Critical temperature control of Bi2Sr2CuOx by quenching

Physica C 167 (1990) North-Holland

258-262

CRITICAL TEMPERATURE T. ISHIDA

CONTROL

OF Bi,Sr,CuO,

BY QUENCHING

and T. SAKUMA

Received 26 February 1990 Revised manuscript received 4 March

I990

We investigated the effect of quenching on the critical temperature 7, of sintered Bi&,CuO, (2201-phase ). The mtdpoint I, of the 2201-phase was raised from 6 to I7 K by quenching. The resistive onset temperature of the quenched sample reached 60 K. The r, behavior was similar to that of Bt2SrzCaCu,0, (22 12 ).

The Bi-based cuprous oxide superconductors can be classified into three types depending on the number of CuOz stackings (Bi,Sr,Ca,_ ,Cu,,O_,, n = 1, 2, 3) [ l-31. The critical temperature T, of the 2223phase (n= 3 ) is 107 K and that of the 22 12-phase (n=2) is 90 K. The T, of the 2201-phase (n= 1 ) was reported to be about 10 K [ 1,2,4]. 7,increases as the number of CuOz stackings increases. but it is not known what determines T,in the Bi compounds. Various experiments have been performed by controlling the doping content and oxygen stoichiometry of the preceding Laz_ .Sr,yCu04 and YBa2Cu307P, systems [ 5,6]. The Bi-based oxides do not contain a T, control parameter explicitly in their chemical formulae. We attempted to find a practical method for controlling the superconducting and normal-state properties of the Bi-based oxides, i.e., quenching from various elevated temperatures into liquid nitrogen [7-l 21. We reported that the T,of Bi,Sr,CaCu,O, ( 22 12 ) increases appreciably by quenching from a certain temperature (T,)to 77 K [ 7-101. In contrast. the T, of Bi 1.92Pbo.48Sr,CazCu3.,0.1 (2223 1 is insensitive to T,below 65O’C [ 11 1. Our suggestions were as follows. ( 1) Holes are supplied from the ( BiO), layer to the CuOz layer. (2) The quenching controls the amount of charge transfer, presumably by inducing atomic disorder, resulting in changes in the superconducting and normal-state properties. ( 3 ) The structural unit CuO,-M-CuO, (M=Ca”+ or Y’+ ) is responsible for the 90 K superconductivity of Bi,SrzCaCuzO, and YBa&u,O, [ lo]. (4) The 0921-4534/90/$03.50

(North-Holland )

0 Elsevier Science Publishers

B.V.

C’uOZ layer located in the center of three C‘uO, stackings in the Bi 2223 oxide is free from the effect of quenching and hence essential for the 107 K superconductivity [ 1 1 1. (5 ) From this idea we predicted that 7, of BilSrzCuO, (2201-phase) would be very sensitive to high temperature quenching [ I 1 ] Buckley et al. [ 121 also reported the 7,-sensitive (/I=1. 2. 3) at tem11. of Biz., (Ca,Sr, ~, ),z+,C~,IO~ peratures above 77 K. They added Ca atoms in theit 2201 samples and reported a small contribution of the 22 12-phase to the X-ray diffraction pattern. Flower et al. [ 131 cited a zero resistance 7;. of 76 K for the composition Bi,,Ca,j,Sr,.,CuO,. regarded as the 2201-phase. This was remarkably high. Thus an experiment with the Ca-free 220 1 sample is required to rule out the possibility that the superconducting property manifested was that of the minor 32 13phase. There are structural similarities between the Bibased and Tl-based oxides. Bi$r&uO, is characterized by a structural modulation along the b-axis [ 41. Recently, a similar quenching technique was applied to TI,Ba,Ca,,+ ,Cu,O, ( II = 1. 2, 3 ) by Shimakawa et al. [ 15.161. The 7, of the Tl 2212-phase also increased with quenching, similar to our results of the Bi 2212-phase. They found that the 7; of Tl,BazCu,O, (220 1) increased up to 87 K. This experiment implied the possibility that high-7, superconductivity above 80 K is essentially due to the single CuOz layers. not to the CuOZ stackings. However. this is exceptionable because the T,'s of other oxide

259

T. Ishida, T. Sakuma / T, control of Bi$rzCuOX by quenching

a=5.42(3) A, b=23.5(3) 8, and c=24.5(2) A. Corresponding peaks are marked by an asterisk in fig. 1. Judging from the similarity of the diffraction pattern and lattice constants, we consider that sample B consisted mainly of Bi17Sr16Cu70r, which was reported by Ikeda et al. [ 41. An attempt to obtain a monophasic 2201 sample was not successful when we started from a stoichiometric mixture. The samples were cut by a diamond microsaw in a rectangular shape ( 1.1 x 2.0 x 8.0 mm3). The resistivity p was measured by the AC four terminal method, where the AC current was 1 mA and lockin detection was employed to recover a voltage drop. The sample was glued onto a Cu plate with GE703 1 and cooled in a helium atmosphere from room temperature down to 1.5 K by using a closed cycle helium refrigerator. The temperature T was measured by a calibrated carbon glass thermometer mounted near the sample. Measurements were carried out in cooling as well as in warming with no temperature hysteresis. The as-prepared sample A showed a superconducting transition at 7.14 K (midpoint ) with a breadth of 0.67 K. The resistivity of sample A was 32 m&m at 25 K and 42 mncm at 300 K. Sample B, prepared by the additional sintering at 830°C was semiconducting (p( 10 K) = 12 Qcm, p( 300 K) = 80

superconductors containing a single CuOZ layer in a unit cell remain low. To check the validity of this hypothesis, it is interesting to examine whether or not the analogous compound Bi2Sr2CuZOx is capable of 80 K superconductivity. In this paper, as a series of experiments, we report an attempt to raise the T, of the Bi 2201 compound by means of quenching. The BiZSrZCuO, samples were prepared by a solid state reaction in air from Bi203, SrC03, and CuO. After pelletizing, the samples were reacted at 760°C for 96 h and cooled in a furnace. We denote this state as sample A. sample A was reground, pelletized, further annealed at 830°C for 37 h, and then cooled in the furnace, resulting in sample B. In fig. 1, we show the X-ray powder diffraction pattern of sample A, where the diffraction peaks are indexed by assuming a tetragonal structure (14/mmm) with a=3.797(8) A and c= 24.46 (6) A. We found that the main phase of the sample A is that of the 2201-phase, but a smaller amount of an impurity phase is also seen in the diffraction pattern (marked by * on fig. 1). In fig. 2, we present the X-ray powder diffraction pattern of sample B. One finds that the impurity phase of sample A becomes dominant in sample B. We tentatively indexed the diffraction peaks of fig. 2 by an orthorhombic structure with lattice constants

I

,I

Fig. 1. The X-ray powder diffraction pattern (14/mmm, a=3.797( 8) 8, and c=24.46(6)

I

I

I

I

I

I

II

I

2

SAMPLE

I

a = 3.797(0) c = 24.46(6)

I

I

I

I

A A A

of sample A (sintered at 760°C). The diffraction peaks are indexed by a tetragonal A). The impurity phase contributes the peaks marked by *.

structure

T. Ishida. T Sakuma / T,. control qfBi$+,C‘uO,

260

Fig. 2. The X-ray powder diffraction rhombic

structure

oflatticeconstants,

hy yucnchrng

SAMPLE

B

a= 5.42(3) b= 23.5(3) c = 24.5(2)

A A A

pattern of sample B (sintered at 830°C). The diffraction a=5.42(3) A, h=23.5( 3) A and c=24.5( 2) A.

mQcm) although a slight decrease of the resistivity was found below 7.4 K. The sintering temperature 76O’C of sample A was essential to obtain the superconducting phase. Care should be taken to avoid deterioration of sample quality when quenching sample A from temperatures higher than 76O’C. We examined the T, of sample A as a function of T,, where the same piece of the sample was repeatedly used. Prior to quenching, the sample was annealed at 700°C for I h in air. This process cancels the effect of any preceding phenolization by GE703 1 varnish [ 171. Subsequently, the sample was held at T, and quenched into liquid nitrogen. In table I, we summarize the quenching temperature T,, the annealing time At at T,, the temperatures of O%, 10%. 50 % and 90% resistivities in transition and the resistivityp (at 25 and 300 K). In fig. 3, we show the temperatures of lo%, 50%, and 90% resistivities of sample A as a function of T,. The midpoint 7; increased from 6 to 17 K. At higher T,, we annealed the sample for a short period (see table I) to minimize the growth of the semiconducting phase (see fig. 2). The T, increase from T,=7OO”C to 820°C is remarkable. T, (90%) is as high as 20.9 K for the sample quenched from 820°C. We consider that the T, decrease at 840°C was due to the deterioration of

peaks are indexed

by assummg

an ortho-

the sample, because the sample 7; was not reproducible by successive quenching from lower TQ. For Tq> 76O’C. the T, enhancement might be in competition with the quality change of the sample. The sample was thoroughly melted at 9OO’C. For La- and Y-based systems, T, can be scaled as a function of hole concentration pc contained in the CuOz layers [ 5,6]. This relation was confirmed to hold for Bi 22 12 and 220 1 systems, where a doping technique was used to control p, [ 181. Figure 3 suggests that holes were overdoped in the as-prepared sample A, resulting in a lower T, and the hole concentration was reduced by quenching. As we predicted [ 81, quenching enhanced the 7; of the Bi 2201 sample. However, T, shown in fig. 3 remains low compared to that of Tl,Ba$Zu,O,. To see if it is possible to achieve a higher T,, WCexamine the superconducting onset temperature of the quenched sample. The resistivity of the 8OO’C quenched sample is almost constant at temperatures above 30 K, but the amplification of the resistivity change (0.95-1.3 times the 25 K resistivity) shown in fig. 4 definitely shows that the resistivity drop started at about 60 K. In table I the onset temperatures To for different Tq’s are also listed. *TOseems to correlate with T, except at Tq=600”C. i.e.. we get

T. Ishida, T. Sakuma

/ T, control of Bi~r&‘uO,

by quenching

Table I The quenching temperature T,, the holding duration At at T,, the superconducting transition the onset temperature T, and the resistivities p (25 and 300 K) of quenched sample A. At

T4 (“C)

T,(K)

T0 (K)

(h)

300 400 500 600 700 750 800 820 840

25 16 5 2.5 1 1 0.17 0.08 0.08

0%

10%

50%

90%

3.39 8.43 11.00 10.00 12.86 12.49 14.00 13.63 8.50

4.88 9.05 11.48 11.81 13.30 14.15 14.98 14.92 11.00

6.15 9.58 12.00 13.06 13.85 14.92 16.00 16.92 13.08

7.86 10.83 12.90 14.00 14.71 15.92 17.84 20.92 16.39

0

15 20 35 17 60 55 60 50 50

50

261

p(mQcm) 25 K

300 K

15 20 16 72 20 24 41 33 70

21 23 23 14 26 28 42 31 48

100

Temperature

5 I 200

I

I 500

Ts

I

I

I 800

("C 1

Fig. 3. The resistive transition temperatures ( lo%, 50% and 90%) of sample A as a function of a quenching temperature T,

higher values of To (50-60K) at higher T,.Comparable onset temperatures were observed for several different samples. Further T, enhancement of Bi2SrZCu,0x would be possible if the 220 1 crystal is stable at higher T,. Recently, Adachi et al. [ 19 ] reported an 80 K on-

T,( O%, IO%, 50% and 90%),

temperatures

150

200

( x

250

300

1

Fig. 4. The resistive transition of Bi,Sr,CuO, (sample A) quenched from 800°C where the vertical scale is exaggerated. The onset temperature (60 K) is indicated by V. The inset is the resistive transition at lower temperatures. The resistivities are normalized at 25 K.

set temperature (the zero resistance at 29 K) for BiSr-Cu-0 thin films. They attributed it to the formation of the Ca-free 22 12-phase. It is not plausible to interpret the 60 K superconductivity of fig. 4 by the presence of the 22 12-phase. Our sample was bulk. It was pointed out that the 22 12-phase is never obtained in the atomic ratio of Ca/(Sr+Ca) ~0.2 by the usual ceramics method [ 19 1. Actually, we could not find a trace of the 22 12-phase contribution in the X-ray diffraction pattern.

262

T. Ishldu,

T. Sakurnu

/ I; conrrol ofH~2S’r,C‘u0,

In conclusion, the midpoint T, of the Bi,Sr,CuO, sample was significantly raised from 6 to 17 K by quenching. Our results suggest that holes are overdoped in the as-prepared Bi 2201 sample. We revealed that the superconducting onset temperature of the Ca-free Bi 220 1 compound reached 60 K. The T, was enhanced but still low compared to the Tc’s of the Tl 2201 oxide (87 K) and the Bi 2212 oxide (90 K). The structural modulation of the Bi double layer may suppress the T, of the Bi 2201 compound. Further efforts are needed to explore higher T,‘s in this system of a single CuOz layer.

Acknowledgements The authors thank T. Sasaki for technical support and M. Takagi, K. Takayama and M. Nojima for experimental assistance. This work was partially supported by a grant-in-aid for scientific research (priority areas) from the Ministry of Education. Science and Culture.

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