Transition from a single-ion to a collective diffusion mechanism in alkali borate glasses

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) x 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 to 0.08 for Na-borate and at X approximate to 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. (C) 2007 Elsevier B.V. All rights reserved.