Vanadium-lead substituted 2223 Bi-Sr-Ca-Cu-O superconductors

Solid State Communications, Vol. 76, No. 12, pp. 1351-1356, 1990. Printed in Great Britain.

0038-1098/90 $3.00 + .00 Pergamon Press plc

V A N A D I U M - L E A D SUBSTITUTED 2223 Bi-Sr-Ca-Cu-O SUPERCONDUCTORS Ying Xin, Z. Z. Sheng and F. T. Chan Physics Department, University of Arkansas, AR, USA and P. C. W. Fung and K. W. Wong* Physics Department, University of Hong Kong, Hong Kong

(Received 20 August 1990 by W. Y. Kuan) Following up our previous work [24] on the fabrication of the Bi2 xVxSr2Ca2Cu3Oy and Bi2_xVxSrzCaCuzO8 superconductors with x ranging from a fraction to unity we report here the fabrication of double-substituted superconducting ceramics satisfying the nominal stoichiometric ratio of Biz_(x+y)VxPbySr2Ca2Cu3Oq, where x, y run from small fractions to 0.7. For most of the samples, the lower T, = 80 K phase disappears, leaving the higher T, = 110 K phase. Since a wide range of combinations of x, y can lead to similar superconductors and that the pure T,. = 110 K phase samples are associated with larger values of (x + y), we believe that the V-Pb substitution is significantly different from the Pb-doped or Pb-Sb-doped cases in the basic crystal structure. Our samples are found to pass through the paramagnetic to the diamagnetic state as the temperature is decreased across the critical temperature region. 1. I N T R O D U C T I O N IT HAS been found that there are quite a large number of crystal structures or phases that can co-exist or exist under separate conditions in the class of Bi-SrC a - C u - O superconductors [1-5]. For examples, there are nominal compositions satisfying Bi : Sr: Ca : Cu = 2 : 2 : 0 : 1 (T,. ~ 10K), l : I : l : 2 ( T , < 8 0 K ) , 2 : 2 : 1 : 2 (T, ~ 80K) and 2 : 2 : 2 : 3 (T, ~ IIOK). While the lowest order phase (2 2 0 1) is mono-clinic, the other phases so far reported seem to have average orthorhombic structures with different crystal parameters. The (220 1), (2212) and (2223) phases have single, double, and triple layers of CuO2 plane in the sub-unit cell respectively, and more CuO2 planes are believed to be associated with higher values of T,. It is therefore important to enhance the (2223) l l 0 K phase by using various methods of preparation [e.g. 6, 7] or by doping. In fact, A1, La, Sb and Pb doped in the Bi-type superconductors [8-21] have been reported and the last member Pb has been found to be a rather good dopant in enhancing the l l 0 K phase. The double doping with Pb-Sb has aroused interest along this line [22, 23]. However, only a very limited amount of the

* Permanent address: Physics Department, University of Kansas, KS 66045, USA.

dopant can replace Bi, in the nominal composition ( < 40%). In order to understand more fully about the role played by dopants of the high 7", phases of the Bi-type superconductors, it is crucial to find out whether other dopant(s), apart from Pb, could also enhance the formation of the high T,. phase(s). Even more important it is crucial to find out whether any other element can have an equal standing as Bi; at the end, we hope that Bi can be replaced completely such that a new superconductor class can be found. The recent band structure calculations [24] on a hypothetical structure same as the bismuth structure but with the composition V2Sr2CatCu208 produce a band structure favouring the formation of condensed charged excitons. According to the EEM theory [25, 26] a superconductor would result if such a condition is satisfied. Thus such an estimation suggests that vanadium might be used to replace Bi, at least partially. With this motivation in mind, we have attempted and have succeeded in replacing Bi by V in the predetermined (2 2 1 2) and (2 2 2 3) superconductors up to at least a ration of r = V/Bi = 1. We have demonstrated that the R-Tcurves show clear sudden drops at about 110 K, though the T, (R = 0) value is only several degrees above that of the pure Bi-type superconductor prepared under same conditions. The readiness in the formation of the

1351

1352

V A N A D I U M - L E A D S U B S T I T U T E D 2223 B i - S r - C a - C u - O

I10K phase is significantly enhanced with large amounts of V-substitution leads us to consider that V might play the role of Bi. In other words, the V-substituted superconductors might form a high T, crystal structure different from that of the corresponding pure Bi-type. In fact a new V2Ca2BizSr3Cu3Oy T, structure has been found [27]; here z is very small. We must note, however, that when the ratio V/Bi is above 1 in our previous work, the sample becomes semi-conductor like. Since the atomic size of V is significantly smaller than that of Bi, with V substituting Bi alone, the crystal structure is bound to distort the a, b axes of the unit cell, although the valence of V and Bi are compactable. On the other hand, the atomic size of Pb is greater than that of Bi, but their valences do not match as well. It is thus natural to expect that with V-Pb doubly substituted for part of Bi, the sample would be more stable in its crystal structure. For the reasons stated above, we have purposely chosen V and Pb to be the substituting elements and report in this investigation the fabrication of the V-Pb double-substituted ( 2 2 2 3 ) nominal ceramic superconductors. We have found that the R - T curve tail disappears for samples covering a wide range of V and Pb quantities in this combined substitutions. 2. SAMPLE P R E P A R A T I O N A N D R E L E V A N T MEASUREMENT First, AR grade powders of Bi203, SrO(SrCO3), CaO(CaCO3), CuO were mixed and ground in an agate mortar in the pre-determined composition of Bit Sr2Ca2Cu308.5, the precursor. The well-mixed powder was preheated in a tube furnace (model 847, Lindberg) with both ends open at a temperature of 820°C for 20 h. The resulting material was taken out of the furnace, cooled in air and ground again. Appropriate amounts of Bi203, V203 and PbO2 powders of AR grade were added to the precursor according to the stoichiometric proportion of Bi2 i.~+~./V~PbySr2Ca2Cu3Oq, where x, y take on various fractions to be specified. The resulting mixture was well ground again, and pressed into pellets of 12mm in diameter, and 2 mm in thickness using a hydromatic press with a pressure of about 7000 kgcm 2. For the purpose of quenching, the pellets were lined up in an alumina cylinder. For sintering, the tube furnace was maintained at a setting temperature of 825°C. The loaded alumina cylinder was introduced into the furnace. The "combined" sintering process consists of several stages as specified below: (i) 825°C for 55 h, then (ii) 845°C for 25 h, then (iii) 860°C for 36 h.

Voi. 76, No. 12

The above heating process A was carried out in air with both ends of the tube open. 2.1. Resistance-temperature measurement The resistance of each sample was measured in the temperature range (16-30K) by the standard fourprobe technique using an a.c. current source of 27 Hz, with a closed cycle APD refrigerator system equipped with a computer control and data processing units. The R - T curves for various samples are presented in Fig. 1(a)-(f). 2.2. A.c. susceptibility measurement A.c. susceptibility Z was measured for each sample using the technique similar to that employed by Norton [28] with an a.c. of frequency 500 Hz, covering a temperature range of (16-250 K). Typical Z- T curve for sample 12 (x = 0.2, y = 0.5) is shown in Fig. 3. Other z - T curves are similar, which are only used to check the R - T results, are not presented here. 3. ANALYSIS The initial nominal compositions of our doublesubstituted samples in this investigation are represented by Bi2 (x+y)VxPbySr2Ca2Cu3Oq. To carry out a relatively systematic analysis, we plot in Fig. l(a) the R - T curves for samples sintered by process A and satisfying x = 0.1, while y runs from 0.1 to 0.7. The observational points are very dense in most part of each curve and will not be shown. The s y m b o l s . , o, [] etc. are added for the convenience of identifying different curves pertaining to various y values. We notice that when y = 0.1, the resistance of the sample is relatively low, being < 1 mf~ even at 275 K. Though the absolute value of R depends on the positions of the four probes, yet we would remark that we have used the same setup in measuring the samples which have very similar sizes after sintering. The R values shown thus represent the relative resistances among the samples. When we enlarge the R - T curve for the x = 0.1, y = 0.1 sample, we find the sample have two phases, one specified by the onset temperature T,? of l I 4 K and the other by about 80K. The T, (R = 0) value is rather low, however, being only 67 K. When y increases to 0.2, the R - T slope for the linear portion increases and the T,. (R = 0) value has increased to 102 K and the lower phase disappeared. As y increases from 0.3, to 0.4 then 0.5, 7", (R = 0) decreases gradually and the R - T slope decreases accordingly. For greater values of y (0.6, 0.7), T, (R = 0) drops to 80K and the sample is semi-conductor like. For clarity in analysis we show in Table 1 the values of x, y, T,~, T, (R = 0), involved for 28 samples reported

Vol. 76, No. 12

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S U B S T I T U T E D 2223 B i - S r - C a - C u - O

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1354

V A N A D I U M - L E A D S U B S T I T U T E D 2223 B i - S r - C a - C u - O

Table 1. Transition temperatures and compositions of samples Sample number l

2 3 4 5 6 7 8 9 10 11

12 13 14 15

16 17

18 19 20 21 22 23 24 25 26 27 28

V (x)

Pb (y)

T, (K) onset

T, (K) zero

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114, 80 114 ll4 114 110 104 104 114 115 116

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114

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here. Fig. l(b) indicates the R - T curves for other combinations of Bi, V, Pb specified with x = 0.2, y = 0.1 . . . . . 0.6. In general, like result shown in Fig. l(a), there is an optimum combination of x and y (in this case x = 0.2, y = 0.5) for which the R - T curve shows the best superconductivity properties. As before, the R - T slope in the range (about 130 to 300K) decreases gradually, eventually to negative values when the amount of Pb is increased. We would remark also that the samples have rather high onset temperatures. Fig. 1(c) shows that all the samples with the range of substitutions specified by (x = 0.3; y = 0.1 . . . . . 0.5) are all metallic like. The best sample is the one satisfying x = 0.3, y = 0.5. Notice also that in the (x = 0.3, y = 0.4) sample, the 110 K phase shows

Vol. 76, No. 12

up clearly. Figures l(d)-(f) demonstrate other R - T curves with various other combinations of x and y for samples sintered by this process. Apart from the features already stated for other earlier figures, it is important to note that the optimum combinations of x and y for the better superconductors T~ (R = 0) > 95K, T~~ > l l 0 K ) do not indicate which is a better dopant, V or Pb. This feature is interesting in our further study and understanding of the structures of the B i - V - S r - C a - C u and B i - P b - S r - C a - C u superconductors. To analyze the samples from another angle, we present the Fig. 2(a)-(f) the same R - T curves but in each figure, the amount of total substitution, i.e. (x + y) is fixed. Comparing these figures, we observe that in general better superconductors are associated with relatively large amount of substitutions, namely x + y = 0.6, 0.7 and 0.8, rather than smaller values provided x > 0.2. The z - T curve in Fig. 3 for the sample x = 0.2, y = 0.5 shows the dominant 110K phase and a very small amount of 80 K magnetic phase. There is one interesting feature we wish to note: the )~-T curve first drops a little at T ~ 109K and then rises, implying that as the temperature is lowered, it first becomes a para-magnetic material and then changes to diamagnetic at lower temperatures. We have found that all the )~-Tcurves indicate such a feature. Such a property deserves further attention. 4. D I S C U S S I O N S A N D C O N C L U S I O N S (1) We have found recently [24] that on replacing part of Bi by V only in the B i - S r - C a - C u ceramic superconductors, the l l 0 K phase is significantly enhanced. However, the T~ (R = 0) value is still around 80 K. Following our previous work, we have fabricated the (V, Pb) double-substituted superconductors based on the reasons stated in the Introduction and have found here that the 8 0 K phase disappears for our better samples. (2) (i) The onset temperatures of our samples cover a narrow range of 110-118 K. (ii) There is a wide range of combinations of V, Pb associated with T, (R = 0) in the range (90-103 K). In particular, the better samples are specified by relatively large amounts of nominal substitutions (x + y = 0.6, 0.7 and 0.8). (3) The V - P b substituted superconductor is different from the Pb doped and P b - S b doped superconductors in the following sense: (i) It has been found previously that Pb (or P b Sb) can substitute up to at most about 40% of Bi present in the predetermined atomic compositions.

Vol. 76, No. 12

VANADIUM-LEAD

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Fig. 2. R - T curves for the 28 samples in Fig. 1 but grouping together curves o f constant x + y: (a) x + y = 0 . 3 ; ( b ) x + y = 0.4;(c) x + y = 0 . 5 ; ( d ) x + y = 0.6;(e) x + y = 0 . 7 ; ( f ) x + y = 0.8.

With V present, very g o o d substituted superconductors can be made readily up to at least 66% o f Bi (x + y = 0.8). Samples with still larger x + y values will be investigated further. (ii) Figures 1 and 2 and Table 1 show that x and y can vary in a wide range. There is no such property in the P b - S b doped samples. (4) Since the substitution can be close to unity, we are led to believe that an entire layer o f Bismuth oxide in the (2 2 2 3) structure might have been replaced by

a layer o f V - P b oxide. Based on our previous work that V/Bi can be as high as 1, V might form a new structure by itself. In view o f these two pieces o f work, we might guess the atomic structure o f the B i - V - S r C a - C u superconductor and pursue to fabricate a class o f superconductors with V replacing Bi completely. Such finding will be reported in a separate article. (5) The resistance o f the better samples is relatively low, being smaller than l mf2 even at r o o m temperature.

VANADIUM-LEAD SUBSTITUTED 2223 Bi-Sr-Ca-Cu-O

1356

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Fig. 3. A.c. susceptibility versus temperature for the sample satisfying x = 0.2, y = 0.5. (6) The superconductivity properties are very sensitive to the temperatures involved in the combined sintering processes. We must explore other effective sintering processes and shall report in the following article the results of other sintering processes. (7) We believe that vanadium has a valency of slightly greater than three in the structure. This is the reason for using V203 in our fabrication and oxidation is found to be favourable for making superconductors. If V205 is used in the preparation, reduction in Ar atmosphere is found to be necessary. Such a crucial result is significantly different from and contrary to the work reported by Che et al [29]. (8) When the temperature is decreased, passing through the critical temperature region, each sample passes through the paramagnetic state, before settling in the diamagnetic state.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

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6. 7. 8.

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Vol. 76, NO. 12

P.A. Lindberg, Z.X. Shen, B.O. Wells, D.S. Dessau, W.P. Ellis, A. Borg, J.K. Kang, D.B. Mitzi, I. Lindau, W.E. Spicer & A. Kapitulnik, Phys. Rev. B40, 8840 (1989). X. Chen, J. Xia, Z. Chen, Y. Qian, C. Fan, L. Yang, C. Chu, C. Xu, Q. Zhar.g & M. Xu, Solid State Commun. 71, 117 (1989). C.J. Huang, T.Y. Tseng, T.S. Heh, F.H. Chen, W.S. Jong, Y.S. Fran & S.M. Shian, Solid State Commun. 72, 563 (1989). S. Sugai & M. Sato, Phys. Rev. 1340, 9292 (1989). Z. Chen, J. Xia, J. Chen, X. Chen, Y. Qian, M. Fan, C. Fan, L. Yang & Q. Zhang, Solid State Commun. 70, 133 (1989). C.G. Cu, J.L. Zhang, S.I. Li, J. Li, F. Shi, S.Z. Shou, Z.H. Shi & J. Dou, Solid State Commun. 71, 287 (1989). W.G. Zeng, P.C.W. Fung, G.M. Lin, J.X. Zhang, G.G. Siu, Z.L. Du & K.F. Liang, Solid State Commun. 71, 949 (1989). M.A. Dinia, O. Pena, C. Perrin & M. Sergent, Solid State Commun. 73, 715 (1990). V. Pleckacek & F. Gomory, Solid State Commun. 73, 349 (1990). B. Gogia, S.C. Kashyap, D.K. Pandya & K.L. Chopra, Solid State Commun. 73, 513 (1990). A.A. Youssef, Y. Horie & S. Mase, Solid State Commun. 73, 771 (1990). A.K. Sarkar, I. Maartense, B. Kumar & T.L. Peterson, Supercond. Sci. Technol. 3, 199 (1990). G. Jasiolek, J. Georecka, J. Majewski, S. Yuan, S. Jin & R. Liang, Supercond. Sci. Technol. 3, 194 (l 990). H.B. Liu, X.N. Zhan, Y.Z. Chao, G.E. Zhou, Y.Z. Ruan, Z.J. Chen & Y.H. Zhang, Physica C156, 804 (1988). H.B. Lin, L.Z. Cao, L. Zhou, I.J. Mao, W.J. Zhang, X.X. Li, Z.D. Yu, B. Xue, X.L. Mao, G.E. Shou, Y.Z. Ruan, Z.J. Chen & Y.H. Zhang, Solid State Commun. 69, 867 (1989). P.C.W. Fung, Z.C. Lin, Z.M. Liu, Ying Xin, Z.Z. Sheng, F.T. Chan, K.W. Wong, YongNian Xu & W.Y. Ching, Solid State Commun. 75, 211 (1990). K.W. Wong & W.Y. Ching, Physica C158, 1 (1989); ibid, 15. K.W. Wong & W.Y. Ching, Physiea C152, 397 (1988). X. Fei, G.F. Sun, D.F. Lu & K.W. Wong (unpublished). M.L. Norton, J. Phys. El9, 268 (1986). G.C. Che, J.K. Liang, W. Chen, D.N. Zhang, S.L. Jia, Y.M. Ni, S.S. Xie & Z.X. Zhao, J. Mater. Sci. 24, (1989).