Chemistry of High-Temperature Superconductors - ACS Publications

Chapter 8 S i n g l e - C r y s t a l G r o w t h of the H i g h - T e m p e r a t u r e Superconductor YBa Cu O 2

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F. Holtzberg, D. L. Kaiser, B. A. Scott, T. R. McGuire, T. N. Jackson, A. Kleinsasser, and S. Tozer

Downloaded by UNIV OF NEW ENGLAND on February 9, 2017 | http://pubs.acs.org Publication Date: August 28, 1987 | doi: 10.1021/bk-1987-0351.ch008

Thomas J. Watson Research Center, IBM, Yorktown Heights, NY 10598

We report a set of conditions for crystal growth of the high-temperature superconductor YBa Cu O . The as-grown single crystals have critical temperatures up to 85 K. Preliminary studies show that the transition temperature can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are of suitable dimensions for definitive magnetic, optical and transport measurements. 2

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After the major discovery of the high temperature superconductor La _ Ba Cu0 . by Bednorz and Muller (1) and the subsequent identification (2) and isolation (3-5) of the higher T YBa Cu O (YBC) phase, there followed a tidal wave of publications. The proliferation of papers attests to the seeming ease of fabrication of ceramic samples which have superconducting properties. It is evident on the other hand that there is a rather limited literature on the growth of single crystals of YBC. Indeed to our knowledge, there are few publications which give serious consideration to the methods and conditions for single crystal growth of YBC having dimensions, purity and homogeneity suitable for determination of the intrinsic properties of this superconductor. The following communication addresses conditions for crystal growth and establishes some relevant aspects of the phase relationships in the system containing the desired YBC compound. Initial crystal growth experiments using the more common low temperature fluxes such as B 0 , KF and PbO were unsuccessful. Subsequent DTA measurements suggested that YBC decomposes peritectically at about 1020°C. To overcome this difficulty, we searched for a liquidus field for crystal growth below the decomposition temperature. We concentrated our efforts on the pseudoternary YBC-BaCu0 -CuO system. 2

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Experimental Procedures Selection of Crucible Material. Initial experiments (6) were carried out in platinum crucibles. Crystals up to 1x2x0.01 mm were found primarily in cavities within the 0097-6156/87/0351-0079$06.00/0 © 1987 American Chemical Society

Nelson et al.; Chemistry of High-Temperature Superconductors ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS

solidified mass.

The dimensions of the crystals appeared to be independent of the

amount of charge. Yield of Y B C crystals was considerably less than 1%. Several problems were encountered with crystal growth from the platinum crucibles. The liquid wet the Pt surface and could not be completely contained. In ad dition, the liquid reacted extensively with the crucible, forming mixed platinates. C o n sequently, the charges were depleted of yttrium and barium oxides, lowering the Y B C crystal yield. A search was therefore made for other crucible materials. Samples of the constituent compounds Y B C , B a C u 0 , and C u O and also compo 2

sition 'a' in Fig. 1 were heated in air at 950°C for 16 hrs on Pt, A u and C u , and single crystals of sapphire, Y-stabilized zirconia (12% Y), magnesia, magnesium

aluminate

( M g A l 0 ) and quartz. A l l of the potential crucible materials showed little or no visible


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reaction with the three compounds. Extensive melting of composition 'a' occurred on all support materials with the exception of Pt. Single crystal magnesia (which is difficult to obtain) and gold showed the least apparent reactivity with the melt. Determination of Melting Behavior.

O n the basis of the above results, gold crucibles

were used in a study of melting and crystal growth in the Y B C - B a C u 0 - C u O 2

pseudoternary system, Fig. 1.

Compositions along the pseudobinaries B a C u 0 - C u O 2

and Y B C - C u O and within the pseudoternary at molar B a / Y ratios of 9/1, 4/1 and 7/3 and C u O mole fractions from 0.5 to 0.8 were studied. The sample with B a / Y = 9/1 and C u O mole fraction = 0.7 (composition 'a' in Fig. 1) showed the greatest degree of melting.

Samples were prepared from mixtures of Y B C , B a C u 0

2

and C u O .

Use of

these compounds, rather than their oxide or carbonate precursors, greatly improved solid state reactivity, allowing complete reaction in a much shorter period of time. Y B C and B a C u 0 were synthesized by heating pressed pellet mixtures of B a C 0 , C u O and 2

Y 0 2

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at 940-950°C in flowing 0 (g) 2

for 16 hrs on a platinum sheet. The pellets were

then ground, remixed, pressed and the heating procedure repeated.

In each case the

samples were fired on a bed of powder of the same mixture. Following the final reaction period, they were cooled at 50°C/hr to 400°C. BaCu0

2

The resulting compounds Y B C and

were examined by x-ray powder diffraction, and found to give single phase

patterns in good agreement with those published in the literature (3,7). T o determine a suitable growth temperature for composition 'a', samples were heated to temperatures between 950 and 1000°C and cooled at 25°C/hr.

Crystal

growth was most favorable at 970°C for this composition. Crystal Growth. For the crystal growth runs, a pellet of composition 'a' weighing 2 gms was placed in a 0.35 c m A u crucible which was supported on a A u sheet on an alumina plate. The sample was heated at 200°C/hr to 970°C in air, held at 970°C for 1.5 hrs and cooled at 25°C/hr to 400°C. 3

The liquid moved along the inner and outer surfaces and therefore could not be contained in the crucible. Free-standing, well-formed Y B C crystals were found in the cavity between the crucible bottom and the support plate where the liquid moved away from the crystals, as can be seen in the micrograph, Fig. 2. Y B C crystals were rectan gular prisms of two types. Thin platelets (up to 1 χ2x0.01 mm) were observed similar to those grown in Pt crucibles. The other type had sharp edges of nearly equal lengths, up to 0.5x0.5x0.2 mm. These crystals were fairly easy to separate from the A u . Smaller crystals (up to 0.2x0.2x0.1 mm) of Y B C also grew on the inner walls of the crucible and in the solidified mass. The latter had residual melt on the surfaces or were embedded in the solid, making it difficult to isolate them for measurement.


8.

HOLTZBERG ET AL.

Single-Crystal Growth of

YBa Cu O 2

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CuO

Figure 1. Pseudoternary system Y B C - B a C u 0 - C u O showing compositions along constant B a / Y lines used in melting studies in A u crucibles at 950°C in air. C o m position 'a' was used in all crystal growth runs. Compositions are given in terms of mole percent of the binary oxides BaO, C u O and Υ θ ! . 2

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Figure 2. Scanning electron micrograph of Y B C crystals grown in a A u crucible. Note the extremely sharp edges and flat facets. (Micrograph courtesy of J. G . Clabes and D. A . Smith)


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CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS

Results


X - R a v and Compositional Analyses. Single crystal x-ray precession photographs of Y B C crystals from the Pt and A u crucible runs confirmed their perovskite-related structure. Crystal composition was determined by electron microprobe analysis. The Y / B a / C u ratios were 1/2/3 in each crystal within the accuracy of the microprobe measurements. However, the crystals grown in A u crucibles contained approximately 0.7 m o l e % A u .

Magnetic Measurements. T o determine superconducting transition temperatures, mag netic measurements were made using a S H E 905 magnetometer equipped with a 40 K G superconducting solenoid. A typical magnetization vs. temperature curve for a crystal from a A u crucible run is shown in Fig. 3. The diamagnetic shielding and Meissner ef fects were observed on heating and cooling respectively in a low field (14 Oe). Rea sonable agreement was obtained between calculated and measured susceptibilities after correction for demagnetization. The low Meissner effect of 2 0 % is typical of these oxide superconductors (8). We estimate the critical temperature for this crystal to be 80 Κ or higher. In general, our crystals have T values in the range of 55 to 85 K. A s in ceramic samples (4,9,10), the critical temperatures are strongly dependent on oxygen concentration and have been increased by annealing in oxygen. We are currently searching for optimal annealing conditions. c

Resistivity Measurements. Four-probe resistivity measurements were also used to de termine critical temperatures. The resistance vs. temperature profile for the crystal shown in Fig. 4 gives a critical temperature of about 85 K. Electrical contact was made to the crystal by wire-bonding aluminum to the surface; however, this method is not always successful. Alternative techniques have given ohmic contacts with low resistance which have eliminated the self-heating problem associated with the aforementioned method. These results will be the subject of another publication.

Conclusions In summary, we have developed a technique for growing single crystals of Y B C reproducibly under carefully controlled conditions. The as-grown crystals are super conducting with transition temperatures in the range 55 to 85 K ; post-annealing in ox ygen increases the T . Detailed transport, magnetic and optical properties of the Y B C c

single crystals are currently under investigation.



8.

HOLTZBERG ET AL.

83

Single-Crystal Growth of YBaiCu^O^

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Temperature (K)

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Figure 3. Temperature-dependent magnetization data at low field for a crystal from a A u crucible run. Diamagnetic shielding and the Meissner effect occur on heating and cooling respectively.

0.12

0.08 0)

ο

ΰ eu 0.04

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Temperature

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(K)

Figure 4. Temperature-dependent resistance for a crystal from a Pt crucible run.


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SUPERCONDUCTORS

Acknowledgments We thank S. LaPlaca for single crystal x-ray diffraction measurements, H . Lilienthal for magnetization measurements and K. Kelleher for microprobe measurements.


Literature Cited 1. Bednorz, J. G.; Muller, Κ. A. Z. Phys. 1986, B64, 189. 2. Wu, M. K.; Ashburn, J. R.; Torng, C. J.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang, Y. Q.; Chu, C. W. Phys. Rev. Lett. 1987, 58, 908. 3. Cava, R. J.; Batlogg, B.; van Dover, R. B.; Murphy, D. W.; Sunshine, S.; Siegrist, T.; Remeika, J. P.; Rietman, Ε. Α.; Zahurak, S.; Espinosa, G. P. Phys. Rev. Lett. 1987, 58, 1676. 4. Grant, P. M.; Beyers, R. B.; Engler, E. M.; Lim, G.; Parkin, S. S. P.; Ramirez, M. L.; Lee, V. Y.; Nazzal, Α.; Vazquez, J. E.; Savoy, R. J. Phys. Rev. Β 1987, 35, 7242. 5. LePage, Y.; McKinnon, W. R.; Tarascon, J. M.; Greene, L. H.; Hull, G. W.; Hwang, D. M. Phys. Rev Β 1987, 35, 7245. 6. Kaiser, D. L.; Holtzberg, F.; Scott, Β. Α.; McGuire, T. R. submitted to Appl. Phys. Lett. 7. Migeon, N.; Jeannot, F.; Zanne, M.; Aubry, J. Rev. Chim. Miner. 1976, 13, 440. 8. Dinger, T. R.; Worthington, T. K.; Gallagher, W. J.; Sandstrom, R. L. submitted to Phys. Rev. Lett. 9. Qadri, S. B.; Toth, L. E.; Osofsky, M.; Lawrence, S.; Gubser, D. U.; Wolf, S. A. Phys. Rev. Β 1987, 35, 7235. 10. Strobel, P.; Capponi, J. J.; Chaillout, C.; Marezio, M.; Tholence, J. L. Nature, 1987, 237, 306. RECEIVED

July 6, 1987


Chemistry of High-Temperature Superconductors - ACS Publications

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