HREM studies of stacking faults in HoBa2Cu3O7-δ superconductors

Physica C 175 ( 1991 ) 651-666 North-Holland

HREM studies of stacking faults in HoBa2fu307_6 superconductors Y. Yan 1 and M.G. Blanchin D~partement de Physique des Mat~riaux (UA CNRS no. 172), Universit~ Claude Bernard, 69622 Villeurbanne Cedex, France

A. Wicker ALCATEL-ALSTHOM Recherche, Route de Nozay, 91460 Marcoussis, France

Received 5 September 1990 Revised manuscript received 18 December 1990

High resolution transmission electron microscopy (HREM) has been used to reveal directly the structure of the metal lattice and its defects in HoBa2Cu3OT_6high-To superconductors. Columns of Ho, Ba and Cu atoms can be discriminated by their differences in image contrast depending on the atomic scattering factors, while the columns Ofoxygen atoms and voids appear as white dots ("tunnels"). Twins have been observed as well as several types of extrinsic planar faults which have been studied through comparison between the experimental HREM images and different structural models. Besides the displacement vectors R= 1/6 [0,0,1 ], R-- 1/6 [3,0,1 ], R= 1/6 [0,3,1 ], proposed previously for translation faults in YBazCu3OT_6materials, a new fault model having a displacement vector R= I/6 [ 3,3,1 ] is discussed here. Other new structural models have been constructed by adding a rotation operation to the models mentioned above, which correspond to permutation planar faults. The transformation from mirror to glide character along such faults, as well as the dislocations produced by the difference between lattice parameters a and b across the interfaces which have been observed on the HREM micrographs, confirm the structure of the permutation faults proposed in the present work.

1. Introduction Since superconductivity above liquid nitrogen temperature was discovered by Wu et al. [ 1 - 3 ] , structural studies of high-T¢ superconductor ceramics have r e m a i n e d an active research field. One of the most i m p o r t a n t parameters for applications of the high-To superconductors is their critical current density (J~) in an applied magnetic field. In sintered ceramics, the current transport is usually limited by the Josephson-coupling between adjacent grains leading to generally rather low " i n t e r g r a i n " critical current densities. All the physical properties of the orthorhombic superconductor compounds RlaECU307 _ 6 (with rare-earth elements R -- Y, Ho... ) are extremely anisotropic. The mechanical properties of these materials, which are poor, are also dep e n d e n t on the density of the structural defects a n d On leave from Laboratory of Solid State Microstructures, Nanjing University, China.

on the structure of the grain boundaries. Therefore, an investigation of the microstructure of these materials appears to be necessary to achieve an improved u n d e r s t a n d i n g of their superconducting properties. In the present work, we report high resolution transmission electron microscopy ( H R E M ) studies of HOBaECU307_a superconductors, which proved to have high Jc values at low temperatures a n d fields in terms of the Bean model [4]. The HoBaECU307 high-To superconductors have an orthorhombic structure (depicted in fig. 1 ) with lattice parameters a - - 0 . 3 8 1 9 nm, b - - 0 . 3 8 9 3 n m a n d c = 1.1652 n m at room temperature [ 5 ], corresponding to an oxygen deficient perovskite with Ho a n d Ba cations ordered over A sites of the ABO3 structure in the sequence B a - H o - B a - B a - H o - B a . Two kinds of oxygen vacancies are ordered in the perfect HoBa2Cu307 structure: one (hereafter referred to as V 1-type) is located at 0, 0, 1/2 in the Ho planes and another one (hereafter referred to as V2-type) is located at 1/2, 0,0 in the Cu( 1 ) planes, which leaves

0921-4534/91/$ 03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

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Y. Yan et aL / H R E M studies of stacking faults in H o B a 2 C u 3 O z_a

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large tunnels along the [010] direction (as shown in fig. l ( c ) ) . The structural p a r a m e t e r s for HoBa2Cu307, as d e t e r m i n e d by neutron diffraction at r o o m temperature, are given in table 1 [ 5 ]. T h e i m p o r t a n t microstructural features within the grains o f these ceramic materials are twin b o u n d a ries ( T B ) , antiphase b o u n d a r i e s ( A P B ) a n d stacking faults. The a t o m i c structure o f these defects a n d the m e c h a n i s m s o f their f o r m a t i o n are expected to shed light on the relationship between the microstructure a n d the superconducting properties. T E M studies can be used to verify the essential defect features from where geometrical modelling can be done. In the isostructural superconductors o f the Y - B a -

C u - O system, insertion o f an Y layer or o f a CuO layer at the position o f the CuO layer ( z = 0 ) was rep o r t e d [ 6 ], as well as defects at the grain b o u n d a r i e s corresponding to a lattice p a r a m e t e r o f 1.38 n m instead o f the 1.16 n m value in the u n d e f o r m e d structure [7]. On the other h a n d the existence o f antiphase boundaries with a c / 3 displacement vector [ 8 10 ], a n d o f ( C u O ) double layers [ 10-14 ] have been reported. In this work, we focused our attention on the H R E M study o f different types o f p l a n a r defects lying in the c-planes o f HoBa2Cu307_a crystals. The structure and mechanism o f formation o f these planar faults will be discussed through c o m p a r i s o n o f experimental images with different structural models.

Table 1 The atomic coordinates and temperature factor parameters B (~,) for HoBa2Cu3OT, where B = x2( u 2), and u 2 is the mean square atomic displacement in ~2

X Y z B

Ba

Ho

Cu(1)

Cu(2)

O(1)

0(2)

0(3)

0(4)

1/2 1/2 0.1844 0.48

1/2 1/2 1/2 0.35

0 0 0 0.46

0 0 0.3559 0.22

0 0 0.1588 0.61

1/2 0 0.3779 0.29

0 1/2 0.3788 0.36

0 1/2 0 0.8

Y. Yan et aL / HREM studies of stacking faults in Ho Ba 2Cu s 0 7_6

2. Experimental procedure High-To HoBa2Cu307_z superconductor ceramic samples were produced in the Laboratoires ALCAT E L - A L S T H O M Recherche, Marcoussis, France. The samples were made from powder synthesized by a solid-state reaction at 950°C; cylinders ( ~ _~ 4.5 mm, 1 _~ 40 m m ) were shaped by isostatic pressing and sintered at 920°C for 3 h under pure oxygen; susceptibility measurements gave a Tc of around 86 K. The specimens for electron microscopy were prepared by the standard technique for ceramic oxides. A suspension of the crystals in alcohol was ground with a pestle in an agate mortar at room temperature. Crystals were recovered from the suspension on a porous carbon film. Ion milling cannot be used to prepare the specimens for electron microscopy due to the heavy irradiation damage introduced by the ion beams. HREM observations were performed using a JEOL 200CX-UHP2S microscope at 200 kV with a top entry stage. The objective lens spherical aberration constant was determined as Cs = 0.89 mm. The simulation of the HREM images was performed

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on a microcomputer using a program based on the multislice theory, as reported in detail previously [15,16].

3. Results 3.1. Overview o f the microstructure o f the m a t e r i a l

Figure 2 is a HREM micrograph along the a- or the b-axis, which reveals three states in the material: the ordered state (in the left area), the amorphous state (in the central part) and the intermediate ordered state (for instance, the area including A and A' or B and B ' ) , For several reasons the HREM images along the a or b direction of the ordered structure cannot be distinguished without difficulty [ 6,15 ]. So unless stated otherwise, we do not differentiate between the [ 100] and the [010] direction in this paper. Despite that difficulty, it will be shown hereafter that HREM imaging provides valuable information on the structure of the crystalline state and the defects that it may contain (see fig. 2).

Fig. 2. HREM micrographalong the a- or b-axisshowingthe crystalline, amorphous and intermediate-orderedstate in the specimen;the slices A, A', B, and B' have the c-axisin common.

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Y. Yan et al. / H R E M studies of stacking faults in HoBa2Cu~07_6

Fig. 3. (a) High-magnification H R E M micrograph of a perfectly ordered region, observed along the a- or b-axis. (b) Optical diffraction pattern from the negative, which allows one to check the defocus value Af - - 50 nm. (c) Simulated HREM image calculated for a crystal thickness t ~ 2 n m and Af= - 50 nm, showing good resemblance with the experimental image in (a).

Figure 3(a) is a [100] or [010] atomic structure image obtained from a region of ordered crystal sufficiently thin near the edge of the specimen. The image was taken close to the Scherzer defocus condition, as shown by the optical diffractogram (fig. 3(h) ), and can be expected to represent faithfully the projected potential of the structure within a good approximation. Image contrast simulations in the [ 100] and [010] projections (see lrtg. 3(c) ) showed that such HREM micrographs from our specimen can be interpreted as the projections of columns of a single species of metal atoms, while the columns of oxygen atoms and voids appear as white dot "tunnels" [ 16 ]. The rows of the brightest spots sandwiched by Ba atoms correspond to the V2-type oxygen vacancies for a [010] projection or occupied oxygen columns for a [ 100] projection; the dots arising from the Vl-type oxygen vacancy columns in Ho planes are seen in fig. 3(a) as brighter than those arising from occupied oxygen positions. Detailed inspection reveals that the distance between Ba-Ba atoms along the c-axis is longer than that between Ba-Ho (or H o -

Ba ) atoms and the distance between Cu ( 2 ) - C u ( 2 ) atoms on both sides of the Ho planes is shorter than that between Cu(1 ) - C u ( 2 ) across the Ba planes. These results agree with the data drawn from neutron diffraction studies [ 5 ]. The orthorhombic phase of the Y - B a - C u - O system was reported to be heavily twinned [ 6,17-19 ]. The same phenomenon was observed for the H o - B a Cu-O system [ 20 ], The HREM image of a ( 110)" twin boundary, viewed along the [ 001 ] direction, is reproduced in fig. 4 with the electron diffraction pattern inset. Measurement of the distance between double spots on the diffraction pattern allows to determine that the (100) and (010) planes are kinked by approximately 1 ° and the (110) planes by about 1.8 ° across the (110) twin plane; this is consistent with the fractional difference between the lattice parameters a and b. Studies of (110) twin boundaries in HoBa2Cu307_6 superconductors were reported previously [20]; but such HREM observations of twin boundaries on (110) planes (fig. 4), together with the structure images of fig. 3, confirm the or-

Y. Yan et al. /HREM studies of stacking faults in HoBazCu~07_6

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Fig. 4. HREM image of a twin boundary as viewed along the [001 ] zone axis. The inset is the diffraction pattern from the area across the twin boundary. thorhombic structure of our specimens.

served

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in the isostructural [10-12,14] and in a YEBa4CUTO~4+x specimen prepared at high pressure [ 13 ]. Analysis of the corresponding HREM micrographs through comparison with simulated images allowed to confirm the intercalation of CuO layers. However, as demonstrated hereafter, identical features observed in two-dimensional projections can possibly result from different three-dimensional defects. In order to interpret the experimental images, different geometrical models were constructed. Figure 6 reproduces the model and the structural projections along different directions of a first type of defect on the c-plane. The structure in one part (crystal I) of the crystal can be derived from that of the other part (crystal II) of the same crystal by means of a translation parallel to the c direction. The two parts exhibit a mirror symmetry with respect to the midplane between the two Cu ( 1 ) layers. Such a defect is obtained if a slab of Cu( I ) plane with thickness t = c / 6 is inserted within the perfect structure nearby the Cu( 1 ) plane at the top of the crystal I. The interface can be characterized by a constant

YBa2Cu307_a compounds

3.2. E x t e n d e d defects on (001) planes

Figure 5(a) is a low magnification electron micrograph of planar faults on (001) planes. High densities of such defects were found locally in the specimen, like in regions D, D' and D". Figure 5(b) is a HREM image along the a- or b-axis from region D of fig. 5 (a) where two types of defects are observed. One (arrowed M) shows mirror symmetry whereas another one (arrowed APB) shows glide symmetry with a shift (in projection) 1/2 [ 100] or 1/2[010]. As mentioned above, the brightest dots in the HREM structure images (taken at, or nearly at, Scherzer defocus) of the ordered crystals correspond to the V2type oxygen vacancy columns, along the b-axis, or to the occupied oxygen columns, along the a-axis, in the Cu ( 1 ) planes. So regarding the image contrast, the planar defects may be thought to consist of a pair of Cu( 1 ) planes, and thus the additional material is expected to have the composition CuO. Extended planar defects exhibiting similar features were ob-

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Y. Yan et al. / HREM studies of stacking faults in HoBa2Cu3Oz_a

Fig. 5. (a) High density of planar faults in a region of the specimen observed at low magnification. (b) HREM micrograph from the region D of (a) viewed along the a- or b-axis at high magnification.

displacement vector R ~ = I / 6 [0,0,1]. Since both projections along the a- and the b-axes exhibit mirror symmetry in this case (figs. 6 ( b ) and 6 ( c ) ) , we label this model MM. A rather similar model was proposed by other authors, but with a different displacement vector R~ = 1 / 3 [ 0,0,1 ] [ 8-10 ]. Measurements on our experimental micrographs (like fig. 5 ( b ) ) show that the distance between the two Cu( 1 )

planes across the interface is 0.19 nm, i.e. c / 6 , as assumed for our model MM. Other displacement vectors have to be considered, which correspond to models discussed by Zandbergen et al. [ I l ] and Matsui et al. [ 12 ]. The first model is MA in which a displacement component along the a-axis is added (R2= 1/6 [3,0,1 ] ), as shown in fig. 7. These defects still exhibit mirror symmetry when

Y. Yan et al. / H R E M studies of stacking faults in HoBa2Cu307_6

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projected along the [ 100] direction, but the interface looks like an antiphase boundary when viewed along the [010] direction, as do the faults marked APB in fig. 5(b). Like the model MM, the inserted

slab has the chemical composition CuO. In the same way, if one part of the crystal is derived from the other part of the same crystal by means of a displacement vector R3 = 1/6 [ 0,3,1 ] having a displace-

Y. Yan et al. / H R E M studies of stacking faults in HoBa2Cu307_6

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ment component along the b-axis, an interface is obtained which appears as an APB when seen along the [ 100 ] direction, but exhibits mirror symmetry when viewed along the [ 010 ] direction. This corresponds

to the model AM (fig. 8). As mentioned above, it is difficult to achieve a reliable distinction between [ 100 ] and [ 010 ] HREM images. Thus in a first step both model MA and AM could be used to interpret

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Y. Yah et al. / H R E M studies o f stacking faults in HoBazCu307_6

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the features of the experimental images like fig. 5 (b). Besides the models mentioned above, a new model can be considered where the displacement vector is R4= 1/6[,3,3,1 ]. The structural model and its pro-

jections along [ 100], [010] and [ 110] are drawn in fig. 9. Since the interface shows antiphase character in both [100] and [100] projections, we name this model AA.

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Y. Yan et al. / HREM studies o f stacking faults in HoBaeCu~07_6

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Clearly it is impossible to assess the structure of stacking faults from the projection along one direction only, since we cannot distinguish between the HREM images along the a- or the b-axis. So HREM

images along [ 110] (fig. 10) w e r e examined for further study of these defects. Although no atomic structure image can be achieved along this direction, due to the limited resolution of the 200 kV electron

Y. Yan et at./HREMstudies of stacking faults in HoBa2Cu~Oz_6

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Fig. 10. High magnificationHREM imagesof (a) MA or AM faults, and (b) AA or MM faults as viewedalong the [ 110] direction.

microscope used for the observation, the lattice fringes in fig. 10(a) reveal a large amount of planar faults exhibiting lattice shift (arrowed in fig. 10(a) ). The distance measured between B and C across such an interface is about 13/12 times that between A and

B in the perfect crystal, like in the images along the [100] or [010] direction (fig. 5 ( b ) ) , and in agreement with the value obtained from the models MM, AM, MA, and AA. According to the projections of the different structural models along the [ 110 ] di-

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Y. Yan et al. / H R E M studies o f stacking faults in HoBa2Cu~07_6

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rection (figs. 6-9), it is quite clear that only the models MA (fig. 7) or AM (fig. 8) can match with the features seen in fig. 10(a). Interfaces exhibiting mirror symmetry in the [ 110] projection were occasionally observed, like M in fig. 10(b), which can

possibly correspond to the projection of models MM (fig. 6) or AA (fig. 9). But most of the faults experimentally observed along [ 110 ] looked like APB, as seen in fig. 10(a).

Y. Yan et al. / HREM studies of stacking faults in HoBaeCu307_~ Table 2 Relationshipsbetweendifferenttypes of permutation planar faults depending on their position in real space x, y

x+ 25a, y

x, y+25

x+ 25a, y+25b

PMM PMA PAM PAA

PMA PMM PAA PAM

PAM PAA PMM PMA

PAA PAM PMA PMM

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3.3. Interaction with permutation twinning A series of new models can be created by adding a rotation operation to the models constructed above. For example, if part I of the crystal in model M M is derived from the other part, II, by m e a n s of a translation 1/6 [ 0,0,1 ] a n d then a rotation o f 90 ° a r o u n d the c-axis, i.e. a- a n d b-axes of the d o m a i n s o n either side of the fault are interchanged whilst c is corn-

Fig. 12. HREM micrograph of a permutation extrinsic planar fault observed (a) along the [ 100] or [010] direction and (b) along the [ 110] direction.

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Y. Yan et al. / HREM studies of stacking faults in HoBa2Cu307_~

men, the model PMM is obtained (fig. 11 ( a ) ) . The latter operation corresponds to the "permutation" (or "rotation") twinning which was observed in the Y - B a - C u - O system [ 15,20-22]. In a similar way, the permutation twin models PMA, PAM, and PAA can be derived from the models MA, AM, and AA by adding the same rotation operation (figs. 11 (bd)). Because of the difference of about 2% between the lattice parameters a and b, there will be a lattice shift, seen along the c direction, across the permutation twin boundaries (models PMM, PAM, PMA and PAA). A displacement vector r = 1/2[100] or 1/2 [010] will be created at regular intervals of 25a or 25b: in this way one type of permutation planar fault can be transformed into another one. For example, a permutation twin boundary corresponding to the model PMM at the point (x, y, z) will be transformed into the model PMA at ( x + 2 5 a , y, z)~ In a similar way PMM would be transformed into PAM at the point (x, y+25b, z), and into PAA at the point ( x + 2 5 a , y + 2 5 b , z ) . (The coordinate system here belongs to, say, part I of the crystal in real space.) The possible transformation relationships for the permutation planar faults PMM, PMA, PAM, and PAA are listed in table 2. The experimental observations reveal a large density of permutation planar faults in oar specimens. A HREM image of such a fault viewed along the aor the b- axis is reproduced in fig. 12(a), where the lattice shift across the interface and the transformation from mirror to antiphase character along the interface are clearly seen. A dislocation produced by the difference between lattice parameters a and b is thus observed (circled in fig. 12(a)). The structure of the permutation planar faults was also confirmed by HREM images along the [110] direction, like fig. 12(b), where regions exhibiting mirror symmetry are marked MB, and regions exhibiting antiphase character are marked APB (seen along the c direction, lattice shifts across the APB interfaces are clearly visible). With respect to figs. 6-9, it is quite clear that the regions M B i n fig. 12(b) correspond to the projection of models PMM or PAA, and the regions APB in fig. 12(b) correspond to that of model PAM or PMA. The HREM image also shows the repeat sequence MB-APB-MB--APB-MB along some interfaces, which corresponds to the

transformation from one type of permutation planar faults to another one in agreement with the relationships of table 2.

4. Discussion

Like the isostructural YBa2Cu307_,~ compounds, the present HoBa2Cu307_ 6 specimens exhibited a large density of extended defects lying in the c-planes. With respect to the normal stacking sequence along the c-axis, these extrinsic faults correspond to the intercalation of a CuO double layer. The presence of these extended faults confirms the highly anisotropic character of the present HoBa2Cu3OT_~ materials which seem to behave more or less like "layer structures" parallel to {001 }. Such an assumption is supported by the features of the HREM micrograph of fig. 2, which apparently documents the contrast in a small region during crystallographic growth. In the left part of fig. 2, slices of crystal can be seen which are parallel to {001 } planes. They exhibit a rather smooth surface in the Cu( 1 ) planes. Such images suggest that the crystallization rate should be higher parallel to the (001} planes than perpendicular to them. In other words, the two-dimensional ordered layer structures can be nucleated within the {001} planes and then get linked together to form the threedimensional blocks of crystalline structure. The formation of the extrinsic planar faults naturally follows from this crystallographic growth process. Depending on the relative orientation of the crystal slices on either side of the Cu( 1 ) planes, different types of planar defects will be formed when the layer structures meet at the interfaces. The HREM image of fig. 2, when seen along the c direction, reveals a displacement vector R = 1/2 [ 1,0,0 ] or 1/2 [ 0,1,0 ] between the slices A and A': thus an antiphase interface may be created when they meet. On the other hand no displacement is observed between the slices B and B': a mirror fault may be created when they meet. If the orientation of the a- and the b-axis at the beginning of the nucleation process is not common to both crystal slices, a permutation fault will be created when crystal layers meet, which have the c-axis in common whilst a- and b-axes are interchanged. According to the structural models considered above, the distances between atoms of the same na-

Y. Yan et al. / H R E M studies of stacking faults in HoBa2Cu307_6

665

Fig. 13. HREM micrographfrom a crystal region (observed along the a- or b-axis) suggestingthe coexistence of AM and MA extrinsic faults. ture Cu( 1 ) - C u ( 1 )' and O ( 4 ) - O ( 4 ) ' across the MM interface are shorter than those between anions and cations, like Cu( 1 ) - O ( 4 ) ' or C u ( l ) ' - O ( 4 ) : thus strong anion-anion and cation-cation repulsive forces can be expected which make MM mirror boundaries energetically less stable than glide boundaries such as AM, MA, or AA. Nevertheless some HREM micrographs, like the [ 110] projection of fig. 10 (b), revealed interfaces exhibiting no lattice shift (i.e. no permutation twinning) over much more than 50 lattice spacings along the interface; this suggests that MM or AA faults can form in these crystals. Any difference in the H R E M contrast features between the same projections of both defects should be hardly detectable on the micrographs, even at 400 kV. It can be expected that the AA configuration is energetically more favourable than the MM one, especially with possible rearrangement of the oxygen atoms in the CuO layers. However, few faults of these types were found in the present specimens. Clearly a much larger density of AM- or MA-type faults was observed (see fig. 10(a), for instance). The displacement vector R = 1/6 [ 0,3,1 ], having the

b / 2 component, could be considered as the most "likely from a crystallographic point of view. In fact it is effectively involved in the formation of YBa2Cu408 and Y2Ba4Cu70~5+x phases [20,24]. Nevertheless, features of some HREM micrographs, like fig. 13, can be hardly interpreted without the possible coexistence of both displacement vectors Rl = 1/613,0,1 ] and R2= 1/610,3,1 ]. In fig. 13 for instance, the MB fault appears to have a pure mirror character, over more than 25 lattice spacings along the fault, which means that no permutation twinning occurs. Consequently the parts of the crystal on either side of the MB fault are seen along the same (a or b) direction of the structure and thus both faults having a mirror character and faults having a glide character are found in the same crystal region for the same projection of the structure. With respect to figs. 7 and 8, this suggests that faults having the R~ = 1/6 [3,0,1] displacement vector may exist very locally, provided that some rearrangement of the oxygen atoms (in the positions arrowed in fig. 7 ( a ) ) take place, as pointed out by Zandbergen et al. [ 11 ].

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5. Conclusion

The H R E M contrast features of extrinsic planar faults on c-planes have been systematically studied in the present HoBa2CuaO7_a superconductors having the o r t h o r h o m b i c structure. C o m p a r i s o n between the experimental a n d the simulated H R E M images of the perfect structure indicates that the chemical composition of the slab inserted is CuO. With respect to symmetry operations, these faults can be divided into two groups: the translation faults a n d the p e r m u t a t i o n faults. Besides the translation faults MM, MA a n d AM having displacement vector R = 1/610,0,1], R = 1 / 6 1 3 , 0 , 1 ] , R = 1 / 6 [ 0 , 3 , 1 ] respectively, proposed previously for YBa2Cu307_6 materials, a new fault model AA with a displacement vector R = 1 / 6 1 3 , 3 , 1 ] has been proposed here. H R E M micrographs o b t a i n e d along the [ 100] or [ 010 ] as well as the [ 110 ] direction confirm that M M or AA faults may have formed locally in our specimens, but the M M fault should be energetically less stable. The H R E M observations also suggest that a few MA-type faults could exist in the specimens, implying rearrangement of the oxygen atoms in the CuO layers. New structural models PMM, PMA, PAM, a n d PAA have been constructed by adding a rotation operation to the models m e n t i o n e d above, which correspond to p e r m u t a t i o n planar faults. The transformation from mirror to glide character along the corresponding interfaces, as well as the dislocations produced by the difference between lattice parameters a a n d b across the faults, which are observed on the micrographs, confirm the structure of the p e r m u t a t i o n faults proposed in the present work. Nevertheless details of the quite complicated crystallographic growth m e c h a n i s m in the specimens have to be investigated further. Systematic studies of the nature of the structural defects observed in various samples of the H o - B a - C u - O system are presently u n d e r way, related to variations in the parameters controlling the elaboration process. The influence of such defects on superconducting properties is also u n d e r study. The results will be reported in the near future. Acknowledgement

We wish to t h a n k Dr. B. Poumellec (Laboratoire des Compos6s Non-Stoechiom6triques, Universit6

Paris-Sud, Orsay, F r a n c e ) a n d Dr. P. Dubois (ALCATEL-ALSTHOM Recherche, Marcoussis, France) for stimulating discussions on the subject. References

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