Enhanced ionic conductivity and mesoscopic size effects in heterostructures of BaF2 and CaF2

Ionic heterolayers of CaF2/BaF2/CaF2 have been prepared by molecular beam epitaxy (MBE). The spacing has been varied from ∼1 nm to ∼1 μm. In the range of 1 μm to 5 nm, the conductivity (measured effective conductivity parallel to the interface) increases progressively with the increased interfacial density. This is even true for spacings in the sub-Debye range. For comparatively large spacings (>50 nm), semi-infinite space charges provide a quantitative description, while the behavior at smaller spacings reveals nano-size anomalies. At spacings smaller than ∼5 nm, the conductivity decreases. The results clearly demonstrate the possibility to prepare artificial ion conductors and the potential of mesoscopic ion conduction. The paper describes transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray pole figure, secondary ion mass spectrometry (SIMS) and impedance spectroscopic characterization.