Book Received

Superionic conductors have high ionic conductivity because of extensive disorder in a component sublattice of the solid. Knowledge of the nature and extent of the disorder and its dynamics is essential for detailed understanding of superionic behaviour. The present article contains an outline of experimental techniques used to obtain this knowledge and describes results of their application to a variety of superionic materials.     

Superionic conductors

The so-called superionic conductors represent a class of solid materials showing an unusually high ionic conductivity. Their structure is characterized by strong disorder in the sublattice of conducting ions. We shall describe the dynamic properties of the partially disordered state on the basis of two classes of theoretical models. For stochastic lattice gases we discuss the behavior of various time-correlation functions relevant for inelastic neutron- and light-scattering experiments and for tracer-diffusion measurements. In addition we study a system of interacting Brownian particlés in the presence of a periodic potential as a model for optimized (AgI-type) ionic conductors. A mean-field theory is developed which implies a relationship between the conductivity and structural properties.     

A model for the heat of transport in ionic conductors

The ion flow caused by a temperature gradient originates the ionic thermopower which is quantified by the heat of transport. Experimentally, it is known that in superionic conductors, the heat of transport Q is nearly equal to the activation energy for ion transport E_a. In the present paper, a model for the heat of transport in ionic conductors has been proposed based on a lattice dynamical theory of diffusion. We have shown that the relationship between Q and E_a is determined by the participation degree of different phonon modes, in particular the short wavelength phonons to the atomic jump processes. The implication of this finding to the transport properties of superionic conductors has been discussed, and it is suggested that the degree of the collective motion in ionic conductors increases with the increase in Q/E_a. The model predicts that good ionic conductors will show large value of Q/E_a. The importance of the acoustic phonons in the ion transport processes has been also pointed out.     

The ionic Seebeck effect and heat of cation transfer in Cu2−δSe superionic conductors

The ionic Seebeck coefficients of Cu_2?δSe superionic conductors are measured in the temperature range 340–380°C. The data obtained are used to determine the heat of transfer Q _i of copper ions as a function of the degree of nonstoichiometry and the temperature. The heat of transfer of copper cations increases from 0.19 to 0.22 eV as the degree of nonstoichiometry δ increases from 0.015 to 0.050. It is noted that the heat of transfer Q _i tends to increase with an increase in the temperature. Assumptions regarding the specific features of the cation diffusion in the Cu_2?δSe superionic conductors are made from the observed closeness of the heat of transfer and the activation energy for ionic conduction.     

Liquid-like Raman scattering by superionic materials

The Raman spectrum of a superionic conductor, α-AgI, is compared with the spectra of the melt of AgC$\text{l}$ and AgBr, in order to determine the responsible Raman processes. We conclude that because of the strong anharmonicity and high translational disorder in superionic conductors, spectra analyses based on the concept of harmonic lattice are unsuitable. A dynamical picture of ions similar to that of liquids appears to be more useful than the phonon concepts.     

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.     

Breakdown of absolute rate theory and prefactor anomalies in superionic conductors

We suggest that the anomalously low prefactors observed for the ionic hopping rate in defect-structure superionic conductors are due to a breakdown of absolute rate theory. A treatment that takes dissipative processes into account is introduced and a new expression for the prefactor is obtained. We present NMR data on the superionic conductor Li₂Ti₃O₇ that supports these ideas and extract the value of the elemental energy transfer per collision.     

A theory of the microwave ac conductivity of superionic conductors with randomly modulated structure

Localized resonant absorbing states of ions due to a random potential modulating the structure of superionic conductors are shown to be responsible for ac current response maxima in high frequency tails. The theory allows for interpretation of the microwave maxima found in samples of α-AgI and β-Cu Br exhibiting strain-induced texture, and also in some polycrystalline ionic structures.     

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.     

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.     

Superionic transition in ice

The possibility of the proton system in ice to change its configuration is determined by the existence of Bjerrum and ionic defects. In this paper the concentrations of ionic defects are studied as a function of temperature. It is shown that at enough high temperatures the proton system undergoes a first order phase transition into the state with high concentrations of ionic defects. This transition is analogous to the transitions is ordinary superionic conductors. The possibility of experimental observation of this transition is discussed.     

Molecular dynamics simulation of the superionic conductor BaF2

Molecular dynamics simulations of the BaF₂ fluoride crystal were carried out over a wide range of temperatures in order to study structural and transport characteristics in the low-temperature, the high-temperature superionic, and the molten state. The experimental temperature dependence of the lattice constant was taken into account. A sharp change in total energy of the system in the vicinity of T=1200 K indicates a phase transition to the high-temperature state with a transition energy U=(18.8±0.2) kJ/mol which is close to the value of the latent heat Q=18.36 kJ/mol obtained experimentally at 1275 K. Calculation of the radial distribution functions g(r) shows that in the high-temperature superionic state the F^– sublattice is already disordered while the Ba²⁺ sublattice stays regularly ordered, which keeps the crystal in the solid state. In the low-temperature state both sublattices are regularly ordered, and in the molten state both sublattices are disordered. The calculated diffusion constants of F^– in the superionic state is about 10^–9m²/s which is a typical value for superionic conductors. The temperature dependence of the diffusion constant obeys the Arrhenius equation. Higher statistical moments of the trajectories are used to characterise the type of ion movement.     

Frequency-dependent spin-lattice relaxation study of transport processes in superionic conductors

The sensitivity of the¹⁹F spin-lattice relaxation dispersion, T₁,(ω), to motional disorder in crystalline superionic conductors of the type La_1?xSr_xF_3?x (x = 0; 0.03) is shown. T₁ times are measured in the frequency range from 90 kHz to 370 MHz using standard techniques in combination with field-cycling. The relaxation dispersion shows qualitative differences from the standard Bloembergen-Purcell-Pound behavior. At low frequencies a relaxation model using a distribution of correlation times for diffusing ions is found to be consistent with the experimental results. At frequencies higher than 50 MHz another process of the Debye type which is not induced by ionic hopping dominates the relaxation.     

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.     

A liquid-like picture for the interpretation of Raman scattering in superionic conductors

Raman spectra of superionic conductors are studied in a model which takes into account polarizability changes due to ionic motions between lattice sites. The scattered intensity is determined by a generalized dynamical structure factor at wave vectors depending on the periodicity of the lattice. The structure factor is evaluated in a model which for the mobile ions assumes Brownian motion in a sinusoidal potential. The results are compared with the spectrum of α-AgI. The observed broad Rayleigh wings can be explained by quasielastic scattering due to diffusive motions.     

Entropy of crystallization of materials from a “molten” sublattice of superionic conductors

It is shown that the entropy of crystallization of materials (such as Cu, Ag, Au, and Li) in a “molten” sublattice of superionic conductors→crystal system may substantially exceed the entropy of crystallization of the same materials from a melt. The observed behavior explains the known tendency of various superionic conductors to form filamentary crystals (whiskers) in the solid phase, together with the completely different structure of these whiskers. Fiz. Tverd. Tela (St. Petersburg) 40, 227–228 (February 1998)     

The effect of ion-ion correlations on the ionic hall effect in crystals

Abstract

Because an external magnetic field does no work on moving charges, it is argued that independently migrating point defects in an ionic crystal cannot exhibit any Hall effect, to first order. This is in sharp contrast to the cooperative motions displayed by superionic conductors. Experimental results on interstitial migration in AgBr and α—AgI agree with these expectations.     

Collective excitations in fast ion conductor superlattices

Within the framework of the electromagnetic theory, the collective modes in the superlattice system composed of superionic conductors and ionic crystals are studied. The superionic conductor is described by the hydrodynamical model in which the anion cage is immersed in the cation liquid. The behavior of the modes are analysed in terms of the coupling strength between excitations pertaining to different layers. The coupling strength is controlled by varying the slab thicknesses. An interesting behavior in which the diffusion mode transforms to the relaxation mode when the coupling strength is varied from strong to weak is obtained. Also, the effect of the coupling strength on the acoustical and optical modes are shown.     

On the relation between the collective modes of ionic liquids and superionic conductors

A recent discussion concerning the effects of the Coulomb forces on the spectrum of hydrodynamic modes of superionic conductors is updated. It is pointed out that in general the low-frequency charge-conduction mode cannot be disentangled from the high-frequency charge oscillation mode. For moderate Coulomb effects the chargedensity fluctuation spectrum of superionic-conductors should reveal an optic quasihydrodynamic peak analogous to the one already found in ionic liquids.Association Euratom-Etat Belge