Sonderforschungsbereiche
Sonderforschungsbereich 458 - Ionenbewegung in Materialien mit ungeordneten Strukturen -
vom Elementarschritt zum makroskopischen Transport -
C1 - Prof. Dr. A. Heuer

Complex dynamic processes in ion conductors: mixed alkali effect and ionic

Understanding the complex nature of ion dynamics in disordered systems, inparticular in the presence of an electric field, is one of the majorchallenges in the field of ion conductors, even for relatively simple systems like single alkali silicates. One general strategy to gain new insight into complex process is to modify the problem and to analyse the effect of this modification.

The mixed alkali problem is a challenging problem in the field of iondynamics. In the mixed alkali system, the dynamics slows down dramatically when one species is partially replaced by a second species. The dynamicstructure model explains this effect by postulating the generation of species-specific sites giving rise to different diffusion pathways for bothalkali species.

We elucidate these phenomena via computer simulations. Only recently it has become possible to simulate at sufficiently low temperatures the singlealkali silicate glasses where the frequency dependent conductivity displays a significant dispersion due to jump processes. With our parallelworkstation cluster we started to perform analogous simulations for mixedalkali silicate system. Given the wealth of the microscopic informationfrom simulations our work is guided by the following questions:

1. What is the role of the network at low temperatures?
2. How can the complexity of the ion dynamics be expressed in terms of correlated back- and forthjumps and dynamic heterogeneities?
3. Do spatial or dynamic correlations exist between unlike ions?
4. Is it possible to map the ion dynamics on adiscrete hopping model?

In the last two years, we have developed several concepts which may help to quantify these questions in an appropriate way.

For all these aspects we will compare mixed alkali systems to single alkalisystems for the same concentration of the respective ion. In this way one can check the degree to which partial replacement of one ion species by a different species is similar to a reduction of its concentration.

Drittmittelgeber:

Deutsche Forschungsgemeinschaft

Beteiligte Wissenschaftler:

A. Heuer, M. Kunow, M. Vogel, R.D. Banhatti, J. Reinisch

Veröffentlichungen:

R. Banhatti, A. Heuer, Structure and dynamics of lithium silicate melts:molecular dynamics simulations, Phys. Chem. Chem. Phys. 3, 5104 - 5108 (2001).

A. Heuer, M. Kunow, M. Vogel, R.D. Banhatti, Characterization of thecomplex ion dynamics in lithium silicate glasses via computer simulations,Phys. Chem. Chem. Phys. 4, 3185 -  3192 (2002).

J. Reinisch, A. Heuer, Coulomb Interactions in simple Lattice-Models: AComparison between Counter Ion Model and Random Energy Model, Phys. Rev. B 66, 064301 (2002).

A. Heuer, M. Kunow, M. Vogel, R.D. Banhatti, Backward correlations anddynamic heterogeneities: a computer study of ion dynamics, Phys. Rev. B 66, 224201/1 - 224201/11 (2002).