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Origin of the mixed alkali effect from RMC and bond-valence calculations

Outline. The new RMC constraint:- Calculation of bond-valence sum- Effects on the structureThe mixed alkali effect:- Introduction- Experimental results from neutron diffraction- Results from RMC based on neutron and x-ray diffraction data- Results from bond-valence calculations of cond

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Origin of the mixed alkali effect from RMC and bond-valence calculations

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    1. Origin of the mixed alkali effect from RMC and bond-valence calculations Jan Swenson1 and Stefan Adams2 1Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden 2GZG, Abt. Kristallographie, Universität Göttingen, Germany

    2. Outline

    3. A new bond-valence sum constraint in RMC

    4. Effect on bond-valence sum distribution Chemically more plausible local environments for the mobile Ag ions with the inclusion of the bond-valence constraint.

    5. The mixed alkali effect

    8. Bond distances and coordination numbers

    9. Insights from Raman scattering

    10. Insights from RMC modelling

    11. Structural model of LixRb1-xPO3

    13. Bond valence analysis of RMC models RMC models of about 4000 atoms are subdivided into ca. 4*106 volume elements. Each volume element is counted as “accessible” for a mobile ion A if the valence sum mismatch |?V(A)| at its center is below a given threshold value or if ?V(A) changes its sign across the volume element. Restricted clusters of “accessible” sites constitute ac conduction pathways. Percolating pathway cluster(s) contribute to the dc conductivity.

    14. Conductivity and activation energy determined from the volume fraction of the percolating pathway cluster

    15. Prediction of the mixed alkali effect

    23. The main reason for the mixed alkali effect Blue: Li+ pathways in Li0.5Rb0.5PO3 Magenta: Regions blocked by Rb ions but otherwise would have been Li+ conduction pathways.

    24. Influence of differences in local alkali environment on the mixed alkali effect

    25. Why is the blocking by unlike alkali ions so effective?

    26. Fractal dimension of pathways For a wide range of bond-valence mismatch thresholds the fractal dimension of the infinite pathway for Li ions in LiPO3 remains £ 2 on the length scales of individual hops. A low number of Rb ions would be sufficient to block the most energetically favorable Li pathways.

    27. Conclusions The inclusion of a soft bond-valence constraint in RMC makes it possible to improve the correctness of the local environment around atoms or ions for which no rigid bonding constraints can be used. Can be used for all chemical components provided that reliable bond-valence parameters are available. Most useful for chemical components having low neutron and x-ray scattering factors. Bond-valence analysis of RMC models can provide new important insights. The mixed alkali effect is reproduced and understood directly from the RMC models of LixRb1-xPO3 glasses. The two types of alkali ions are randomly mixed. The two types of alkali ions have distinctly different pathways of low dimensionality. Pathways for A ions coincide with channels of high concentration of both mobile A ions and immobile B ions. This implies that immobile B ions tend to block the pathways for mobile A ions and vice versa, and this is the main reason for the mixed alkali effect.

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