Physica C 185-189 (1991) 601-602 North-Holland

SUPERCONDUCTIVITY AT 60K IN L a 2 . x S r x C a C u 2 0 O I0<x '%0,4} SYNTHESIZED USING AN 0 2 .

HIP T E C H N I Q U E

Takeshi SAKURAI, Toru YAMASHITA', H. YAMAUCHi and Shoji TANAKA

Superconductivity Research Laboratory, ISTEC. 10-13 Shinonome 1-chome, Koto-ku, Tokyo ! 35, JAPAN

Superconducting La2.xSrxCaCu206 1326 phase) samples (with x in the range of 0 < x <0.4) exhibiting Tc °nset at 60K

were synthesized using an O2-H1P (hot isostatic press) technique. The HIP-processing temperature, Ta. was found to be

the key parameter for obtaining superconductivity Csuperconductor-izalion") in the 326 samples, as long as the other

parameters unchanged. A cation ordering between two cation sites, i.e. 4e and 2a sites, was concluded in the

superconducting 326 sample with x=O,2 by structural refinement. The cation ordering is thought to be one of the major

reasons for the superconductor-ization of 326 samples.

I: INTRODUCTION

As the 326 compound posses crystallographic as well as

chemical features common with conventional high Tc

superconducting cuprates, it had been thought that

superconductivity should occur by chemical dopingll-5l.

Nonetheless, no one had been successful up until Cava.

et.al[6] recently reported that they had successfully

synthesized Lal.6Sro.4CaCu206 with Tc °riser at 60K,

They employed a long-time annealing under a high 0 2

pressure of 20arm. In this present work, we attempt to

search for the optimum condition to "superconductor-ize"

326 samples using an O2-HIP technique and discuss the

reasons for such superconductor-ization.

2: EXPERIMENTAL PROCEDURE

The samples were prepared by a conventional solid-state

reaction method plus an O2-HIP process[7]. Samples

sintered at a variety of temperatures, Ts, (900 °C < Ts~_ 1070

°C), were HIP-processed at various temperatures, Ta, 1920

C _ Ta_ 1270 C), for various periods of time, t, up to

10Oh. For HIP-processing, a mixture of 80%-Ar gas and

20%-0 2 gas of the toL-'d pressure of 1000atm was utilized

The crystallographic structure of the samples were

analyzed by x-ray diffraction. A neutron diffraction was

performed for the structural refinement. Electrical

resistivity was measured as a function of temperature using a

conventional four-probe techmque. The magnetic

susceptibility was measured in a d.c. magnetic field of 10 Oe

using a SQUID magnetometer.

3: RESULTS AND DISCUSSION

Superconducting 326 phase was contained in all samples

with compositions, x=0.1, 0,2.0.3 and 0.4. It seems that

the formed phases and supercondcting properties depend on

Ta. but only a little on Ts. As Ta increases, the normal state

resistivity gets lower, while Tc °nsct remains near 60K for

all the samples except one that HiP-processed at g29 °C.

Vvq~en Ta exceeds 1070 °C, Tc°nSet's gets deteriorated. Ta

higher than 970 ~ is required in order to superconductor-ize

!he 37~6 pha~e. The optimum value for Ta is found zo be

1070'~" for the superconductor-ization. Note that sampte~

with Ta betv~een 970 ~ ar.,d 1170'(" are ~1 single pha~e.

Figure 1 show's the electrical resistivity -vs- temperature

curves for the samples witr~ x=0,2 which were HIF-

processed at various period~ up ~.o ' ~, , =

' Present address: Electron Microscope' Centre, University of Queensland, St. Lucia. Brisbane, Queensland 4G72.

AUSTRALIA

0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V, All rights rcsc~'ed.

602 7". Sakurai et aL / Superconductivi~ at 60K in La2aSrxCaCu206 (O <x~0.4) synthesized

100 . . . . , . . . . , . . . . , . . . . , . . . . , . . . .

• Oh; I s i lnlerl ld

,o :\

a 6 ~ , , a ~ a611'mb '

0 5 0 1 0 0 150 2 0 0 2 5 0 3 0 0

Temperature (K)

Fig.l. Electrical resistivity -vs- temperature for

the "326" samples, La 1.8Sr0.2CaCu206,

in terms of the HIP-process periods, t

=6h, 27h, 50h and 100h, when (Ts, Ta) =

(950 ~ , 1020 L').

(950 ~, 1020 L'-'). As t increases, the quality of the sample gets better. Another evidence was detected in the magnetic susceptibility measurement. The longer t is, the larger the Meissner volume fraction is.

The crystal structure refinement was made for the

superconducting La 1.8Sr0.2CaCu206.01 +0.01 [81. In the

final refinement, a cation ordering was concluded. That is,

Sr ions occupy the 4e sites exclusively and Ca ions occupy

the 4e sites with a very low value of occupancy. This result agrees with other refinement data[9](Table. I ).

There seems to be two independent effects in the O2-HIP-

process. One is the total pressure effect, which should act a

driving force for the cation ordering. The other is the

oxygen pressure effect, that may control the oxygen content. It is suspected that a high oxygen pressure effectively

introduces oxygen ions into the defect sites around the cation

at 4e site, Therefore, in the 326 sample O2-HIP-processed ,

the Sr ion substituting only for La ion should be effective as a hole donor to the CuO 2 planes.

4: CONCLUSION

The "superconductor-ized" 326 compounds were

successfully obtained by means of an O2-HIP technique.

The superconducting 326 compounds were obtained in the

range of 0<x~<0.4, when Ta>970~C. The optimum

value of Ta was determined to be 1070~, regardless the

Table.l. Refined structure parameters for the

superconducting "326" samples.

3ornpot,,~l 3ond length 3aAa.O I xa

This work Lal .SSdL2CaCu206.O,I

2.4934(~ 2.6530(9), 2.1537(31

,2.345911si

,,,,t/..~ a-O 1 )(4 La/Sr/Cs-02 )(4 L.a/S~/Ca-02 xl Cu-O],y4, 1.911510' ) Cu-02 xt 2.3068(20) ~u-03 I(1 1.6704~1 C a t i o n . r a t i o

Laur~/Cil (2a} La/Sr/Ca (4e) Occupancy 01 02 03 Lattice parameter a c

0.14(010.~4~i

CavJ's 9roupe 191 ,. ka1.6.SrO.4C&Cu205.94

2.348(~ 1.91201 ' 2.293(61

0.1gl I0 I 0.8'15 0.63 / 0.1, / 0.07 0.708 / 0.2 10,096

1.02 1.02 0.97 0.03

0.955

3.0207 3.8208 19.5434 19.5993

2.11 2.14

value of Ts. Single phase compounds were obtained for the Sr content in the range of 0.1 < x _ 0 . 3 5 .

A cation ordering was determined by a joint x-ray/neutron

diffraction technique for the superconducting sample. Cation ordering occurred under a high pressure, i.e. during

HIP-processing, was likely to be one of the reasons for the superconductor-ization of the 326 compounds.

ACKNOWLEDGEMENT

Authors acknowledge to Dr. J. O. Willis of SRL-ISTEC

and Dr. G. H. Kwei of Los Alamos National Laboratory for

their helpful discussions. This work was supported by

New Energy and Industry Technology Development

Organization as a part of program for R&D of Basic Technology for Future Industries

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