Structure images ofa- andb-axis projections in YBa₂Cu₃O7-δsuperconductors observed by HREM at 200 kV

Studies of oxygen vacancies in high-Tc YBa2Cu3O7-δ superconductors have been attempted by high-resolution electron microscopy (HREM). HREM images taken near Scherzer defocus show a good correspondence with the projection of the structure proposed from neutron and X-ray diffraction results. Y, Ba and Cu atoms are seen as dark spots with different contrast depending on the atomic scattering factors. The spots arising from the positions of oxygen vacancies located at (0,0,1/2) in Y planes and (½, 0, 0) in Cu(1) planes are brighter than those corresponding to occupied oxygen positions. The simulated images confirm that the brightest spots which appear at unoccupied atomic oxygen positions (½ 0,0) in Cu(1) planes appear not only because of the existence of the oxygen vacancies themselves, but also due to the shift of the Ba atoms away from the Cu(1) planes; on the other hand the brighter spots located at (0,0,½) in Y planes are due merely to the existence of oxygen vacancies.