HREM STUDIES OF SURFACE-CHEMISTRY OF SUPERCONDUCTING YBA2CU3O7-DELTA

We have investigated the surface chemistry of superconducting YBa2Cu3O7-delta using high-resolution electron microscopy. Specimens crushed in air showed a thick 'amorphous' coating, which would not recrystallize into the bulk structure under electron beam annealing. Instead oxide microcrystals formed within the coating. The amorphous coating layers were reduced to less than half of their original thickness when the specimens were crushed in argon, and were found to recrystallize into the YBa2Cu3O7-delta structure under the same conditions, with atomically clean surfaces produced and observed directly in the profile images. It was also found that the (001) surface terminated with the Cu-O chain. Surface reconstruction with a double atomic layer of Cu covering the (113) and (112) surfaces was confirmed by computer simulation of the profile images. Possible mechanisms of formation of the coating and clean surfaces are discussed.