Structural features of defect cascades in YBaZCu30, as a function of oxygen stoichiometry

Single crystals of YBa2Cu30,, of various oxygen stoichiometries, were irradiated with 50 keV Kr' to create defect cascades similar to those resulting from fast neutron irradiation. Two-beam dark-field transmission electron microscopy in weak dynamic conditions was used to measure the defect cascade size distribution in the principal axes for four different oxygen stoichiometries, and high-resolution electron microscopy verified the mean size of these cascades. The mean size of the defect cascades generated was independent of oxygen stoichiometry from x = 6.35 to x = 6.9, which indicates that magnetic flux pinning does not vary because of any association between the oxygen concentration and the cascade size. For Kr' incident near the c axis the defect cascades have on average a circular cross-section of 3.5 nm diameter perpendicular to the ion beam, while an elliptical cross-section was found for ions incident near the x axis. The minor axis (3.5nm) was in the [OOl] direction and the major axis (5.5nm) was in the [OlO] direction. This aspect ratio of 1.6 matches that found for high-energy heavy-ion track damage in YBa2Cu3Ox, which implies that the shape is determined not by the recoil damage mechanism but in the cooling or relaxation stage where the anisotropy in thermal conductivity determines the defect shape. 5