Transition from a Single Ion to a Collective Diffusion Mechanism in Alkali Borate Glasses

Imre Á, Berkemeier F, Mehrer H, Gao Y, Cramer C, Ingram M

Abstract

Two distinct regions of the dc conductivity and its temperature dependence as a function of the mol fraction of alkali oxides, X, are observed in Na- and Li-borate glasses. At low alkali contents the dc conductivity [sigma]dc increases only moderately with X. However, at higher alkali contents, log [sigma]dc increases linearly and the activation enthalpy [Delta]H of [sigma]dc × T decreases linearly with log X, i.e. the dc conductivity reveals an effective power-law behavior. The transition between low alkali and high alkali behavior takes place at X [approximate] 0.08 for Na-borate and at X [approximate] 0.09 for Li-borate glasses. This behavior suggests that the diffusion mechanism changes at these alkali contents. The results are discussed in terms of ion separations and the transition from a single-ion jump to a collective diffusion mechanism. The vanishing of the mixed-alkali peak in Na-Rb borate and alumino-germanate glasses at sodium contents similar to that observed for the change in slope of [sigma]dc(X) in this work suggests that both phenomena share the same origin.