Cooperative diffusion in one dimensional ionic conductors and soliton brownian motion

It is shown that under simplifying assumptions cooperative diffusion in one dimensional ionic conductors can be approached in terms of soliton brownian motion. At thermal equilibrium the cooperative ionic diffusion coefficient is simply estimated versus the diffusion coefficient of a free ion.     

Dynamics of collective excitations in one dimensional superionic conductors

We study the dynamics of collective excitations of a chain of interacting ions in a periodic potential including the effects of discreteness and of thermal damping. We describe a method to discriminate between the various possible cases ranging from soliton like propagation to the hopping of the collective coordinate. The frequency dependent mobility is computed and specific applications to materials are discussed.     

Collective motion in superionic conductors

A theory of collective motion in superionic conductors is described by the use of a model of a crystalline cage immersed in a viscous liquid. The viscoelastic force and the interionic Coulomb force are considered as the cage—liquid interaction. The density-correlation functions and the frequency-dependent conductivity are calculated. The calculated conductivities for α-AgI are in good agreement with experiments. It is concluded that the structure in a.c. conductivity experimentally observed for α-AgI at frequencies below 10 cm^-1 can be ascribed to acoustic phonons.     

PAC study of ionic motion in silver compound superionic conductors

Ionic motion in superionic conductors, Ag₂S, Ag₂Se and Ag₃SI was investigated by γ-γ PAC on¹¹¹Cd. Diffusion constant measurements showed that probe ions migrate almost as fast as Ag⁺ ions above 500K in Ag₂S and Ag₂Se and above 700K in Ag₃SI. Multivalent impurities were found to be unstable in AgI and Ag₂Te. The correlation time of ionic motion was deduced from the observed relaxation rate together with the diffusion constants. The correlation time and its activation energy increase in order of Ag₂S, Ag₂Se and Ag₃SI.The flight distance of Ag⁺ ions remains almost constant in the measured temperature range.     

Sedimentation in superionic conductors

The homogeneous distribution of mobile atoms in a simple of a nonstoichiometric superionic conductor with mixed (ionic and electronic) conductivity is changed when the sample is placed in a centrifugal field. The steepness of the resulting equilibrium gradient and the rate at which the equilibrium is attained depend upon the reduced mass of atoms and their ambipolar diffusivity.     

Particle correlation in superionic conductors

A molecular-dynamics method is applied to study the particle correlation in superionic conductor α-AgI. It is found that there is a close connection between the particle correlation λ_i,j (i,j=I or Ag) and the radial distribution function. The values of λ_II and λ_IAg at the nearest neighbor site are below 0.3. The values of λ_AgAg at the nearest neighbor site and λ_II at the next nearest neighbor site are about 1/2 ≈ 1/3 of it of λ_II (and λ_IAg) at the nearest neighbor site.     

Thermal conductivity in superionic conductors

The contribution of mobile ions to the thermal conductivity in superionic conductors is investigated by making use of the lattice gas model with a hopping term. The thermal conductivity is obtained as a function of an ion concentration and a repulsive interaction energy between the nearest neighbors. It is shown that the contribution of mobile ions to the thermal conductivity is of an Arrhenius type in the usual temperature region in which measurements are conducted.     

Neutron scattering in superionic conductors

The coherent dynamical structure factor S(q,ω) of one-dimensional superionic conductors is studied in a model of Brownian particles with harmonic interactions moving in a periodic potential. We present results for the q-dependence of the halfwidth of the quasielastic peak, which reflects the commensurability ratios of the periodic potential and the molten sublattice. Furthermore the influence of the periodic potential on the collective excitations of the mobile ions is discussed.     

Electronic structure and covalency in superionic conductors

The electronic states of noble metal halides and alkali halides are calculated by the DVXα cluster method to get more microscopic evidence for the p — d hybridization and the covalency in noble metal halides. It is found that both components of anti-bonding and bonding exist in the diagram of overlap population (DOP) for AgX (X=halogen) and these two components are made up of the 4d band of Ag ion and the p band of halogen ion, which form the p — d hybridization. The covalency of noble metal halides is in the border between that of the fourfold coordinated compounds such as AgI and that of the sixfold coordinated compounds such as AgCl. These calculation results on the covalency are compared with the Phillips's ionicity. Paper presented at the Patras Conference on Solid State Ionics, Patras, Greece, Sept. 14 – 18, 2004.     

Long-wavelength density fluctuations in superionic conductors

In order to study the long-wavelength collective motions in superionic conductors we propose a continuum model in which the relative motion of the two ionic components is governed by a viscoelastic force and the interionic Coulomb forces. We predict that under the influence of the Coulomb forces the diffusion mode is converted into a relaxation mode which has a finite frequency fork0.     

Ionic plasma oscillations in superionic conductors

Some superionic conductors behave as if their charge was carried by nearly free charged particles. If this is the case, these free particles should undergo plasma oscillations. The plasma oscillations may already have been observed in superionic AgI and CuI. Similar oscillations might also be observable in ionic liquids such as melted AgI.     

A theoretical study of edge effects on conductivities in superionic conductors

Summary A theoretical examination of the effects of sample sizes on the conductivities of certain superionic compounds is presented. It is observed that conductivities (0.49≤σ≤2.63) for which correction factor values are included are lower than the measured values (0.66≤σ≤3.0). The effect of this correction, however, decreases with decrease in sample sizes. The author of this paper has agreed to not receive the proofs for correction.     

Specific heat and thermal conductivity of superionic conductors in the superionic phase

The temperature dependences of the specific heat and the thermal conductivity of crystalline superionic conductors LnF₃ (Ln = La, Ce, Pr), Li₂B₄O₇ and α-LiIO₃ in the superionic phase have been investigated experimentally. The specific heat C _p and the thermal conductivity K are observed to increase monotonically over a wide range of temperatures above the Debye temperature Θ_D. This increase is attributed to the relaxational interaction of high-frequency phonons with two-level systems. Fiz. Tverd. Tela (St. Petersburg) 39, 1548–1553 (September 1997)     

Correlation effects on frequency dependent conductivity: Application to superionic conductors

The Scher-Lax model for the calculation of a.c. conductivity is modified to allow for temporal and spatial correlations. If, due to, say, lattice relaxation effects, a charge is most likely to continue to move in the same direction (caterpillar mechanism), then the real part of the conductivity will saturate, at large frequencies, below the d.c. level. The reverse will be true if a bounce-back mechanism dominates. These two cases of correlation can be related to the behavior of the superionic conductors AgI and Na β-alumina, respectively. The degree of disorder in the hopping sites determines the distribution of ionic transition rates which in turn governs the details of the conductivity curve. The frequency dependent conductivity due to the motion of an ionic cluster (dimer) is also analyzed.     

Bistability effects of the brownian motion in periodic potentials

In the stationary state the equation of motion for particles moving in a periodic potential has two solutions, a locked one and a running one, for low and intermediate damping constants and for suitable external forces. The effect of an additional Langevin force to this bistable behaviour is investigated. For finite noise strength, the mobility depends continuously on the external force, whereas in the limit of vanishing strength of the noise force one gets a sharp transition between the locked and the running solution at a critical external force. This critical force is calculated exactly in the low friction limit and approximately for intermediate friction constants. Furthermore the temperature dependence for various forces including the critical one is shown in the low friction limit.     

The role of disorder in superionic conductors

This paper illustrates from a phenomenological point of view why the study of superionic conductors is essentially a study of disorders. Crystals that are good ionic conductors lack a long-range order in their mobile-ion sublattice. Moreover, low-temperature anomalies typical of amorphous materials appear to be rather common in superionic crystals. In several solid electrolytes the coupling between “disorder modes” typical of glasses and translational degrees of freedom of the ions can be shown to enhance ionic diffusion. The observed, or expected, properties of these superionic conductors are briefly discussed. The hypothesis that disorder may often play a dynamic role in ion transport in solids suggests ways to synthesize materials of technological interest.     

Correlation effects in superionic conductors: Model calculations for the thermoelectric power

A Holstein Hamiltonian in which the interaction between neighbouring ions is included in a simple way is used to describe superionic conductors. For this model the thermoelectric power is calculated as a function of temperature for different ion densities.     

Phase transitions and dynamics of superionic conductors

A Hamiltonian is constructed for superionic conductors taking into account the mutual interactions and transports of the mobile ions, and their interactions with the phonons of the cage. Under special conditions we recover the phenomenological results of Rice et al. and Huberman in regard to phase transitions. Two transition points are shown to be possible in the presence of the mutual interactions of cations (as observed in RbAg₄I₅ and CuBr). Structural phase transitions involving the cage when exist are found to occur at the same temperature at which the conductivity becomes critical, in agreement with experiments. Dynamical aspects of our Hamiltonian are also discussed. The collective modes of the phonon-cation system are calculated and used to explain the abrupt disappearance of certain modes above the critical temperature as observed in the Raman spectra of RbAg₄I₅, KAg₄I₅ and AgI. Our theory does contain the possibility that there is no soft mode with nonzero frequency, in accordance with the existing experimental situation. Our Hamiltonian is compared to others. The similarities between superionic conductors and other systems (Hubbard model, ferroelectrics, Jahn-Teller systems, molecular crystals) are emphasized.     

Are the prefactor anomalies in superionic conductors due to 1-D effects?

We have examined both the ionic conductivity and the NMR line narrowing in two channel-structured superionic conductors in the light of Richards' one-dimensional theory. For Li₂Ti₃O₇ the ionic conductivity is not sufficiently anisotropic to explain the observed small NMR prefactor in terms of i-D effects. For β-LiAlSiO₄ the NMR line narrowing in the crossover region does not yield the predicted large change in the activation energy given by the 1-D theory.     

Brillouin scattering from thermal fluctuations in superionic conductors

We present a theory of light scattering from long-wavelength thermal fluctuations in superionic conductors using a model of a crystalline cage immersed in a viscous liquid. The local or internal degrees of freedom of the liquid are taken into account by the addition of a frequency dependent contribution to the viscosity. The differences between our theory and that of Huberman and Martin are discussed.