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High-pressure synthesis of superconducting Sr-Ca-Cu-O samples
Physiea C 208 (1993) 226-230 North-Holland
High-pressure synthesis of superconducting Sr-Ca-Cu-O samples Seiji Adachi
a,
H. Y a m a u c h i a, Shoji T a n a k a
a
and Nobuo M6ri b
a Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo 135, Japan b Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan Received 22 january 1993
Samples with nominal compositions of Sro.6Cao.333CuO2.oO+z(z=0.00-0.20) were prepared using a high-pressure technique. The crystallographic structure of the major phase in the sample with z = 0.00 was an "infinite-layer" type. With increasing z, the "infinite-layer" phase decomposed and new compounds having periodic structures of 13.6 A and 10.3 A along the c-axis appeared. The samples with z > 0.10, which contained little of the "infinite-layer" phase, exhibited bulk superconductivity. This suggests the presence of new superconducting phases in addition to the "infinite-layer" phases in the Sr-Ca-Cu-O system.
1. Introduction
2. Experimental
Since the discovery of a (Sr, Ca)CuO 2 compound consisting of stacked pairs of a Cu-O2 sheet and a single cationic layer, known as the "infinite-layer" structure [ 1,2 ], many researchers have attempted to make the compound superconductive. In 1991, the first "infinite-layer" superconductors, (Srl_xLnx)CuO2 ( L n = N d [3], La [4] ), having Tcs of ~ 40 K were synthesized using high-pressure techniques. These superconductors are n-type. Later, Azuma et al. [ 5 ] reported superconductivity at 110 K for (Sro.7Cao.3)o.9CuO2. They claimed that this superconductor had an "infinite-layer" structure and was the first p-type cuprate without Cu-O5 pyramidal sheets. However, they did not succeed in synthesizing pure single-phase samples which should have shown single-step superconducting transitions in both diamagnetic and electrical resistivity measurements. Previously, we prepared (Sr, Ca)o.9sCuO2.oo+z ( S r : C a ~ 1 : 1, z = 0 - 0 . 2 0 , nominally) samples and observed superconductivity at 110 K (diamagnetic onset temperature) in a Sro.5oCao.45CuO2.o5 sample. However, it was difficult to prepare single-phase samples [ 6 ]. In this paper, we have obtained evidence for the presence of new phases in the S r - C a - C u - O system.
The S r - C a - C u - O samples were prepared using a high-pressure technique, described below. S r 2 f u O 3 , Ca2CuO3, CaO2 and CuO powders were used as starting materials. These powders were mixed in the appropriate ratios. The powder mixture was charged in a gold cell surrounded by a BN separator and compressed up to 5 GPa using a cubic-anvil-type apparatus. Pyrophyllite was used for the pressure transmitting medium. During compression, the powder mixture was heated to about 930°C for 30 min by passing an electrical current through a carbon heater. Generally, KC103 or KCIO4 is used as an oxidizer and the oxidizer is charged in a gold cell together with the powder mixture. To avoid the inclusion of KC1 in the product, the oxidizer is usually put in the gold cell unmixed with the charged powder. In the present experiment, when KCIO3 or KC104 was used, it was difficult to avoid inhomogeneous oxidation of the sample powder mixture. To circumvent this problem, we utilized CaO2 as one of the starting materials; CaO2 was expected to play the role not only of the oxidizer but also of the Ca source. In this way we could anticipate obtaining homogeneously oxidized samples. The effects of an oxidizing atmosphere on both the structural and superconducting properties were in-
0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
S. Adachi et al. / High-pressure synthesis of superconducting SCCO
vestigated. Samples with the nominal composition of Sro.6Cao.aa3CuO2.oo+z were prepared. The oxidizing atmosphere was controlled by varying the oxygen content, z, in the range of 0.00-0.20. The crystal structures of these samples were examined by powder X-ray diffraction (XRD; using Cu Kct radiation) and transmission electron microscopy (TEM). In order to detect superconductivity, electrical resistivity was measured by the conventional four-probe method. Furthermore, the DC magnetic susceptibility was monitored using a SQUID magnetometer in the "filed-cooling" mode employing an external field of 10 Oe.
3. Results and discussion Figure 1 shows X R D patterns of the prepared samples. For the sample with z = 0.00, the main phase was of the "infinite-layer" structure. Some reflection peaks form CuO and some unknown phases were also present. With increasing z, the "infinite-layer" phase
227
CuK~ h
5
.
°
o
z=O.O0
v
m
co (-
0.05 0.10 0.15 [cu%
0.20 10
20
30
40
50
20 (deg) Fig. 1. Powder X-ray diffraction patterns for samples of Sro.6Cao.3a3CuO2.oo+z (z=0.00-0.20). (Indices are for the "infinite-layer" phase. )
seemed to decompose and different phases appeared. Generally, two intense reflection peaks from CuO are seen at 20=35.6 ° and 38.8 °, and the intensities of both peaks are nearly the same. (Strictly
Fig. 2. Transmission electron microscope image for the sample of Sro.6Cao.333CuO2.~.
S. Adachi et al. / High-pressure synthesis of superconducting SCCO
228
speaking, the former peak is slightly stronger than the latter one.) The 101 reflection from the "infinite-layer" structure is the strongest for this phase and is expected to appear at 2 0 ~ 3 5 ° [7]. Unfortunately, this diffraction angle coincides with the 20 for the strongest reflection from CuO. However, by comparing the intensities of the peaks at 20= 35-36 ° and at about 39 ° (shown in fig. 1 ), the "infinitelayer" phase seemed to decompose as z increased and practically disappeared for samples with z_> 0.10. Some reflections clearly appeared at 20 lower than 10 ° fro samples with z > 0.05, indicating long-range ordered structures. Peaks at 20= 6.5 ° and 8.5 ° were observed for samples with z=0.05-0.15 and with z=0.20, respectively. These reflections correspond to long-range periodicities of 13.6 A and 10.3 /k, respectively. Figure 2 shows a TEM image for the sample with z=0.15. A periodic structure of 13.6/~ was clearly observed. Furthermore, each 13.6/k stacking could be divided into three parts, with thicknesses of 5.1 A/3.4 A/5.1 A. another type of stacking of 17.0 A width was also observed. It should be notes that the difference in width between the two types of stack-
ing, i.e. 17.0-13.6 = 3.4/k, is equal to the c-axis length of the known "infinite-layer" structure. These results suggested that there was a new series of compounds with stacking periods of ( 10.2 + 3.4n)A where n = 0, 1, 2.... It was likely that the crystal structure of the compounds was composed of two structural blocks which were the "infinite-layer" block and an unknown one. Assuming that the new compounds would have a crystal structure related to the "infinite-layer one, we speculated that the crystal structure was as shown in fig. 3(c). In the figure, the structures for Bi- or Tl-based cuprate superconductors (a) and the "infinite-layer" compound (b) are also drawn for comparison. This proposed structure resembles those for Bi- or Tl-based superconductors. If such compounds existed, they would become new high-T¢ superconductors after appropriate introduction of holes. Figure 4 shows the temperature dependence of DC magnetic susceptibility for the prepared samples. Meissner signals were detected for all the samples, especially the samples with z=0.05-0.15. These showed fairly large Meissner signals which indicated superconducting volume fractions of ~ 20% at 5 K.
--0 0
0
0
-
© ~
0
~
CuO2 sheet
~5.1 L [:it~1: ~t~,~
~F
- - unknown O
-o 0
0
"
O
O
(a) Bi, Tl-based cuprate superconductors
~
O
0
0
O
O
O
O
0
"~ : 0
-3.4 A ~
--
--0
©
i
A W
(b)
(c)
the "infinite-layer" compounds
~
CuO2 sheet - -
the __"infinite-layer" structure
this work
Fig. 3. Illustration o f crystal structures for (a) Bi- or Tl-based cuprate superconductors, (b) the "infinite-layer" compounds, and (c) new compounds.
S. Adachi et aL / High-pressure synthesis of superconducting SCCO
Field Cooled
: H=IO
and sharp transition to the superconducting state) and the appearance of periodic structures with stackings of 13.6 or 10.3 ,~. (Those phases shown schematically in fig. 3 (c) seemed to exist. )
Oe
0.0@0
z=O.O0
== - 0 . 0 0 1
,,A
• • • •
°
°
229
"
° oo
°
.
..
=+
.0
=o +
/-o,o
~- 0 . 0 5
,, -0oo
/
÷ ÷÷÷÷÷÷*÷+
": . . . .
4. S u m m a r y
+ +o
:/:: .... .
Samples
u t= a = e n a ~ t= = = e = ~ e 0 1 5. \ -
.0"0030
=
i
40
6o
Temperature
=
=
eo
lOO
12o
(K)
Fig. 4. Temperaturedependencesof DC magnetic susceptibility for samples of Sro.6Cao.333CuO2.oo+, (z = 0.00-0.20).
The electrical resistivity data are shown in fig. 5. The samples with z>0.05 showed metallic-like conduction in the normal state and sharp transitions to zero resistivity at temperatures above liquid N2 temperature. These results indicated that the samples with z> 0.10, which contained little of the "infinite-layer" phase, exhibited bulk superconductivity. This further suggested the presence of new superconducting phases, besides the "infinite-layer" phases, in the SrCa-Cu-O system. These experimental results also indicated a correlation between the occurrence of good superconducting properties (i.e. large Meissner signal, low electrical resistivity in the normal state
0.010
,.
z=O.O0 (X 1 / 1 0 ) , ~
~
0.005
~
0.000 [
. . . . . . . 50
_1~
nominal composition of ( z = 0 . 0 0 ~ 0 . 2 0 ) were prepared under a high pressure of 5 GPa. The effects of oxidizing atmosphere on both structural and superconducting properties were investigated. The major phase in the sample with z = 0.00 had an "infinitelayer" structure. As z increased, the "infinite-layer" phase started to decompose and new compounds having structures with periodicities of 13.6 and 10.3 appeared along the c-axis. The samples with z>_ 0.10, which contained little of the "infinite-layer" phase, exhibited Meissner volume fractions of ~ 20%. It appeared that new superconducting phases other than the "infinite-layer" phases were present in the Sr-Ca-Cu-O system. Tentative crystal structures for these new phases were suggested.
Acknowledgements
The authors would like to thank T. Yagi, H. Takahashi, W. Utsumi, T. Uchida and T. Hishinuma (of the University of Tokyo) for helpful advice in the high-pressure syntheses. They also thank N. Koshizuka, M. Murakami, A. Tokiwa-Yamamoto, N. Sugii, K. Kubo, N. Seiji and N. Watanabe (of SRLISTEC) for their helpful discussions. This work was supported by NEDO in the program for R&D of Basic Technology for Future Industries.
f o lo (x 1/2) 0.20 (x v4)
with
Sro.6Cao.333CuO2.oo+z
0.15(x 112)
.
. . . . . . .
100
150
Temperature
References 200
(K)
Fig. 5. Temperaturedependencesof electricalresistivityfor samples of Sro.6Cao.333CuO2.oo+z(z = 0.00-0.20).
[ 1 ] T. Siegrist, S.M. Zahurak, D.W. Murphy and R.S. Roth, Nature 334 (1988) 231.
230
S. Adachi et al. / High-pressure synthesis of superconducting SCCO
[2] H. Yamane, Y. Miyazaki and T. Hirai, J. Ceram. Soc. Jpn. 97 (1989) 143 (in Japanese) [3] M.G. Smith, A. Manthiram, J. Zhou, J.B. Goodenough and J.T. Markert, Nature 351 ( 1991 ) 549. [4] G. Er, Y. Miyamoto, F. Kanamaru and S. Kikkawa, Physica C 181 (1991) 206.
[ 5 ] M. Azuma, Z. Hiroi, M. Takano, Y. Bando and Y. Takeda, Nature 356 (1992) 775. [6] S. Adachi, T. Sakurai, H. Yamauchi, H. Takahashi and N. M6ri, Proc. of ISS'92 (in press). [7 ] M. Takano, Y. Takeda, H. Okada, M. Miyamoto and T. Kusaka, Physica C 159 (1989) 375.
High-pressure synthesis of superconducting Sr-Ca-Cu-O samples Seiji Adachi
a,
H. Y a m a u c h i a, Shoji T a n a k a
a
and Nobuo M6ri b
a Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo 135, Japan b Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan Received 22 january 1993
Samples with nominal compositions of Sro.6Cao.333CuO2.oO+z(z=0.00-0.20) were prepared using a high-pressure technique. The crystallographic structure of the major phase in the sample with z = 0.00 was an "infinite-layer" type. With increasing z, the "infinite-layer" phase decomposed and new compounds having periodic structures of 13.6 A and 10.3 A along the c-axis appeared. The samples with z > 0.10, which contained little of the "infinite-layer" phase, exhibited bulk superconductivity. This suggests the presence of new superconducting phases in addition to the "infinite-layer" phases in the Sr-Ca-Cu-O system.
1. Introduction
2. Experimental
Since the discovery of a (Sr, Ca)CuO 2 compound consisting of stacked pairs of a Cu-O2 sheet and a single cationic layer, known as the "infinite-layer" structure [ 1,2 ], many researchers have attempted to make the compound superconductive. In 1991, the first "infinite-layer" superconductors, (Srl_xLnx)CuO2 ( L n = N d [3], La [4] ), having Tcs of ~ 40 K were synthesized using high-pressure techniques. These superconductors are n-type. Later, Azuma et al. [ 5 ] reported superconductivity at 110 K for (Sro.7Cao.3)o.9CuO2. They claimed that this superconductor had an "infinite-layer" structure and was the first p-type cuprate without Cu-O5 pyramidal sheets. However, they did not succeed in synthesizing pure single-phase samples which should have shown single-step superconducting transitions in both diamagnetic and electrical resistivity measurements. Previously, we prepared (Sr, Ca)o.9sCuO2.oo+z ( S r : C a ~ 1 : 1, z = 0 - 0 . 2 0 , nominally) samples and observed superconductivity at 110 K (diamagnetic onset temperature) in a Sro.5oCao.45CuO2.o5 sample. However, it was difficult to prepare single-phase samples [ 6 ]. In this paper, we have obtained evidence for the presence of new phases in the S r - C a - C u - O system.
The S r - C a - C u - O samples were prepared using a high-pressure technique, described below. S r 2 f u O 3 , Ca2CuO3, CaO2 and CuO powders were used as starting materials. These powders were mixed in the appropriate ratios. The powder mixture was charged in a gold cell surrounded by a BN separator and compressed up to 5 GPa using a cubic-anvil-type apparatus. Pyrophyllite was used for the pressure transmitting medium. During compression, the powder mixture was heated to about 930°C for 30 min by passing an electrical current through a carbon heater. Generally, KC103 or KCIO4 is used as an oxidizer and the oxidizer is charged in a gold cell together with the powder mixture. To avoid the inclusion of KC1 in the product, the oxidizer is usually put in the gold cell unmixed with the charged powder. In the present experiment, when KCIO3 or KC104 was used, it was difficult to avoid inhomogeneous oxidation of the sample powder mixture. To circumvent this problem, we utilized CaO2 as one of the starting materials; CaO2 was expected to play the role not only of the oxidizer but also of the Ca source. In this way we could anticipate obtaining homogeneously oxidized samples. The effects of an oxidizing atmosphere on both the structural and superconducting properties were in-
0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
S. Adachi et al. / High-pressure synthesis of superconducting SCCO
vestigated. Samples with the nominal composition of Sro.6Cao.aa3CuO2.oo+z were prepared. The oxidizing atmosphere was controlled by varying the oxygen content, z, in the range of 0.00-0.20. The crystal structures of these samples were examined by powder X-ray diffraction (XRD; using Cu Kct radiation) and transmission electron microscopy (TEM). In order to detect superconductivity, electrical resistivity was measured by the conventional four-probe method. Furthermore, the DC magnetic susceptibility was monitored using a SQUID magnetometer in the "filed-cooling" mode employing an external field of 10 Oe.
3. Results and discussion Figure 1 shows X R D patterns of the prepared samples. For the sample with z = 0.00, the main phase was of the "infinite-layer" structure. Some reflection peaks form CuO and some unknown phases were also present. With increasing z, the "infinite-layer" phase
227
CuK~ h
5
.
°
o
z=O.O0
v
m
co (-
0.05 0.10 0.15 [cu%
0.20 10
20
30
40
50
20 (deg) Fig. 1. Powder X-ray diffraction patterns for samples of Sro.6Cao.3a3CuO2.oo+z (z=0.00-0.20). (Indices are for the "infinite-layer" phase. )
seemed to decompose and different phases appeared. Generally, two intense reflection peaks from CuO are seen at 20=35.6 ° and 38.8 °, and the intensities of both peaks are nearly the same. (Strictly
Fig. 2. Transmission electron microscope image for the sample of Sro.6Cao.333CuO2.~.
S. Adachi et al. / High-pressure synthesis of superconducting SCCO
228
speaking, the former peak is slightly stronger than the latter one.) The 101 reflection from the "infinite-layer" structure is the strongest for this phase and is expected to appear at 2 0 ~ 3 5 ° [7]. Unfortunately, this diffraction angle coincides with the 20 for the strongest reflection from CuO. However, by comparing the intensities of the peaks at 20= 35-36 ° and at about 39 ° (shown in fig. 1 ), the "infinitelayer" phase seemed to decompose as z increased and practically disappeared for samples with z_> 0.10. Some reflections clearly appeared at 20 lower than 10 ° fro samples with z > 0.05, indicating long-range ordered structures. Peaks at 20= 6.5 ° and 8.5 ° were observed for samples with z=0.05-0.15 and with z=0.20, respectively. These reflections correspond to long-range periodicities of 13.6 A and 10.3 /k, respectively. Figure 2 shows a TEM image for the sample with z=0.15. A periodic structure of 13.6/~ was clearly observed. Furthermore, each 13.6/k stacking could be divided into three parts, with thicknesses of 5.1 A/3.4 A/5.1 A. another type of stacking of 17.0 A width was also observed. It should be notes that the difference in width between the two types of stack-
ing, i.e. 17.0-13.6 = 3.4/k, is equal to the c-axis length of the known "infinite-layer" structure. These results suggested that there was a new series of compounds with stacking periods of ( 10.2 + 3.4n)A where n = 0, 1, 2.... It was likely that the crystal structure of the compounds was composed of two structural blocks which were the "infinite-layer" block and an unknown one. Assuming that the new compounds would have a crystal structure related to the "infinite-layer one, we speculated that the crystal structure was as shown in fig. 3(c). In the figure, the structures for Bi- or Tl-based cuprate superconductors (a) and the "infinite-layer" compound (b) are also drawn for comparison. This proposed structure resembles those for Bi- or Tl-based superconductors. If such compounds existed, they would become new high-T¢ superconductors after appropriate introduction of holes. Figure 4 shows the temperature dependence of DC magnetic susceptibility for the prepared samples. Meissner signals were detected for all the samples, especially the samples with z=0.05-0.15. These showed fairly large Meissner signals which indicated superconducting volume fractions of ~ 20% at 5 K.
--0 0
0
0
-
© ~
0
~
CuO2 sheet
~5.1 L [:it~1: ~t~,~
~F
- - unknown O
-o 0
0
"
O
O
(a) Bi, Tl-based cuprate superconductors
~
O
0
0
O
O
O
O
0
"~ : 0
-3.4 A ~
--
--0
©
i
A W
(b)
(c)
the "infinite-layer" compounds
~
CuO2 sheet - -
the __"infinite-layer" structure
this work
Fig. 3. Illustration o f crystal structures for (a) Bi- or Tl-based cuprate superconductors, (b) the "infinite-layer" compounds, and (c) new compounds.
S. Adachi et aL / High-pressure synthesis of superconducting SCCO
Field Cooled
: H=IO
and sharp transition to the superconducting state) and the appearance of periodic structures with stackings of 13.6 or 10.3 ,~. (Those phases shown schematically in fig. 3 (c) seemed to exist. )
Oe
0.0@0
z=O.O0
== - 0 . 0 0 1
,,A
• • • •
°
°
229
"
° oo
°
.
..
=+
.0
=o +
/-o,o
~- 0 . 0 5
,, -0oo
/
÷ ÷÷÷÷÷÷*÷+
": . . . .
4. S u m m a r y
+ +o
:/:: .... .
Samples
u t= a = e n a ~ t= = = e = ~ e 0 1 5. \ -
.0"0030
=
i
40
6o
Temperature
=
=
eo
lOO
12o
(K)
Fig. 4. Temperaturedependencesof DC magnetic susceptibility for samples of Sro.6Cao.333CuO2.oo+, (z = 0.00-0.20).
The electrical resistivity data are shown in fig. 5. The samples with z>0.05 showed metallic-like conduction in the normal state and sharp transitions to zero resistivity at temperatures above liquid N2 temperature. These results indicated that the samples with z> 0.10, which contained little of the "infinite-layer" phase, exhibited bulk superconductivity. This further suggested the presence of new superconducting phases, besides the "infinite-layer" phases, in the SrCa-Cu-O system. These experimental results also indicated a correlation between the occurrence of good superconducting properties (i.e. large Meissner signal, low electrical resistivity in the normal state
0.010
,.
z=O.O0 (X 1 / 1 0 ) , ~
~
0.005
~
0.000 [
. . . . . . . 50
_1~
nominal composition of ( z = 0 . 0 0 ~ 0 . 2 0 ) were prepared under a high pressure of 5 GPa. The effects of oxidizing atmosphere on both structural and superconducting properties were investigated. The major phase in the sample with z = 0.00 had an "infinitelayer" structure. As z increased, the "infinite-layer" phase started to decompose and new compounds having structures with periodicities of 13.6 and 10.3 appeared along the c-axis. The samples with z>_ 0.10, which contained little of the "infinite-layer" phase, exhibited Meissner volume fractions of ~ 20%. It appeared that new superconducting phases other than the "infinite-layer" phases were present in the Sr-Ca-Cu-O system. Tentative crystal structures for these new phases were suggested.
Acknowledgements
The authors would like to thank T. Yagi, H. Takahashi, W. Utsumi, T. Uchida and T. Hishinuma (of the University of Tokyo) for helpful advice in the high-pressure syntheses. They also thank N. Koshizuka, M. Murakami, A. Tokiwa-Yamamoto, N. Sugii, K. Kubo, N. Seiji and N. Watanabe (of SRLISTEC) for their helpful discussions. This work was supported by NEDO in the program for R&D of Basic Technology for Future Industries.
f o lo (x 1/2) 0.20 (x v4)
with
Sro.6Cao.333CuO2.oo+z
0.15(x 112)
.
. . . . . . .
100
150
Temperature
References 200
(K)
Fig. 5. Temperaturedependencesof electricalresistivityfor samples of Sro.6Cao.333CuO2.oo+z(z = 0.00-0.20).
[ 1 ] T. Siegrist, S.M. Zahurak, D.W. Murphy and R.S. Roth, Nature 334 (1988) 231.
230
S. Adachi et al. / High-pressure synthesis of superconducting SCCO
[2] H. Yamane, Y. Miyazaki and T. Hirai, J. Ceram. Soc. Jpn. 97 (1989) 143 (in Japanese) [3] M.G. Smith, A. Manthiram, J. Zhou, J.B. Goodenough and J.T. Markert, Nature 351 ( 1991 ) 549. [4] G. Er, Y. Miyamoto, F. Kanamaru and S. Kikkawa, Physica C 181 (1991) 206.
[ 5 ] M. Azuma, Z. Hiroi, M. Takano, Y. Bando and Y. Takeda, Nature 356 (1992) 775. [6] S. Adachi, T. Sakurai, H. Yamauchi, H. Takahashi and N. M6ri, Proc. of ISS'92 (in press). [7 ] M. Takano, Y. Takeda, H. Okada, M. Miyamoto and T. Kusaka, Physica C 159 (1989) 375.