Study of space charge and local lattice distortion in ionic conductors by artificially synthesized superlattice structures in nanometer-scale

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In this study, artificial layers are synthesized to tune dopant distribution in proton-conducting perovskite-type oxides in order to investigate influence of the dopant distribution on proton dynamics. SrZrO3 doped with trivalent rare earth metal has been chosen as a model material, as its properties of bulk crystal is well known. In this type of proton conductors, it is suggested that the dopant might have a significant effect on the proton dynamics.The artificial layers, in which the dopant is distributed periodically in two-dimensional plane, are formed by Pulsed Laser Deposition (PLD). The host material, SrZrO3 (SZO) and the dopant layer, Y2O3 are alternately deposited on (001)MgO substrate. The thickness of SZO and Y2O3 layers are 5-20 and 0.25-1nm, respectively, and the total thickness of the film is about 200nm. HREM cross sectional views of the thin films show that very thin and flat interfacial layers of Y2O3 are formed and there is almost no difference in atomic arrangements … More between SZO and Y2O3 in HREM image, though their structure in bulk crystals are different. XRD analysis revealed that the SZO lattice is slightly distorted due to a large difference in thermal expansion coefficient. The epitaxial relationship of the artificial layer and the MgO substrate is (001)//(001) and [100]//[100].In-plane electrical conductivity in wet air saturated with H2O and D2O at 20℃ has been obtained by impedance spectroscopy. An isotope effect is observed and it is confirmed that the main carrier in the artificial layers is proton below 400℃. It is found that the conductivity is the highest and the activation energy is the lowest when the SZO layer thickness is about 10nm. This result suggests that there is a preferable arrangement of the dopant distribution in this material, however, the conductivity was much low and the activation energy was high compared to those of bulk crystals. The electronic structures around the band gap of artificial layers are the same irrespective of the layer structure and are different from those of SZO and SZY thin films. Different mechanisms of proton conduction must be considered to elucidate the results for the artificial layers. Studies by hetero-layered structures of proton conductors might give important information to solve the problem of very low conductivity at the grain boundary region in their bulk crystals. Less