摘要:

AbstractStructurally disordered solid electrolytes, both crystalline and glassy, as well as ionic melts, exhibit a set of spectroscopic peculiarities that are at variance with the predictions of simple random‐hopping models. Typically, stretched exponential functions are encountered where ordinary exponential functions are expected. The effect has come to be called “universal dynamic response.” In this paper, it is traced back to a mean square displacement which, in a broad time regime, varies as time to the power ofβ, with 0<β<1. Computer simulations as well as model considerations show that this particular shape of the mean square displacement is caused by the frequent occurrence of correlated forward‐backward hopping sequences. The latter processes result from the mutual repulsive Coulomb interaction of the mo

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950901

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractA microscopic mechanism for understanding fractional power‐laws in the ac‐response of ionic conductors is discussed in terms of hopping of charged particles in disordered lattices. We review recent Monte‐Carlo simulations for lattices with Gaussian disorder in the local site energies, which show that “anomalous diffusion” occurs at intermediate time‐scales as a result of the interplay of Coulomb‐interactions and structural disorder. A new model pertaining to ionic transport in oxid glasses is proposed, which emphasises the role of long‐range Coulomb‐potentials due to counter‐ions. Implications with respect to other dynamic probes e.g. inelastic neutron scattering and ultrasonic attenuation are als

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950902

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractThe first results of an extensive simulation study of diffusion in sodium beta”‐alumina are presented. The simulations are performed by the molecular dynamics technique on a large periodic system of 3213 ions, including all the ions in the spinel blocks, with a realistic model for the interactions between the ions. From simulations performed at a series of temperatures between 200 and 1200 K, we obtain the ionic conductivity, which, in accord with experiments, shows non‐Arrhenius behaviour. The frequency‐dependent conductivity shows marked dispersion at low temperatures, which agrees qualitatively with recent measurements. The reasons for this dispersion are di

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950903

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractA time correlation function based on the Tricomi function with two shape parameters is proposed For special values of these parameters the Debye and Cole‐Davidson functions are recovered. The Tricomi function can be Fourier transformed analytically and an equation for the spectral density is derive

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950904

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractWe present a novel model for ionic transport in glasses which we call diffusion limited percolation. The model focuses on the fact that each mobile ion prefers and maintains its own distinct characteristic environment in the glassy network. This leads to mismatch effects and to the existence of a memory effect which strongly influences the process of ion migration. We show that by our simple model a variety of so far imperfectly understood phenomena can be explained, among them the drastic increase of the dc conductivity of oxide glasses with the modifier content and the mixed alkali effect.

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950905

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractThe a.c. response due to jump diffusion in systems with fluctuating energy barriers is calculated in effective medium approximation. The crossover from a frequency independent to a frequency dependent a.c. conductivity is shown to be related to a transition from normal to anomalous diffusion. To extract the non‐analytical frequency dependence of the dynamic conductivity for ω → 0 we study the loss function which – depending on the use of the proper local Green's function in the EMA equation – reveals a √ω dependence in the region of normal diffusion as it should. The loss function looks like ω−αwith 0<α<1 in the region of anomalous diffusion, but the details of the frequency dependence are different for different models, i.e. the curves a

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950906

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractA theoretical mechanism is presented for nuclear spin relaxation in ionically‐conducting glasses (or disordered crystals) caused by ionic motion. This is based on the diffusion‐controlled relaxation model previously used to account for electrical relaxation in such materials: relaxation is assumed to occur at particular (interstitialcy) sites, triggered by the arrival of other diffusing ions. The experimental dependence of the spin‐lattice relaxation rate on temperature and ionic concentration can be understood on the basis of this

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950907

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractWith a 6‐centre Lennard‐Jones potential, we have studied thermal collective transport coefficients of the benzene model liquid by molecular dynamics (MD) calculations. Shear and bulk viscosity as well as thermal conductivity were determined for 8 different states of liquid benzene. Agreement between experimental values and MD results is surprisingly good for the shear viscosity and thermal conductivity. Unfortunately there are no experimental data available for the bulk viscosity. In the wider neighbourhood of the triple point of benzene, the time correlation functions for the shear and the bulk viscosity were found to have an extreme form, namely an initial short‐time decay within 0.5 ps followed by a long positive branch of very small amplitude up to about 4–6 ps. A comparable long time behaviour was also found in the reorientational correlation fu

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950908

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractMolecular dynamics simulations were used to calculate the depolarized Rayleigh light scattering spectra of CS2/CCI4liquid mixtures at 298 K modelling the intermolecular interactions by atom‐atom Lennard‐Jones potentials. Approximating each molecule as a point scatterer these spectra were obtained using the dipole‐induced dipole mechanism for the interaction‐induced polarizability. Intensities and lineshapes of the first‐order perturbation theory for the system polarizability of this model are compared to the exact all‐order results. The agreement of both models with experimental spectra is in some respects satisfactory, but also displays characteristic

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950909

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractIonic conductivities in glass depend very strongly on temperature, frequency and glass composition. In the absence of well‐defined structure, the behaviour of vitreous electrolytes may be strongly influenced by various “memory effects”. These include the classical ion‐atmosphere effect which tends to return ions to their starting positions – and hence causes a reduction in ion mobilities – and a newly formulated site memory effect, which effectively creates “cation vacancies”, and leads to conductivity enhancement. The site memory effect gives a straightforward explanation for compositional variations in conductivity (including the mixed alkali effect) and points up a connection with “strength” and “fragility” concepts

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950910

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY