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Defect structures preceding amorphization by ion irradiations in YBa2Cu3O7-†
Physica C 162-164 (1989) 532-533 North-Holland
DEFECT STRUCTURES PRECEDING AMORPHIZATION BY ION IRRADIATIONS IN YBa2Cu307-8* M. A. KIRK, M. C. FRISCHHERZ, J. Z. LIU, L. L. FUNK, L. J. THOMPSON, E.A. RYAN, S. T. OCKERS, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA, and H. W. WEBER, Atominstitut der 5sterreischischen Universit&ten, A-1020 Wien, Austria. Transmission electron microscopy was used to investigate defect microstructures in single crystals of YBa2Cu307-8 as a function of ion irradiation dose at 300 K. Two qualitatively different defect microstructures were observed to precede amorphization. The transition between microstructures occurs at a damage level where published resistance versus temperature measurements indicate a transition from superconducting to insulating behavior. 1. INTRODUCTION The study of irradiation effects in high Tc superconductors is expected to yield insight into intrinsic defect properties of fundamental interest and into irradiation modification of properties of interest to device applications. Recent studies of ion irradiation effects in the high Tc superconductor YBa2Cu307-8 have demonstrated unusual changes in both the normal state and superconducting properties. Specifically, changes in electrical resistance versus temperature as a function of ion dose display progressive reduction and broadening of the superconducting Tc until a transition to insulating behavior is observed at a damage level near 0.07 dpa (displacement per atom). 1,2 The purpose of our work was to investigate the development of the defect microstructure under similar irradiation conditions. Our results yielded a possible explanation for the transition from superconducting to insulating behavior. 2. EXPERIMENTAL PROCEDURES Samples were produced from small single crystals. Their inductive transitions were measured
to be sharp with onset at 91 K. They were crushed and deposited on gold grids. Ion irradiation and electron microscopy were both performed at ambient temperature in the High-Voltage Electron Microscope (HVEM)-Accelerator Facility at Argonne National Laboratory. The irradiation conditions were 1.5 MeV Ne+ and 1.7 MeV Kr+ at a dose rate of 2.6 x 1011 ions cm-2 s-1. The ranges of both ions greatly exceeded the sample thicknesses under observation (~100nm). The HVEM was operated at 120 keV which is below the threshold energy for electron damage 3. Two beam dark field images were taken employing g = 200. 3. RESULTS AND DISCUSSION Selected electron micrographs from one irradiation are displayed in Fig. 1. A continuous buildup in density of small defects ( 5 nm black-white or black spot contrast features) was observed in exactly the same sample area through a dose of 3.5x1014 Ne+ cm -2 (Fig. lc). Also, through this dose, no change in diffraction patterns was observed. However, at higher doses (Fig. ld and f) the onset of a partially amorphous structure was observed as a faint ring
*Work supported by the U.S.Department of Energy, Basic Energy Sciences--Materials Science under contract #W-31-109-Eng-38 (MAK, JZL, LJT, JRW, EAR, STO) and the National Science Foundation--Office of Science and Technology Centers under contract #STC-8809854 (MCF). MCF on leave from Atominstitut der 5sterr. UniversitSten, partly supported by Fonds zur FSrderung wissenschaftlicher Forschung under contract # 6837.
M.A. Kirk et al. / Defect structures preceding amorphization
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FIGURE 1 Series of electron micrographs of 1.5 MeV Ne + irradiation of YBa2Cu307-5 at 300 K for doses (a) Preirradiation, (b) 3x1013 ions cm -2, (c) 3.5x1014 ions cm -2, (d) 6x1014 ions cm -2, (e) same as (d) at higher magnification, (e) 1.2xl 015 ions cm -2. in the diffraction patterns. A transition to a cellular microstructure appeared at a dose of 6.0x1014 Ne + cm "2 (0.08 dpa) (Fig. l e). The bright areas are "good" crystal and the dark areas are predominantly highly defective or amorphous. Thus, cells of "good" crystal may be enclosed by walls of apparently amorphous structure, and the wall volume increased at the expense of the "good" crystal with increasing ion dose (Fig. l f). Full amorphization was not reached in the Ne + irradiation through a dose of 1.2x1015 ions cm -2 (0.16 dpa). The same transition from an isolated defect structure at low doses to a cellular, partially amorphous structure was observed with 1.7 MeV Kr + irradiations at a dose of 4.0 x 1013 ions cm -2 (0.07 dpa) in three
different crystals with orientations near [100] and [001]. Full amorphization was observed by a dose of 1.6x1014 Kr+ cm -2 (0.28 dpa). These microstructural results, therefore, depended only on damage level, at least over a mass range from Ne to Kr. The results did not depend on the irradiation direction, and no apparent interaction with existing crystal defect structure was observed. REFERENCES 1. A.E. White, et al, Phys. Rev. B37 (1988) 3755. 2. G.J. Clark, et al, Appl. Phys. Lett. 51 (1987) 1462. 3. M.A. Kirk, et al, in High-Tc Superconductors, edited by H. W. Weber (Plenum, New York, 1988), p.59.
DEFECT STRUCTURES PRECEDING AMORPHIZATION BY ION IRRADIATIONS IN YBa2Cu307-8* M. A. KIRK, M. C. FRISCHHERZ, J. Z. LIU, L. L. FUNK, L. J. THOMPSON, E.A. RYAN, S. T. OCKERS, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA, and H. W. WEBER, Atominstitut der 5sterreischischen Universit&ten, A-1020 Wien, Austria. Transmission electron microscopy was used to investigate defect microstructures in single crystals of YBa2Cu307-8 as a function of ion irradiation dose at 300 K. Two qualitatively different defect microstructures were observed to precede amorphization. The transition between microstructures occurs at a damage level where published resistance versus temperature measurements indicate a transition from superconducting to insulating behavior. 1. INTRODUCTION The study of irradiation effects in high Tc superconductors is expected to yield insight into intrinsic defect properties of fundamental interest and into irradiation modification of properties of interest to device applications. Recent studies of ion irradiation effects in the high Tc superconductor YBa2Cu307-8 have demonstrated unusual changes in both the normal state and superconducting properties. Specifically, changes in electrical resistance versus temperature as a function of ion dose display progressive reduction and broadening of the superconducting Tc until a transition to insulating behavior is observed at a damage level near 0.07 dpa (displacement per atom). 1,2 The purpose of our work was to investigate the development of the defect microstructure under similar irradiation conditions. Our results yielded a possible explanation for the transition from superconducting to insulating behavior. 2. EXPERIMENTAL PROCEDURES Samples were produced from small single crystals. Their inductive transitions were measured
to be sharp with onset at 91 K. They were crushed and deposited on gold grids. Ion irradiation and electron microscopy were both performed at ambient temperature in the High-Voltage Electron Microscope (HVEM)-Accelerator Facility at Argonne National Laboratory. The irradiation conditions were 1.5 MeV Ne+ and 1.7 MeV Kr+ at a dose rate of 2.6 x 1011 ions cm-2 s-1. The ranges of both ions greatly exceeded the sample thicknesses under observation (~100nm). The HVEM was operated at 120 keV which is below the threshold energy for electron damage 3. Two beam dark field images were taken employing g = 200. 3. RESULTS AND DISCUSSION Selected electron micrographs from one irradiation are displayed in Fig. 1. A continuous buildup in density of small defects ( 5 nm black-white or black spot contrast features) was observed in exactly the same sample area through a dose of 3.5x1014 Ne+ cm -2 (Fig. lc). Also, through this dose, no change in diffraction patterns was observed. However, at higher doses (Fig. ld and f) the onset of a partially amorphous structure was observed as a faint ring
*Work supported by the U.S.Department of Energy, Basic Energy Sciences--Materials Science under contract #W-31-109-Eng-38 (MAK, JZL, LJT, JRW, EAR, STO) and the National Science Foundation--Office of Science and Technology Centers under contract #STC-8809854 (MCF). MCF on leave from Atominstitut der 5sterr. UniversitSten, partly supported by Fonds zur FSrderung wissenschaftlicher Forschung under contract # 6837.
M.A. Kirk et al. / Defect structures preceding amorphization
533
FIGURE 1 Series of electron micrographs of 1.5 MeV Ne + irradiation of YBa2Cu307-5 at 300 K for doses (a) Preirradiation, (b) 3x1013 ions cm -2, (c) 3.5x1014 ions cm -2, (d) 6x1014 ions cm -2, (e) same as (d) at higher magnification, (e) 1.2xl 015 ions cm -2. in the diffraction patterns. A transition to a cellular microstructure appeared at a dose of 6.0x1014 Ne + cm "2 (0.08 dpa) (Fig. l e). The bright areas are "good" crystal and the dark areas are predominantly highly defective or amorphous. Thus, cells of "good" crystal may be enclosed by walls of apparently amorphous structure, and the wall volume increased at the expense of the "good" crystal with increasing ion dose (Fig. l f). Full amorphization was not reached in the Ne + irradiation through a dose of 1.2x1015 ions cm -2 (0.16 dpa). The same transition from an isolated defect structure at low doses to a cellular, partially amorphous structure was observed with 1.7 MeV Kr + irradiations at a dose of 4.0 x 1013 ions cm -2 (0.07 dpa) in three
different crystals with orientations near [100] and [001]. Full amorphization was observed by a dose of 1.6x1014 Kr+ cm -2 (0.28 dpa). These microstructural results, therefore, depended only on damage level, at least over a mass range from Ne to Kr. The results did not depend on the irradiation direction, and no apparent interaction with existing crystal defect structure was observed. REFERENCES 1. A.E. White, et al, Phys. Rev. B37 (1988) 3755. 2. G.J. Clark, et al, Appl. Phys. Lett. 51 (1987) 1462. 3. M.A. Kirk, et al, in High-Tc Superconductors, edited by H. W. Weber (Plenum, New York, 1988), p.59.