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Polytypoidic structures in high temperature oxide superconductors
POL~POIDIC
June 1990
MATERIALS LETTERS
Volume 9, number 10
SQUIRES
IN HIGH TEMP~~TURE
OXIDE SUPERCONDU~ORS
R. RAMESH, D.M. HWANG, T.S. RAVI, C.C. CHANG, A. INAM, X.D. WU, T. VENKATESAN, J.M. TARASCON Bellcore, Red Bank, NJ 07701, USA
and S.M. GREEN Centerfor Superconductivity Research, Department ofPhysics and Astronomy, University of Maryland, College Park, MD 20752. USA
Received 1March 1990
High resolution transmission electron microscopy has been used to reveal the cationic structure of the high temperature oxide superconductors. Of the many new systems discovered recently, the Bi-Sr-Ca-G-0, Tl-Ba-Ca-Cu-0 and Y-Ba-Cu-0 systems have been extensively studied. Compositional variations are accommodated by the formation of polytypoidic stacking units whose cationic stoichiomet~es are in specific fixed ratios. Exampfes from the Bi cuprate bulk superconductors and Y-Ba-Cu-0 thin films are presented. Identification of the different polytypoids, each of which has a specific transition temperature, r,, can shed light on the electrical properties of the mixed bulk or thin films. Polytypism and polytypoidism also provide a unified approach to the structural and chemical analysis of these oxide superconductors.
1. In~~uction
Many naturally occurring complex minerals are layered structures. One of the most fascinating aspects of such layered structures is the frequent occurrence of variations in the.crystal structure with or without a change in composition. When the change in crystal structure occurs without any change in composition, such as in Sic or in many chalcogenides, it is known as polytypism, e.g., Sic, chalcogenides. On the other hand, when the change in crystal structure (or stacking sequence of different Iayers) occurs due to a change in composition, it is known as polytypoidism. A large variety of natural as well as man made materials exhibit polytypoidism. Typical examples include Al-O-N [ 1 ] and other oxynitride ceramics and complex minerals such as mica [ 2,3 1, etc. Several techniques have been used to understand the structure of these complex materials [ 31. However, high resolution transmission electron micros-
copy (HREM) at resolutions better than 2 8, (i.e. ~2 A) is particularly useful in this respect, since it enables the direct observation of the cationic stacking sequence and defects in the structure, as well as enabling the characterization of local variations in structure and composition. For example, in many of these materials, complex periodic or quasi-periodic structures have been known to form [ 41. These ordered structures can have either long range structural coherence or can be localized. Ordered structures that do not exist over a significant volume of the sample are likely not to be detected by X-ray diffraction or neutron diffraction techniques. However, the high spatial resolution in HREM enables the detection and characterization of such local ordered structures. When carried out in conjunction with detailed image simulations and image processing, HREM can provide si~i~c~t info~ation about the structure and composition of the material, that cannot be obtained by any other technique. The discovery of high temperature superconduc-
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MATERIALS LETTERS
tivity in certain classes of oxides [ 6-8 1, mainly cuprates, has opened up a new area of materials physics and many potential high technology applications. Since these cuprates are layered structures, HREM studies have proven to be of great use in understanding their structure, chemistry and defect microstructure. These superconductors exhibit polytypoidism, with each polytypoid having a different superconducting transition temperature, T,.The Bi- and Tlbased cuprates typify this behaviour. These cuprates have the general composition of BizSr2Cu,Ca,_ ,O,, or T1,BazCu,Ca,_ i0, where n= 1, 2, 3 and in some cases 4, and x= 1 or 2. In the Bi cuprate system, the n = 1 phase has a T,in the range of 7-22 K, the n = 2 phaseaT,of85Kandthen=3phaseaT,ofllO K. The structures of these phases are illustrated schematically in fig. 1. In the Y-Ba-Cu-0 system, there are at least three superconducting polytypoids, with
the possibility for the existence of a more extensive range of either stable or metastable polytypoidic phases. Some of the possible polytypoidic variants in the Y-Ba-Cu-0 system are shown in fig. 2. In this paper, results of HREM studies of the polytypoidic microstructures in bulk Bi cuprates and in Y-Ba-Cu-0 thin films are described.
2. Experimental Appropriate amounts of Biz03 (99.9%), (99.999%), SrC03 (99.9%), or SrOz (99.5%, overnight at 12O”C), CaCO, (99.5%) or (99.5%, dried at 200°C) and CuO (99.5%) ground together until no visual evidence of mogeneities was observed. After calcining the ders overnight at 800°C the samples were
Fig. 1. Schematic illustrations of the crystal structures of the n = I,2 and 3 polytypoids in the Bi cuprate system.
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PbO dried CaO were inhopowthor-
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June 1990
exposure to the electron beam to minimize the beam induced damage. The interpretation of the experimental images was verified by carrying out simulations using the MacTempas@ #’ software.
3. Results and discussion
YBa2Cu307
YBa2Cu408
Y2Ba2Cu409
‘123’
- 124’ or ‘248’
‘224’
Fig. 2. Schematic illustrations of the “l-2-3”, “2-4-8” and “2-24” phases, that are poiytypoidi~ variants in the Y-Ba-Cu-0 system.
oughly ground and pressed into pellets and sintered at 850°C for 60 h, with one intermediate grinding [ 9 1. The samples were fired in air and were furnace cooled to room temperature. Resistivity-temperature plots were measured on bars approximately 10x 2 x 1 mm3 from room temperature down to 77 K in a liquid nitrogen thermos. Contacts consisted of spring-loaded Rh-plated steel pogos (Augat-pylon) pressed onto silver paint pads in the standard four-point con~guration. About i mA rms currrent was supplied at 40 Hz with the sample’s voltage drop being detected by a lock-in amplifier. The zero resistance temperature was defined as the temperature below which the amplifier could not detect a signal. The detection limit was typically 0.5 @2 cm. Details of the thin film laser deposition process have already been reported elsewhere [ lo]. Bi cuprate samples for transmission electron microscopy were prepared by mechanically grinding to about 50 urn thickness followed by argon ion milling to electron transparency in a liquid nitrogen holder at 6 kV. For the thin films, cross-section samples were prepared and were ion milled for a few minutes prior to examination so as to reduce the degradation due to moisture in the atmosphere. High resolution electron microscopy (HREM) was carried out in the Berkeley Atomic Resolution Microscope (ARM) at a point-to-point resolution of 1.6 A. HREM images were obtained under conditions close to Scherzer defocus. Images were obtained within a few minutes of
In the Bi cuprates, the best imaging conditions are obtained in the [ 1lo] zone axis o~entation, in which the different cationic layers appear as columns [ 111. Fig. 3 shows one typical HREM image of the n = 2 structure in this zone axis orientation. The interpretation of this HREM image in terms of the projection of the different cationic columns is verified by carrying out detailed image simulations. In fig. 4a the projected potential for this structure in the [I lo] zone axis is shown. The corresponding simulated image for a foil thickness of 30 A and objective lens defocus of - 550 A is shown in fig. 4b and confirms the inte~retation of the experimental image in fig. 3. The n = 3 compound contains three CuOZ layers, as is illustrated in the [ 11O] zone axis atomic structure image in figs. 5a and 1. Fig. 5a, along with fig. 5b, illustrates the beneficial role of image processing in improving the quality of the information available in the image. This is especially true in the case of these oxides. which undergo electron beam induced damage. In fig. 5a, the clarity of the experimental image is reduced due to the amorphous background. By carrying out the Fourier transform and filtering available in the SEMPER@#’ program, the image in fig. 5b is obtained. The amorphous background in this image is considerably reduced compared to that in fig. 5a. Such image processing techniques are extremely valuable when the periodic part of the information in the image is less pronounced compared to the random part. The n = 4 structure in the Bi cuprates is only observed as isolated stacking defects and attempts to synthesize this phase in bulk form have so far been unsuccessful. The formation and microstructural distribution of ” MacTempas@ is a program for the simulation of HRTEM images and is a registered trademark of R. Kilaas, c/o Total Resolution, Berkeley, CA. ” SEMPER@ is a program for the processing of experimental HRTEM images.
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Fig. 3. Atomic re: :,&ion image in the [ 1 lo] zone axis o~entation of the n=2 polytypoid illust~ting the different cationic layers.
Fig. 4. (a) Projected potential for the n=2 structure in the [ 1lo] zone. (b) Simulated image for a foil thickness of 30 A and objective lens defocus of - 550 A.
such polytypoidic structures is crucial in determining the transport properties of the superconductor [ 12 1. Since each polytypoid has differing electronic properties (i.e. superconducting with different transition temperatures, insulating, semiconducting, etc. ) their spatial distribution in the microst~cture is important in understanding the origin of the superconducting properties, such as the critical current densities. Since the experimental conditions required to form each of these polytypoids are different from one another, it is essential to carry out systematic studies 360
of the effect of the relevant processing parameters on the transport properties and the microstructure. Such studies have been carried out in the case of the sintered Bi cuprates. In these materials, the growth of lower T, polytypoids at the grain boundaries of the n = 3 grains, fig. 6, causes the occurrence of a knee in the resistive transition at I 10 K. The formation of the n = 2 or n = 1 polytypoids at the grain boundaries shows a systematic dependence on the starting composition. For example, partial substitution of Bi by Pb is helpful in avoiding this. The
Volume 9, number 10
MATERIALS LETTERS
June 1990
Fig. 6. Lattice fringe image of the grain boundary region in the unleaded Bi cuprate superconductor, showing the formation of the n=2 phase (indicated by arrows) adjacent to the grain boundaries due to the segregation of Bi to the grain boundary regions.
boundaries, leading to the formation of the Bi-rich lower 7+,polytypoids adjacent to the grain boundaries. One reason for this could be the possible enrichment of the grain boundary regions with oxygen, due to the higher diffusivity along grain boundaries. In the case of sintered Y-Ba-Cu-0 superconductors, such a polytypoidic growth adjacent to the grain boundaries has not been observed. This may explain why it is easier to obtain zero resistance in the YBa-Cu-0 ceramics without a low temperature knee in the resistive transition. Fig. 5. (a) Unprocessed atomic resolution image of the n= 3 polytypoid in the Bi cuprate system. (b) Same image as in (a), but SEMPER processed to reduce the background amorphous noise. Note the increase in clarity of the image in (b) over that in (a).
starting composition also has a distinct effect on the transport properties and the microstructure. For the synthesis of the n = 3 compound, it is found that Ca enrichment at the expense of Bi is beneficial. On the other hand, when the alloy is B&rich and Ca-deficient, the lower T, poIytypoids have been consistently observed at the grain boundaries, even when lead is added. This suggests that there is a fundamental tendency for Bi to segregate to the grain
3.1. Thin films It is clear that the earliest applications of these materials are likely to be in the area of thin film microelectronics. Hence, although there is much to be learned about the materials science of bulk superconductors, it is essential to understand the microstructure, defects and the processing issues involved in thin film materials. Among several processing techniques for thin film deposition, it has been found that laser deposition using a pulsed excimer laser provides excellent quality films. Films deposited under optimum conditions exhibit a superconducting
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transition temperature in the range of 88-9 1 K, with a transition width of less than 1 K [ 13 1. Thin films deposited on perovskite substrates such as LaAIOJ, possess critical currents, J,, in zero magnetic fields, of better than 5x lo6 A/cm’, at 77 K. These values of transport critical currents are much higher than that obtained in single crystals or in the bulk materials. These films also exhibit extremely low surface resistivities [ 141, so that they are excellent candidates for microwave applications. Pinning of the fluxons is thought to be important for obtaining high critical currents. It is well known that structural defects can interact with the moving fluxons and effectively pin them. Since very large quenching rates are involved in the laser deposition process, it is also conceivable that considerable structural disorder is “frozen” into the thin films, in a manner similar to that observed in rapid solidification technology. To understand the origin of the higher critical currents in the thin films, we have initiated a detailed and systematic study of the microstructure and transport properties of the films prepared by laser deposition. Fig. 7 shows a typical high resolution, low magnification image of a thin film sample. The microstructure typically consists of a complex intergrowth
June 1990
of the “l-2-3” structure, the “2-4-8” structure and another structure corresponding to the cationic composition of “2-2-4” [ 15,161. Stacking faults along the c-direction are to be expected, since the bonding in these materials is generally weaker along that direction than in the a-b plane. However, in laser deposited films structural (and possibly compositional) changes occur not only along the c-direction, but also in the a-b plane. In the u-b plane, the film undergoes frequent changes in structure and possibly in composition. This produces the wavy microstructure observed in the low magnification image in fig. 7. In addition, in order to accommodate the changes in composition and structure along the u-b plane, dislocations and antiphase boundaries are introduced. The density and type of such stacking defects is likely to be systematically dependent upon the deposition conditions such as the substrate temperature, the oxygen partial pressure, etc. The “2-4-8” structure forms as defects when the deposition temperature is lower than the optimum value for the growth of the “l-2-3” structure. The insertion of another CuO chain layer into the “1-23” structure that is glide plane related to the CuO chain layer in the “l-2-3” structure doubles the unit cell dimensions along the c-direction. The glide plane
Fig. 7. Low magnification lattice fringe image illustrating the defect microstructure of laser deposited thin films. Some of the defects are identified in this image.
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is along the [ 1001 direction. Since this phase has a lower T, compared to YBazCusO,, the insertion of this structural unit as stacking defects in the “l-2-3” lattice can be a potential source of flux pinning in the thin films. It is to be noted that in good quality “ l2-3” films, this phase exists only as stacking defects and such extended three-dimensional regions are rare. Another type of stacking defect that is observed in the laser-deposited thin films has the structure corresponding to the cationic stoichiometry of “2-2-4”. Fig. 8 shows an atomic resolution image of this defect phase in the [ 1001 zone axis. As illustrated schematically in figs. 2a-2c, the only difference between these polytypoidic variants in the Y-Ba-Cu-0 system is in the layers that are inserted between two perovskite blocks, P. For example, in the case of the “2-2-4” structure. in addition to two CuO chain layers, a YO layer is present between them. The experimental image, fig. 8, and the results of detailed image simulations and matching indicate that there is strong likelihood for the existence of a YO layer, rather than a Y layer. The YzBazCu409 (2-2-4) phase appears to be charge balanced and hence might be insulating. It is,
Fig. 8.
[ 1001 atomic resolution
image of the “2-2-4”
structure
LETTERS
June 1990
however, important to note that charge imbalance and possibly superconductivity can be induced by suitable doping using either the oxygen content as the variable or by cationic substitutions. Clearly, it is also important to synthesize such new phases in bulk form in order to characterize their structural and physical properties in greater detail. 3.2. General discussion and conclusions The generic feature to the results presented in this paper is polytypoidism in these different superconducting oxides. In the case of the Bi cuprates, polytypoidism is introduced due to the change in the composition of the perovskite blocks. This is directly related to a change in the T, of that compound. It is seen that the T, increases as the number of Cu-0 planes in the structure increases (up to n = 3 for the Bi cuprates). The stability of the different polytypoids is influenced by changes that take place in the BiO double layers. For example, substitution of Bi by Pb stabilizes the n = 3 structure relative to the n = 2 structure. However, it is not clear at the present time whether this is due to a change in the kinetics of the
illustrating
the different
cationic
layers in this new structure.
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formation of the different phases (through the formation of a Pb-rich amorphous phase) or due to a direct influence on the relative stability of the different polytypoidic superconducting phases. This is in direct contrast to the Y-Ba-Cu-0 system, in which polytypoidic superconducting phases with different T, values are introduced through changes in the composition of the layers that intercalate between two perovskite blocks. These layers are also the locations at which oxygen can be reversibly intercalated. The changes in T, are in all likelihood due to the changes in the nature of these doping layers, due to the changes in the oxygen content. At the present time, it is not possible to synthesize the n = 1 and n= 3 analogs of the Bi cuprates in the Y-Ba-Cu-0 system. It is, however, very likely that such phases can be synthesized in metastable, thin film form using the laser deposition technique. Indeed, the large deviation of this process from thermodynamic equilibrium, suggests that many more phases such as the “2-2-4” phase can be synthesized in either stable or metastable form. The structural characteristics of the Tl-based cuprates bear similarities to both the Y-Ba-Cu-0 and the Bi cuprates systems. For example, the n= 1, 2, 3 and 4 polytypoids can be synthesized with relative ease in this system. So also, structures with one and two TlO layers can be synthesized (similar to the I‘ l2-3” and “2-4-8” phases respectively). The “2-2-4” structure in the Y-Ba-Cu-0 system does not have a corresponding phase in either the Bi cuprates or the Tl cuprates, but its cationic stacking sequence is in some ways similar to the structure of the superconducting phase in the Pb-Sr-(Ca,Y )-Cu-0 system. In this system, the two PbO layers are separated by a Cu atom that is in the + 1 oxidation state and has twofold oxygen coordination. It is evident that the polytypoid/polytype concept allows us to describe the structural and physical properties of these superconductors in a unified manner. Hence, from the structural chemistry point of view, one of the objectives should be to synthesize as many compounds as possible (both superconducting as well as otherwise) in order to fully understand the structural chemistry and the origin of superconductivity in these materials. The pulsed laser
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June 1990
deposition technique is an attractive synthesis tool to achieve this because the non-equilibrium conditions prevailing allow more phases to form. High resolution electron microscopy is a necessary tool for the study of these phases because many of them occur in small quantities that cannot be studied using macroscopic characterization techniques such as Xray diffraction.
Acknowledgement The authors wish to acknowledge the support and encouragement of P.L. Key, J.H. Wernick, M.J. Bowden and P.F. Liao. The support of Professor G. Thomas and the staff of the National Center for Electron Microscopy is also gratefully acknowledged.
References K.H. Jack, J. Mater. Sci. 11 ( 1976) 1135. P. Krishna, ed., Progress in crystal growth and characterization, Vol. 7. Polytype structures (Pergamon Press, Oxford, 1983). C.N.R. Rao and J. Gopalakrishnan, Recent trends in solid state chemistry (Academic Press, New York, 1988). J.M. Cowley, J.B. Cohen, M.B. Salamon and B.J. Wuensch, eds., Modulated structures (Am. Inst. Phys., New York, 1979). 1P. Buseck, J. Cowley and L. Eyring, High resolution transmission electron microscopy and related techniques (Oxford Univ. Press, Oxford, 1981). J.G. Bednorz and K.A. Miiller, Z. Physik B 64 ( 1986) 189. 1C. Michel, M. Hervieu, M.M. Bore], A. Grandin, F. Deslandes, J. Provost and B. Raveau, Z. Physik B 68 ( 1987) 421. [8] Z.Z. Sheng and A.M. Hermann, Nature 332 (1988) 138. [ 91 S.M. Green, Y. Mei, A.E. Manzi, H.L. Luo, R. Ramesh and G. Thomas, J. Appl. Phys. 66 (1989) 728. [ lo] A. Inam, MS. Hedge, X.D. Wu, T. Venkatesan, P. England, P.F. Miceli, E.W. Chase, C.C. Chang, J.M. Tarascon and J.B. Wachtman, Appl. Phys. Letters 53 (1988) 908. [11 R. Ramesh, S.M. Green and G. Thomas, in: Studies of high temperature superconductors, ed. A.V. Narlikar (NOVA, New York, 1990). 112 R. Ramesh, G. Thomas, SM. Green, C. Jiang, Y. Mei, M.L. Rudee and H.L. Luo, Phys. Rev. B 38 (1988) 7070. 113 A. lnam et al., Appl. Phys. Letters, in press. [l4 R. Ramesh et al., Science 247 (1990) 57. [l5 R. Ramesh et al., J. Mater. Res., in press.
June 1990
MATERIALS LETTERS
Volume 9, number 10
SQUIRES
IN HIGH TEMP~~TURE
OXIDE SUPERCONDU~ORS
R. RAMESH, D.M. HWANG, T.S. RAVI, C.C. CHANG, A. INAM, X.D. WU, T. VENKATESAN, J.M. TARASCON Bellcore, Red Bank, NJ 07701, USA
and S.M. GREEN Centerfor Superconductivity Research, Department ofPhysics and Astronomy, University of Maryland, College Park, MD 20752. USA
Received 1March 1990
High resolution transmission electron microscopy has been used to reveal the cationic structure of the high temperature oxide superconductors. Of the many new systems discovered recently, the Bi-Sr-Ca-G-0, Tl-Ba-Ca-Cu-0 and Y-Ba-Cu-0 systems have been extensively studied. Compositional variations are accommodated by the formation of polytypoidic stacking units whose cationic stoichiomet~es are in specific fixed ratios. Exampfes from the Bi cuprate bulk superconductors and Y-Ba-Cu-0 thin films are presented. Identification of the different polytypoids, each of which has a specific transition temperature, r,, can shed light on the electrical properties of the mixed bulk or thin films. Polytypism and polytypoidism also provide a unified approach to the structural and chemical analysis of these oxide superconductors.
1. In~~uction
Many naturally occurring complex minerals are layered structures. One of the most fascinating aspects of such layered structures is the frequent occurrence of variations in the.crystal structure with or without a change in composition. When the change in crystal structure occurs without any change in composition, such as in Sic or in many chalcogenides, it is known as polytypism, e.g., Sic, chalcogenides. On the other hand, when the change in crystal structure (or stacking sequence of different Iayers) occurs due to a change in composition, it is known as polytypoidism. A large variety of natural as well as man made materials exhibit polytypoidism. Typical examples include Al-O-N [ 1 ] and other oxynitride ceramics and complex minerals such as mica [ 2,3 1, etc. Several techniques have been used to understand the structure of these complex materials [ 31. However, high resolution transmission electron micros-
copy (HREM) at resolutions better than 2 8, (i.e. ~2 A) is particularly useful in this respect, since it enables the direct observation of the cationic stacking sequence and defects in the structure, as well as enabling the characterization of local variations in structure and composition. For example, in many of these materials, complex periodic or quasi-periodic structures have been known to form [ 41. These ordered structures can have either long range structural coherence or can be localized. Ordered structures that do not exist over a significant volume of the sample are likely not to be detected by X-ray diffraction or neutron diffraction techniques. However, the high spatial resolution in HREM enables the detection and characterization of such local ordered structures. When carried out in conjunction with detailed image simulations and image processing, HREM can provide si~i~c~t info~ation about the structure and composition of the material, that cannot be obtained by any other technique. The discovery of high temperature superconduc-
Volume 9, number 10
MATERIALS LETTERS
tivity in certain classes of oxides [ 6-8 1, mainly cuprates, has opened up a new area of materials physics and many potential high technology applications. Since these cuprates are layered structures, HREM studies have proven to be of great use in understanding their structure, chemistry and defect microstructure. These superconductors exhibit polytypoidism, with each polytypoid having a different superconducting transition temperature, T,.The Bi- and Tlbased cuprates typify this behaviour. These cuprates have the general composition of BizSr2Cu,Ca,_ ,O,, or T1,BazCu,Ca,_ i0, where n= 1, 2, 3 and in some cases 4, and x= 1 or 2. In the Bi cuprate system, the n = 1 phase has a T,in the range of 7-22 K, the n = 2 phaseaT,of85Kandthen=3phaseaT,ofllO K. The structures of these phases are illustrated schematically in fig. 1. In the Y-Ba-Cu-0 system, there are at least three superconducting polytypoids, with
the possibility for the existence of a more extensive range of either stable or metastable polytypoidic phases. Some of the possible polytypoidic variants in the Y-Ba-Cu-0 system are shown in fig. 2. In this paper, results of HREM studies of the polytypoidic microstructures in bulk Bi cuprates and in Y-Ba-Cu-0 thin films are described.
2. Experimental Appropriate amounts of Biz03 (99.9%), (99.999%), SrC03 (99.9%), or SrOz (99.5%, overnight at 12O”C), CaCO, (99.5%) or (99.5%, dried at 200°C) and CuO (99.5%) ground together until no visual evidence of mogeneities was observed. After calcining the ders overnight at 800°C the samples were
Fig. 1. Schematic illustrations of the crystal structures of the n = I,2 and 3 polytypoids in the Bi cuprate system.
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PbO dried CaO were inhopowthor-
Volume 9. number 10
MATERIALS LETTERS
June 1990
exposure to the electron beam to minimize the beam induced damage. The interpretation of the experimental images was verified by carrying out simulations using the MacTempas@ #’ software.
3. Results and discussion
YBa2Cu307
YBa2Cu408
Y2Ba2Cu409
‘123’
- 124’ or ‘248’
‘224’
Fig. 2. Schematic illustrations of the “l-2-3”, “2-4-8” and “2-24” phases, that are poiytypoidi~ variants in the Y-Ba-Cu-0 system.
oughly ground and pressed into pellets and sintered at 850°C for 60 h, with one intermediate grinding [ 9 1. The samples were fired in air and were furnace cooled to room temperature. Resistivity-temperature plots were measured on bars approximately 10x 2 x 1 mm3 from room temperature down to 77 K in a liquid nitrogen thermos. Contacts consisted of spring-loaded Rh-plated steel pogos (Augat-pylon) pressed onto silver paint pads in the standard four-point con~guration. About i mA rms currrent was supplied at 40 Hz with the sample’s voltage drop being detected by a lock-in amplifier. The zero resistance temperature was defined as the temperature below which the amplifier could not detect a signal. The detection limit was typically 0.5 @2 cm. Details of the thin film laser deposition process have already been reported elsewhere [ lo]. Bi cuprate samples for transmission electron microscopy were prepared by mechanically grinding to about 50 urn thickness followed by argon ion milling to electron transparency in a liquid nitrogen holder at 6 kV. For the thin films, cross-section samples were prepared and were ion milled for a few minutes prior to examination so as to reduce the degradation due to moisture in the atmosphere. High resolution electron microscopy (HREM) was carried out in the Berkeley Atomic Resolution Microscope (ARM) at a point-to-point resolution of 1.6 A. HREM images were obtained under conditions close to Scherzer defocus. Images were obtained within a few minutes of
In the Bi cuprates, the best imaging conditions are obtained in the [ 1lo] zone axis o~entation, in which the different cationic layers appear as columns [ 111. Fig. 3 shows one typical HREM image of the n = 2 structure in this zone axis orientation. The interpretation of this HREM image in terms of the projection of the different cationic columns is verified by carrying out detailed image simulations. In fig. 4a the projected potential for this structure in the [I lo] zone axis is shown. The corresponding simulated image for a foil thickness of 30 A and objective lens defocus of - 550 A is shown in fig. 4b and confirms the inte~retation of the experimental image in fig. 3. The n = 3 compound contains three CuOZ layers, as is illustrated in the [ 11O] zone axis atomic structure image in figs. 5a and 1. Fig. 5a, along with fig. 5b, illustrates the beneficial role of image processing in improving the quality of the information available in the image. This is especially true in the case of these oxides. which undergo electron beam induced damage. In fig. 5a, the clarity of the experimental image is reduced due to the amorphous background. By carrying out the Fourier transform and filtering available in the SEMPER@#’ program, the image in fig. 5b is obtained. The amorphous background in this image is considerably reduced compared to that in fig. 5a. Such image processing techniques are extremely valuable when the periodic part of the information in the image is less pronounced compared to the random part. The n = 4 structure in the Bi cuprates is only observed as isolated stacking defects and attempts to synthesize this phase in bulk form have so far been unsuccessful. The formation and microstructural distribution of ” MacTempas@ is a program for the simulation of HRTEM images and is a registered trademark of R. Kilaas, c/o Total Resolution, Berkeley, CA. ” SEMPER@ is a program for the processing of experimental HRTEM images.
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June 1990
Fig. 3. Atomic re: :,&ion image in the [ 1 lo] zone axis o~entation of the n=2 polytypoid illust~ting the different cationic layers.
Fig. 4. (a) Projected potential for the n=2 structure in the [ 1lo] zone. (b) Simulated image for a foil thickness of 30 A and objective lens defocus of - 550 A.
such polytypoidic structures is crucial in determining the transport properties of the superconductor [ 12 1. Since each polytypoid has differing electronic properties (i.e. superconducting with different transition temperatures, insulating, semiconducting, etc. ) their spatial distribution in the microst~cture is important in understanding the origin of the superconducting properties, such as the critical current densities. Since the experimental conditions required to form each of these polytypoids are different from one another, it is essential to carry out systematic studies 360
of the effect of the relevant processing parameters on the transport properties and the microstructure. Such studies have been carried out in the case of the sintered Bi cuprates. In these materials, the growth of lower T, polytypoids at the grain boundaries of the n = 3 grains, fig. 6, causes the occurrence of a knee in the resistive transition at I 10 K. The formation of the n = 2 or n = 1 polytypoids at the grain boundaries shows a systematic dependence on the starting composition. For example, partial substitution of Bi by Pb is helpful in avoiding this. The
Volume 9, number 10
MATERIALS LETTERS
June 1990
Fig. 6. Lattice fringe image of the grain boundary region in the unleaded Bi cuprate superconductor, showing the formation of the n=2 phase (indicated by arrows) adjacent to the grain boundaries due to the segregation of Bi to the grain boundary regions.
boundaries, leading to the formation of the Bi-rich lower 7+,polytypoids adjacent to the grain boundaries. One reason for this could be the possible enrichment of the grain boundary regions with oxygen, due to the higher diffusivity along grain boundaries. In the case of sintered Y-Ba-Cu-0 superconductors, such a polytypoidic growth adjacent to the grain boundaries has not been observed. This may explain why it is easier to obtain zero resistance in the YBa-Cu-0 ceramics without a low temperature knee in the resistive transition. Fig. 5. (a) Unprocessed atomic resolution image of the n= 3 polytypoid in the Bi cuprate system. (b) Same image as in (a), but SEMPER processed to reduce the background amorphous noise. Note the increase in clarity of the image in (b) over that in (a).
starting composition also has a distinct effect on the transport properties and the microstructure. For the synthesis of the n = 3 compound, it is found that Ca enrichment at the expense of Bi is beneficial. On the other hand, when the alloy is B&rich and Ca-deficient, the lower T, poIytypoids have been consistently observed at the grain boundaries, even when lead is added. This suggests that there is a fundamental tendency for Bi to segregate to the grain
3.1. Thin films It is clear that the earliest applications of these materials are likely to be in the area of thin film microelectronics. Hence, although there is much to be learned about the materials science of bulk superconductors, it is essential to understand the microstructure, defects and the processing issues involved in thin film materials. Among several processing techniques for thin film deposition, it has been found that laser deposition using a pulsed excimer laser provides excellent quality films. Films deposited under optimum conditions exhibit a superconducting
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transition temperature in the range of 88-9 1 K, with a transition width of less than 1 K [ 13 1. Thin films deposited on perovskite substrates such as LaAIOJ, possess critical currents, J,, in zero magnetic fields, of better than 5x lo6 A/cm’, at 77 K. These values of transport critical currents are much higher than that obtained in single crystals or in the bulk materials. These films also exhibit extremely low surface resistivities [ 141, so that they are excellent candidates for microwave applications. Pinning of the fluxons is thought to be important for obtaining high critical currents. It is well known that structural defects can interact with the moving fluxons and effectively pin them. Since very large quenching rates are involved in the laser deposition process, it is also conceivable that considerable structural disorder is “frozen” into the thin films, in a manner similar to that observed in rapid solidification technology. To understand the origin of the higher critical currents in the thin films, we have initiated a detailed and systematic study of the microstructure and transport properties of the films prepared by laser deposition. Fig. 7 shows a typical high resolution, low magnification image of a thin film sample. The microstructure typically consists of a complex intergrowth
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of the “l-2-3” structure, the “2-4-8” structure and another structure corresponding to the cationic composition of “2-2-4” [ 15,161. Stacking faults along the c-direction are to be expected, since the bonding in these materials is generally weaker along that direction than in the a-b plane. However, in laser deposited films structural (and possibly compositional) changes occur not only along the c-direction, but also in the a-b plane. In the u-b plane, the film undergoes frequent changes in structure and possibly in composition. This produces the wavy microstructure observed in the low magnification image in fig. 7. In addition, in order to accommodate the changes in composition and structure along the u-b plane, dislocations and antiphase boundaries are introduced. The density and type of such stacking defects is likely to be systematically dependent upon the deposition conditions such as the substrate temperature, the oxygen partial pressure, etc. The “2-4-8” structure forms as defects when the deposition temperature is lower than the optimum value for the growth of the “l-2-3” structure. The insertion of another CuO chain layer into the “1-23” structure that is glide plane related to the CuO chain layer in the “l-2-3” structure doubles the unit cell dimensions along the c-direction. The glide plane
Fig. 7. Low magnification lattice fringe image illustrating the defect microstructure of laser deposited thin films. Some of the defects are identified in this image.
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is along the [ 1001 direction. Since this phase has a lower T, compared to YBazCusO,, the insertion of this structural unit as stacking defects in the “l-2-3” lattice can be a potential source of flux pinning in the thin films. It is to be noted that in good quality “ l2-3” films, this phase exists only as stacking defects and such extended three-dimensional regions are rare. Another type of stacking defect that is observed in the laser-deposited thin films has the structure corresponding to the cationic stoichiometry of “2-2-4”. Fig. 8 shows an atomic resolution image of this defect phase in the [ 1001 zone axis. As illustrated schematically in figs. 2a-2c, the only difference between these polytypoidic variants in the Y-Ba-Cu-0 system is in the layers that are inserted between two perovskite blocks, P. For example, in the case of the “2-2-4” structure. in addition to two CuO chain layers, a YO layer is present between them. The experimental image, fig. 8, and the results of detailed image simulations and matching indicate that there is strong likelihood for the existence of a YO layer, rather than a Y layer. The YzBazCu409 (2-2-4) phase appears to be charge balanced and hence might be insulating. It is,
Fig. 8.
[ 1001 atomic resolution
image of the “2-2-4”
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however, important to note that charge imbalance and possibly superconductivity can be induced by suitable doping using either the oxygen content as the variable or by cationic substitutions. Clearly, it is also important to synthesize such new phases in bulk form in order to characterize their structural and physical properties in greater detail. 3.2. General discussion and conclusions The generic feature to the results presented in this paper is polytypoidism in these different superconducting oxides. In the case of the Bi cuprates, polytypoidism is introduced due to the change in the composition of the perovskite blocks. This is directly related to a change in the T, of that compound. It is seen that the T, increases as the number of Cu-0 planes in the structure increases (up to n = 3 for the Bi cuprates). The stability of the different polytypoids is influenced by changes that take place in the BiO double layers. For example, substitution of Bi by Pb stabilizes the n = 3 structure relative to the n = 2 structure. However, it is not clear at the present time whether this is due to a change in the kinetics of the
illustrating
the different
cationic
layers in this new structure.
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formation of the different phases (through the formation of a Pb-rich amorphous phase) or due to a direct influence on the relative stability of the different polytypoidic superconducting phases. This is in direct contrast to the Y-Ba-Cu-0 system, in which polytypoidic superconducting phases with different T, values are introduced through changes in the composition of the layers that intercalate between two perovskite blocks. These layers are also the locations at which oxygen can be reversibly intercalated. The changes in T, are in all likelihood due to the changes in the nature of these doping layers, due to the changes in the oxygen content. At the present time, it is not possible to synthesize the n = 1 and n= 3 analogs of the Bi cuprates in the Y-Ba-Cu-0 system. It is, however, very likely that such phases can be synthesized in metastable, thin film form using the laser deposition technique. Indeed, the large deviation of this process from thermodynamic equilibrium, suggests that many more phases such as the “2-2-4” phase can be synthesized in either stable or metastable form. The structural characteristics of the Tl-based cuprates bear similarities to both the Y-Ba-Cu-0 and the Bi cuprates systems. For example, the n= 1, 2, 3 and 4 polytypoids can be synthesized with relative ease in this system. So also, structures with one and two TlO layers can be synthesized (similar to the I‘ l2-3” and “2-4-8” phases respectively). The “2-2-4” structure in the Y-Ba-Cu-0 system does not have a corresponding phase in either the Bi cuprates or the Tl cuprates, but its cationic stacking sequence is in some ways similar to the structure of the superconducting phase in the Pb-Sr-(Ca,Y )-Cu-0 system. In this system, the two PbO layers are separated by a Cu atom that is in the + 1 oxidation state and has twofold oxygen coordination. It is evident that the polytypoid/polytype concept allows us to describe the structural and physical properties of these superconductors in a unified manner. Hence, from the structural chemistry point of view, one of the objectives should be to synthesize as many compounds as possible (both superconducting as well as otherwise) in order to fully understand the structural chemistry and the origin of superconductivity in these materials. The pulsed laser
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deposition technique is an attractive synthesis tool to achieve this because the non-equilibrium conditions prevailing allow more phases to form. High resolution electron microscopy is a necessary tool for the study of these phases because many of them occur in small quantities that cannot be studied using macroscopic characterization techniques such as Xray diffraction.
Acknowledgement The authors wish to acknowledge the support and encouragement of P.L. Key, J.H. Wernick, M.J. Bowden and P.F. Liao. The support of Professor G. Thomas and the staff of the National Center for Electron Microscopy is also gratefully acknowledged.
References K.H. Jack, J. Mater. Sci. 11 ( 1976) 1135. P. Krishna, ed., Progress in crystal growth and characterization, Vol. 7. Polytype structures (Pergamon Press, Oxford, 1983). C.N.R. Rao and J. Gopalakrishnan, Recent trends in solid state chemistry (Academic Press, New York, 1988). J.M. Cowley, J.B. Cohen, M.B. Salamon and B.J. Wuensch, eds., Modulated structures (Am. Inst. Phys., New York, 1979). 1P. Buseck, J. Cowley and L. Eyring, High resolution transmission electron microscopy and related techniques (Oxford Univ. Press, Oxford, 1981). J.G. Bednorz and K.A. Miiller, Z. Physik B 64 ( 1986) 189. 1C. Michel, M. Hervieu, M.M. Bore], A. Grandin, F. Deslandes, J. Provost and B. Raveau, Z. Physik B 68 ( 1987) 421. [8] Z.Z. Sheng and A.M. Hermann, Nature 332 (1988) 138. [ 91 S.M. Green, Y. Mei, A.E. Manzi, H.L. Luo, R. Ramesh and G. Thomas, J. Appl. Phys. 66 (1989) 728. [ lo] A. Inam, MS. Hedge, X.D. Wu, T. Venkatesan, P. England, P.F. Miceli, E.W. Chase, C.C. Chang, J.M. Tarascon and J.B. Wachtman, Appl. Phys. Letters 53 (1988) 908. [11 R. Ramesh, S.M. Green and G. Thomas, in: Studies of high temperature superconductors, ed. A.V. Narlikar (NOVA, New York, 1990). 112 R. Ramesh, G. Thomas, SM. Green, C. Jiang, Y. Mei, M.L. Rudee and H.L. Luo, Phys. Rev. B 38 (1988) 7070. 113 A. lnam et al., Appl. Phys. Letters, in press. [l4 R. Ramesh et al., Science 247 (1990) 57. [l5 R. Ramesh et al., J. Mater. Res., in press.