Bond valence analysis of ion transport in reverse Monte Carlo models of mixed alkali glasses
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Bond valence analysis of ion transport in reverse Monte Carlo models of mixed alkali glasses Stefan Adams1 and Jan Swenson2 1 GZG, Abt. Kristallographie, Universität Göttingen, D-37077 Göttingen (Germany) 2 Department of Applied Physics, Chalmers University of Technology, S-412 96 Göteborg, Sweden
ABSTRACT An analysis of RMC structure models of ion conducting glasses in terms of our bond softness sensitive bond-valence method enables us to identify the conduction pathways for a mobile ion as regions of sufficiently low valence mismatch. The strong correlation between the volume fraction F of the "infinite pathway cluster" and the transport properties yields a prediction of both the absolute value and activation energy of the dc ionic conductivities directly from the structural models. Separate correlations for various types of mobile cations can be unified by employing the square root of the cation mass as a scaling factor. From the application of this procedure to RMC models of mixed alkali glasses, the mixed alkali effect, i.e. the extreme drop of the ionic conductivity when a fraction of the mobile ions is substituted by another type of mobile ions may be attributed mainly to the blocking of conduction pathways by unlike cations. The high efficiency of the blocking can be explained by the reduced fractal dimension of the pathways on the length scale of individual ion transport steps.
INTRODUCTION Solid electrolytes presently attract considerable scientific interest because of their potential applications in electrochemical devices. While investigations mostly focus on the optimization of conductivities in superionic phases, it is as important to understand, why the high selective ionic mobility through the nearly frozen matrix in a solid electrolyte declines dramatically when a fraction of the mobile ions is substituted by another type of mobile ions. As this well-known but hardly understood conductivity drop is most pronounced for solid electrolytes containing alkali ions, it is generally referred to as the mixed alkali effect (MAE) [1-3]. The here studied MAE in glasses is connected to large non-linear changes in those properties that are directly linked to ionic transport (ionic conductivity, ionic diffusion, dielectric relaxation etc.) while macroscopic properties such as density, elastic moduli, refractive index etc. exhibit only small deviations from the normal gradual variations with composition. One of the main obstacles for an understanding of ionic conduction in glasses in general and particularly of the MAE is the lack of detailed knowledge on the local structure and thus on the conduction pathways for the mobile ions. As the structural information from diffraction data of amorphous solids is not sufficiently detailed to grant for unique structure solutions, RMC fits represent the only viable technique to convert the experimental data into local structure models. From these RMC structure models transport pathways for the mobile ions are identified by a
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modified bond valence approa
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