytical method. Both the partial pair distribution function gij(r) and the partial pair distribution function [PDF] of the metal non-bridging oxygen gij(r) M-nbo are also calculated by the same method. The composition dependence of these functions shows a maximum devia-tion around the alkali mole fraction (Li/Li+K) of 0.5. These results could be compared with those obtained for the activation energies Eσ and ED of con-ductivity and diffusion respectively which show a maximum deviation around the same alkali mole fraction. They also could be compared with the results on conductivity which show a minimum deviation at the same ratio. These results lead to the conclusion that there is a certain relation between the static structure [g(r)] and the mixed alkali effect (MAE). On the other hand the results show a maximum deviation in the values of PDF’s (at first neighbor distance) as function of Li/Li+K in the case of K-nbo and Li-nbo (potassium and lithium non-bridging oxygen). Whereas no deviation is observed in the case of K-bo and Li-bo (the case of bridging oxygen). The discussion of the results shows that the static structure (distribution of atoms) and the alkali non-bridging oxygen concentration are strongly related to the mixed alkali effect (MAE) phenomenon. The results are discussed in comparison with various MAE theories