Recently, artificial ion conductors have been prepared by growing
epitaxial heterolayers consisting of BaF2-CaF2 using molecular beam
epitaxy. The ionic conductivity of these heterolayers shows a strong
dependence on the layer thickness [N. Sata, S. Eberman, K. Eberl, and
J. Maier, Nature 408, 996 (2000)]. In this paper three such
heterolayers with different spacings (sample A: 80 nm, sample B: 10 nm,
sample C: 1 nm) are investigated by conventional transmission electron
microscopy and high-resolution transmission electron microscopy. The
spacings are chosen such that they fall into the three conductivity
regimes observed in N. Sata (l>50 nm; 8<l<50 nm; l<8 nm). In accordance
with conductivity studies, the samples with spacings of 10 nm or
greater (A,B) are epitaxial and continuous, whereas in the case of
extremely small spacing (C) the continuity of the layers is destroyed
by formation of a column-like structure. Analytical electron microscopy
reveals that, instead of forming multilayers, Ca and Ba separate in
different columns in sample C. The structure properties of sample A
(large l) are quite ideal: Planar interfaces with regular arrays of
misfit dislocations with their Burgers vectors on the interface are
observed. In the case of sample B (medium l) the lattice misfit is
accommodated, in addition, by wavy interfaces associated with
dislocations characterized by a Burgers vector that makes a large angle
to the interfaces. The (111) lattice spacing very close to the
interfaces is markedly changed due to this novel relaxation mechanism
in the multilayer. The influences of the crystallographic defects on
the ionic conductivity are also discussed. (C) 2004 American Institute
of Physics.</textarea>
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