To study the ion dynamics in an amorphous mixed alkali system, frequency-dependent conductivities of 0.3[xLi 2 OÁ(1 À x)Na 2 O]Á0.7B 2 O 3 glasses have been measured by impedance spectroscopy in a wide temperature range. The conductivities show a transition from their dc values into a dispersive regime where they increase continuously with frequency, tending towards a linear frequency dependence at sufficiently low temperatures. In addition to the '' classical '' mixed alkali effect, i.e. the occurrence of a minimum in the dc conductivity, we also observe the following new mixed alkali effect. In contrast to conductivity spectra of single cation glasses which follow the time-temperature superposition principle, featuring a temperature-invariant shape, the shapes of the conductivity spectra of the mixed alkali glasses studied here are found to change with temperature. To explain the effect, we suggest differently activated mobilities of the two different ionic species. The spectra are discussed in the framework of the concept of mismatch and relaxation (CMR).

We have determined the first wide-range conductivity spectra of a mixed alkali glass system, extending from 10(-3) Hz to 1.3 THz. We report on a new mixed alkali effect in the ac conductivity that is evident even if the low-frequency contributions (linked to hopping motions of the mobile cations) are removed from the experimental spectra. From our results we conclude that the nonhopping contributions to the ac conductivity involve both the mobile ions and the glassy network.

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