High resolution electron microscopy of ion-irradiated GdBa 2 Cu 3 O 7
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M-O. Ruault and H. Bernas Centre de Spectrometrie Nucleaire et Spectrometrie de Masse, IN2Ps CNRS Bat. 108, 91405 Campus Orsay, France
M. Gasgnier UPR 210, CNRS, 1 Place A. Briand, 92195 Meudon, France (Received 28 June 1990; accepted 11 December 1990)
GdBa2Cu3O7 crystals were irradiated at room temperature with 200 keV Ne ions and 300 keV Xe ions. In situ standard TEM and further HREM studies show two types of extended defects: (i) mobile extended defects, which account for the preferential defect pinning to twin boundaries reported earlier. These defects are rapidly recovered and difficult to observe by HREM investigations; (ii) stable amorphous areas which are clearly identified by HREM observations. Their overlapping and aggregation conceivably lead to amorphization of the sample.
I. INTRODUCTION
II. EXPERIMENTAL
Considerable effort has been put into the investigation of the effect of electron and ion irradiation on the new high Tc superconductors. The YBa 2 Cu 3 O 7 material has been studied intensively (for reviews see, for example, Refs. 1 and 2). The main result on light ion irradiation (damage produced uniformly throughout the foil) is the preferential oxygen redistribution in the basal a-b plane at the beginning of the irradiation which leads to the Tc reduction and to the orthorhombic to tetragonal transformation.3 In irradiations with energetic heavy ions in the 30-300 keV range, the ion energy loss leads to a locally highly disordered region (i.e., displacement cascade —10 nm). Clustering and recombination may occur within these regions (see the work of Kirk et al.4'5 and Ruault et al.6'1). In some cases, small defect clusters with an asymmetrical black-white contrast (whatever their nature) were observed in thin samples for low dose irradiation (^1012 ions/cm2). From dynamical observation on tape recording during 15 K and room temperature irradiation, Ruault et al.6'1 surmised that two types of extended defects were probably formed during Kr ion irradiation: dislocations lying in the basal CuO plane as well as very disordered (possibly amorphous) zones. The latter lead to amorphization of the material, as shown by Kirk et a/.4'5 However, to our knowledge, no direct determination of the nature of the observed extended defects has been published so far. In this work, using both HREM and in situ TEM, we show evidence of the presence of both defects similar to dislocation loops in metals and amorphous regions in a Ne (or Xe) irradiated GdBa2Cu3O7 thin sample.
Crushed small GdBaCuO single superconductor crystals8 are deposited on holey carbon grids for microscopy investigations. 300 keV Ne and 100 KeV Xe irradiations were performed at room temperature in situ in a Philips EM400 electron microscope on line with the ion implantor using a standard double tilt sample holder.9 Video recording was performed during irradiation. From time to time the ion irradiation was interrupted and dark-field images were taken under different diffraction conditions (particularly near the {001} basal plane with the [200],
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