Relationship between crystal growth and physical properties in the high temperature superconductor YBa2Cu3O7-δ

Journal of Crystal Growth 99 (1990) 915-921 North-Holland

915

RELATIONSHIP BETWEEN CRYSTAL GROWTH AND PHYSICAL PROPERTIES IN THE HIGH TEMPERATURE SUPERCONDUCTOR YBa ~Cu307_

F. HOLTZBERG and C. FEILD I B M Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598, USA

The availability of single crystals of superconducting cuprates has made measurements of a number of physical properties of these highly anisotropic systems possible. However, in spite of major developments in crystallization techniques and a massive literature on the growth process, there is generally considerable variation in the critical temperature obtained from transport, magnetic or optical measurement. Certain persistent problems arising during the crystal growth process are examined and analyzed in an attempt to isolate factors influencing physical properties. In this context we review the growth of YBa2Cu307_ ~ (123) crystals from nonstoichiometric melts, emphasizing impurity effects and complexities of the oxygenation process, using as an example the substitution of Ca in the 123 single crystals. Finally we look critically at the measurements themselves as a means of evaluating crystal quality.

1. Introduction

Following the discovery of superconductivity above 30 K by Bednorz and Mtiller [1], in the perovskite cuprate oxide La2_xBaxCuO4, much effort has been expended towards raising the superconducting critical temperature, T~. Indeed, attempts to increase the transition temperature have met with much success as evidenced by the discovery of superconductivity above liquid nitrogen temperature in YBa2Cu3OT_~ [2-5], Bi2Sr 2 Can_lCunO2n+4 [6], T 1 2 B a 2 C a n _ l C u n O 2 n + 4 [7], and related compounds and recently Pb2Sr2Ln0.sAE0.sCu308 [8], where Ln is a lanthanide and AE an alkaline earth ion. Perhaps equally exciting is the prospect of superconductivity above 30 K in non-copper based materials such as Ba0.6K0.4BiO3 [9,10]. These various structures have proven to be quite versatile in accommodating a wide variety of other ions. Unfortunately, most substitutions have a deleterious effect on superconducting properties. Some ions quickly quench superconductivitY even in relatively small amounts, while others are incorporated in large amounts with little or no effect on Tc. Most of these effects can be explained on the basis of size and electron count arguments which affect the tuning of the physical and electronic structure. 0022-0248/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

A vast body of data now exists on these materials through which the theorist must sift to extract reliable results. Variation in results is understandable, since all of the known materials superconducting above 30 K are non-stoichiometric, thermodynamically metastable compounds. In addition to variable cation ratios, some materials exhibit intergrowth and oxygen defects restricting preparation conditions, making synthesis of the compounds extremely difficult and characterization frustrating. In order to examine the relationship between the crystal growth process of a high temperature superconductor and its physical properties, we have elected to study the 123 compound, which does not suffer from many of the complications mentioned above. However, the twinning of the ab plane and the oxygen defects still present problems that must be addressed, There still exists an uncomfortable variation in the results of certain measured physical properties in spite of the general availability of single crystals. In the following we attempt to identify the common sources of discrepancies in measurements linked to crystal quality, and propose some methods by which samples can be standardized and thereby compared. By studying the effect of impurities introduced into crystals, both intention-

916

F. Holtzberg~ C. Feild / Relationships between growth and properties in YBa 2Cu 307_ 8

ally and inadvertently, we hope to see the correspondence between impurity concentration and superconducting properties.

2. Flux growth of crystals The flux method of crystal growth [11-15] takes advantage of a low melting region of a subsystem of the pseudoternary BaO-CuO-Y203. The area of interest is bounded by BaCuO2-CuO and 123 [16,17]. Crystallization occurs along a path connecting the incongruent 123 decomposition point and the ternary eutectic. The crystallization path is not very well determined because melting temperatures depend on partial pressures of oxygen, impurities in the source materials and solubility of crucible materials in the melt; and according to Roth et al. [16], the solubility of CO 2 in the liquid phase. We had prepared our charges from the precursors BaCuO 2, CuO and 123 made from the constituent oxides and carbonates (Johnson Matthey BaCO3 99.999% CuO 99.999% and Y203 99.999%) in order to reduce carbon contamination from unreacted carbonate. The reactants were pressed into pellets and placed in a gold crucible folded into shape from five rail gold sheet. The crucible was supported on a gold sheet on alumina. As detailed in earlier publications [11-13], we had chosen gold as the container since we had determined that the melt reacted more aggressively with all other materials except single crystalline MgO. The finite concentration of gold (0.007 at%) found in the crystals appeared to have little effect on the superconducting properties. The thermal cycle and growth process were carried out as previously described [13]. Nucleation of 123 appears to take place on the gold and as the cooling cycle proceeds, the surface tension of the flux on the metal drains the remaining liquid away, leaving the crystals essentially free of flux. When small crucibles (about l c m diameter) and small charges (about 2 g) are used the crystal perfection is excellent as seen in fig. 1. Even under these conditions crystals must be carefully selected to avoid samples with intergrowths and satellites. Defects such as residual flux, para-

Fig. 1. Scanning electron micrograph of an as-grown 123 crystal on gold coveredwith residual flux (micrographcourtesy of F.W. Gayle).

sitic crystallization and bicrystal formation can readily be seen in the polarizing microscope. This is the first stage of crystal selection. We note here, that as the crystals become larger they tend to develop more imperfections. As-grown crystals were superconducting with transition temperatures of about 80-85 K with broad transitions. Selected crystals were annealed at 4 2 0 ° C for ten days which brought critical temperatures to 91-93.5 K depending on temperature calibration and the relative positions of the thermometer and sample in different equipment. In any one apparatus relative values are meaningful. We have found a steady increase in Tc with minor refinements in the crystal growth process. With the above procedures, crystals can be reproducibly grown with transition widths of 0.2-0.5 K; measured resistively [18] (fig. 2a), inductively [19] (fig. 3), or in a squid magnetometer in low magnetic fields [20]. The electrical resistivity of 123 in the normal state is extremely anisotropic. The resistivity perpendicular to the C u - O planes increases with decreasing temperature, indicating incipient localization. This upturn has been confirmed by several authors [21-25]. On the other hand, Iye et al. report that they only see the semiconductor-like behavior in poorer samples [26]. The quality of samples was judged by the transition temperature

917

F, Holtzberg, C. FeiM / Relationships between growth and properties in YBa 2Cu~O7_ 8

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and the low value of O~b. In a recent paper [27], the best sample had O~b = 150 /~12 cm with T~ = 91 K and z~T~---1 K. A similar result was obtained by Hidaka et al. [28] In that crystal T~ = 90 K with Oab = 400 /~£2 cm, just above T~. These results can be compared with the measurements of Yeh shown in fig. 2a. For this sample Tc = 93.1 K and AT~ = 0.2 K and 0~b = 60/~12 cm just above the critical temperature [29]. In spite of this low value, pc shows evidence of localization (fig. 2b). Clearly, from the review of the resistivity data, samples of 123 have continued to improve with time. It is also apparent that the question of metallic or semicon-

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ducting behavior in the c-direction still needs resolution.

3. T w i n structure

Under certain conditions, crystals which are relatively highly twinned before annealing, develop large twin free areas during the annealing process as shown in fig. 4 (see also ref. [30]). The twin structure in most cases is continuous around the six sides of well formed crystals as is readily observed in the thicker sample shown in fig. 5 for two surfaces. The observation of the continuous twin domains for all surfaces infers that there are no trapped non-superconducting phases in the interior. We have convinced ourselves that this is true by observing the same twin pattern on interior crystal surfaces after cleaving. The continuous twin pattern for all surfaces is another measure of crystal quality. One can go further. The contrast between the reflectivity of light polarized in the a and b directions, the optical bireflectance [31], is greatest for 0 7 and decreases as the tetragonal 06 composition is approached. Twin boundaries have been shown by decoration experiments to act as pinning centers [32]. This reference also proposes that a large untwinned region in which a vortex lattice is formed, implies that there is another pinning mechanism

918

F. Holtzberg, C. FeiM / Relationships between growth and properties in YBa 2Cu307 _ 8

Fig. 4. Comparison of the twin structure before and after annealing showing the growth of a single domain area.

in which defects occur at or near the level of molecular densities. If there are other pinning centers their nature is not understood. This problem needs more research so that one can gain

further insite into the macroscopic properties of the superconducting materials.

4. Impurities

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a,:b Fig. 5. Optical micrograph showing the ab plane/c direction twin structure continuity.

The level of sophistication approached in the crystal growth of the 123 has been somewhat impeded by the accidental introduction of defects and impurities. Tests done in the early stages of crystal growth clearly indicated crucible materials could be the source of serious impurity contamination [12]. Crystals grown in A1 crucibles were thought to have depressed critical temperatures. This was confirmed in a study by Siegrist et al. [33] who correlated the decrease in T~ with increasing A1 concentration. Through full structure X-ray diffraction analysis they also demonstrated that A1 substitution was confined to the copper chain sites. The transition temperatures tend to be lower and widths are generally broader under such contamination, even after lengthy thermal annealing in oxygen. It is now known that many impurities can affect crystal perfection and

F. Holtzberg, C. Feild / Relationships between growth and properties in YBa 2Cu307_

superconducting properties. It is, therefore, important to use the highest purity reactants for synthesis and recognize that most ceramic crucibles contain other impurities which can come from the crucible source material or from the binders introduced during the fabrication process. As an example of intentional doping several reports have appeared on the effects of calcium substitution in 123. Some reported that substitution took place on the barium site [34]. Others reported that calcium preferred the yttrium site. The latter seems more reasonable since the size is quite similar [35-42]. The allure here was the prospect that the transition temperature could be raised by further oxidizing the copper oxide network. Regrettably, materials carefully prepared to insure high oxygen content with copper valences above 7 / 3 + s h o w e d reduced T~ [35,38]. Two papers, somewhat at variance with each other, reported on a tetragonal superconductor Y0.aCa0.2 Ba aCu 306.1. One indicated a To = 50 K [40], while the other showed a two-stepped transition at 84 K and 50 K, resulting from sample inhomogeneity [39]. Studies of the effect of oxygen concentration on Tc and structure were inconclusive. To elucidate the role of Ca in 123 as presented in the previous papers on polycrystalline materials, we have prepared small single crystals in the Y - C a - B a - C u - O system. Crystals were obtained from the following Y / C a ratios: 9/1, 5/1, 1/1, 1 / 2 in a BaCuO2-CuO pseudobinary eutectic solvent. Crystal growth techniques were the same as for the pure compound [13]. Plate-like crystals ---500 /~m in size were obtained. Electron microprobe analysis has shown that the Y and Ca fractions add up to unity when the data is normalized to a copper ratio of three. Furthermore, in samples prepared with high calcium concentrations, there appears to exist a limiting composition of Yo.vsCa 0.25Ba2Cu 3Qv. Microscopic examination under polarized light showed that samples prepared from low calcium concentrations had the usual twin patterns; however, some crystal prepared from calcium rich melts appeared to be isotropic. Preliminary X-ray studies on a crystal of nominal composition Y 0 . v 5 C a 0 . 2 5 B a 2 C u 3 O y gave a tetragonal unit cell with lattice constants a = 3.852 A, c = 11.823 *

919

with a cell volume of 175.4 ~3. This large c parameter and cell volume is consistent with Jir~k et al.'s powder data for a tetragonal unit cell [39]. Surprisingly, AC susceptibility measurements yielded a T~ = 85 K with a AT~ = 2 K. The high T~ is in striking contrast to what has been observed with as-grown samples of the unadulterated 123. Our result is in agreement with those of Manthiram et al. and Jirhk et al. but at variance with McCarron et al.'s 50 K T~ [40]. Examination of their resistivity and susceptibility data reveals a broad transition starting around 80 K which is perhaps a consequence of oxygen inhomogeneity. By annealing the crystals at various low temperatures and partial pressures of oxygen, changes in T~ as a function of oxygen content were observed. Manthiram et al. [35] show the uptake of oxygen to be complete around 350 ° C at 1 bar of 02 . Loss of 02 begins for temperatures above 450 o C. Our crystal was annealed in an 02 atmosphere at 600 o C, cooled at 100 ° C / h to 420 ° C and held for 250 h. Some surface degradation occurred which may have been caused by the initial 600 ° C heating. Interestingly though, annealing in 02 induced twin patterns consistent with an orthorhombic structure which could clearly be seen under polarized light. Unexpectedly, the transition temperature had decreased to 54 K with a AT~ = 1 K (fig. 6a).

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920

F. Holtzberg C. Feild / Relationships between growth and properties in YBa eCu307_

Attempting to observe reversibility of the oxygen intercalation, oxygen was removed by heating the crystal for 12 h in air at 420 ° C. The transition temperature increasing to 68 K with A T~ = 1 K (fig. 6b). To check if an equilibrium concentration of oxygen had been obtained, we reheated the sample for 18 h under identical conditions. A n increase of approximately 0.2 K was seen. Twin patterns of the crystal were still visible but appeared more diffuse. Subsequent annealing in 10% 0 2 in N 2 at 420 ° C for 12 h further increased T~ about 2 K to 70 K, AT~ = 1 K. Some twinning was still present. Additional annealing with the same gas mixture for 12 h at 5 0 0 ° C removed the twins and the transition temperature was once again restored to 85 K with a ATc = 1 K (fig. 6c). The qualitative aspects of this behavior are unusual. The transition temperature appears to vary continuously across the range of oxygen concentrations, in contrast to pure 123 which shows a discontinuous two stepped behavior [43]. With Ca substitution we find that the width of the transition remains about 1 K for all concentrations of oxygen. F r o m these results on single crystals the following scenario can be envisioned: Ca introduced into the Y - B a - C u - O system substitutes for yttrium in the triple perovskite structure. Solid solubility extends over the range 0 _ < x < 0 . 2 5 in Y t _ x C a x B a z C u 3 O y . The two variables in this experiment are the concentrations of calcium and oxygen. Both quantities affect the charge balance. In order to examine the the relationship between the two variables, we consider the effective oxidation state of the copper oxide sublattice. The copper oxide lattice maintains its degree of oxidation by compensating an increase of Ca with a concomitant loss of oxygen. Therefore, the compensated Ca d o p e d material with disordered oxygen in the basal plane is almost equivalent to fully oxidized 123. The decrease in Tc with the addition of oxygen to the Ca doped material is not fully resolved. The results, however, is consistent with the model of Shafer et al. relating hole concentrations to superconducting transition temperatures [38].

5. Conclusion We have touched on the flux method of crystal growth. Optical characterization, AC and DC susceptibility, and transport measurements of the superconducting critical temperature were considered. We have attempted to identify some key factors involved in the the growth process related to crystal perfection. The effects of impurities are discussed through the example of the introduction of analyzable quantities of Ca into 123 single crystals, and dependence of Tc on oxygen concentration and ordering. Improvement in the crystal growth process will depend more and more on the determination of the effects of remaining impurities and defects on the superconducting properties.

Acknowledgements The authors are indebted to N.-C. Yeh for permitting us to use unpublished resistivity data, K.H. Kelleher for microprobe analysis, R.F. Boehme for single crystal X-ray measurements, and T.K. Worthington for the gift of an AC susceptibility apparatus.

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Yu.A. Ossipyan, I.F. Chegolev, LJ. Buravov and V.N. Laukhin, Physica C 153-155 (1988) 1539. [26] Y. Iye, T. Tamegai, H. Takeya and H. Takei, Japan. J. Appk Phys. 26 (1987) L1941. [27] Y. Iye, T. Tamagai, H. Takeya and H. Takei, Japan. J. Appl. Phys. 27 (1988) L658. [28] Y. Hidaka, Y. Enomoto, M. Suzuki and T. Morikami, Rev. Elec. Commun. Lab. Nippon Telegraph and Telephone Corp. 36 (1988) 567. [29] N.-C. Yeh, private communication. [30] A recent publication describes the detwinning of 123 by annealing single crystals under uniaxial stress: H. Schmid, E. Burkhardt, B.N. Sun and J.-P. Rivera, Physica C 157 (1989) 555. [31] H. Schmid and J.-P. Rivera, Physica C 153-155 (1988) 1748. [32] G.J. Dolan, G.V. Chandrashekhar, T.R. Dinger, C. Feild and F. Holtzberg, Phys. Rev. Letters 62 (1989) 827. [33] T. Siegrist~ L.F. Schneemeyer, J.V. Waszczak, N.P. Singh, R.L. Opila, B. Batlogg, L.W. Rupp and D.W. Murphy, Phys. Rev. B36 (1987) 8365. [34] M. Kosuge, B. Okai, K. Takahashi and M. Ohta, Japan. J. Appl. Phys. 27 No. 6 (1988) L1022. [35] A. Manthiram, S.-J. Lee and J.B. Goodenough, J. Solid State Chem. 73 (1988) 278. [36] Y. Tokura, J.B. Torrance, T.C. Huang and A.I. Nazzal, Phys. Rev. B38 (1988) 7156. [37] A. Tokiwa, Y. Syono, M. KStkuchi, R. Suzuki, T. Kajitani, N. Kobayashi, T. Sasaki, O. Nakatsu and Y. Muto, Japan. J. Appl. Phys. 27 (1988) L1009. [38] M.W. Shafer, T. Penney, B.L. Olson, R.L. Greene and R.H. Koch, Phys. Rev. B39 (1988) 2914. [39] Z. Jir~tk, J. Hejtm~mek, E. Pollert, A Triska and P. Vasek, Physica C 156 (1988) 750. [40] E.M. McCarron III, M.K. Crawford and J.B. Parise, J. Sofid State Chem. 78 (1989) 192. [41] Y. Zhang, N. Karpe, M. Pont, R. Puzniak, V. Skurmyev and K.V. Rao, Bull. Am. Phys. Soc. 34 (1989) 888. [42] T. Bj~Srnholm, M.B. Maple, I.K. Schuller and J.D. Jorgensen, Bull. Am. Phys. Soc. 34 (1989) 889. [43] R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Nature 329 (1987) 423.