Dynamic Pathway Models for Ion Transport in Nanostructured Heterolayers
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1023-JJ01-06
Dynamic Pathway Models for Ion Transport in Nanostructured Heterolayers Stefan Adams, and Esther S. Tan Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore, 117574, Singapore ABSTRACT The influence of local structure variations on the charge transport properties are still not well understood at an atomic level. In this work the experimentally observed drastic conductivity enhancement in epitactic stacks of BaF2:CaF2 heterolayers compared to any of the two fluoride ion conducting phases is reproduced by molecular dynamics simulations and analyzed in detail with particular emphasis on the variation of properties with the distance to the two-phase boundary. Ion mobility varies with the distance to the interface but remains significantly enhanced throughout the modeled layers when compared to bulk materials. The bond valence method is utilized to study correlations between the conductivity enhancement and the microstructure. A time-averaged violation of local electroneutrality postulated in the mesoscopic multiphase model is verified by the bond valence analysis of the molecular dynamics simulation trajectories. The variation of the ion mobility can be related to the extension of clusters of unoccupied accessible pathway regions.
INTRODUCTION Ion transport in nanostructured materials has attracted significant interest both for its practical applications, the enhancement of conductivity in solid electrolytes, and from a scientific point of view, because transport behavior changes qualitatively, when the spacing between adjacent interfaces becomes comparable to the order of magnitude of the space charge layer thickness. In our previous work we had found indications of such a ìmesoscopic multiphase effectî [1] in periodic sequences of thin sub-Debye length thin β- and γ-AgI like layers. Soon thereafter Sata et al. [2] have been able to synthesize nanometer-scale planar BaF2:CaF2 heterostructures by molecular beam epitaxy and found an enhancement of the ionic conductivity with decreasing period length (i.e. with decreasing thickness of the individual layers) compatible with predictions based on the mesoscopic size effect. In this work we investigate such BaF2:CaF2 heterostructures by molecular dynamics simulations using dedicated empirical force field parameters from [3, 4] and various boundary conditions. Details of the dynamic ion transport pathways in the local structure models can be extracted by applying our transport pathway analysis method - that utilizes the bond valence (BV) approach [5-7] - to series of instantaneous configurations extracted from the molecular dynamics trajectories. This will allow to assess the influence of local strains, dislocations and potential local deviations from electroneutrality at the interfaces of the heterostructures and to compare them with the expectations for the presumed mesoscopic size effect, allowing to discuss the microstructural origin of the conductivity enhancement.
EXPERIMENT In this work we performed molec
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