Quantitative Analysis of Local Structure in High Tc Superconductors by Energy Filtering Convergent-Beam Electron Diffraction

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Improvement of the critical current density, Jc is one of the important subjects for practical application of high Tc superconductor YBa_2Cu_3O_<7-x> (YBCO). The crystal structure and superconducting properties sensitively depend on oxygen concentration that easily changes depending on atmosphere. Convergent beam electron diffraction has been used to extract information on oxygen concentration and/or charge distribution and/or oxygen. The observed diffraction intensities are analyzed with the dynamical theory of electron diffraction. Large angle CBED patterns were observed with a FE-TEM : JEM-2010FEF equipped with an energy filter of omega-type.The accuracy of the measured oxygen concentration depends on the reliability of temperature factors (B-factors) used in the analysis. An actual temperature of a specimen under electron beam irradiation is estimated with anisotropic temperature factors. The local oxygen concentration determined vary from point to point. This result shows that oxygen concentration is not uniform in YBCO and oxygen deficiency fluctuates locally. No super lattice spots were observed in [001] diffraction patterns indicating that oxygen deficiency occurs with random depopulation of oxygen along a Cu-O chain. Fluctuation of oxygen deficiency is the most probable reason for the peak-effect in YBCO. Information on charge distribution on CuO_2 and CuO planes are extracted by more precise analysis of CBED patterns.