Ionic Motion in Materials with Disordered Structures
Diffusion and Ionic Conduction in Borate Glasses
Ion-conducting materials are of great technolgical interest as solid electrolytes in batteries, fuel cell, and censors.
Our project is basic in nature and aims at a better understanding of charge and mass transport processes in
such materials: We have investigated a series of
0.2[XNa2O(1-X)Rb2O]·0.8B2O3
ion conducting glasses as a function of composition and temperature by means of frequency-dependent
impedance spectroscopy and the tracer diffusion technique. The temperature dependence of the dc conductivity
and the tracer diffusion coefficients of 22Na and 86Rb follow Arrhenius functions.
The activation enthalpies and pre-exponential factors are extracted and compared. The following features of the
mixed-alkali effect were observed as a function of the relative alkali content X: i) a minimum in glass
transition temperature near X=0.2, ii) a minimum in conductivity near X=0.4, iii) a corresponding
maximum in activation enthalpy of conductivity near X=0.4, iv) a crossover of 22Na and
86Rb diffusivities near X=0.2. In contrast to 22Na, the 86Rb
diffusivity has nearly an exponential X dependence in the mixed-alkali range at a constant temperature.
This points to a major difference in diffusion of the smaller sodium and the larger rubidium ions. The diffusivity
crossover composition is temperature independent. The charge and mass transport diffusivities are found to be
different which is expressed by a the Haven ratio smaller than unity. The Haven ratio of the pure sodium-borate
glass is independent of temperature, however a temperature dependent Haven ratio was observed for the
rubidium-borate glasses. The Haven ratio decreases with decreasing temperature, which is ascribed to an
increased correlation and/or collectivity between jumps of adjacent ions.
Activation enthalpies of ionic
conductivities in Na - Rb alumino-germanate and borate glasses are reviewed. Correlations
between the activation enthalpy and the ratio of average distances between like alkali ions,
<dion>, to the average distances between network-former atoms,
<dnetwork>, are elucidated for single- and mixed-alkali glasses. The Haven ratio is shown to
decrease with decreasing <dion>/<dnetwork>. Interstitial-like and
substitutional-like subnetworks of ion sites are suggested. The experimentally observed dependence of the
Haven ratio on <dion>/<dnetwork> is consistently reproduced by a
Monte Carlo simulation of ion dynamics on this random network including single and collective ion
jumps.
Atomic free volumes of
0.2[XNa2O·(1-X)Rb2>=]·0.8B2O3
glasses and of amorphous B2O3 have been studied by positron annihilation
spectroscopy. By adding Na2O and Rb2O to pure B2O3
the free volumes are reduced in size.
The influence of hydrostatic pressure on the ionic conductivity of
several glassy electrolytes has been studied. A new model is described for cation transport in glass, in which
below the glass transition temperature a dynamic equilibrium exists between 'open' and 'closed' sites in the
glass network. Work on the influence of pressure on tracer diffusion is in progress.
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