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Superconducting properties of the lead-containing 2212 phase Bi1.75Pb0.25Sr2CaCu2O8 + z with Tc in the range 70–98 K
Journalofthe
Less-Common
Merals, 171(1991)
337
337-344
Superconducting properties of the lead-contai~ng 22 12 phase Bi,.,,Pb,.*,Sr,CaCu,0, +z with Cr;,in the range 70-98 K M. Ryguia, T. Rentschler,
M. Schlichenmaier,
W. Wischert and S. Kemmler-Sack
l~s~itl~t~r An~r~a~iseh~ Chemie der U~~~ersi~~t,Auf der Mo~e~stelie
IS, W-TWX?Tiibingen IF. R.G.1
(Received February 4,199 I )
Abstract The lead-containing, nearly single-phase 22 12 material of nominal composition Bil,,Pb,,&SrzCaCuzOn+: has a r, of about 70 K after air quenching. By controlling post-treatment, the transition temperature can be raised to about 98 K. Since no significant changes in the oxygen content are observed, we conclude that the function of post-treatment is to produce a more favorable ordering of oxygen in the lattice. Accordingly, all dimensions are influenced by the post-treatment. Additional results for air-quenched 2212 material with variations in the Bi:Pb:Sr:Ca ratio show that differences in the position of T, are also reflected in variations of the cell dimension, thus pointing to an extended field of existence for the 22 I2 phase in the system Bi-Pb-Sr-Ca-Cu-0 with phase boundaries which are still unknown.
1. Introduction In the Bi-Sr-Ca-Cu-0 system a series of stacking polytypes of idealized composition (BiO),Sr,Ca,, _ , Cu,,OZ,,+7+_ is existent and members with n = 1 (2201), II = 2 (2212) and IZ= 3 (2223j have been isolated. For these three cases, introduction of lead in the crystal structure was reported [l-5] and at least a part of the lead is substituted for bismuth, thus influencing the modulation of the incommensurate superstructures of the pure bismuth compounds [6, 71. Additionally, for the lead-containing n = 3 phase, a partial substitution of lead for calcium was observed [7]. For the three members with y1= 1, 2 and 3, the correlation between the true chemical composition, their crystal structure and the superconducting properties are not fully understood. For n = 1, the composition of the superconducting phase is not included by the above idealized composition 18, 91. Lead-free samples with n = 3 consist of an irregular intergrowth of lamellae with n= 1, 2, ...) 6 [lo] and the II = 3 structure must be stabilized by an admixture of lead. In the lead-free n = 2 samples, stacking faults due to intergrowth of lamellae with n ic 2 are also abundant, reducing significantly with increasing lead content f 1 l]. For the lead-containing fz= 2 samples a pseudotetragonal, orthorhombic as well as monoclinic distorted subcell was observed, the degree of distortion being influenced by the lead content and/or the conditions of preparation [2, 3, 51. The transition temperature to superconductivity is influenced by the lead content and the conditions of preparation, and values for T, between approxi0022-%X38/9 I /$X50
0 Elsevier Sequoia/P~nted
in The Netherlands
338
mately 60 and 95 K are observed. However, no clear correlation between T,, the structure of the subcell and the lead content has yet been found. The purpose of the present investigation is to study the correlation between the superconducting properties, the structure of the subcell and the total oxygen content for a fixed lead content. For this reason the composition Bi,,,,Pb,,,Sr,CaCu,O, + z was chosen, because of the existence of a nearly single-phase 2212 material [5]. To exclude the influence of smaller fluctuations of the sample composition, a batch of approximately 90 g was synthesized and all studies described here were performed with identical material. To demonstrate the influence of the nominal composition of the samples on the superconductivity, additional results for 22 12 materials with variations in the Bi : Pb : Sr : Ca ratio are also included. It is shown that by careful control of post-treatment conditions, the transition temperature can be raised to about 98 K. Since significant changes in the oxygen content are absent, we conclude that the function of post-treatment is mainly to produce a more favorable ordering of oxygen in the lattice.
2. Experimental
details
A 90 g batch of nominal composition Bi,,,,Pb,,,,,Sr,CaCu,O,+, was prepared from Bi(NO,),*5H,O (DAB6, Merck), PbO, Sr(NO,),, CaCO, (all p.A., Merck) and CuO (puriss.p.A., Fluka). The materials were mixed in an agate mortar and prefired in a corundum crucible (Degussit A123) in air at 710 “C for 18 h. The product was mixed in a ball mill (agate beaker and balls) and heated between 845
0
7
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
‘;rT
t’ -0.002 .?z! +2- -0.004 i m _ -0.006 z ; Y VI (a)
-0.008 -0.0,
(b)
Fig. 1. x vs. T for powder samples of Bi,,,, Pb,,,2sSrzCaCu20x+z: (a) air quenched, quenched, (c) post-annealed in argon at 650 “C for 15 h.
(b) liquid NZ
339
and 860 “C for 260 h. The reaction was interrupted several times for regrindings in the ball mill and X-ray analysis (Philips powder diffractometer, Cu Ka radiation, gold standard). After the final step the relative intensities and positions of the peaks of the X-ray diffraction pattern became invariant and the annealing was stopped. The resulting air-quenched material was split into several parts. In one series of experiments the results of post-treatment on powder samples were studied. For the second set several pellets (diameter approximately 13 mm, thickness approximately 1 mm, density 85% of the X-ray density) were pressed and submitted to different post-treatments. The conditions of preparation for samples of nominal composition Bi, ,,Pb,,,dSr,CaCu,O, + i and Bi,,,Pb,,,Sr,CaCu,O, + i were identical with the exception that the batches were smaller (approximately 15 g) and ground exclusively in an agate mortar. The products were examined by X-ray diffraction (the degree of crystalline orientation was calculated according to ref. 12), chemical analysis (according to ref. 13), measurement of the electrical resistivity (standard four-probe d.c. technique) and d.c. magnetic susceptibility (SQUID magnetometer, magnetic field 10 - 3 T ).
3. Results and discussion The powder X-ray diffraction pattern of the as-prepared air-quenched sample of nominal composition Bi,,,,Pb,,,,,Sr,CaCu,0, +z is in agreement with the results of ref. 5, demonstrating the existence of a nearly single-phase 22 12 material with a slightly orthorhombic distorted subcell (a = 5.398( 5), b = 5.4 18( 5), c = 30.86( 5) A). No diffraction lines corresponding to a phase of Ca,PbO, type are visible. In the susceptibility x vs. temperature T signal (Fig. 1 (a)) of the airquenched powder the onset of a very small diamagnetic signal due to a small 2223 content is observed at about 110 K. At about 70 K a pronounced second step is developed. Since the Meissner fraction of this phase is about 50% at 20 K (compared with a perfect diamagnet with a susceptibility of 4~cx = - l), we conclude that the superconductivity is bulk and results from the majority air-quenched 22 12 phase. Quenching with liquid N, did not affect the purity of the sample. The lattice parameters of the 2212 subcell are slightly reduced (a= 5.382(5), b = 5.398(5), c = 30.84(5) A) and the extent of the orthorhombic distortion (a - b)/(a + h) = - 1.5 x 10 m3 is slightly inferior (compared with the value of - 1.9 x 10-jfor the air-quenched sample). In the susceptibility vs. temperature signal (Fig. l(b)) the presence of a small amount of 2223 phase is again detectable by the appearance of a small negative signal at about 110 K. In this case a pronounced second step develops at about 95 K. The Meissner fraction of this phase is about 50% at 20 K and we conclude that the superconductivity is bulk and results from the majority liquid N, quenched 22 12 phase. After post-annealing in an argon stream (at 650 “C for 15 h) a nearly singlephase 2212 material is conserved. However, the orthorhombic distortion of the
340
22 12 subcell (a = 5.394( 5), b = 5.420( 5), c = 30.90( 5) A) is more pronounced ((a - b)/( a + b) = - 2.4 X 10-j). The susceptibility measurement (Fig. l(c)) again indicates the presence of a small amount of 2223 phase. Furthermore, in this case the second step develops at a still higher temperature of about 98 K. From a Meissner fraction of about 50% at 20 K we conclude likewise that the superconductivity is bulk and results from the majority argon-treated 2212 phase. Pressed and identically post-treated pellets of composition Bi,,,,Pb,,l,Sr,CaCu208+, show a degree of preferential orientation of about 60%. A comparison between the temperature dependence of the resistivity for the air-quenched, liquid N2 quenched and argon-treated pellets is shown in Fig. 2. The quenching with liquid N, does not appreciably alter either the room temperature resistance or the temperature dependence in the normal conducting region, but it moves the transition temperature from about 62 K to 88 K. However, argon treatment results in a drastic increase in the room temperature resistance and a nearly semiconductorlike temperature dependence in the normal conducting region, most probably owing to a partial formation of a semiconducting byproduct. The observed T, of about 87 K is in disagreement with the susceptibility data for the corresponding pellet (see below) and powder with a T, of about 98 K. The susceptibility measurements for identically post-treated pellets of composition Bi, ,,,Pb,,,,,Sr,CaCu,O, +1, however, uniformly give values for T, (onset of the Meissner signal) in good agreement with the data for the powdered samples (approximately 65 K for air quenching, approximately 90 K for quenching with liquid N,, approximately 98 K for argon treatment). Thus, the observed depression of T, in the resistivity measurements of the argon-treated pellet is not a bulk effect but the result of a superficial decomposition. From the susceptibility measurements of the powders and pellets we conclude that T, is reproducibly raised to about 90 K by quenching with liquid N, and to as high as around 98 K by post-annealing in an argon stream. Since changes in oxygen content or ordering seemed the most likely explanation for the increase in T, on post-treatment, we examined the oxygen content of these samples. For the three powders we find a very similar oxygen content of 8.14 (air quenched), 8.08 (quenched with liquid N,) and 8.08 (argon-treated). The corresponding values for a formal copper valence of 2.26( 3), 2.20( 3) and 2.20( 3) are nearly identical within the limit of experimental error. Interestingly, for the n = 1 phase of nominal composition Bi,,(,, Sr,,,,CuO,, similar results have been obtained recently [9]. By controlled post-annealing, the transition temperature can be raised from 4 K to about 15 K and in some cases as high as 18 K without changing the oxygen content of the samples. From the observed minor changes in the oxygen content of the present 2212 samples we conclude that the increase in T, from around 70 K for the airquenched sample to approximately 90 or 98 K after post-treatment is mainly the result of the production of a more favorable ordering of oxygen in the lattice. This assumption is supported by the observed variations in the lattice parameters and the extent of orthorhombic distortion of the 2212 subcell for the different posttreated samples. Furthermore, the experiments point to the existence of a more favorable crystalline order at higher temperatures. This ordered state can be
341
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. . kw....r;l. . . , . 50 100
. ‘ I . , . I . . . , . : 200 258 150
T CKI
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,...‘l*,-.‘...,‘..+.‘.,..‘..,.
0
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150
200
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. 300
300 T CKI
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Fig. 2. Temperature dependence of the resistivity for pellets of composition Bi,,,,Pb,,zsSrzCaCu,0, +;: (a) air quenched, (b)liquid Nz quenched, (c) post-annealed in argon at 650 “C for 1.5h.
342
conserved at room temperature by rapid quenching with liquid N, but not by the more slow air quenching. Additionally, the development of a favorable crystalline order is expected through the applied controlled argon treatment. Careful structural investigations in combination with transmission electron microscopy measurements will allow the expected differences to be revealed more precisely. These results will be highly interesting for the understanding of the superconducting properties of the bismuth family of cuprate superconductors. Since the cell parameters of the 22 12 subcell are likewise influenced by the Bi:Pb: Sr:Ca ratio of the samples, additional experiments were performed to support the present findings. Air-quenched powders of nominal composition Bi, ,,Pb,,$r,CaCu,O, +z consist of a 2212 majority phase. The lattice constants of the 2212 subcell (a=5.367(5), b=5.397(5), c=30.75(5)& (a-b)/(a+b)= -2.8x 10-3) show deviations from the values of the air-quenched Bi,,,,Pb,,,,,Sr,CaCu,O,+, samples and the orthorhombic distortion has increased. However, an additional marked difference between the samples lies in the weak but clearly visible appearance of the diffraction lines of a phase of Ca,PbO, type in the X-ray diffraction pattern. From the observed differences we conclude that the true composition of the 2212 phase in the sample of nominal composition Bi,,Pb,,,Sr,CaCu,O, +z is no longer described by the general formula Bi,_.Pb,Sr,CaCu,O,+, and deviations in the Bi : Pb: Sr:Ca ratio are expected. In fact, the composition of this 2212 phase is unknown. Susceptibility measurements of air-quenched powders of nominal composition Bi,,Pb,,4Sr,CaCu,0,+z (Fig. 3(a)) show a Meissner fraction of about 50% at 20 K. Correspondingly, the superconductivity is bulk and mainly due to the 2212 majority phase. Similar to Bi,,,,Pb,,,,Sr,CaCu,O,+z (Fig. l), a small 2223 fraction (T, = 110 K) is equally present. The transition temperature of the 2212 phase is about 75 K and is not identical with the slightly smaller value for the corresponding sample Bi,,,,Pb,,,,,Sr,CaCu,O,+; (Fig. l(a)), thus reflecting the influence of subcell dimensions and/or chemical composition on the position of T,. The observed tendencies are enforced by further decreasing the formal Bi : Pb ratio. Air-quenched powders of nominal composition Bi,,Pb,,6Sr,Cu,0,+, con-
0
20
40
60
80
100
120
140
; -0.008 El =: 2
-0.01 1
lb)
(4 Fig. 3. x vs. T for air-quenched BI, ,Pb,, ,Sr,CaCuzO,
+ :.
powder
samples
of compositions
(a) Bi,.,Pb,,.4SrZCaCuZOx+L,
and (b)
343
tain a 2212 majority phase (a=5.378(5), b=5.414(5), c=30.‘79(5)& (a-b)! (a + b) = - 3.3 x 10p3) with a still stronger orthorhombic distortion. In addition, an increased amount of a phase of CazPbO, type is present compared with the above described Bi,.6Pb0,4SrZCaCu20R+c sample. Owing to the formation of a byproduct, the true composition of this 22 12 phase is also unknown. From the susceptibility measurements of the air-quenched powder of nominal composition Bi,,,Pb,,6Sr,CaCu,0,+t a Meissner fraction of about 50% at 20 K is found (Fig. 3(b)). Correspondingly, the 2212 majority phase shows bulk superconductivity. In addition, a small 2223 contribution ( T, = 110 K) is detected. Interestingly, r, of the 2212 majority phase is as high as 80 K and has consequently increased by about 5 IS compared with the identically prepared sample of nominal composition Bi,,~Pb~~,~Sr~CaCu~O~+:. From the observed X-ray and susceptibility data of the 2212 phases in the three air-quenched samples of nominal compositions Bi,,,,Pb,,,,,Sr,CaCu20,+z, and Bi,,,Pb0,hSr2CaCu208+z, we conclude that for the Bi,,,Pb,,Sr,CaCu,O,+, superconducting 2212 phase there is an extended field of compositions in the system Bi-Pb-Sr-Ca-Cu-0, easily detectable by the differences between the lattice parameters of the 22 12 subcell. For identical conditions of preparation, the positions of T, are simultaneously influenced, thus reflecting the assumed differences in composition and structural order.
Acknowledgments This work was supported by the Bundesministerium fur Forschung und Technologie (FKZ 13N5482/0), the Verbundprojekt Supraleitungsforschung des Landes Baden-Wiirttemberg and the Verband der Chemischen Industrie. T. Rentschler thanks the Landesgraduiertenforderung of Baden-Wurttemberg for granting a scholarship. The help of Mrs. A. Ehmann and Mrs. E, Niquet is much appreciated.
References 1 M. Schlichentnaier, S. L&h and S. Kemmler-Sack, J. Less-Common Mer., 162 (1990) 155. 2 A. Ehmann, S. Kemmler-Sack, R. Kiemel, S. Losch, W. Schafer, M. Schlichenmaier, L. Kan, B. Elschner, R. Gross, K. Hipler, R. P. Huebener, H. R. Khan and Ch. J. Raub, J. Less-Common Met., 137( 1989) L25. 3 A. Ehmann, S. Kemmler-Sack, R. Kiemel, S. Liisch, W. Schafer, M. Schlichenmaier, L. Kan, B. Elschner, R. Gross, K. Hipler, R. P. Huebener, S. Dottinger, W. Forkel, C. Scholl. H. R. Khan and Ch. J. Raub, J. Less-Common Met., 151(1989) 55. 4 S. Losch, M. Schlichenmaier, S. Kemmler-Sack, C. Scholl, S. Dottinger, W. Forked, D. Kolle, R. Gross and R. P. Huebener, J. Less-Common Mer., 159 (1990) 26 1. 5 M. Schlichenmaier, S. Liisch, S. Kemmler-Sack, D. Kolle. H. Hartmann and R. P. Huebener, J. Less-Common Met.. I60 ( 1990) Li . 6 H. W. Zandbergen, W. A. Green, A. Smit and G. van Tendeloo. Physica C, 163 ( 1990 j 426. 7 G. Miehe, T. Vogt, H. Fuers and M. Wilhelm. Physica C, f71(1990) 339.
344 8 S. A. Sunshine, L. F. Schneemeyer, R. M. Fleming, A. T. Fiory, S. Martin and S. H. Glarum, Mol. Cry% L.iq. Cvst., I84 ( 1990) 9. 9 L. F. Schneemeyer, S. A. Sunshine, R. M. Fleming, S. H. GJarum, R. B. van Dover, P. Marsh and J. V. Waszcak, Appi. Phys. Lett., 57( 1990) 2362. 10 0. Eibl, Physica C, I# ( 1990) 249. 11 M. Saunke, B. Kabius, S. Thierfeldt, R. R. Arons, H. Maletta, J. Bock and E. Preisler, presented at Hochtemperatursupraleiter
und Kristallchemie IV, Bad HonneJ November
7-9, IWO.
12 N. Enomoto, H. Kikuchi, N. Uno, H. Kumakura, K. Togano and K. Watanabe, Jpn. J. Appi. Phys., 29 (1990) fA47. i 3 W. Schafer, J. Maier-Rosenkranz, S. Losch, R. Kiemel, W. Wischert and S. Kemmler-Sack, 3. Less-
Common Mel.., I42 (1988) L5.
Less-Common
Merals, 171(1991)
337
337-344
Superconducting properties of the lead-contai~ng 22 12 phase Bi,.,,Pb,.*,Sr,CaCu,0, +z with Cr;,in the range 70-98 K M. Ryguia, T. Rentschler,
M. Schlichenmaier,
W. Wischert and S. Kemmler-Sack
l~s~itl~t~r An~r~a~iseh~ Chemie der U~~~ersi~~t,Auf der Mo~e~stelie
IS, W-TWX?Tiibingen IF. R.G.1
(Received February 4,199 I )
Abstract The lead-containing, nearly single-phase 22 12 material of nominal composition Bil,,Pb,,&SrzCaCuzOn+: has a r, of about 70 K after air quenching. By controlling post-treatment, the transition temperature can be raised to about 98 K. Since no significant changes in the oxygen content are observed, we conclude that the function of post-treatment is to produce a more favorable ordering of oxygen in the lattice. Accordingly, all dimensions are influenced by the post-treatment. Additional results for air-quenched 2212 material with variations in the Bi:Pb:Sr:Ca ratio show that differences in the position of T, are also reflected in variations of the cell dimension, thus pointing to an extended field of existence for the 22 I2 phase in the system Bi-Pb-Sr-Ca-Cu-0 with phase boundaries which are still unknown.
1. Introduction In the Bi-Sr-Ca-Cu-0 system a series of stacking polytypes of idealized composition (BiO),Sr,Ca,, _ , Cu,,OZ,,+7+_ is existent and members with n = 1 (2201), II = 2 (2212) and IZ= 3 (2223j have been isolated. For these three cases, introduction of lead in the crystal structure was reported [l-5] and at least a part of the lead is substituted for bismuth, thus influencing the modulation of the incommensurate superstructures of the pure bismuth compounds [6, 71. Additionally, for the lead-containing n = 3 phase, a partial substitution of lead for calcium was observed [7]. For the three members with y1= 1, 2 and 3, the correlation between the true chemical composition, their crystal structure and the superconducting properties are not fully understood. For n = 1, the composition of the superconducting phase is not included by the above idealized composition 18, 91. Lead-free samples with n = 3 consist of an irregular intergrowth of lamellae with n= 1, 2, ...) 6 [lo] and the II = 3 structure must be stabilized by an admixture of lead. In the lead-free n = 2 samples, stacking faults due to intergrowth of lamellae with n ic 2 are also abundant, reducing significantly with increasing lead content f 1 l]. For the lead-containing fz= 2 samples a pseudotetragonal, orthorhombic as well as monoclinic distorted subcell was observed, the degree of distortion being influenced by the lead content and/or the conditions of preparation [2, 3, 51. The transition temperature to superconductivity is influenced by the lead content and the conditions of preparation, and values for T, between approxi0022-%X38/9 I /$X50
0 Elsevier Sequoia/P~nted
in The Netherlands
338
mately 60 and 95 K are observed. However, no clear correlation between T,, the structure of the subcell and the lead content has yet been found. The purpose of the present investigation is to study the correlation between the superconducting properties, the structure of the subcell and the total oxygen content for a fixed lead content. For this reason the composition Bi,,,,Pb,,,Sr,CaCu,O, + z was chosen, because of the existence of a nearly single-phase 2212 material [5]. To exclude the influence of smaller fluctuations of the sample composition, a batch of approximately 90 g was synthesized and all studies described here were performed with identical material. To demonstrate the influence of the nominal composition of the samples on the superconductivity, additional results for 22 12 materials with variations in the Bi : Pb : Sr : Ca ratio are also included. It is shown that by careful control of post-treatment conditions, the transition temperature can be raised to about 98 K. Since significant changes in the oxygen content are absent, we conclude that the function of post-treatment is mainly to produce a more favorable ordering of oxygen in the lattice.
2. Experimental
details
A 90 g batch of nominal composition Bi,,,,Pb,,,,,Sr,CaCu,O,+, was prepared from Bi(NO,),*5H,O (DAB6, Merck), PbO, Sr(NO,),, CaCO, (all p.A., Merck) and CuO (puriss.p.A., Fluka). The materials were mixed in an agate mortar and prefired in a corundum crucible (Degussit A123) in air at 710 “C for 18 h. The product was mixed in a ball mill (agate beaker and balls) and heated between 845
0
7
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
‘;rT
t’ -0.002 .?z! +2- -0.004 i m _ -0.006 z ; Y VI (a)
-0.008 -0.0,
(b)
Fig. 1. x vs. T for powder samples of Bi,,,, Pb,,,2sSrzCaCu20x+z: (a) air quenched, quenched, (c) post-annealed in argon at 650 “C for 15 h.
(b) liquid NZ
339
and 860 “C for 260 h. The reaction was interrupted several times for regrindings in the ball mill and X-ray analysis (Philips powder diffractometer, Cu Ka radiation, gold standard). After the final step the relative intensities and positions of the peaks of the X-ray diffraction pattern became invariant and the annealing was stopped. The resulting air-quenched material was split into several parts. In one series of experiments the results of post-treatment on powder samples were studied. For the second set several pellets (diameter approximately 13 mm, thickness approximately 1 mm, density 85% of the X-ray density) were pressed and submitted to different post-treatments. The conditions of preparation for samples of nominal composition Bi, ,,Pb,,,dSr,CaCu,O, + i and Bi,,,Pb,,,Sr,CaCu,O, + i were identical with the exception that the batches were smaller (approximately 15 g) and ground exclusively in an agate mortar. The products were examined by X-ray diffraction (the degree of crystalline orientation was calculated according to ref. 12), chemical analysis (according to ref. 13), measurement of the electrical resistivity (standard four-probe d.c. technique) and d.c. magnetic susceptibility (SQUID magnetometer, magnetic field 10 - 3 T ).
3. Results and discussion The powder X-ray diffraction pattern of the as-prepared air-quenched sample of nominal composition Bi,,,,Pb,,,,,Sr,CaCu,0, +z is in agreement with the results of ref. 5, demonstrating the existence of a nearly single-phase 22 12 material with a slightly orthorhombic distorted subcell (a = 5.398( 5), b = 5.4 18( 5), c = 30.86( 5) A). No diffraction lines corresponding to a phase of Ca,PbO, type are visible. In the susceptibility x vs. temperature T signal (Fig. 1 (a)) of the airquenched powder the onset of a very small diamagnetic signal due to a small 2223 content is observed at about 110 K. At about 70 K a pronounced second step is developed. Since the Meissner fraction of this phase is about 50% at 20 K (compared with a perfect diamagnet with a susceptibility of 4~cx = - l), we conclude that the superconductivity is bulk and results from the majority air-quenched 22 12 phase. Quenching with liquid N, did not affect the purity of the sample. The lattice parameters of the 2212 subcell are slightly reduced (a= 5.382(5), b = 5.398(5), c = 30.84(5) A) and the extent of the orthorhombic distortion (a - b)/(a + h) = - 1.5 x 10 m3 is slightly inferior (compared with the value of - 1.9 x 10-jfor the air-quenched sample). In the susceptibility vs. temperature signal (Fig. l(b)) the presence of a small amount of 2223 phase is again detectable by the appearance of a small negative signal at about 110 K. In this case a pronounced second step develops at about 95 K. The Meissner fraction of this phase is about 50% at 20 K and we conclude that the superconductivity is bulk and results from the majority liquid N, quenched 22 12 phase. After post-annealing in an argon stream (at 650 “C for 15 h) a nearly singlephase 2212 material is conserved. However, the orthorhombic distortion of the
340
22 12 subcell (a = 5.394( 5), b = 5.420( 5), c = 30.90( 5) A) is more pronounced ((a - b)/( a + b) = - 2.4 X 10-j). The susceptibility measurement (Fig. l(c)) again indicates the presence of a small amount of 2223 phase. Furthermore, in this case the second step develops at a still higher temperature of about 98 K. From a Meissner fraction of about 50% at 20 K we conclude likewise that the superconductivity is bulk and results from the majority argon-treated 2212 phase. Pressed and identically post-treated pellets of composition Bi,,,,Pb,,l,Sr,CaCu208+, show a degree of preferential orientation of about 60%. A comparison between the temperature dependence of the resistivity for the air-quenched, liquid N2 quenched and argon-treated pellets is shown in Fig. 2. The quenching with liquid N, does not appreciably alter either the room temperature resistance or the temperature dependence in the normal conducting region, but it moves the transition temperature from about 62 K to 88 K. However, argon treatment results in a drastic increase in the room temperature resistance and a nearly semiconductorlike temperature dependence in the normal conducting region, most probably owing to a partial formation of a semiconducting byproduct. The observed T, of about 87 K is in disagreement with the susceptibility data for the corresponding pellet (see below) and powder with a T, of about 98 K. The susceptibility measurements for identically post-treated pellets of composition Bi, ,,,Pb,,,,,Sr,CaCu,O, +1, however, uniformly give values for T, (onset of the Meissner signal) in good agreement with the data for the powdered samples (approximately 65 K for air quenching, approximately 90 K for quenching with liquid N,, approximately 98 K for argon treatment). Thus, the observed depression of T, in the resistivity measurements of the argon-treated pellet is not a bulk effect but the result of a superficial decomposition. From the susceptibility measurements of the powders and pellets we conclude that T, is reproducibly raised to about 90 K by quenching with liquid N, and to as high as around 98 K by post-annealing in an argon stream. Since changes in oxygen content or ordering seemed the most likely explanation for the increase in T, on post-treatment, we examined the oxygen content of these samples. For the three powders we find a very similar oxygen content of 8.14 (air quenched), 8.08 (quenched with liquid N,) and 8.08 (argon-treated). The corresponding values for a formal copper valence of 2.26( 3), 2.20( 3) and 2.20( 3) are nearly identical within the limit of experimental error. Interestingly, for the n = 1 phase of nominal composition Bi,,(,, Sr,,,,CuO,, similar results have been obtained recently [9]. By controlled post-annealing, the transition temperature can be raised from 4 K to about 15 K and in some cases as high as 18 K without changing the oxygen content of the samples. From the observed minor changes in the oxygen content of the present 2212 samples we conclude that the increase in T, from around 70 K for the airquenched sample to approximately 90 or 98 K after post-treatment is mainly the result of the production of a more favorable ordering of oxygen in the lattice. This assumption is supported by the observed variations in the lattice parameters and the extent of orthorhombic distortion of the 2212 subcell for the different posttreated samples. Furthermore, the experiments point to the existence of a more favorable crystalline order at higher temperatures. This ordered state can be
341
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...' *.". ..:.
.5
........
.4 .3
.A .:-t'. +.\__. _-+
.2
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.I 0 0
. . kw....r;l. . . , . 50 100
. ‘ I . , . I . . . , . : 200 258 150
T CKI
(4 rho
[mOhm*cml
,...‘l*,-.‘...,‘..+.‘.,..‘..,.
0
50
0
I00
150
200
250
(b) rho
. 300
300 T CKI
CmOhm*cml 9 a Y--:'
7 6 5 4 3 2 I 0 0
(c)
~~~~'~~-l.'~~~.l.~~~'..l.'.... 50 100 t50
200
250
300 T EKI
Fig. 2. Temperature dependence of the resistivity for pellets of composition Bi,,,,Pb,,zsSrzCaCu,0, +;: (a) air quenched, (b)liquid Nz quenched, (c) post-annealed in argon at 650 “C for 1.5h.
342
conserved at room temperature by rapid quenching with liquid N, but not by the more slow air quenching. Additionally, the development of a favorable crystalline order is expected through the applied controlled argon treatment. Careful structural investigations in combination with transmission electron microscopy measurements will allow the expected differences to be revealed more precisely. These results will be highly interesting for the understanding of the superconducting properties of the bismuth family of cuprate superconductors. Since the cell parameters of the 22 12 subcell are likewise influenced by the Bi:Pb: Sr:Ca ratio of the samples, additional experiments were performed to support the present findings. Air-quenched powders of nominal composition Bi, ,,Pb,,$r,CaCu,O, +z consist of a 2212 majority phase. The lattice constants of the 2212 subcell (a=5.367(5), b=5.397(5), c=30.75(5)& (a-b)/(a+b)= -2.8x 10-3) show deviations from the values of the air-quenched Bi,,,,Pb,,,,,Sr,CaCu,O,+, samples and the orthorhombic distortion has increased. However, an additional marked difference between the samples lies in the weak but clearly visible appearance of the diffraction lines of a phase of Ca,PbO, type in the X-ray diffraction pattern. From the observed differences we conclude that the true composition of the 2212 phase in the sample of nominal composition Bi,,Pb,,,Sr,CaCu,O, +z is no longer described by the general formula Bi,_.Pb,Sr,CaCu,O,+, and deviations in the Bi : Pb: Sr:Ca ratio are expected. In fact, the composition of this 2212 phase is unknown. Susceptibility measurements of air-quenched powders of nominal composition Bi,,Pb,,4Sr,CaCu,0,+z (Fig. 3(a)) show a Meissner fraction of about 50% at 20 K. Correspondingly, the superconductivity is bulk and mainly due to the 2212 majority phase. Similar to Bi,,,,Pb,,,,Sr,CaCu,O,+z (Fig. l), a small 2223 fraction (T, = 110 K) is equally present. The transition temperature of the 2212 phase is about 75 K and is not identical with the slightly smaller value for the corresponding sample Bi,,,,Pb,,,,,Sr,CaCu,O,+; (Fig. l(a)), thus reflecting the influence of subcell dimensions and/or chemical composition on the position of T,. The observed tendencies are enforced by further decreasing the formal Bi : Pb ratio. Air-quenched powders of nominal composition Bi,,Pb,,6Sr,Cu,0,+, con-
0
20
40
60
80
100
120
140
; -0.008 El =: 2
-0.01 1
lb)
(4 Fig. 3. x vs. T for air-quenched BI, ,Pb,, ,Sr,CaCuzO,
+ :.
powder
samples
of compositions
(a) Bi,.,Pb,,.4SrZCaCuZOx+L,
and (b)
343
tain a 2212 majority phase (a=5.378(5), b=5.414(5), c=30.‘79(5)& (a-b)! (a + b) = - 3.3 x 10p3) with a still stronger orthorhombic distortion. In addition, an increased amount of a phase of CazPbO, type is present compared with the above described Bi,.6Pb0,4SrZCaCu20R+c sample. Owing to the formation of a byproduct, the true composition of this 22 12 phase is also unknown. From the susceptibility measurements of the air-quenched powder of nominal composition Bi,,,Pb,,6Sr,CaCu,0,+t a Meissner fraction of about 50% at 20 K is found (Fig. 3(b)). Correspondingly, the 2212 majority phase shows bulk superconductivity. In addition, a small 2223 contribution ( T, = 110 K) is detected. Interestingly, r, of the 2212 majority phase is as high as 80 K and has consequently increased by about 5 IS compared with the identically prepared sample of nominal composition Bi,,~Pb~~,~Sr~CaCu~O~+:. From the observed X-ray and susceptibility data of the 2212 phases in the three air-quenched samples of nominal compositions Bi,,,,Pb,,,,,Sr,CaCu20,+z, and Bi,,,Pb0,hSr2CaCu208+z, we conclude that for the Bi,,,Pb,,Sr,CaCu,O,+, superconducting 2212 phase there is an extended field of compositions in the system Bi-Pb-Sr-Ca-Cu-0, easily detectable by the differences between the lattice parameters of the 22 12 subcell. For identical conditions of preparation, the positions of T, are simultaneously influenced, thus reflecting the assumed differences in composition and structural order.
Acknowledgments This work was supported by the Bundesministerium fur Forschung und Technologie (FKZ 13N5482/0), the Verbundprojekt Supraleitungsforschung des Landes Baden-Wiirttemberg and the Verband der Chemischen Industrie. T. Rentschler thanks the Landesgraduiertenforderung of Baden-Wurttemberg for granting a scholarship. The help of Mrs. A. Ehmann and Mrs. E, Niquet is much appreciated.
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