Crossover from ionic hopping to nearly constant loss in the fast ionic conductorLi0.18La0.61TiO

Abstract

Electrical conductivity measurements of the fast ionic conductor Li 0.18 La 0.61 TiO 3 have been conducted at temperatures ranging from 8 to 300 K and frequencies between 20 Hz and 5 MHz. A detailed analysis of the ac conductivity shows the existence of a crossover between two different regimes. At high temperatures and/or low frequencies correlated ion hopping is responsible for a power-law frequency dependent and thermally activated ac conductivity. On the other hand, at sufficiently low temperatures and/or high frequencies, the ions do not have enough thermal energy or time to hop between neighboring sites, and remain caged. The ac conductivity is then characterized by a linear frequency dependence ͑i.e., the equivalent of a nearly constant loss͒ and by a weak exponential temperature dependence of the form exp(T/T 0 ). A crossover between the two regimes is found, which is thermally activated with an activation energy Eϭ0.17 eV, significantly lower than that observed for the dc conductivity, E ϭ0.4 eV. From this result, it is shown that the so-called ''augmented Jonscher expression'' fails to describe the ac conductivity in the whole frequency and temperature ranges. All these findings suggest that the nearly constant loss originates from electrical loss occurring during the time regime while the ion is still confined in the potential-energy minimum. Further, it is proposed that the loss mechanism involves some type of process where the potential-energy minimum relaxes in time on a time scale much shorter than the ionic hopping time scale. At longer times, as soon as the ion has significant probability of being thermally activated out of the potential well, the nearly constant loss terminates and correlated ion hopping becomes the only contribution to the ac conductivity.

Electrical conductivity relaxation in lithium doped silver iodide

“…Disordered materials like ionically conducting glasses and polymers show this "stretched" exponential behavior in the electrical conductivity relaxation, showing the importance of ion-ion correlations in the ion hopping process. 18,19 The parameter n has been proposed to be close to 1 for strongly correlated ion motion and equals to 0 for completely random and independent Debye-like ion hops. 20 Fig.…”

Nearly constant loss effect in sodium borate and silver meta-phosphate glasses: New insights

“…The question considered in Section 2.3, i.e., whether the hopping part and the NCL part are additive constituents to the total conductivity, was examined also in earlier works by Ngai et al [26][27][28]. Their formal ingredients for describing the two parts were different from ours and, as a consequence, the two components were found to be non-additive [28].…”

Crossover to nearly constant loss in ac conductivity of highly disordered pyrochlore-type ionic conductors

“…4,10 Based on this fact and on several other properties of the NCL experimentally observed in ionic conductors, Ngai proposed its physical origin might be due to the displacement of the mobile ions in their local vibrational motion at low temperature and/or high frequency. León et al [11][12][13] later found experimental evidence, in two different lithium ionic conductors, consistent with such an origin for the NCL contribution to the ac conductivity. From the measurement of the crossover temperature ͑frequency͒ at which the NCL behavior terminates at different fixed frequencies ͑temperatures͒, they were able to determine an activation energy E NCL for the crossover which was found to be much lower than the activation energy E dc for the dc conductivity, governed by ion hopping dynamics, and in fact E NCL was found to be similar to the value estimated for the energy barrier for the ions to leave their cages, E a .…”