Dynamics of mobile ions in single- and mixed-cation glasses

Abstract

To study the ion dynamics of several inorganic glasses, including single- andmixed-cation glasses, we have determined conductivity spectra over wide ranges infrequency. In the case of the single-cation glasses, these spectra extend from a fewhertz up to the terahertz regime. The spectra show a transition from their dcvalues to a dispersive regime where the conductivity increases continuously withfrequency, tending towards a linear frequency dependence at sufficiently lowtemperatures. At high frequencies the dynamic conductivity is governed byvibrational contributions, whereas ionic hopping sequences determine thelow-frequency part of the spectra. In an intermediate-frequency regime, bothhopping and vibrational contributions contribute to the dynamic conductivity.The shape of the high-frequency conductivity spectra is discussed for variousglasses. The low-frequency spectra are discussed in the framework of the conceptof mismatch and relaxation.For the mixed-cation glasses where spectra have beentaken by impedance spectroscopy, we report on a new kind of mixed-alkali effect.In contrast to conductivity spectra of single-cation glasses which follow thetime–temperature superposition principle, featuring a temperature-invariantshape, the shapes of the conductivity spectra of the mixed-alkali glasses studiedhere are found to change with temperature. To explain this effect, wesuggest differently activated mobilities of the two different ionic species.