Low temperature ac conductivity in the fast ionic conductor Li0.18La0.61TiO3

We have performed ac conductivity measurements of the ionically conducting Li0.18La0.61TiO3 in the frequency range 100 Hz-30 MHz and at temperatures r anging from 8 to 300 K. The power law frequency dependence, usually ascribe d to ionic hopping, crosses over to a linear frequency dependence as temper ature is decreased, due to a near constant loss contribution to the ac cond uctivity. This crossover is thermally activated, and the activation energy is related to the energy barrier that ions must overcome to jump outside th eir wells. This fact is discussed in terms of the vibrational mean-square d isplacement of ions inside their wells, at times shorter than the onset of the intersite hopping process, This points to the vibrational origin of the near constant loss term. (C) 2001 Elsevier Science B.V. All rights reserve d.