Crystal growth of Bi-Sr-Ca-Y-Cu-O by the travelling solvent floating zone method

Physiea C 222 (1994) 252-256

ELSEVIER

Crystal growth of Bi-Sr-Ca-Y-Cu-O by the travelling solvent floating zone method Dong Han Ha *, In Seon Kim, Yong Ki Park Korea Research Institute of Standards and Science, P.O. Box 3, Taedok, Taejon 305-606, Korea

Kunihiko Oka, Yoshikazu Nishihara Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba 305, Japan

Received 17 November 1993

Abstract

We have grown Bi2Sr2Cal_xYxCu2Os (x--0, 0.2, 0.4) single crystals by the travelling solvent floating zone method. The size of the single crystal decreases due to Y-substitution. The crystal without Y-substitution (x = 0) consists of nearly Bi2Sr2CalCu2Os single phase and shows a sharp superconducting transition, but Y-substituted crystals show phase segregation which leads to a broad superconducting transition. The lattice constant along the c-axis decreases with Y-concentration, but those along the a-, baxis do not show any noticeable change.

1. Introduction

The substitution effect in high-To superconductors has been extensively studied up to the present. For instance, charge carrier concentration and lattice parameter, can be controlled by proper substitution. It is known that the Cu site can be substituted with Ni or Fe [ 1-3 ] ions within a limited range in the Bisystem, but Sr or Ca can be substituted with La or Y in nearly the whole range of stoichiometry. The substitution of Ca with Y was performed on polycrystals [ 4-6 ], films [ 7 ] and single crystals [ 8-10 ]. Generally the lattice constant along the c-axis decreases with Y-concentration. The superconducting transition temperature slightly increases for a low Y-substitution level up to about 20%, but the resistivity does not drop to zero until above ~ 50%.

In most studies the crystal growth of Y-substituted Bi-systems has been performed by the flux method [ 8-10 ]. However, the results are different from each other suggesting Y-atoms do not uniformly substitute at Ca sites which induces not only some spatial distribution of the composition in the Bi2Sr2Cal_xYxCu2Os crystal but also phase segregation. Recently Takamuku et al. [ 11,12 ] have grown the Bi2Sr2Cal_xY~Cu2Os single crystal by the travelling solvent floating zone (TSFZ) method. They reported that the lattice constant along the c-axis decreased with Y-concentration, but the grown crystal did not show Meissner transition in crystals with x>_-0.2, although those grown by the flux method showed superconducting properties up to x~, 0.5. In this work, we have grown the Bi2Sr2Cal_xY~Cu2Os ( x = 0 , 0.2, 0.4) crystals by the TSFZ method and measured the superconducting properties of thc

* Corresponding author. 0921-4534/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved SSDIO921-4534(93)EIO91-J

D.H. Ha et al. / Physica C 222 (1994) 252-256 .....

grown crystals by X-ray diffraction (XRD), magnetization and resistivity measurements.

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CuO

3. Results and discussion

Powder XRD patterns of as-grown crystals were taken with Cu Ka radiation. Fig. 1 confirms that the x - - 0 crystal consists of nearly the Bi(2212) single phase, but the Bi2Sr2CuO~ (hereafter, Bi(2201 )) phase is also distinctly observed in Y-substituted crystals which show that phase segregation occurred

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powders were used as starting materials. Although the phase diagram of the Bi-Sr-Ca-Cu-O system [ 13,14] is known and many groups [ 15,16 ] have reported on the nominal compositions of the feed and solvent for the crystal growth of Bi2Sr2CazCu2Os (hereafter, Bi (2212) ), those for the Y-substituted case were not known at all until now. So we tried to mix the starting materials in the nominal compositions of Bi2.2Srl.sCa]_xYxCU20 s ( x = 0 , 0.2, 0.4) for feeds and Bi2.4SrLsCa]_zYxCul.sOs (x--O, 0.2, 0.4) for solvents by referring to previous reports [ 15,17 ] on the crystal growth of the Bi(2212) system. The mixed powders were calcined at 810°C for 12 h, then pulverized and pressed into rods of 8 mm diameter and 100 mm length. The feed rods were sintered in air at 850-900 ° C for 62 h with intermediate grinding, and solvent rods were sintered in air at 850°C for 12 h. Single-crystal growth was performed in an infrared radiation furnace (NEC, SC-35HD) equipped with double halogen lamps through a twice-scanning process. The rate of the first-scan was 7-8 ram/h, and that of crystal growth (second-scan) was 0.5 m m / h for x = 0 and 0.4 m m / h for Y-substituted cases. A small size single crystal of good quality was used as a seed for x = 0, but a part of the first-scanned rod was used as a seed for Y-substituted cases for the crystal growth. The size of grown single crystal decreased by Y-substitution. We easily obtained single crystals up to an area of 5 × 3 mm 2 for x f O , but could obtain only a few single crystals of about 2×0.5 mm 2 for Ysubstituted cases. Bi203, SrCO3, CaCO3,

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in those crystals. In Fig. I, it can be noticed that all of the (00l) peaks for Y-substituted crystals seem to be superpositions of two or more peaks. We may suppose that the separation of (00l) peaks for Y-substituted crystals are superpositions of (00I) peaks and other peak such as ( 111 ) or (119) peaks. However, computer simulation using the powder X-ray Rietreid analysis program (RIETAN) [ 18] confirmed that the separation of (00l) peaks cannot be ascribed to the superposition of (001) and other peaks. The positions of small shoulder peaks, marked by an open circle, are nearly the same as those of the (001) peaks for the x = 0 crystal. So we consider that there is some spatial distribution in the composition of the Bi2Sr2Ca~_xYxCu2Os phase which induces separation of (00l) peaks, namely, Y-atoms did not uniformly substitute at the Ca site, which shows the same tendency as polycrystals or single crystals by many groups [6,8,10]. The high-resolution electron microscopy image confirmed that the x = 0 crystal consisted of the homogeneous Bi(2212) single phase, but in Y-substituted crystals not only many distortions are involved but also even the amorphous phase. However, it is considered that Y-atoms distribute uniformly in the major part of the grown crystal because the intensities of the main (00l) peaks are very large compared with those of shoulder peaks and the width of each main (001) peak is nearly the same as that for the x = O crystal. There is no evidence of superposition in other peaks because lattice constants along the a-, and

254

D.H. Ha et al. / Physica C 222 (1994) 252-256

b-axes hardly changed by Y-substitution up to x = 0.4 as shown in Fig. 2. It can be noticed that the lattice constant along the c-axis decreases more rapidly at lower nominal concentrations, suggesting that Y-atoms are preferentially substituted at the Ca sites in that region. Temperature dependence of the resistivity for each crystal is shown in Fig. 3. Electromagnetic properties were measured post-annealing. Specimens were first beat-treated in air at 840°C for 30 rain, cooled to 400°C at a rate of 12°C/h, then furnace-cooled to below 200 ° C. Electrical resistivity was measured using the conventional four-probe method with a current of 1 mA along the a-axis for each crystal. Both the resistivity and the superconducting transition width increased with Y-substitution. Various parts separated from the grown crystal boule of x = 0 showed the same resistivity vs. temperature relation, but those for Y-substituted crystals have a different resistivity behavior from each other as shown in Fig. 3b. Fig. 3b shows the temperature dependence of resistivity for various specimens obtained from the same part of the grown crystal boule of x = 0.4. In Fig. 3b one specimen shows semiconducting resistivity behavior but the others show metallic behavior. However, every specimen shows nearly the same superconducting onset transition temperature. The DC magnetization measurement was carried out using a Quantum Design SQUID magnetometer, Fig. 4 shows field-cooled superconducting transitions taken at a field of 10 G parallel to the c-axis for each crystal, which was a stack of very thin single crystals. The superconducting onset transition tern-

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Temperature (K) Fig. 4. Field-cooled superconductin8 transitions of the Bi2Sr2Cal_xYxCu208 (x=0, 0.2, 0.4) crystals taken using a 10 G field parallel to the c-axis.

perature slightly increases for a low Y-concentration level of x = 0 . 2 , but it decreases at x=0.4, which is consistent with the previous results [ 4,8 ] for the Ysubstituted Bi-system. The superconducting onset

D.H. Ha et aL I Physica C 222 (1994) 252-256

transition temperatures are 84, 90, 80 K for x=0, 0.2 and 0.4 respectively. The superconducting transition width was greatly broadened by Y-substitution suggesting nonuniform substitution of Y-atoms which leads to the spatial distribution in the composition of the Bi2Sr2Cal_xY~Cu208 phase. It also leads to phase segregation and defects. We have measured the magnetic hysteresis curves for each crystal in magnetic fields up to 5 T. It is known that defects [ 19 ] or secondary phase particles [ 20 ] may enhance the pinning force. However, the magnetic hysteresis greatly decreases with Y-concentration as shown in Fig. 5, although Y-substituted crystals involve many defects and secondary phase particles compared with the x = 0 crystal. The decrease of magnetic hysteresis with Y-concentration can be ascribed to the decrease of the single crystal size to a certain extent [21 ]. However, it is considered that the decrease of the single crystal size is not a principal parameter for the variation of magnetic hysteresis because there is not remarkable difference in single crystal size between the x=0.2 and the x = 0.4 crystals although the magnetic hysteresis for x = 0.4 is 5-10 times smaller than that for x = 0.2. The magnetic hysteresis behavior with substituent concentration is currently under investigation. In summary, we have grown Bi2Sr2Ca~_~YxCu208 (x=0, 0.2, 0.4) single crystals by the TSFZ method. The structural and superconducting properties of the grown crystals show the same tendency reported for single crystals grown by the flux method. The size of the single crystals decreases due to Y-substitution. All 80

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255

the grown crystals show superconducting transitions. In Y-substituted crystals, the superconducting transition was broadened and each specimen showed a different resistivity vs. temperature relation, even though they were obtained from the same part of the grown crystal boule, suggesting that there is a secondary phase like the Bi(2201 ) phase as well as some spatial distribution in the composition of the Bi2Sr2Cal_xYxCu2Os phase. Information on the phase diagram of the Y-substituted system and more stable crystal growth conditions will be necessary in order to obtain more homogeneous Y-substituted single crystals. The lattice constant along the c-axis decreases with Y-concentration, but those along the a- and b-axes do not show any noticeable change. Magnetic hysteresis was rapidly decreased with the increase in Y-concentration.

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