Application of convergent beam electron diffraction in the structural study of high-temperature superconducting oxides.

Convergent beam electron diffraction (CBED) is a powerful technique for symmetry study of crystal. It has widespread application in physics and material sciences, as demonstrated in a recent superconducting oxide study. Using this technique, we have studied Ba-La-Cu-O superconductors with a transition temperature of about 40°K and Ba-Y-Cu-O superconductors with a critical temperature (Tc) of about 90°K. We have found that in Ba-La-Cu-O superconductors the superconducting phase La2−xBaxCuO4−y has a distorted K2NiF4-type structure and the space group Fmmm. The two other phases in Ba-La-Cu-O superconductors have also been studied. In our Ba-Y-Cu-O superconductors, the Ba2YCu3O7−x compound, which is responsible for 90°K superconductivity, has two different space groups: An orthorhombic space group Pmmm and a tetragonal space group P4mm or P4/mmm.