We have analyzed the constant loss contribution to the ac conductivity in the frequency range 10 Hz -1 MHz and temperatures down to 8 K, for two Li ionic conductors, one crystalline (Li 0.18 La 0.61 TiO 3 ) and the other glassy (61SiO 2 ? 35Li 2 O ? 3Al 2 O 3 ? P 2 O 5 ). As temperature is increased a crossover is observed from a nearly constant loss to a fractional power law frequency dependence of the ac conductivity. At any fixed frequency v, this crossover occurs at a temperature T such that v ഠ n 0 exp͑2E m ͞k B T͒, where n 0 is the attempt frequency and E m is identified with the barrier for Li 1 ions to leave their wells. DOI: 10.1103/PhysRevLett.86.1279 Much effort has been devoted during the last few decades to understand the dynamics of ionic transport in ionically conducting materials. In spite of the advances made, there is still no general agreement on interpretation of the experimental data [1][2][3][4][5][6][7][8][9][10][11]. Most research activity in this field has focused on the origin and properties of the long-range ion motion, and electrical relaxation is the most commonly used experimental tool to access the ion dynamics. The frequency dependence of the ionic conductivity can be usually well described by using Jonscher's expression [12]where s 0 is the dc conductivity, v p is a characteristic relaxation frequency, and n is a fractional exponent. Both s 0 and v p are thermally activated with about the same activation energy, indicating that the dispersive conductivity, s ‫ء‬ ͑v͒, originates from migration of ions. However, there is another ubiquitous contribution to dispersive conductivity that has received much less attention so far. This contribution consists of a nearly frequency independent dielectric loss,´0 0 ͑v͒ ഠ A, which corresponds to an almost linear frequency dependent term of the form s 0 ͑v͒ v´0 0 ͑v͒ ഠ Av in the real part of the complex conductivity. At sufficiently low temperature or high frequencies, the Av term dominates over the power law dependence of exponent n. The existence of this nearly constant loss (NCL) was suggested more than 20 years ago and subsequently verified [13,14]. Since then, few investigations of its properties have been made [15][16][17] and low temperature data with its dominant contribution are still scarce.Although, ultimately, mobile ions seem to be responsible also for the NCL, the experimental facts including its dependence on temperature and the effect of mixed alkalis point to a different origin than ionic hopping [18]. Experimentally, A is not thermally activated and has temperature dependence much milder than s 0 or v p [18][19][20]. Partial replacement by alkali ions of a different kind has the effect of reducing the NCL [3,18,21,22], but the reduction in A is much smaller than the decrease in s 0 due to the well-known mixed alkali effect [3]. From these facts, it has been very recently proposed [18] that local vibrational relaxation reflected in the mean-square displacement of ions could be the origin of the constant loss in ionic conductors...

The authors investigate the dynamics of a series of room temperature ionic liquids, based on the same 1-butyl-3-methylimidazolium cation with different anions, by means of broadband (10(-6)-10(9) Hz) dielectric spectroscopy and depolarized light scattering in the temperature range from 400 K down to 35 K. Typical ionic conductivity is observed above the glass transition temperature Tg. Below Tg the authors detect relaxation processes that exhibit characteristics of secondary relaxations, as typically observed in molecular glasses. At high temperatures, the characteristic times of cation reorientation, deduced from the light scattering data, are approximately equal to the electric modulus relaxation times related to ionic conductivity. In the supercooled regime and close to Tg, the authors observe decoupling of conductivity from structural relaxation. Overall, room temperature ionic liquids exhibit typical glass transition dynamics, apparently unaltered by Coulomb interactions.

We investigated the dynamics of a series of room temperature ionic liquids based on the same 1-butyl-3-methylimidazolium (BMIM) cation and different anions by means of broadband dielectric spectroscopy covering 15 decades in frequency, and in the temperature range from 400 K down to 10 K. A dc conductivity is observed in these systems above Tg with a typical ionic conductor behavior. Below, two relaxation processes appear, with the same characteristics as the secondary relaxations typically observed in glasses. The activation energy of the secondary processes and their dependence on the anion are different. The fast relaxation process is attributed to conformational changes in the butyl group of the BMIM cation and the slower process shows the characteristics of a genuine JG relaxation.The glass transition involves a dramatic slowing down of the structural relaxation in supercooled liquids from the ps time scale towards diverging times which ultimately brings the liquid into the glassy state. The only technique that can follow the evolution of the dynamics of the system in this huge time scale window is dielectric spectroscopy, and studies covering the full dynamic range (over 15 decades) of some paradigmatic glass formers are available [1][2][3]. Dielectric relaxation showed that secondary relaxations appear at frequencies higher than the main relaxation process, the importance of these processes being already pointed out more than three decades ago by Johary and Goldstein (JG) [4]. They proposed that these processes appear as a consequence of the glassy state, and they demonstrated that one type of process, commonly termed now "JG β relaxation", occurs in liquids of simple rigid molecules and does not involve intramolecular motion. The fundamental origin of secondary relaxations in supercooled liquids is a matter of current attention and dispute, with much effort being devoted to clarifying its true nature [5][6][7][8].Secondary relaxations have been studied in many systems, but the search of these processes in new types of materials should help to obtain a better understanding of their origin. A particular insight was obtained recently with binary systems, showing a continuous transformation between a high frequency wing and a JG relaxation when the composition of the mixture is changed [9]. Ionic systems, formed by anions and cations have been studied to a very low extent. The most common ionic materials are salts, like the NaCl, which have usually melting temperatures around 800• C. An exception is calcium potassium nitrate Ca 0.4 K 0.6 (NO 3 ) 1.4 (CKN), with a rather low T g at 333 K [10,11]. This so called molten salt was shown to exhibit a secondary mechanical relaxation, but it was never clearly observed in dielectric spectroscopy [4,12]. So the existence of the secondary relaxations in systems with free charges, and in particular, in purely ionic systems is an open question.A new class of chemicals has been discovered in the last decades, the room temperature ionic liquids (RTIL). They are molten salt...

The dependence of ionic transport properties on the structure and composition of perovskites Li0.5 - x Na x La0.5TiO3 (0 ≤ x ≤ 0.5) has been analyzed by means of ND, XRD, NMR, and impedance spectroscopy. Local lithium mobility is shown to decrease progressively by 2 orders of magnitude along the series; however, long-range dc conductivity decreases sharply at x = 0.2 more than 6 orders of magnitude. The decrease of dc conductivity from values typical of fast ionic conductors, 10-3 S/cm at room temperature, to values of insulators, 10-10 S/cm, is discussed in terms of a three-dimensional percolation model for lithium diffusion. As deduced from XRD and ND data, the number of vacant sites in conduction pathways is controlled by the amount of Na and La in the perovskite.