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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...

Electrical conductivity and NMR relaxation times (T 1 and T 2 ) have been determined in the Li 0.18 La 0.61 TiO 3 perovskite. At room temperature, the unit cell constants are a ) 3.865-(1), b ) 3.876(1), and c ) 7.788(2) Å and the space group Pmmm (orthorhombic). In this doubled perovskite, the Rietveld analysis of the X-ray powder pattern showed that La ions occupy preferentially one type of sites (z/c ) 0), and Li and vacancies accommodate with the remaining La at the second site (z/c ) 0.5). From this fact, Li motion should be favored in the plane ab; however, exchanges of Li between contiguous layers are detected above 200 K by NMR spectroscopy. From T 1 and T 2 NMR data, two main relaxation mechanisms have been detected, which have been ascribed to localized exchanges (200-273 K) and extended motions of Li (above 273 K). The dc conductivity shows a non-Arrhenius temperature dependence, and local activation energies of 0.41 and 0.26 eV were obtained in the low-and high-temperature ranges, respectively. Both NMR and electrical conductivity relaxations are described by "stretched exponential" functions, characteristic of correlated ion motions.

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