DETERMINATION OF THE LOCAL OXYGEN STOICHIOMETRY IN YBA2CU3O7-DELTA BY ELECTRON-ENERGY LOSS SPECTROSCOPY IN THE SCANNING-TRANSMISSION ELECTRON-MICROSCOPE

By using parallel detection electron energy loss spectroscopy (PEELS) in the scanning transmission electron microscope (STEM), the oxygen K-absorption edge spectrum is used to determine delta in YBa2CU3O7-delta to an accuracy of < 5%. The instability of YBa2Cu3O7-delta to the electron beam enables oxygen reduction experiments to be performed in situ in the microscope. Results show a "two plateau" behaviour similar to that observed in transition temperature versus oxygen stoichiometry measurements, and can be interpreted in terms of oxygen vacancy ordering and charge distribution between the CuO chains and CuO2 planes. In addition, the sub-nanometer probe of the STEM enables changes in oxygen deficiency to be observed with high spatial resolution. The high spatial resolution has been used to observe fluctuations in oxygen stoichiometry in the region of grain boundaries in bulk sintered samples.