Universal scaling of the conductivity relaxation in crystalline ionic conductors (original) (raw)
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Effects of reduced dimensionality in the relaxation dynamics of ionic conductors
Europhysics Letters (EPL), 2005
We report on the dispersive ionic conductivity in Li0.5 - xNaxLa0.5TiO3 (0 <= x <= 0.5), where the number of available positions for the mobile Li ions is reduced by introducing immobile Na ions. At high frequency the conductivity is power law dependent with an exponent which increases as the number of accessible neighboring sites decreases. This result is quantitatively
Solid State Ionics, 1999
The question of how to relate the macroscopic conductivity relaxation measurement (´*(v), s*(v) or M*(v)) to the microscopic movement of the ions is a problem that must be resolved. Comparing with the results of a stochastic transport theory of charged carriers, we find that among the other choices the electric modulus, M*(v), is the most appropriate representation of the macroscopic data to describe the microscopic movement of the ions. It is found that M*(v) faithfully reproduces the shape of the dispersion of the microscopic ionic movement except that the entire electric modulus relaxation time spectrum is shifted uniformly away from the microscopic ionic hopping relaxation time spectrum by a calculable frequency-independent factor. Nuclear spin relaxation is a microscopic probe of ionic movement. A combined study of ionic motion using electrical relaxation and nuclear spin relaxation in a crystalline ionic conductor provides the means to verify the theoretical relation between the macroscopic electric modulus spectrum and the microscopic ionic hopping relaxation spectrum. Finally, we present some recent advances in our understanding of the experimental data that indicate the importance of ion-ion interactions in many ionically conducting crystals, glasses and melts.
Journal of Non-crystalline Solids, 2002
We have investigated the crossover in the ac conductivity of ionic conductors from the linear frequency dependence (near-constant loss) found at low temperatures, to the power law dependence characteristic of ionic hopping at higher temperatures. We have analyzed electrical conductivity measurements of the fast ionic conductor Li0.18La0.61TiO3 in the frequency range 20 Hz–5 MHz, and from room temperature down to 8 K. Our results show the failure of the so-called `augmented Jonscher's expression' to describe the ac conductivity in the whole frequency and temperature ranges, and give further support to the origin of the near-constant loss in the vibrational relaxation of ions within their cages.
Journal of Non-crystalline Solids, 2006
We present Admittance Spectroscopy measurements on two different ionically conducting materials, Gd 2 Ti 2Ày Zr y O 7 and Li 0.5Àx Na x -La 0.5 TiO 3 . Electrical relaxation data have been found to be well described by stretched exponential functions of the form U(t) = exp(À(t/ s) 1Àn ). In Gd 2 Ti 2Ày Zr y O 7 the concentration of mobile ions in the whole series is very low, from 1% to 0.02 %, and we find that by strongly decreasing the density of mobile ions the value of the exponent n shows a significant decrease from 0.44 ± 0.01 to 0.16 ± 0.01. In Li 0.5Àx -Na x La 0.5 TiO 3 we have investigated changes in ion dynamics when approaching the percolation threshold for lithium diffusion by increasing the number of immoble sodium ions, and found that the exponent n increases towards a value of 0.81 ± 0.01 as the mobility of lithium ions becomes more and more constrained close to the percolation threshold. Both results are discussed in terms of the importance of ion-ion correlations in the dispersive behavior of the electrical conductivity.
Evidence of secondary relaxations in the dielectric spectra of ionic liquids
Physical Review B, 2006
We investigated the dynamics of a series of room temperature ionic liquids based on the same 1-butyl-3-methyl imidazolium cation and different anions by means of broadband dielectric spectroscopy covering 15 decades in frequency (10 −6 -10 9 Hz), and in the temperature range from 400 K down to 35 K. An ionic conductivity is observed above the glass transition temperature Tg with a relaxation in the electric modulus representation. Below Tg, two relaxation processes appear, with the same features as the secondary relaxations typically observed in molecular glasses. The activation energy of the secondary processes and their dependence on the anion are different. The slower process shows the characteristics of an intrinsic Johari-Goldstein relaxation, in particular an activation energy E β =24k B T g is found, as observed in molecular glasses.
Electrical and acoustical relaxation in fast ionic conductors
Solid State Ionics, 1990
The ultrasonic velocity, attenuation coefficient, electric conductivity and dielectric perminivity have been measured in wide temperature and frequency ranges in various crystalline, polycrystalline and glassy solid electrolytes. The high relaxational ultrasonic attenuation maxima and corresponding velocity dispersion have been observed. Such behaviour is supposed to be determined by acoustoionic interactions of two types: (i) the piezoelectric interaction in the piezoelectric superionic; (ii) interaction caused by acoustic wave modulation of chemical potential felt by mobile ions in the centrosymmetric materials. In polycrystalline and glassy superionics the non-Debye behaviour for ultrasonic relaxation has been established. In such materials the characteristic frequency dependencies of ionic conductivity and dielectric permittivity were obtained. A simple theoretical model describing both frequency and temperature dependencies of electrical and acoustical properties of solid electrolytes is proposed. This model is based on continuous distribution of activation energies in a limited interval and describes the experimental data fairly well.
Near constant loss regime in fast ionic conductors analyzed by impedance and NMR spectroscopies
Physical Chemistry Chemical Physics, 2014
Universal dielectric response (UDR) and nearly constant loss (NCL) dispersive regimes have been investigated in fast ion conductors with perovskite and NASICON structure by using NMR and impedance spectroscopy (IS). In this study, the electrical behavior of La 0.5 Li 0.5 TiO 3 (LLTO-05) perovskite and Li 1.2 Ti 1.8 Al 0.2 (PO 4 ) 3 (LTAP0-02) NASICON compounds was investigated. In both systems a three-dimensional network of conduction paths is present. In the Li-rich LLTO-05 sample, lithium and La are randomly distributed on A-sites of perovskites, but in LTAP0-02 Li and cation vacancies are preferentially disposed at M1 and M2 sites. In perovskite compounds, local motions produced inside unit cells are responsible for the large ''near constant loss'' regime detected at low temperatures, however, in the case of NASICON compounds, local motions not participating in long-range charge transport were not detected. In both analyzed systems long-range correlated motions are responsible for dc-conductivity values of ceramic grains near 10 À3 S cm À1 at room temperature, indicating that low-temperature local motions, producing large NCL contribution, are not required to achieve the highest ionic conductivities.