- Email: [email protected]
Neutron- and electron-irradiation effects on the microstructure of YBa2Cu4O8 observed by HREM
Physica C 338 Ž2000. 103–109 www.elsevier.nlrlocaterphysc
Neutron- and electron-irradiation effects on the microstructure of YBa 2 Cu 4O 8 observed by HREM Masafumi Akiyoshi a , Kazuaki Hashimoto b, Toyohiko Yano a,) a
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan b Department of Industrial Chemistry, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
Abstract Neutron-irradiated and unirradiated YBa 2 Cu 4O 8 superconductor ceramics were observed by high-resolution electron microscopy ŽHREM.. Defects around the double Cu–O chain were frequently observed. Besides already reported single and triple Cu–O chain defects, small, white dot-like spot defects, were observed in the a–c plane, and relatively large ‘W letter-like zigzag’ defects were observed in the b–c plane. A structure-model of the latter defect was proposed. All these defects were observed in the materials irrespective of neutron irradiation. Furthermore, an electron-beam-irradiation effect was observed by long time illumination of the same area by HREM to confirm that these defects were not induced by electron irradiation. Electron irradiation induced mainly sputtering of surface atoms. Defects associated with the Cu–O chain were not easily created by the 300 keV electron irradiation. q 2000 Elsevier Science B.V. All rights reserved. Keywords: YBa 2 Cu 4 O 8 ; HREM; Neutron irradiation; Electron irradiation; Defect structure
1. Introduction YBa 2 Cu 4 O 8 is known as an 80 K superconductor. Unlike YBa 2 Cu 3 O 7yx Ž60–90 K superconductor., the oxygen content of YBa 2 Cu 4 O 8 is stable up to 8008C w1,2x. Crystal structure of YBa 2 Cu 4 O 8 belongs to an A-centered orthorhombic cell. The space group is Ammm with lattice constants of a s 0.384 nm, b s 0.387 nm, and c s 2.72 nm w3x. There is a w0 1r2 1r2x diagonal glide in this structure. Planary four-coordinated edge shared double Cu–O chains are running parallel to the b-axis. These Cu–O double chains are the typical feature of the crystal structure of YBa 2 Cu 4 O 8 . )
Corresponding author. E-mail address: [email protected] ŽT. Yano..
Wisniewski et al. w4x reported that neutron-irradia´ ted YBa 2 Cu 4 O 8 and YBa 2 Cu 3 O 7yx showed significant enhancement of the Jc . The intragrain critical current Jc was inferred from M–H hysteresis loops. An accurate determination of the absolute values of Jc was difficult, because detailed information on the sizes and shapes of the grains had not been available. However, irradiation effects on Jc could obtained from comparing the widths of the hysteresis loops before and after irradiation. In the case of YBa 2 Cu 4 O 8 of the present study, JcirrrJcunirr was up to a factor of 2 ; 7.5, Žmaximized at 40 K and 1 T.. It is known that if size of defects match with the diameter of the normal core of the vortex Ž, 2 j ., it works as pinning centers efficiently. The diameter of vortex core varies with temperature and magnetic field, so the relation between the JcirrrJcunirr factor
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 2 1 0 - 0
104
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
and temperature andror magnetic field depends on the distribution of the diameter of defects. There are only a few reports on structural defects in YBa 2 Cu 4 O 8 w5,6x. Furthermore, it is known that electron-beam irradiation during electron microscopy may induce structural defects into materials. In this paper, therefore, structural defects in YBa 2 Cu 4 O 8 is further observed by high-resolution electron microscopy ŽHREM., and also electron-irradiation effect on structure is examined.
2. Experimental procedures The specimen used in this study was kindly supplied by A. Wisniewski. The sample was prepared ´ by the pyrolytic method and the X-ray diffraction analysis and its sharp superconducting transition confirmed that the sample was single phase of YBa 2 Cu 4 O 8 w4x. The specimen was irradiated up to a fluence of 1.3 = 10 21 nrm2 Ž E ) 0.1 MeV. at room temperature in the EWA reactor at the Institute of Atomic Energy, Swierk, Poland. The specimen that was not irradiated by neutron was also examined. The foils for transmission electron microscopy ŽTEM. observation were prepared by the crushing method. YBa 2 Cu 4 O 8 bulk ceramic was crushed in methanol using an agate mortar and dispersed by an ultrasonic vibrator. The suspension was dropped onto a carbon-reinforced lacy microgrid over a filter paper. The transmission electron microscope used in this study was Hitachi H-9000, having a point resolution of 0.20 nm and the operation was performed at an accelerating voltage of 300 kV.
3. Results and discussion 3.1. Effect of neutron irradiation Neutron-irradiated YBa 2 Cu 4 O 8 was not shown distinct amorphous region unlike heavy-ion-irradiated damage in YBa 2 Cu 3 O 7yx w7,8x. Structural defects around the double Cu–O chain were frequently observed both in the as-synthesized and the neutronirradiated specimens w6x. We cannot detect significant change of the size or the density of the defect between before and after irradiation from low Žaround
=100 000. to high Ž; =1 000 000. magnification observation. But, enhancement of critical current density after neutron irradiation of the specimen means that some flux pinning centers are created by the neutron irradiation of which the size in the diameter is around 10 nm in the a–b plane or 1 nm along the c-axis. More accurate scrutiny from several directions may show the change induced by the neutron irradiation. The defects observed along the w100x Ž b–c plane. and along the w010x Ž a–c plane. were classified into some types. First, the double Cu–O chain changed into a triple or a single Cu–O chain w6x. At the former type, the double Cu–O chain changed into a triple Cu–O chain running along the b-axis, thus, a corresponding local structure is expressed as YBa 2 Cu 5 O 9 . To the contrary, the double Cu–O chain changed into a single Cu–O chain running along the b-axis, corresponding to YBa 2 Cu 3 O 7yx in the same way. The single chain and the triple chain containing structures have no w0 1r2 1r2x diagonal glide shift. So a layer in which the double Cu–O chain changes into a triple or a single chain induces half unit cell discrepancy along the b-axis. Furthermore, the triple or the single chain have a different unit length along the c-axis from that of the double Cu–O chain structure, so when the double chain changes into another type of chain in a crystal, it introduces lattice distortion. The defect size of this type was almost 4 ; 5 nm, and the strain field around the defect has about the same size of the defect, so the effective defect size was estimated to be 8 ; 10 nm. on the other hand, the diameter of the normal core of the vortex is about 2 j , and the coherence length of YBa 2 Cu 4O 8 in the a–b plane, ja – b Ž0. is reported to be 5 nm at 0 K w9x. Therefore the effective defect size matched well to the diameter of the normal core of the vortex. It is known that YBa 2 Cu 4 O 8 decomposes into YBa 2 Cu 3 O 7yx and CuO above 8508C in air, and YBa 2 Cu 3 O 7yx is more stable than YBa 2 Cu 4 O 8 at temperatures higher than 10008C and oxygen pressure below 2 = 10 7 Pa w1x. Then the single Cu–O chain observed in the unirradiated YBa 2 Cu 4 O 8 w1,6x is believed to be introduced during the synthesis process. The second type, a small area of lattice was twisted, and an area of the b–c plane changes into
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
105
Fig. 1. White spot type defects observed in the a–c plane of YBa 2 Cu 4 O 8 which was neutron-irradiated up to 1.3 = 10 21 nrm2 Ž E ) 0.1 MeV. at room temperature; Ža. and Žb. are taken by different imaging conditions.
Fig. 2. ‘W letter-like zigzag’ defects observed in the b–c plane of unirradiated YBa 2 Cu 4 O 8 .
106
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
an area of the a–c plane. There is a thin 908 rotation twins about the c-axis. The double Cu–O chain expected to be observed as a ‘stair-rod’ like image viewed along the w010x direction Ž a–c plane. and as ‘zigzag’ image viewed along the w100x direction Ž b–c plane.. YBa 2 Cu 4 O 8 contains a w0 1r2 1r2x diagonal glide caused by the double Cu–O chain. Therefore, a change in lattice direction induces a Ž1r2. b mismatch around the double Cu–O chain. These defects mentioned above were described in the previous reports w6x. The third type, double white spots across the double Cu–O chain were observed in the a–c plane projected images. This defect is shown with white circles in Fig. 1a. The image shown in Fig. 1b with white arrows probably corresponds with the same defect shown in Fig. 1a, taking by different imaging conditions. To estimate a defect structure, several models such as elimination of oxygen atoms, which are located on the top of CuO5 pyramids, were
applied to clarify the observed images. Image simulations using a multi-slice simulation program were conducted, but we could not obtain a proper model, which satisfy the observed images. The size of this white spot-type defect observed in the a–c plane was smaller than 1 nm. It is close to the coherence length in the plane along the c-axis Ž j c Ž0. s 0.5 nm w9x.. There is a possibility that the defect stretches along the b-axis. At last, relatively large ‘W letter-like zigzag’ image was observed in the projection onto the b–c plane, as shown in Fig. 2. The Cu–O band width of this defect was the same as that of the triple Cu–O chain mentioned above, but it contains a w0 1r2 1r2x unit shift like the double Cu–O chain structure while the triple chain does not have this shift. A unit cell of this structure extended five times as long as that of YBa 2 Cu 4 O 8 along the b-axis. Fig. 3 shows schematic illustrations of structural models of Ža. the double Cu–O chain structure ŽYBa 2 Cu 4 O 8 ., Žb. the triple
Fig. 3. Structural models of YBCO family projected onto the Ž100. plane. Ža. Up-right: the double Cu–O chain structure ŽYBa 2 Cu 4 O 8 .. Žb. Bottom-right: the triple chain structure ŽYBa 2 Cu 5 O 9 .. Žc. Bottom-left: base model of the ‘W letter-like zigzag’ defect. Žd. Up-left: structural model of the ‘W letter-like zigzag’ defect.
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
107
Fig. 4. HREM images taken along the b-axis after prolonged observation of the YBa 2 Cu 4 O 8 which was not irradiated by neutron. Each of the captions means illumination time of the electron beam; Ža. 10 min, Žb. 82 min, Žc. 157 min.
108
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
Fig. 4 Ž continued ..
Cu–O chain structure ŽYBa 2 Cu 5 O 9 ., Žc. a base structure of the ‘W letter-like zigzag’ defect, and Žd. a structural model of the ‘W letter-like zigzag’ defect. This defect structure includes unusual cation polyhedra which contains two Cu atoms. We think this model represents an averaged structure along the a-axis. In one b–c plane, the CuO polyhedron contains Cu atom at one of the two positions of the large hexagon Žmiddle-filled circle., whereas the next b–c plane, the Cu atom at the center of the polyhedron locates the other position in the hexagon Žmiddleopen circle.. If one oxygen is positioned at an alternated site within the hexagon in one b–c plane, octahedron will be constructed around Cu atom. These two octahedrons may share an edge, so this structure contains insecurity. Furthermore, in this case, the local packing of oxygen atom is too dense. Then, it is supposed that the Cu atom is located in planaly six-coordinated oxygens as shown in Fig. 3c. If atomic coordination changed the model as shown in Fig. 3d, majority of CuO polyhedron changed to five-membered polyhedron, which is closer to fourmembered polyhedron. Image simulations were con-
ducted based on this model, but we could not obtain a proper image, which matches with the observed image satisfactorily. 3.2. Effect of electron-beam irradiation Electron-beam-irradiation effect was observed by long time illumination of the same area by TEM to confirm these defects mentioned above were not induced by electron-beam irradiation during observation. It was reported that low energy electron beam causes thermal effects to make single chain type defect in YBa 2 Cu 4 O 8 w1x. In the contrary, high energy electron beam Ž, 1 MeV. forms an amorphous structure around the Cu–O chain at first and then it extends outwards w10x. Electron irradiation mainly caused sputtering of surface atoms. Weak-bonded atoms in an amorphous-like surface were easily sputtered. Fig. 4 shows high-resolution images after prolonged electron illumination of the a–c plane of YBa 2 Cu 4 O 8 which was not irradiated by neutrons. Comparing the first image ŽFig. 4a, irradiation time: 10 min. with the
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
second image ŽFig. 4b, irradiation time: 82 min., the following changes are observed. The amorphous surface region over the edge of the specimen shrank and almost disappeared by surface sputtering effect. Furthermore, spot-type defects in the a–c plane, the same as in Fig. 1a and b disappeared at an initial stage of the electron irradiation. After long illumination, atomic arrangement around the double Cu–O chain was gradually disordered. In Fig. 4b, structural images of some areas near the double Cu–O chain were clouded, and at the last image ŽFig. 4c, irradiation time: 157 min. structural images were distorted more severely. It indicates that local amorphization is initiated gradually by the 300 keV electron irradiation. But in any case, the triple or single chain structures, the a–c b–c transition, the W-letterlike zigzag structure defects were not created by the electron irradiation under the present conditions.
l
4. Conclusion Neutron-irradiated YBa 2 Cu 4 O 8 superconductor was observed by HREM. Several types of structural defect were observed in the as-synthesized and the neutron-irradiated specimens, and we cannot detect a significant change of the size or the density of these defects by the neutron irradiation. But enhancement of critical current density after neutron irradiation of the specimen means some flux pinning centers are created by the neutron irradiation. Besides, already
109
reported defects Žsuch as the change of the double Cu–O chain into triple or single chains., dot-like spot defects and relatively large ‘W letter-like zigzag’ defects, were observed in the a–c plane and b–c plane, respectively. The structural model corresponding to the latter defect was proposed. Furthermore, it was clarified that electron irradiation during observation induced mainly sputtering of surface atoms, and defects associated with the Cu–O chain were not easily created by the 300 keV electron irradiation.
References w1x K. Yamaguchi, T. Miyatake, T. Takata, S. Gotoh, N. Koshizuka, S. Tanaka, Jpn. J. Appl. Phys. 28 Ž1989. 1942. w2x D.E. Morris, A.G. Markelz, B. Fayn, J.H. Nickel, Physica C 168 Ž1990. 153. w3x Y. Yamada, J.D. Jorgensen, S. Pei, P. Lightfoot, Y. Kodama, T. Matsumoto, F. Izumi, Physica C 173 Ž1991. 185. w4x A. Wisniewski, G. Brandstatter, C. Czurda, H.W. Weber, A. ´ ¨ Morawski, T. Lada, Physica C 220 Ž1994. 181. w5x T. Yano, M. Pekala, A.Q. He, A. Wisniewski, M.L. Jenkins, ´ Physica C 247 Ž1995. 55. w6x K. Hashimoto, M. Akiyoshi, A. Wisniewski, M.L. Jenkins, ´ Y. Toda, T. Yano, Physica C 269 Ž1996. 139. w7x Y. Zhu, H. Zhang, M. Suenaga, D.O. Welch, Philos. Mag. A 68 Ž1993. 1079. w8x Y. Zhu, Z.X. Cai, R.C. Budhani, M. Suenaga, D.O. Welch, Phys. Rev. B 48 Ž1993. 6436. w9x J.C. Martinez, O. Laborde, J.J. Prejean, C. Chappert, J.P. Renard, J. Karpinski, E. Kaldis, Europhys. Lett. 14 Ž1991. 693. w10x Y. Matsui, K. Yanagisawa, Jpn. J. Appl. Phys. 31 Ž1992. 29.
Neutron- and electron-irradiation effects on the microstructure of YBa 2 Cu 4O 8 observed by HREM Masafumi Akiyoshi a , Kazuaki Hashimoto b, Toyohiko Yano a,) a
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan b Department of Industrial Chemistry, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
Abstract Neutron-irradiated and unirradiated YBa 2 Cu 4O 8 superconductor ceramics were observed by high-resolution electron microscopy ŽHREM.. Defects around the double Cu–O chain were frequently observed. Besides already reported single and triple Cu–O chain defects, small, white dot-like spot defects, were observed in the a–c plane, and relatively large ‘W letter-like zigzag’ defects were observed in the b–c plane. A structure-model of the latter defect was proposed. All these defects were observed in the materials irrespective of neutron irradiation. Furthermore, an electron-beam-irradiation effect was observed by long time illumination of the same area by HREM to confirm that these defects were not induced by electron irradiation. Electron irradiation induced mainly sputtering of surface atoms. Defects associated with the Cu–O chain were not easily created by the 300 keV electron irradiation. q 2000 Elsevier Science B.V. All rights reserved. Keywords: YBa 2 Cu 4 O 8 ; HREM; Neutron irradiation; Electron irradiation; Defect structure
1. Introduction YBa 2 Cu 4 O 8 is known as an 80 K superconductor. Unlike YBa 2 Cu 3 O 7yx Ž60–90 K superconductor., the oxygen content of YBa 2 Cu 4 O 8 is stable up to 8008C w1,2x. Crystal structure of YBa 2 Cu 4 O 8 belongs to an A-centered orthorhombic cell. The space group is Ammm with lattice constants of a s 0.384 nm, b s 0.387 nm, and c s 2.72 nm w3x. There is a w0 1r2 1r2x diagonal glide in this structure. Planary four-coordinated edge shared double Cu–O chains are running parallel to the b-axis. These Cu–O double chains are the typical feature of the crystal structure of YBa 2 Cu 4 O 8 . )
Corresponding author. E-mail address: [email protected] ŽT. Yano..
Wisniewski et al. w4x reported that neutron-irradia´ ted YBa 2 Cu 4 O 8 and YBa 2 Cu 3 O 7yx showed significant enhancement of the Jc . The intragrain critical current Jc was inferred from M–H hysteresis loops. An accurate determination of the absolute values of Jc was difficult, because detailed information on the sizes and shapes of the grains had not been available. However, irradiation effects on Jc could obtained from comparing the widths of the hysteresis loops before and after irradiation. In the case of YBa 2 Cu 4 O 8 of the present study, JcirrrJcunirr was up to a factor of 2 ; 7.5, Žmaximized at 40 K and 1 T.. It is known that if size of defects match with the diameter of the normal core of the vortex Ž, 2 j ., it works as pinning centers efficiently. The diameter of vortex core varies with temperature and magnetic field, so the relation between the JcirrrJcunirr factor
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 2 1 0 - 0
104
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
and temperature andror magnetic field depends on the distribution of the diameter of defects. There are only a few reports on structural defects in YBa 2 Cu 4 O 8 w5,6x. Furthermore, it is known that electron-beam irradiation during electron microscopy may induce structural defects into materials. In this paper, therefore, structural defects in YBa 2 Cu 4 O 8 is further observed by high-resolution electron microscopy ŽHREM., and also electron-irradiation effect on structure is examined.
2. Experimental procedures The specimen used in this study was kindly supplied by A. Wisniewski. The sample was prepared ´ by the pyrolytic method and the X-ray diffraction analysis and its sharp superconducting transition confirmed that the sample was single phase of YBa 2 Cu 4 O 8 w4x. The specimen was irradiated up to a fluence of 1.3 = 10 21 nrm2 Ž E ) 0.1 MeV. at room temperature in the EWA reactor at the Institute of Atomic Energy, Swierk, Poland. The specimen that was not irradiated by neutron was also examined. The foils for transmission electron microscopy ŽTEM. observation were prepared by the crushing method. YBa 2 Cu 4 O 8 bulk ceramic was crushed in methanol using an agate mortar and dispersed by an ultrasonic vibrator. The suspension was dropped onto a carbon-reinforced lacy microgrid over a filter paper. The transmission electron microscope used in this study was Hitachi H-9000, having a point resolution of 0.20 nm and the operation was performed at an accelerating voltage of 300 kV.
3. Results and discussion 3.1. Effect of neutron irradiation Neutron-irradiated YBa 2 Cu 4 O 8 was not shown distinct amorphous region unlike heavy-ion-irradiated damage in YBa 2 Cu 3 O 7yx w7,8x. Structural defects around the double Cu–O chain were frequently observed both in the as-synthesized and the neutronirradiated specimens w6x. We cannot detect significant change of the size or the density of the defect between before and after irradiation from low Žaround
=100 000. to high Ž; =1 000 000. magnification observation. But, enhancement of critical current density after neutron irradiation of the specimen means that some flux pinning centers are created by the neutron irradiation of which the size in the diameter is around 10 nm in the a–b plane or 1 nm along the c-axis. More accurate scrutiny from several directions may show the change induced by the neutron irradiation. The defects observed along the w100x Ž b–c plane. and along the w010x Ž a–c plane. were classified into some types. First, the double Cu–O chain changed into a triple or a single Cu–O chain w6x. At the former type, the double Cu–O chain changed into a triple Cu–O chain running along the b-axis, thus, a corresponding local structure is expressed as YBa 2 Cu 5 O 9 . To the contrary, the double Cu–O chain changed into a single Cu–O chain running along the b-axis, corresponding to YBa 2 Cu 3 O 7yx in the same way. The single chain and the triple chain containing structures have no w0 1r2 1r2x diagonal glide shift. So a layer in which the double Cu–O chain changes into a triple or a single chain induces half unit cell discrepancy along the b-axis. Furthermore, the triple or the single chain have a different unit length along the c-axis from that of the double Cu–O chain structure, so when the double chain changes into another type of chain in a crystal, it introduces lattice distortion. The defect size of this type was almost 4 ; 5 nm, and the strain field around the defect has about the same size of the defect, so the effective defect size was estimated to be 8 ; 10 nm. on the other hand, the diameter of the normal core of the vortex is about 2 j , and the coherence length of YBa 2 Cu 4O 8 in the a–b plane, ja – b Ž0. is reported to be 5 nm at 0 K w9x. Therefore the effective defect size matched well to the diameter of the normal core of the vortex. It is known that YBa 2 Cu 4 O 8 decomposes into YBa 2 Cu 3 O 7yx and CuO above 8508C in air, and YBa 2 Cu 3 O 7yx is more stable than YBa 2 Cu 4 O 8 at temperatures higher than 10008C and oxygen pressure below 2 = 10 7 Pa w1x. Then the single Cu–O chain observed in the unirradiated YBa 2 Cu 4 O 8 w1,6x is believed to be introduced during the synthesis process. The second type, a small area of lattice was twisted, and an area of the b–c plane changes into
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
105
Fig. 1. White spot type defects observed in the a–c plane of YBa 2 Cu 4 O 8 which was neutron-irradiated up to 1.3 = 10 21 nrm2 Ž E ) 0.1 MeV. at room temperature; Ža. and Žb. are taken by different imaging conditions.
Fig. 2. ‘W letter-like zigzag’ defects observed in the b–c plane of unirradiated YBa 2 Cu 4 O 8 .
106
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
an area of the a–c plane. There is a thin 908 rotation twins about the c-axis. The double Cu–O chain expected to be observed as a ‘stair-rod’ like image viewed along the w010x direction Ž a–c plane. and as ‘zigzag’ image viewed along the w100x direction Ž b–c plane.. YBa 2 Cu 4 O 8 contains a w0 1r2 1r2x diagonal glide caused by the double Cu–O chain. Therefore, a change in lattice direction induces a Ž1r2. b mismatch around the double Cu–O chain. These defects mentioned above were described in the previous reports w6x. The third type, double white spots across the double Cu–O chain were observed in the a–c plane projected images. This defect is shown with white circles in Fig. 1a. The image shown in Fig. 1b with white arrows probably corresponds with the same defect shown in Fig. 1a, taking by different imaging conditions. To estimate a defect structure, several models such as elimination of oxygen atoms, which are located on the top of CuO5 pyramids, were
applied to clarify the observed images. Image simulations using a multi-slice simulation program were conducted, but we could not obtain a proper model, which satisfy the observed images. The size of this white spot-type defect observed in the a–c plane was smaller than 1 nm. It is close to the coherence length in the plane along the c-axis Ž j c Ž0. s 0.5 nm w9x.. There is a possibility that the defect stretches along the b-axis. At last, relatively large ‘W letter-like zigzag’ image was observed in the projection onto the b–c plane, as shown in Fig. 2. The Cu–O band width of this defect was the same as that of the triple Cu–O chain mentioned above, but it contains a w0 1r2 1r2x unit shift like the double Cu–O chain structure while the triple chain does not have this shift. A unit cell of this structure extended five times as long as that of YBa 2 Cu 4 O 8 along the b-axis. Fig. 3 shows schematic illustrations of structural models of Ža. the double Cu–O chain structure ŽYBa 2 Cu 4 O 8 ., Žb. the triple
Fig. 3. Structural models of YBCO family projected onto the Ž100. plane. Ža. Up-right: the double Cu–O chain structure ŽYBa 2 Cu 4 O 8 .. Žb. Bottom-right: the triple chain structure ŽYBa 2 Cu 5 O 9 .. Žc. Bottom-left: base model of the ‘W letter-like zigzag’ defect. Žd. Up-left: structural model of the ‘W letter-like zigzag’ defect.
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
107
Fig. 4. HREM images taken along the b-axis after prolonged observation of the YBa 2 Cu 4 O 8 which was not irradiated by neutron. Each of the captions means illumination time of the electron beam; Ža. 10 min, Žb. 82 min, Žc. 157 min.
108
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
Fig. 4 Ž continued ..
Cu–O chain structure ŽYBa 2 Cu 5 O 9 ., Žc. a base structure of the ‘W letter-like zigzag’ defect, and Žd. a structural model of the ‘W letter-like zigzag’ defect. This defect structure includes unusual cation polyhedra which contains two Cu atoms. We think this model represents an averaged structure along the a-axis. In one b–c plane, the CuO polyhedron contains Cu atom at one of the two positions of the large hexagon Žmiddle-filled circle., whereas the next b–c plane, the Cu atom at the center of the polyhedron locates the other position in the hexagon Žmiddleopen circle.. If one oxygen is positioned at an alternated site within the hexagon in one b–c plane, octahedron will be constructed around Cu atom. These two octahedrons may share an edge, so this structure contains insecurity. Furthermore, in this case, the local packing of oxygen atom is too dense. Then, it is supposed that the Cu atom is located in planaly six-coordinated oxygens as shown in Fig. 3c. If atomic coordination changed the model as shown in Fig. 3d, majority of CuO polyhedron changed to five-membered polyhedron, which is closer to fourmembered polyhedron. Image simulations were con-
ducted based on this model, but we could not obtain a proper image, which matches with the observed image satisfactorily. 3.2. Effect of electron-beam irradiation Electron-beam-irradiation effect was observed by long time illumination of the same area by TEM to confirm these defects mentioned above were not induced by electron-beam irradiation during observation. It was reported that low energy electron beam causes thermal effects to make single chain type defect in YBa 2 Cu 4 O 8 w1x. In the contrary, high energy electron beam Ž, 1 MeV. forms an amorphous structure around the Cu–O chain at first and then it extends outwards w10x. Electron irradiation mainly caused sputtering of surface atoms. Weak-bonded atoms in an amorphous-like surface were easily sputtered. Fig. 4 shows high-resolution images after prolonged electron illumination of the a–c plane of YBa 2 Cu 4 O 8 which was not irradiated by neutrons. Comparing the first image ŽFig. 4a, irradiation time: 10 min. with the
M. Akiyoshi et al.r Physica C 338 (2000) 103–109
second image ŽFig. 4b, irradiation time: 82 min., the following changes are observed. The amorphous surface region over the edge of the specimen shrank and almost disappeared by surface sputtering effect. Furthermore, spot-type defects in the a–c plane, the same as in Fig. 1a and b disappeared at an initial stage of the electron irradiation. After long illumination, atomic arrangement around the double Cu–O chain was gradually disordered. In Fig. 4b, structural images of some areas near the double Cu–O chain were clouded, and at the last image ŽFig. 4c, irradiation time: 157 min. structural images were distorted more severely. It indicates that local amorphization is initiated gradually by the 300 keV electron irradiation. But in any case, the triple or single chain structures, the a–c b–c transition, the W-letterlike zigzag structure defects were not created by the electron irradiation under the present conditions.
l
4. Conclusion Neutron-irradiated YBa 2 Cu 4 O 8 superconductor was observed by HREM. Several types of structural defect were observed in the as-synthesized and the neutron-irradiated specimens, and we cannot detect a significant change of the size or the density of these defects by the neutron irradiation. But enhancement of critical current density after neutron irradiation of the specimen means some flux pinning centers are created by the neutron irradiation. Besides, already
109
reported defects Žsuch as the change of the double Cu–O chain into triple or single chains., dot-like spot defects and relatively large ‘W letter-like zigzag’ defects, were observed in the a–c plane and b–c plane, respectively. The structural model corresponding to the latter defect was proposed. Furthermore, it was clarified that electron irradiation during observation induced mainly sputtering of surface atoms, and defects associated with the Cu–O chain were not easily created by the 300 keV electron irradiation.
References w1x K. Yamaguchi, T. Miyatake, T. Takata, S. Gotoh, N. Koshizuka, S. Tanaka, Jpn. J. Appl. Phys. 28 Ž1989. 1942. w2x D.E. Morris, A.G. Markelz, B. Fayn, J.H. Nickel, Physica C 168 Ž1990. 153. w3x Y. Yamada, J.D. Jorgensen, S. Pei, P. Lightfoot, Y. Kodama, T. Matsumoto, F. Izumi, Physica C 173 Ž1991. 185. w4x A. Wisniewski, G. Brandstatter, C. Czurda, H.W. Weber, A. ´ ¨ Morawski, T. Lada, Physica C 220 Ž1994. 181. w5x T. Yano, M. Pekala, A.Q. He, A. Wisniewski, M.L. Jenkins, ´ Physica C 247 Ž1995. 55. w6x K. Hashimoto, M. Akiyoshi, A. Wisniewski, M.L. Jenkins, ´ Y. Toda, T. Yano, Physica C 269 Ž1996. 139. w7x Y. Zhu, H. Zhang, M. Suenaga, D.O. Welch, Philos. Mag. A 68 Ž1993. 1079. w8x Y. Zhu, Z.X. Cai, R.C. Budhani, M. Suenaga, D.O. Welch, Phys. Rev. B 48 Ž1993. 6436. w9x J.C. Martinez, O. Laborde, J.J. Prejean, C. Chappert, J.P. Renard, J. Karpinski, E. Kaldis, Europhys. Lett. 14 Ž1991. 693. w10x Y. Matsui, K. Yanagisawa, Jpn. J. Appl. Phys. 31 Ž1992. 29.