Pathways for ion transport in nanostructured BaF2:CaF2

The experimentally observed drastic conductivity enhancement in epitactic BaF2:CaF2 heterolayers with respect to any of the two fluoride ion conducting phases is qualitatively reproduced by molecular dynamics simulations and analyzed in detail with particular emphasis on the variation of properties as a function of 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. Moreover the average coordination number of the fluoride ions is significantly reduced around the interface suggesting a redistribution of anions from regular sites to interstitial sites. The variation of the ion mobility can be related to the extension of clusters of unoccupied accessible pathway regions.