Application of nonlinear conductivity spectroscopy to ion transport in solid electrolytes

The field-dependent ion transport in thin samples of different glasses is characterized by means of nonlinear conductivity spectroscopy. Ac electric fields with strengths up to 77 kV/cm are applied to the samples, and the Fourier components of the current spectra are analyzed. In the dc conductivity regime and in the transition region to the dispersive conductivity, higher harmonics in the current spectra are detected, which provide information about higher-order conductivity coefficients. Our method ensures that these higher-order conductivity coefficients are exclusively governed by field-dependent ion transport and are not influenced by Joule heating effects. We use the low-field dc conductivity σ1,dc and the higher-order dc conductivity coefficient σ3,dc to calculate apparent jump distances for the mobile ions, aapp. Over a temperature range from 283 K to 353 K, we obtain values for aapp between 39 Å and 55 Å. For all glasses, we find a weak decrease of aapp with increasing temperature. Remarkably, the apparent jump distances calculated from our data are considerably larger than typical values published in the literature for various ion conducting glasses. These values were obtained by applying dc electric fields. Our results provide clear evidence that the equation used in the literature to calculate the apparent jump distances does not provide an adequate physical description of field-dependent ion transport.