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New superconducting cuprates in the Sr-Ca-Cu-O system
PHYSICA
Physica C 212 (1993) 164-168 North-Holland
New superconducting cuprates in the Sr-Ca-Cu-O system Seiji Adachi ", H. Yamauchi
a
Shoji Tanaka a a n d N o b u o M 6 r i ~
a Superconductivity Research Laboratoo'. International Superconductivity "l~,chnology ( "enter. 1~I O-13 Shinonome, holo-ku, Tokyo 135. Japan b Institute for Solid State Physics, University of 7bkyo. ,'7-22-l Roppongi, Mmato-ku. Tokyo 106, Japan
Received 22 March 1993
Sr-Ca-Cu-O samples which exhibited bulk superconductivity and contained phases of layer structures with long-range periodicities of (10.2+Y4m) A (where m=0, I and 2) were obtained using a high-pressure technique. Phases contained in the samples were investigated by electron diffraction, transmission electron microscopy and X-ray powder diffraction. Members with n = 2, 3 and 4 of a homologous series written as "02 ( n - 1)n" were proposed for these new superconducting phases.
1. Introduction Recently, much attention has been p a i d to highpressure synthesis as a means o f preparing cuprate superconductors o f the "infinite-layer" structure. In 1991, the first "infinite-layer" structural cuprates, (Srl_xLnx)CuO2 ( L n = N d [ 1 ], La [2] ) which were n-type superconductors with T~'s o f ~ 4 0 K, were synthesized by high-pressure techniques. Later, A z u m a et al. [3] reported superconductivity o f 110 K for (Sro.7Cao.3)o.9CuO2 synthesized under high pressure. They claimed that this s u p e r c o n d u c t o r had an "'infinite-layer" structure and was the first p-type cuprate not to contain C u - O s p y r a m i d a l sheets in the crystal. The present authors [4,5] also searched for superconducting cuprates in the S r - C a - C u - O system under high pressures, and p o i n t e d out the presence o f new superconducting phases whose crystal structures had periodicities o f ( 1 0 . 2 + Y 4 m ) (where rn=O, I and 2) along the c-axis, proposing tentative structures, each o f which consisted o f an "'infinite-layer" block and an unknown block. In this paper, we present high-resolution micrographs and propose plausible models for the structures.
2. Experimental The S r - C a - C u - O samples were prepared at 930°C
lor 30 min under a high pressure o f 5 GPa. Details of the sample preparation procedure have been given elsewhere [5]. Various nominal compositions were a t t e m p t e d to obtain samples which contained phases o f layer structures with long-range periodicities of ( 1 0 . 2 + 3 . 4 m ) A where m = 0 . 1. 2 . . . . . When sintercd under the same conditions, phases with periodicities for m = 0 , 1 and 2 were p r e d o m i n a n t in samples (a) Sr0.sCao.lsCUO2.m, ( b ) Sro.6Cao.33~ C u O e m and (c) Sro.6sCao3CuO2.m, respectively. Phases in the o b t a i n e d samples were examined by Xray p o w d e r diffraction (against Cu K a radiation ). Sample ( b ) , Sr0.6Cao.333CuO2.m, containing a phase having l 3.6 A. periodicity, was e m p l o y e d for electron diffraction and transmission electron microscopic studies. F o r the m e a s u r e m e n t o f superconductivity, the DC magnetic susceptibility for obtained samples was m o n i t o r e d using a S Q U I D m a g n e t o m e t e r in the "'field-cooling" m o d e under an external magnetic field o f 10 Oe.
3. Results and discussion An electron diffraction pattern lbr one o f the phases in sample ( b ) is given in fig. 1. The OOI ( / = e v e n ) and hOl ( h + / = e v e n ) spots a p p e a r e d and the lattice constants, a and c, o f the phase were det e r m i n e d to be 3.86 A and 27.2 A, respectively, as-
0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
S. Adachi et al. /New superconducting cuprates in the Sr-Ca-Cu-O system
165
I
Fig. 1. Electron diffraction pattern for one of the phases in sample (b), Sro.6Cao.333CuO2no.
suming that it had a tetragonal symmetry. Figures 2 and 3 are transmission electron micrographs for sample (b). The higher-magnification image given in fig. 2 suggested that there were 7 atomic layers within a 13.6 A stacking. Stacking periods of 10.2 A and 17.0 A were also seen in the lower-magnification image given in fig. 3. It should be noted that the differences in stacking periods, i.e. 17.0-13.6 A = 13.610.2/~=3.4 A, are nearly equal to the c-axis length of the "infinite-layer" structure [ 6-8 ]. Taking these results into account, the structural model shown in fig. 4(iii) is considered to be one of the most plausible structures for the new phase which has a stacking periodicity of 13.6 ~.. This structure is related to the K2NiF4 structure shown in fig. 4(i), and can be understood as the La2SrCu206 structure (as demonstrated in fig. 4 ( i i ) ) with an intercalated "infinite-layer" unit between two facing bottoms of Cu05 pyramids. Employing the conventional way of naming superconducting cuprates, the structure of this new phase with 13.6 A stacking periodicity may be termed the "0223" structure. No cuprates of the "0223" structure have ever been reported. The periodicity of 17.0/I, corresponds to another new phase which can be termed "0234" (as demonstrated in fig. 4(iv)). Therefore, it is likely that there exists a
Fig. 2. Transmission electron micrograph with high magnification for sample (b), S r o . 6 C a o . 3 3 3 C u 0 2 . 1 o .
new homologous series of "02 ( n - 1 )n" layered cuprates in the Sr-Ca-Cu-O system. Figure 5 shows X-ray powder diffraction patterns for samples, (a) Sro.sCao.15CuOz.lo, (b) Sro.6Cao.333CuO2.1o and (c) Sro.65Cao.3CuO2.1o. The peaks at 20= 5.2 °, 6.5 ° and 8.5 ° clearly indicate the presence of phases with long-range periodicities of 10.2 ?,, 13.6 A and 17.0 A, respectively. In the figure, parenthesized indices and asterisks are for the "infinite-layer" phase and CuO, respectively. Most of the other peaks were successfully indexed assuming the "0212", "0223" and "0234" structures of tetragonal symmetry for patterns (a), (b) and (c), respectively. Thus, it is quite conclusive that the predominant phases in the three samples were members of the "02 ( n - 1 ) n" homologous series. The " 0 2 ( n - 1 )n" compounds would become ptype superconductors if appropriate amounts of holes were introduced into the CuO2 planes. If the proposed "0212", "0223" and "0234" phases contain neither vacancies nor interstitial ions, the compounds may be expressed by the chemical formulae
166
S. Adachi et al. /New superconducting cuprates m the Sr-('a-Cu-O s3'stem
•
e;
, - k -;-2-;*. -,-.-;-,-:-;-,-;-;ee+-)';),,,~i,~z; ".<" - -.. . . . . . .
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.
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.
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.= ,,c~'+
.
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-
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~ .v
o
:
.--:
-.• :"
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Fig. 3. Transmission electron micrograph with low magnification for sample ( b ), Sro.6Cao.3 33CuO 2. io.
(Sr, Ca)3Cu206, (Sr. Ca)4Cu308 and (Sr, Ca)5Cu4Olo , yielding average valences o f +3.00, + 2 . 6 7 and + 2 . 5 0 for the Cu ion, respectively. According to a conventional and empirical rule [9], these values are too high for the occurrence of superconductivity. To induce superconductivity, the Cu valency may be reduced by certain methods such as cation substitution and control of oxygen nonstoichiometry. Figure 6 shows the temperature dependences of DC magnetic susceptibility for the three samples, (a),
(b) and (c). Actually all the samples showed bulk superconductivity. The Meissner volume fractions (at 5 K) of these samples were estimated to be ~ 20%, ~ 30% and ~ 15%, respectively• Comparing intensities of the peaks at 2 0 ~ 36 ° and 39 ° in patterns (a) and (b) in fig. 5, samples (a) Sro.sCao.~sCuOzlo and (b) Sro.6Cao.333CuO2.1 o c o n t a i n e d little "infinite-layer" structural phases. (Note that the main peak of the "infinite-layer" structure. i.e. 101 reflection, appears at 2 0 ~ 3 6 ° and the two major peaks of nearly equal intensities from CuO appear at 2 0 ~ 3 6 ° and 39°.) Nonetheless, both samples exhibited fairly large Meissner signals. Therefore, it was most likely that the observed Meissner signals (curves, (a) and (b) in fig. 6) were mainly from the "0212" and "0223" phases, respectively. As seen in pattern (c) in fig. 5 for the sample containing the " 0 2 3 4 " phase, a significant amount of an "infinite-layer" structural phase remained. Thus, it is not possible to exclude the possibility that the Meissner signal from this sample was only from the "infinite-layer" structural phase. However. it appears rather that the " 0 2 3 4 " phase exhibits superconductivity, as do the "'0212" and "0223" phases. It is suspected that, to decrease the Cu valence and have superconductivity occur, these "02 ( n - 1 )n'" phases could contain a certain amount of oxygen vacancies, or more concretely, the apical oxygen ions of Cu-O5 pyramids were partially vacant.
4. Summary The layer structures of superconducting cuprates with periodicities of ( 1 0 . 2 + 3 . 4 m ) ~ (where m = 0 . 1 and 2) in the S r - C a - C u - O system were investigated by means of electron diffraction, transmission electron microscopy and X-ray powder diffraction. The presence of "02 ( n - 1 ) n" phases with n = 2. 3 and 4 was confirmed by high-resohition electron microscopic observations. Each of the " 0 2 ( n - 1 )n'" phases consisted of two different structural blocks, that is, a double-layer rock-salt block, i.e. (Sr, Ca)2O2(~ _~), and an "'infinite-layer" block. The presence of a homologous series consisting of the "02 ( n - 1 )n" (n = 1, 2, 3... ) phases was most plausible.
S. Adachi et al. /New superconducting cuprates in the S r - C a - C u - O system
0
O
~
"0201"
O
0
0
0
0 0 .--..0--© 0
p3.6A~
(ii) "0212"
K2NiF4 structure
La2SrCu206 structure
l 67
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~_.~'~3fiI~ite-'ayC,"
0
i (iv)
(iii) "0223"
"0234"
Fig. 4. Illustrations of crystal structures for (i) KzNiF4, (ii) La2SrCu206 and (iii and iv) new layered cuprates.
0.001
CuKo,
Field Cooled: H=IO Oe 0.000 D
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20
30
40
59
20 (deg) Fig. 5. X-ray powder diffraction patterns for samples (a) Sro.aCao.|5CuO2.1o, (b) Sro.6Cao.3nCuOz.to and (c) Sro.65Cao.3CuO2.,o. (Indices, parenthesized indices and asterisks are for the "02(n- 1)n" phase, the "infinite-layer" phase and CuP, respectively. )
Acknowledgements The authors would like to thank T. Yagi, H. T a b
20
4o 60 Temperature
8o loo ( K )
120
Fig. 6. Temperature dependences of DC magnetic susceptibility for samples (a) Sro.sCao.~sCuO2.m, (b) Sro.6Cao.333CuOzloand ( C) Sro.65Cao.3CuO2. lo.
ahashi, W. Utsurni, T. U c h i d a and T. H i s h i n u m a ( o f the University o f Tokyo) for their helpful advice concerning high-pressure synthesis. They also thank N. Koshizuka, M. M u r a k a m i , A. Tokiwa-Yamarepro, P. Laffez, N. Sugii, K. Kubo, N. Seiji and N. W a t a n a b e ( o f SRL-ISTEC) for their helpful discus-
168
S. Adachi et al. / New superconducting cuprates in the S r - C a - ( ' u - O system
sions. T h i s w o r k was s u p p o r t e d by N E D O in t h e p r o g r a m for R & D Industries.
o f Basic T e c h n o l o g y for F u t u r e
References [ 1] M.G. Smith, A. Manthiram, J. Zhou, J.B. Goodenough and J.T. Mackert, Nature 351 (1991) 549, [2] G. Er, Y. Miyamoto, F. Kanamaru and S. Kikkawa, Physica C 181 (1991) 206.
[3] M. Azuma, Z, Hiroi, M. Takano, Y. Bando and Y. Takeda, Nature 356 (1992) 775. [4] S. Adachi, T. Sakurai, H. Yamauchi, H. Takahashi and N. M6ri, Proc. oflSS'92 (in press). [5] S. Adachi, H. Yamauchi, S. Tanaka and N. M6ri, Physica ( 208 (1993) 226. [6] T. Siegrist, S.M. Zahurak, D.W. Murphy and R.S. Roth, Nature 334 (1988) 231. [7] H. Yamane, Y. Miyazaki and T. Hirai, J. Ceram. Soc Jpn 97 (1989) 143 (in Japanese). [8] M. Takano, Y. Takeda, H. Okada, M. Miyamoto and l Kusaka, Physica C 159 (1989} 375. [9] J.B. Torrance, A. Bezinge, A.I. Nazzal and S.S.P. Parkin. Physica C 162-164 (1989) 291.
Physica C 212 (1993) 164-168 North-Holland
New superconducting cuprates in the Sr-Ca-Cu-O system Seiji Adachi ", H. Yamauchi
a
Shoji Tanaka a a n d N o b u o M 6 r i ~
a Superconductivity Research Laboratoo'. International Superconductivity "l~,chnology ( "enter. 1~I O-13 Shinonome, holo-ku, Tokyo 135. Japan b Institute for Solid State Physics, University of 7bkyo. ,'7-22-l Roppongi, Mmato-ku. Tokyo 106, Japan
Received 22 March 1993
Sr-Ca-Cu-O samples which exhibited bulk superconductivity and contained phases of layer structures with long-range periodicities of (10.2+Y4m) A (where m=0, I and 2) were obtained using a high-pressure technique. Phases contained in the samples were investigated by electron diffraction, transmission electron microscopy and X-ray powder diffraction. Members with n = 2, 3 and 4 of a homologous series written as "02 ( n - 1)n" were proposed for these new superconducting phases.
1. Introduction Recently, much attention has been p a i d to highpressure synthesis as a means o f preparing cuprate superconductors o f the "infinite-layer" structure. In 1991, the first "infinite-layer" structural cuprates, (Srl_xLnx)CuO2 ( L n = N d [ 1 ], La [2] ) which were n-type superconductors with T~'s o f ~ 4 0 K, were synthesized by high-pressure techniques. Later, A z u m a et al. [3] reported superconductivity o f 110 K for (Sro.7Cao.3)o.9CuO2 synthesized under high pressure. They claimed that this s u p e r c o n d u c t o r had an "'infinite-layer" structure and was the first p-type cuprate not to contain C u - O s p y r a m i d a l sheets in the crystal. The present authors [4,5] also searched for superconducting cuprates in the S r - C a - C u - O system under high pressures, and p o i n t e d out the presence o f new superconducting phases whose crystal structures had periodicities o f ( 1 0 . 2 + Y 4 m ) (where rn=O, I and 2) along the c-axis, proposing tentative structures, each o f which consisted o f an "'infinite-layer" block and an unknown block. In this paper, we present high-resolution micrographs and propose plausible models for the structures.
2. Experimental The S r - C a - C u - O samples were prepared at 930°C
lor 30 min under a high pressure o f 5 GPa. Details of the sample preparation procedure have been given elsewhere [5]. Various nominal compositions were a t t e m p t e d to obtain samples which contained phases o f layer structures with long-range periodicities of ( 1 0 . 2 + 3 . 4 m ) A where m = 0 . 1. 2 . . . . . When sintercd under the same conditions, phases with periodicities for m = 0 , 1 and 2 were p r e d o m i n a n t in samples (a) Sr0.sCao.lsCUO2.m, ( b ) Sro.6Cao.33~ C u O e m and (c) Sro.6sCao3CuO2.m, respectively. Phases in the o b t a i n e d samples were examined by Xray p o w d e r diffraction (against Cu K a radiation ). Sample ( b ) , Sr0.6Cao.333CuO2.m, containing a phase having l 3.6 A. periodicity, was e m p l o y e d for electron diffraction and transmission electron microscopic studies. F o r the m e a s u r e m e n t o f superconductivity, the DC magnetic susceptibility for obtained samples was m o n i t o r e d using a S Q U I D m a g n e t o m e t e r in the "'field-cooling" m o d e under an external magnetic field o f 10 Oe.
3. Results and discussion An electron diffraction pattern lbr one o f the phases in sample ( b ) is given in fig. 1. The OOI ( / = e v e n ) and hOl ( h + / = e v e n ) spots a p p e a r e d and the lattice constants, a and c, o f the phase were det e r m i n e d to be 3.86 A and 27.2 A, respectively, as-
0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
S. Adachi et al. /New superconducting cuprates in the Sr-Ca-Cu-O system
165
I
Fig. 1. Electron diffraction pattern for one of the phases in sample (b), Sro.6Cao.333CuO2no.
suming that it had a tetragonal symmetry. Figures 2 and 3 are transmission electron micrographs for sample (b). The higher-magnification image given in fig. 2 suggested that there were 7 atomic layers within a 13.6 A stacking. Stacking periods of 10.2 A and 17.0 A were also seen in the lower-magnification image given in fig. 3. It should be noted that the differences in stacking periods, i.e. 17.0-13.6 A = 13.610.2/~=3.4 A, are nearly equal to the c-axis length of the "infinite-layer" structure [ 6-8 ]. Taking these results into account, the structural model shown in fig. 4(iii) is considered to be one of the most plausible structures for the new phase which has a stacking periodicity of 13.6 ~.. This structure is related to the K2NiF4 structure shown in fig. 4(i), and can be understood as the La2SrCu206 structure (as demonstrated in fig. 4 ( i i ) ) with an intercalated "infinite-layer" unit between two facing bottoms of Cu05 pyramids. Employing the conventional way of naming superconducting cuprates, the structure of this new phase with 13.6 A stacking periodicity may be termed the "0223" structure. No cuprates of the "0223" structure have ever been reported. The periodicity of 17.0/I, corresponds to another new phase which can be termed "0234" (as demonstrated in fig. 4(iv)). Therefore, it is likely that there exists a
Fig. 2. Transmission electron micrograph with high magnification for sample (b), S r o . 6 C a o . 3 3 3 C u 0 2 . 1 o .
new homologous series of "02 ( n - 1 )n" layered cuprates in the Sr-Ca-Cu-O system. Figure 5 shows X-ray powder diffraction patterns for samples, (a) Sro.sCao.15CuOz.lo, (b) Sro.6Cao.333CuO2.1o and (c) Sro.65Cao.3CuO2.1o. The peaks at 20= 5.2 °, 6.5 ° and 8.5 ° clearly indicate the presence of phases with long-range periodicities of 10.2 ?,, 13.6 A and 17.0 A, respectively. In the figure, parenthesized indices and asterisks are for the "infinite-layer" phase and CuO, respectively. Most of the other peaks were successfully indexed assuming the "0212", "0223" and "0234" structures of tetragonal symmetry for patterns (a), (b) and (c), respectively. Thus, it is quite conclusive that the predominant phases in the three samples were members of the "02 ( n - 1 ) n" homologous series. The " 0 2 ( n - 1 )n" compounds would become ptype superconductors if appropriate amounts of holes were introduced into the CuO2 planes. If the proposed "0212", "0223" and "0234" phases contain neither vacancies nor interstitial ions, the compounds may be expressed by the chemical formulae
166
S. Adachi et al. /New superconducting cuprates m the Sr-('a-Cu-O s3'stem
•
e;
, - k -;-2-;*. -,-.-;-,-:-;-,-;-;ee+-)';),,,~i,~z; ".<" - -.. . . . . . .
'
/
"
"
:
"
.
7
"
-+'
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~'2 ~ % , ' .
>
' ' . . -
.
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. :
:)i
_
.
.-
. ::
,:.:
. . . . .
.= ,,c~'+
.
-::"-:--"-
f-.:5";:"5
-.---
.
-.= t..-:..-
•-"-.. .
-
-. . . . . .
l e ~ ..% ~ .
~ .v
o
:
.--:
-.• :"
"
: ; o
-
. . . . . . .
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..
-';-3
. . .
Fig. 3. Transmission electron micrograph with low magnification for sample ( b ), Sro.6Cao.3 33CuO 2. io.
(Sr, Ca)3Cu206, (Sr. Ca)4Cu308 and (Sr, Ca)5Cu4Olo , yielding average valences o f +3.00, + 2 . 6 7 and + 2 . 5 0 for the Cu ion, respectively. According to a conventional and empirical rule [9], these values are too high for the occurrence of superconductivity. To induce superconductivity, the Cu valency may be reduced by certain methods such as cation substitution and control of oxygen nonstoichiometry. Figure 6 shows the temperature dependences of DC magnetic susceptibility for the three samples, (a),
(b) and (c). Actually all the samples showed bulk superconductivity. The Meissner volume fractions (at 5 K) of these samples were estimated to be ~ 20%, ~ 30% and ~ 15%, respectively• Comparing intensities of the peaks at 2 0 ~ 36 ° and 39 ° in patterns (a) and (b) in fig. 5, samples (a) Sro.sCao.~sCuOzlo and (b) Sro.6Cao.333CuO2.1 o c o n t a i n e d little "infinite-layer" structural phases. (Note that the main peak of the "infinite-layer" structure. i.e. 101 reflection, appears at 2 0 ~ 3 6 ° and the two major peaks of nearly equal intensities from CuO appear at 2 0 ~ 3 6 ° and 39°.) Nonetheless, both samples exhibited fairly large Meissner signals. Therefore, it was most likely that the observed Meissner signals (curves, (a) and (b) in fig. 6) were mainly from the "0212" and "0223" phases, respectively. As seen in pattern (c) in fig. 5 for the sample containing the " 0 2 3 4 " phase, a significant amount of an "infinite-layer" structural phase remained. Thus, it is not possible to exclude the possibility that the Meissner signal from this sample was only from the "infinite-layer" structural phase. However. it appears rather that the " 0 2 3 4 " phase exhibits superconductivity, as do the "'0212" and "0223" phases. It is suspected that, to decrease the Cu valence and have superconductivity occur, these "02 ( n - 1 )n'" phases could contain a certain amount of oxygen vacancies, or more concretely, the apical oxygen ions of Cu-O5 pyramids were partially vacant.
4. Summary The layer structures of superconducting cuprates with periodicities of ( 1 0 . 2 + 3 . 4 m ) ~ (where m = 0 . 1 and 2) in the S r - C a - C u - O system were investigated by means of electron diffraction, transmission electron microscopy and X-ray powder diffraction. The presence of "02 ( n - 1 ) n" phases with n = 2. 3 and 4 was confirmed by high-resohition electron microscopic observations. Each of the " 0 2 ( n - 1 )n'" phases consisted of two different structural blocks, that is, a double-layer rock-salt block, i.e. (Sr, Ca)2O2(~ _~), and an "'infinite-layer" block. The presence of a homologous series consisting of the "02 ( n - 1 )n" (n = 1, 2, 3... ) phases was most plausible.
S. Adachi et al. /New superconducting cuprates in the S r - C a - C u - O system
0
O
~
"0201"
O
0
0
0
0 0 .--..0--© 0
p3.6A~
(ii) "0212"
K2NiF4 structure
La2SrCu206 structure
l 67
|17OA0 "
~_.~'~3fiI~ite-'ayC,"
0
i (iv)
(iii) "0223"
"0234"
Fig. 4. Illustrations of crystal structures for (i) KzNiF4, (ii) La2SrCu206 and (iii and iv) new layered cuprates.
0.001
CuKo,
Field Cooled: H=IO Oe 0.000 D
5
o
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'~l
0 0 0
-o.001
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•
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•."
n uunmullnl O
{a) OOgtlIOOOD9016
-0.00% 10
20
30
40
59
20 (deg) Fig. 5. X-ray powder diffraction patterns for samples (a) Sro.aCao.|5CuO2.1o, (b) Sro.6Cao.3nCuOz.to and (c) Sro.65Cao.3CuO2.,o. (Indices, parenthesized indices and asterisks are for the "02(n- 1)n" phase, the "infinite-layer" phase and CuP, respectively. )
Acknowledgements The authors would like to thank T. Yagi, H. T a b
20
4o 60 Temperature
8o loo ( K )
120
Fig. 6. Temperature dependences of DC magnetic susceptibility for samples (a) Sro.sCao.~sCuO2.m, (b) Sro.6Cao.333CuOzloand ( C) Sro.65Cao.3CuO2. lo.
ahashi, W. Utsurni, T. U c h i d a and T. H i s h i n u m a ( o f the University o f Tokyo) for their helpful advice concerning high-pressure synthesis. They also thank N. Koshizuka, M. M u r a k a m i , A. Tokiwa-Yamarepro, P. Laffez, N. Sugii, K. Kubo, N. Seiji and N. W a t a n a b e ( o f SRL-ISTEC) for their helpful discus-
168
S. Adachi et al. / New superconducting cuprates in the S r - C a - ( ' u - O system
sions. T h i s w o r k was s u p p o r t e d by N E D O in t h e p r o g r a m for R & D Industries.
o f Basic T e c h n o l o g y for F u t u r e
References [ 1] M.G. Smith, A. Manthiram, J. Zhou, J.B. Goodenough and J.T. Mackert, Nature 351 (1991) 549, [2] G. Er, Y. Miyamoto, F. Kanamaru and S. Kikkawa, Physica C 181 (1991) 206.
[3] M. Azuma, Z, Hiroi, M. Takano, Y. Bando and Y. Takeda, Nature 356 (1992) 775. [4] S. Adachi, T. Sakurai, H. Yamauchi, H. Takahashi and N. M6ri, Proc. oflSS'92 (in press). [5] S. Adachi, H. Yamauchi, S. Tanaka and N. M6ri, Physica ( 208 (1993) 226. [6] T. Siegrist, S.M. Zahurak, D.W. Murphy and R.S. Roth, Nature 334 (1988) 231. [7] H. Yamane, Y. Miyazaki and T. Hirai, J. Ceram. Soc Jpn 97 (1989) 143 (in Japanese). [8] M. Takano, Y. Takeda, H. Okada, M. Miyamoto and l Kusaka, Physica C 159 (1989} 375. [9] J.B. Torrance, A. Bezinge, A.I. Nazzal and S.S.P. Parkin. Physica C 162-164 (1989) 291.