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Bulk superconductivity observed in (Co,Cu)(Sr,Ba)2(Y,Ca)Cu2Oy
Physica C 426–431 (2005) 487–491 www.elsevier.com/locate/physc
Bulk superconductivity observed in (Co,Cu)(Sr,Ba)2(Y,Ca)Cu2Oy Masahiro Shiraki a, Jun-ichi Shimoyama b
a,b,*
, Shigeru Horii a, Kohji Kishio
a
a Department of Superconductivity, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan PRESTO, Japan Science and Technology Corporation (JST), 4-1-8 Honcho, Kawaguchi, Staitama 332-0012, Japan
Received 23 November 2004; accepted 14 April 2005 Available online 19 July 2005
Abstract Effects of cobalt substitution for copper in Cu(Ba,Sr)2(Y,Ca)Cu2Oy [Cu1212] on its superconducting properties were systematically studied. Bulk superconductivity was confirmed in wide compositional range of the (Cu,Co)(Ba,Sr)2(Y,Ca)Cu2Oy system and their TcÕs were enhanced by annealing under high oxygen pressure. In addition, it was indicated that partial strontium substitution for barium suppresses the incorporation of cobalt into the CuO2 planes. Hole doping with moderate concentration and suppression of cobalt incorporation in the CuO2 planes are believed to result in the new superconducting compositions showing high TcÕs exceeding 70 K. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.72.Bk Keywords: Novel superconductor; Co-doped YBCO; High Tc
1. Introduction Since the discovery of the well-known 90 Kclass high Tc superconductors REBa2Cu3O7d * Corresponding author. Address: Department of Superconductivity, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel.: +81 3 5841 7705; fax: +81 3 5802 2908. E-mail address: [email protected] (J.-i. Shimoyama).
(CuBa2RECu2O7d: Cu1212, various element substitutions have been extensively attempted to improve superconducting properties particularly for enhancement of Tc. One of the guiding principles for enhancement of Tc is to form flat CuO2 planes. Among various 1212-type cuprate superconductors, the HgBa2CaCu2O6+d, which has almost completely flat CuO2 planes, shows the highest Tc 128 K [1], while lower TcÕs were observed for CuBa2RECu2O7d and CuSr2RECu2O7d due to their less flat CuO2 planes. From a view point
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.04.034
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M. Shiraki et al. / Physica C 426–431 (2005) 487–491
of flattening the CuO2 plane in the CuBa2RECu2O7d compounds by chemical substitution, moderately long a- and b-axes lengths are desirably ˚ in average. Although achieved to be 3.85 A many elements were substituted for copper at the blocking layer in CuBa2YCu2O7d [2,3], a few elements, such as cobalt, were found to keep the moderate lattice size of the ab-plane. In the case of (Cu1xMx)Ba2YCu2O7+d (M: transition metals), however, Tc decreases rapidly with an increase of x due to decrease of hole carrier concentrations and/or contamination of M into the CuO2 planes. In the (Cu1xCox)Ba2YCu2O7+d system, cobalt ions are believed to preferentially occupy the copper site at the blocking layer when x is smaller than 0.6 [4], while its superconductivity disappears at x 0.5 due to decrease of hole carriers [5]. Partial substitution of calcium for yttrium was effective for increasing hole carriers in this system, resulting in slightly improved Tc [6]. On the other hand, all the cobalt ions were confirmed to locate at the blocking layer in CoSr2YCu2O7+d [7]. This suggests that partial substitution of strontium for barium is effective for preventing incorporation of cobalt ions into the CuO2 planes. Based on the background described above, we have attempted to synthesize (Cu,Co)(Ba,Sr)2(Y, Ca)Cu2O7+d compounds with various nominal compositions and investigated their superconducting properties mainly on Tc in the present study.
X-ray diffraction method. Magnetic susceptibility was measured using a SQUID magnetometer under an applied field of 1 Oe. Resistivity measurements were performed by the standard d.c. fourprobe method.
3. Results and discussion Powder X-ray diffraction analysis revealed that samples with (Cu,Co)1212 single phase were obtained from wide nominal compositions of (Cu1xCox)(Ba1ySry)2YCu2O7+d. Variation of lattice parameters with substitution levels of cobalt and strontium for oxygen annealed (Cu1xCox)(Ba1ySry)2YCu2O7+d samples are summarized in Fig. 1. Systematic decrease of lattice parameters by strontium substitution was confirmed in the (Cu,Co) 1212 compounds. In addition, with an increase of x under the constant y, the c-axis lengths decreased, while the a-axis lengths increased except for the samples near x 1. For strontium-free samples y = 0, any traces of superconducting transition were not observed down to 4.2 K when x > 0.5. This behavior is consistent with the previous results [5]. On the other hand, a strontium substituted sample, (Cu0.5Co0.5)(Ba0.5Sr0.5)2YCu2O7+d,
2. Experimental Sintered bulk samples with nominal compositions of (Cu1xCox)(Ba1ySry)2(Y1zCaz)Cu2O7+d [x, y = 0–1 and z = 0, 0.3, 0.5] were synthesized by conventional solid-state reactions starting from high purity powders of CuO, Co3O4, BaCO3, SrCO3, Y2O3 and CaCO3. Powder mixtures were calcined at 890 °C in flowing oxygen for 12 h, pressed into pellets and sintered at 920–940 °C for 24 h under flowing oxygen. The sintered bulk samples were annealed in flowing oxygen down to 200 °C for long time and kept for 24 h. Some samples were annealed under a high oxygen pressure of 35 MPa at 200 °C for 24 h. Constituent phases of the samples were analyzed by the powder
Fig. 1. Lattice parameters for (Cu1xCox)(Ba1ySry)2YCu2O7+d after annealed in flowing oxygen at 200 °C. Circles, a triangle and squares represent a, b and c-axes lengths, respectively. Filled symbols correspond to the data sited from Ref. [3].
M. Shiraki et al. / Physica C 426–431 (2005) 487–491
showed apparent diamagnetism with Tc(onset) of 40 K as shown in Fig. 2. Furthermore, partially calcium substituted samples, (Cu0.5Co0.5) (Ba0.5Sr0.5)2(Y1zCaz)Cu2O7+d with z = 0.3 and 0.5, exhibited enhanced Tc(onset) up to 68 K with large diamagnetism at low temperatures. This indicates that hole carrier was successfully introduced by calcium substitution for yttrium. In other words, decrease of hole carrier concentration accompanied by substitution of high valence cobalt for copper is well compensated by divalent calcium substitution for trivalent yttrium. The observed dull magnetic transitions near Tc(onset) are believed due to granular nature of sintered bulks in the carrier underdoped state, which have serious weak-links associated with grain boundaries and long penetration depth in small grains with 2 lm in size. Since the co-substitution by strontium and calcium for barium and yttrium, respectively, was suggested to be effective for improving superconducting properties of (Cu,Co)1212, various samples with nominal compositions of (Cu1xCox)(Ba1ySry)2(Y0.5Ca0.5)Cu2O7+d were synthesized. Although small amount of secondary phases were found in some samples, the (Cu,Co)1212 phase was confirmed to generate from wide compositional range similarly in the case of the calcium-free (Cu,Co)1212 compounds. Note that all the cobalt-substituted samples were of tetragonal
Fig. 2. Temperature dependence of magnetization for sintered bulks of (Cu0.5Co0.5)(Ba0.5Sr0.5)2(Y1zCaz)O7+d [z = 0, 0.3, 0.5] annealed in flowing oxygen at 200 °C.
489
showing sharp powder X-ray diffraction peaks without any peaks due to presence of other superconducting phase, such as CuBa2YCu2O7d. The compositional range showing superconductivity as functions of x and y was found to largely expand particularly towards cobalt-rich compositions by the calcium substitution. Fig. 3 shows temperature dependence of resistivity for (Cu1xCox)(Ba0.75Sr0.25)2(Y0.5Ca0.5)Cu2O7+d with x = 0.5 and 0.75. For comparison, resistivity curves of (Cu1xCox)(Ba0.75Sr0.25)2YCu2O7+d with x = 0.5 and 0.75 and (Cu0.5Co0.5)Ba2(Y0.5Ca0.5)Cu2O7+d are also shown. It is clear that the co-substitution of calcium and strontium is quite effective for decreasing resistivity in the normal state accompanying apparent superconducting transition even for large amount of cobalt containing samples with x = 0.75. Effect of calcium and strontium substitutions can be explained by hole doping and suppression of cobalt incorporation to the CuO2 plane as mentioned above [1], respectively.
Fig. 3. Temperature dependence of resistivity for sintered bulks of (Cu1xCox)(Ba0.75Sr0.25)2(Y1zCaz)O7+d [x = 0.5, 0.75; z = 0, 0.5] and (Cu0.25Co0.75)Ba2(Y0.5Ca0.5)O7+d annealed in flowing oxygen at 200 °C. Data for (Cu0.25Co0.75)(Ba0.75Sr0.25)2(Y0.5Ca0.5)O7+d annealed under P O2 ¼ 3.5 MPa at 200 °C for 24 h is also shown.
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Slightly lowered Tc observed for (Cu0.25Co0.75)(Ba0.75Sr0.25)2(Y0.5Ca0.5)Cu2O7+d compared to that of (Cu0.5Co0.5)(Ba0.75Sr0.25)2(Y0.5Ca0.5)Cu2O7+d suggested the former sample was carrier underdoped state. In fact, its Tc(onset) was enhanced up to 76 K after annealing under high oxygen pressure. Further enhanced Tc can be expected in the heavily cobalt-doped region through additional hole carrier doping by an increase of calcium and/or oxygen contents. Variation of Tc(onset)Õs for samples of (Cu1xCox)(Ba1ySry)2(Y0.5Ca0.5)Cu2O7+d after annealing under high oxygen pressure as functions of x and y is summarized in Fig. 4. It is clearly seen that the new cation composition showing 70 K-class superconductivity widely expanded by the oxygen loading. A gradual decrease of Tc with an increasing y for samples of x = 0.5 is considered to be due to the distortion of CuO2 planes corresponding to the shrinkage of a- and b-axes by substitution of strontium having smaller ionic size than that of barium similarly in the case of Cu(Ba1ySry)2YCu2O7d [8].
Fig. 4. Tc(onset)Õs of the samples with nominal compositions of (Cu1xCox)(Ba1ySry)2(Y0.5Ca0.5)O7+d annealed under P O2 ¼ 3.5 MPa at 200 °C for 24 h determined by the resistivity measurements. Number in each circle represents Tc(onset)/K and filled circles correspond to the non-superconducting samples. Simple asterisk and double asterisks mean samples with small and large amount of impurity phases, respectively.
A sample with a nominal composition of CoBa2(Y0.5Ca0.5)Cu2O7+d showed superconducting transition with Tc(onset) of 40 K both in resistivity and magnetization measurements. Observed large diamagnetism (v 1 at 5 K) and zero resistance achieved at 32 K guaranteed its bulk superconductivity. This particular sample, however, was found to contain considerable amount of an impurity phase, (Y,Ca)Ba2Co3O7+d, through the powder X-ray diffraction analysis. Therefore, the actual composition of the superconducting phase of this sample is supposed to be expressed as (Co,Cu)Ba2(Y,Ca)Cu2O7+d. Optimization of sintering conditions, such as temperature and partial pressure of oxygen, to obtain single phased CoBa2(Y0.5Ca0.5)Cu2O7+d samples is undergoing.
4. Conclusions In order to develop new superconducting compositions in the (Cu,Co)1212 system, we have synthesized sintered bulk samples with nominal compositions of (Cu1xCox)(Ba1ySry)2(Y1zCaz)Cu2O7+d[x, y = 0–1 and z = 0, 0.3, 0.5] by the conventional solid-state reaction method and evaluated their resistivity and magnetization as a function of temperature. The (Cu,Co)1212 phase was found to form from wide compositional range through the synthesis under an ambient pressure except cobalt-poor and strontium-rich compositions. Bulk superconductivity accompanying large diamagnetism and zero resistance were observed in the samples with wide range nominal compositions including new cation ratio. In particular, the calcium substituted samples, (Cu1xCox)(Ba1ySry)2(Y0.5Ca0.5)Cu2O7+d, exhibited 70 K-class superconductivity after annealing under a high oxygen pressure of 35 MPa at 200 °C even for the cobalt-rich samples. A high Tc(onset) of 76 K was recorded by a sample with a nominal composition of (Cu0.25Co0.75)(Ba0.75Sr0.25)2(Y0.5Ca0.5)Cu2O7+d. The largely expanded superconducting compositions showing 70 K-class superconductivity in the present system are explained by the co-operative effects of strontium and calcium substitutions, which affect suppression of cobalt incorporation in the CuO2 plane and carrier doping state, respec-
M. Shiraki et al. / Physica C 426–431 (2005) 487–491
tively. Effect of cobalt substitution on lattice expansion in the ab-plane is also believed to contribute high TcÕs. Further enhancement of Tc in the (Cu,Co)1212 system can be expected in the cobalt-rich compositions by increasing excess oxygen and/or calcium substitution levels, which are also effective for improving flatness of the CuO2 planes. References [1] A. Schilling, M. Cantoni, J.D. Guo, H.R. Ott, Nature 363 (1993) 56.
491
[2] T. Den, T. Kobayashi, Physica C 196 (1992) 141. [3] M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz, S.A. Sunshine, Phys. Rev. B 37 (1988) 7458. [4] H. Renevier, J.L. Hodeau, M. Marezio, A. Santoro, Physica C 220 (1994) 143. [5] J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, Phys. Rev. B 37 (1988) 7458. [6] E. Suard, A. Maignan, V. Caignaert, B. Raveau, Physica C 200 (1992) 43. [7] Q. Huang, R.J. Cava, A. Santoro, J.J. Krajewski, W.F. Peck, Physica C 193 (1992) 196. [8] A. Ono, T. Tanaka, H. Hozaki, Y. Ishizawa, Jpn. J. Appl. Phys. 26 (1987) L1687.
Bulk superconductivity observed in (Co,Cu)(Sr,Ba)2(Y,Ca)Cu2Oy Masahiro Shiraki a, Jun-ichi Shimoyama b
a,b,*
, Shigeru Horii a, Kohji Kishio
a
a Department of Superconductivity, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan PRESTO, Japan Science and Technology Corporation (JST), 4-1-8 Honcho, Kawaguchi, Staitama 332-0012, Japan
Received 23 November 2004; accepted 14 April 2005 Available online 19 July 2005
Abstract Effects of cobalt substitution for copper in Cu(Ba,Sr)2(Y,Ca)Cu2Oy [Cu1212] on its superconducting properties were systematically studied. Bulk superconductivity was confirmed in wide compositional range of the (Cu,Co)(Ba,Sr)2(Y,Ca)Cu2Oy system and their TcÕs were enhanced by annealing under high oxygen pressure. In addition, it was indicated that partial strontium substitution for barium suppresses the incorporation of cobalt into the CuO2 planes. Hole doping with moderate concentration and suppression of cobalt incorporation in the CuO2 planes are believed to result in the new superconducting compositions showing high TcÕs exceeding 70 K. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.72.Bk Keywords: Novel superconductor; Co-doped YBCO; High Tc
1. Introduction Since the discovery of the well-known 90 Kclass high Tc superconductors REBa2Cu3O7d * Corresponding author. Address: Department of Superconductivity, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel.: +81 3 5841 7705; fax: +81 3 5802 2908. E-mail address: [email protected] (J.-i. Shimoyama).
(CuBa2RECu2O7d: Cu1212, various element substitutions have been extensively attempted to improve superconducting properties particularly for enhancement of Tc. One of the guiding principles for enhancement of Tc is to form flat CuO2 planes. Among various 1212-type cuprate superconductors, the HgBa2CaCu2O6+d, which has almost completely flat CuO2 planes, shows the highest Tc 128 K [1], while lower TcÕs were observed for CuBa2RECu2O7d and CuSr2RECu2O7d due to their less flat CuO2 planes. From a view point
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.04.034
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M. Shiraki et al. / Physica C 426–431 (2005) 487–491
of flattening the CuO2 plane in the CuBa2RECu2O7d compounds by chemical substitution, moderately long a- and b-axes lengths are desirably ˚ in average. Although achieved to be 3.85 A many elements were substituted for copper at the blocking layer in CuBa2YCu2O7d [2,3], a few elements, such as cobalt, were found to keep the moderate lattice size of the ab-plane. In the case of (Cu1xMx)Ba2YCu2O7+d (M: transition metals), however, Tc decreases rapidly with an increase of x due to decrease of hole carrier concentrations and/or contamination of M into the CuO2 planes. In the (Cu1xCox)Ba2YCu2O7+d system, cobalt ions are believed to preferentially occupy the copper site at the blocking layer when x is smaller than 0.6 [4], while its superconductivity disappears at x 0.5 due to decrease of hole carriers [5]. Partial substitution of calcium for yttrium was effective for increasing hole carriers in this system, resulting in slightly improved Tc [6]. On the other hand, all the cobalt ions were confirmed to locate at the blocking layer in CoSr2YCu2O7+d [7]. This suggests that partial substitution of strontium for barium is effective for preventing incorporation of cobalt ions into the CuO2 planes. Based on the background described above, we have attempted to synthesize (Cu,Co)(Ba,Sr)2(Y, Ca)Cu2O7+d compounds with various nominal compositions and investigated their superconducting properties mainly on Tc in the present study.
X-ray diffraction method. Magnetic susceptibility was measured using a SQUID magnetometer under an applied field of 1 Oe. Resistivity measurements were performed by the standard d.c. fourprobe method.
3. Results and discussion Powder X-ray diffraction analysis revealed that samples with (Cu,Co)1212 single phase were obtained from wide nominal compositions of (Cu1xCox)(Ba1ySry)2YCu2O7+d. Variation of lattice parameters with substitution levels of cobalt and strontium for oxygen annealed (Cu1xCox)(Ba1ySry)2YCu2O7+d samples are summarized in Fig. 1. Systematic decrease of lattice parameters by strontium substitution was confirmed in the (Cu,Co) 1212 compounds. In addition, with an increase of x under the constant y, the c-axis lengths decreased, while the a-axis lengths increased except for the samples near x 1. For strontium-free samples y = 0, any traces of superconducting transition were not observed down to 4.2 K when x > 0.5. This behavior is consistent with the previous results [5]. On the other hand, a strontium substituted sample, (Cu0.5Co0.5)(Ba0.5Sr0.5)2YCu2O7+d,
2. Experimental Sintered bulk samples with nominal compositions of (Cu1xCox)(Ba1ySry)2(Y1zCaz)Cu2O7+d [x, y = 0–1 and z = 0, 0.3, 0.5] were synthesized by conventional solid-state reactions starting from high purity powders of CuO, Co3O4, BaCO3, SrCO3, Y2O3 and CaCO3. Powder mixtures were calcined at 890 °C in flowing oxygen for 12 h, pressed into pellets and sintered at 920–940 °C for 24 h under flowing oxygen. The sintered bulk samples were annealed in flowing oxygen down to 200 °C for long time and kept for 24 h. Some samples were annealed under a high oxygen pressure of 35 MPa at 200 °C for 24 h. Constituent phases of the samples were analyzed by the powder
Fig. 1. Lattice parameters for (Cu1xCox)(Ba1ySry)2YCu2O7+d after annealed in flowing oxygen at 200 °C. Circles, a triangle and squares represent a, b and c-axes lengths, respectively. Filled symbols correspond to the data sited from Ref. [3].
M. Shiraki et al. / Physica C 426–431 (2005) 487–491
showed apparent diamagnetism with Tc(onset) of 40 K as shown in Fig. 2. Furthermore, partially calcium substituted samples, (Cu0.5Co0.5) (Ba0.5Sr0.5)2(Y1zCaz)Cu2O7+d with z = 0.3 and 0.5, exhibited enhanced Tc(onset) up to 68 K with large diamagnetism at low temperatures. This indicates that hole carrier was successfully introduced by calcium substitution for yttrium. In other words, decrease of hole carrier concentration accompanied by substitution of high valence cobalt for copper is well compensated by divalent calcium substitution for trivalent yttrium. The observed dull magnetic transitions near Tc(onset) are believed due to granular nature of sintered bulks in the carrier underdoped state, which have serious weak-links associated with grain boundaries and long penetration depth in small grains with 2 lm in size. Since the co-substitution by strontium and calcium for barium and yttrium, respectively, was suggested to be effective for improving superconducting properties of (Cu,Co)1212, various samples with nominal compositions of (Cu1xCox)(Ba1ySry)2(Y0.5Ca0.5)Cu2O7+d were synthesized. Although small amount of secondary phases were found in some samples, the (Cu,Co)1212 phase was confirmed to generate from wide compositional range similarly in the case of the calcium-free (Cu,Co)1212 compounds. Note that all the cobalt-substituted samples were of tetragonal
Fig. 2. Temperature dependence of magnetization for sintered bulks of (Cu0.5Co0.5)(Ba0.5Sr0.5)2(Y1zCaz)O7+d [z = 0, 0.3, 0.5] annealed in flowing oxygen at 200 °C.
489
showing sharp powder X-ray diffraction peaks without any peaks due to presence of other superconducting phase, such as CuBa2YCu2O7d. The compositional range showing superconductivity as functions of x and y was found to largely expand particularly towards cobalt-rich compositions by the calcium substitution. Fig. 3 shows temperature dependence of resistivity for (Cu1xCox)(Ba0.75Sr0.25)2(Y0.5Ca0.5)Cu2O7+d with x = 0.5 and 0.75. For comparison, resistivity curves of (Cu1xCox)(Ba0.75Sr0.25)2YCu2O7+d with x = 0.5 and 0.75 and (Cu0.5Co0.5)Ba2(Y0.5Ca0.5)Cu2O7+d are also shown. It is clear that the co-substitution of calcium and strontium is quite effective for decreasing resistivity in the normal state accompanying apparent superconducting transition even for large amount of cobalt containing samples with x = 0.75. Effect of calcium and strontium substitutions can be explained by hole doping and suppression of cobalt incorporation to the CuO2 plane as mentioned above [1], respectively.
Fig. 3. Temperature dependence of resistivity for sintered bulks of (Cu1xCox)(Ba0.75Sr0.25)2(Y1zCaz)O7+d [x = 0.5, 0.75; z = 0, 0.5] and (Cu0.25Co0.75)Ba2(Y0.5Ca0.5)O7+d annealed in flowing oxygen at 200 °C. Data for (Cu0.25Co0.75)(Ba0.75Sr0.25)2(Y0.5Ca0.5)O7+d annealed under P O2 ¼ 3.5 MPa at 200 °C for 24 h is also shown.
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Slightly lowered Tc observed for (Cu0.25Co0.75)(Ba0.75Sr0.25)2(Y0.5Ca0.5)Cu2O7+d compared to that of (Cu0.5Co0.5)(Ba0.75Sr0.25)2(Y0.5Ca0.5)Cu2O7+d suggested the former sample was carrier underdoped state. In fact, its Tc(onset) was enhanced up to 76 K after annealing under high oxygen pressure. Further enhanced Tc can be expected in the heavily cobalt-doped region through additional hole carrier doping by an increase of calcium and/or oxygen contents. Variation of Tc(onset)Õs for samples of (Cu1xCox)(Ba1ySry)2(Y0.5Ca0.5)Cu2O7+d after annealing under high oxygen pressure as functions of x and y is summarized in Fig. 4. It is clearly seen that the new cation composition showing 70 K-class superconductivity widely expanded by the oxygen loading. A gradual decrease of Tc with an increasing y for samples of x = 0.5 is considered to be due to the distortion of CuO2 planes corresponding to the shrinkage of a- and b-axes by substitution of strontium having smaller ionic size than that of barium similarly in the case of Cu(Ba1ySry)2YCu2O7d [8].
Fig. 4. Tc(onset)Õs of the samples with nominal compositions of (Cu1xCox)(Ba1ySry)2(Y0.5Ca0.5)O7+d annealed under P O2 ¼ 3.5 MPa at 200 °C for 24 h determined by the resistivity measurements. Number in each circle represents Tc(onset)/K and filled circles correspond to the non-superconducting samples. Simple asterisk and double asterisks mean samples with small and large amount of impurity phases, respectively.
A sample with a nominal composition of CoBa2(Y0.5Ca0.5)Cu2O7+d showed superconducting transition with Tc(onset) of 40 K both in resistivity and magnetization measurements. Observed large diamagnetism (v 1 at 5 K) and zero resistance achieved at 32 K guaranteed its bulk superconductivity. This particular sample, however, was found to contain considerable amount of an impurity phase, (Y,Ca)Ba2Co3O7+d, through the powder X-ray diffraction analysis. Therefore, the actual composition of the superconducting phase of this sample is supposed to be expressed as (Co,Cu)Ba2(Y,Ca)Cu2O7+d. Optimization of sintering conditions, such as temperature and partial pressure of oxygen, to obtain single phased CoBa2(Y0.5Ca0.5)Cu2O7+d samples is undergoing.
4. Conclusions In order to develop new superconducting compositions in the (Cu,Co)1212 system, we have synthesized sintered bulk samples with nominal compositions of (Cu1xCox)(Ba1ySry)2(Y1zCaz)Cu2O7+d[x, y = 0–1 and z = 0, 0.3, 0.5] by the conventional solid-state reaction method and evaluated their resistivity and magnetization as a function of temperature. The (Cu,Co)1212 phase was found to form from wide compositional range through the synthesis under an ambient pressure except cobalt-poor and strontium-rich compositions. Bulk superconductivity accompanying large diamagnetism and zero resistance were observed in the samples with wide range nominal compositions including new cation ratio. In particular, the calcium substituted samples, (Cu1xCox)(Ba1ySry)2(Y0.5Ca0.5)Cu2O7+d, exhibited 70 K-class superconductivity after annealing under a high oxygen pressure of 35 MPa at 200 °C even for the cobalt-rich samples. A high Tc(onset) of 76 K was recorded by a sample with a nominal composition of (Cu0.25Co0.75)(Ba0.75Sr0.25)2(Y0.5Ca0.5)Cu2O7+d. The largely expanded superconducting compositions showing 70 K-class superconductivity in the present system are explained by the co-operative effects of strontium and calcium substitutions, which affect suppression of cobalt incorporation in the CuO2 plane and carrier doping state, respec-
M. Shiraki et al. / Physica C 426–431 (2005) 487–491
tively. Effect of cobalt substitution on lattice expansion in the ab-plane is also believed to contribute high TcÕs. Further enhancement of Tc in the (Cu,Co)1212 system can be expected in the cobalt-rich compositions by increasing excess oxygen and/or calcium substitution levels, which are also effective for improving flatness of the CuO2 planes. References [1] A. Schilling, M. Cantoni, J.D. Guo, H.R. Ott, Nature 363 (1993) 56.
491
[2] T. Den, T. Kobayashi, Physica C 196 (1992) 141. [3] M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz, S.A. Sunshine, Phys. Rev. B 37 (1988) 7458. [4] H. Renevier, J.L. Hodeau, M. Marezio, A. Santoro, Physica C 220 (1994) 143. [5] J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, Phys. Rev. B 37 (1988) 7458. [6] E. Suard, A. Maignan, V. Caignaert, B. Raveau, Physica C 200 (1992) 43. [7] Q. Huang, R.J. Cava, A. Santoro, J.J. Krajewski, W.F. Peck, Physica C 193 (1992) 196. [8] A. Ono, T. Tanaka, H. Hozaki, Y. Ishizawa, Jpn. J. Appl. Phys. 26 (1987) L1687.