Electrical conductivity measurements of the fast ionic conductor, Li0.18La0.61TiO3, were performed at 8 to 300K, using frequencies of 20 to 5MHz. A detailed analysis of the alternating-current conductivity revealed the existence of a cross-over between 2 regimes. At high temperatures, and/or low frequencies, correlated ion hopping led to a power-law frequency-dependent thermally activated alternating-current conductivity. At sufficiently low temperatures, and/or high frequencies, the ions did not have sufficient thermal energy or time to hop between neighboring sites; and remained caged. The alternating-current conductivity was characterized by a linear frequency-dependence and by a weak exponential temperature dependence. A cross-over between the 2 regimes was found which was thermally activated, with an activation energy of 0.17eV. This was significantly lower than that (0.4eV) observed for direct-current conductivity. It was shown that the so-called augmented Jonscher expression failed to describe the alternating-current conductivity over the entire frequency and temperature ranges. The results suggested that a nearly constant loss originated from the electrical loss which occurred during the time when the ion was still confined within the potential-energy minimum. It was proposed that the loss mechanism involved a process in which the potential-energy minimum relaxed over a time-scale which was much shorter than the ionic-hopping time-scale. At longer times, as soon as the ion had a significant probability of being thermally activated out of the potential well, the nearly constant loss ceased and correlated ion-hopping became the only contribution to the alternating-current conductivity.

Crossover from Ionic Hopping to Nearly Constant Loss in the Fast Ionic Conductor Li0.18La0.61TiO3. A.Rivera, C.León, J.Sanz, J.Santamaria, C.T.Moynihan, K.L.Ngai: Physical Review B, 2002, 65[22], 224302 (6pp)