Superconductivity in Tl2Ba2−xSrxCaCu2O8 solid solutions: Tc≈44 K for a composition with x=2.0

PhysicaC 156 (1988) 775-780 North-Holland,Amsterdam

SUPERCONDUCrIVITY IN TI2Ba=_~Sr~CaCn=Os SOLID SOLUTIONS: Tc~.44 K FOR A C O M P O S I T I O N W I T H xffi2.0 Eric A. HAYRI and M. GREENBLATT Department of Chemistry, Rutgers. The State University of New Jersey. New Brunswick, NJ 08903. USA Received26 October 1988 Revised manuscript received8 November 1988

Solid solutions in the Tl2Ba2_xSrxCaCu20 s system were prepared and characterizedby X-ray powder diffraction, resistivity and magnetic susceptibilitymeasurements. Systematicchanges in T¢and lattice parameters with x are reported. A sample with nominal compositionofTl2Sr2CaCu20$ (x=2.0) iSsuperconductingwith Tc~ 44 K.

Since their discovery, the so-called "'second generation" high-T¢ superconductors have been the subject of extensive research. The discovery of superconductivity in Bi-Sr--Cu-O [ 1 ] and T1-BaCu-O [2 ] has led to the synthesis and characterization of the Bi2Sr2CanCun+lO6+2n ( n=O,1,2 ) [ 1,3-7] and TlmBa2CanCu,+ tO4 + 2n+m ( m = 1,2; n=0,1,2,3) [2,8-11 ] series of compounds. Both of these systems are characterized by layered type structures with substantial amounts of defects and intergrowths [ 12 ]. Because of the complexity of the structure of these phases due to the defects and intergrowths o f one type of compound with another, single phase materials are difficult or impossible to obtain. For this reason, few substitutional studies of these phases have been reported. Recent results of substitutional studies include the replacement of Ba by Pb [ 13 ] and TI by K [ 14 ] in the T1-Ba-Ca-CuO compounds; Ca by Y and rare earths [ 15,16 ], Bi by Pb [ 17 ] and Sr and Ca by K [ 18 ] in the Bi-SrC a - C u - O compounds. Sheng and Hermann [ 19 ] have observed resistive anomalies between 110 and 200 K in TI-Sr-Cu-O. Nagashima et al. [ 20 ] report resistive anomalies at ~ 100 K in T1-Sr-Ca-Cu-O phases which appear to be multiphase; they observed a Meissaer effect at ~ 40 K for a starting composition TILsSrCaCu2Ox. In this communication we report superconductivity in the Tl2Ba2_zSrxCaCu208 system. Superconductivity with T¢-~ 44 K is observed in a

mixed phase sample with the nominal composition of TI2Sr2CaCu208. Moreover, diamagnetic anomalies in the susceptibility at higher temperature suggest that small amounts o f the higher order member o f this system, T]2Sr2Ca2Cu3Oio, may also form. Samples were prepared from stoichiomctric quantities of T1203, BaO2, SrO, CaO2 and CuO. Approximately 0.4 g of starting mixture was pelletized under 3.5 kbar of pressure, wrapped in gold foil and sealed in a quartz tube in air. The tubes were then placed in a preheated muffle furnace at 865°C for two hours and slow cooled to room temperature over a period of several hours. In our experiment we observe some reaction of the quartz tubes with T]203 vapor. The weakened tubes, under pressure from the decomposed peroxides, often explode during cooling, therefore, appropriate precautions should be taken. X-ray powder diffraction patterns were recorded on a Scintag PAD V diffractomcter using monochromatized CuK~ radiation; correction for background scattering and CuK~2 were applied. Lattice parameters were calculated by fitting the observed diffraction data by a least-squares program; KCI was used as an internal standard. Resistivity was measured on brick shaped samples from 300 to 15 K in an APD DE202 cryostat using the standard four probe technique. Contacts were made by soldered indium which was whetted with silver paint. Magnetic susceptibility data were obtained on samples

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776

E.A. Hayri and M. Greenblatt / Superconductivity in TI~Ba2 ~rxCaCusOs solid solutions -

cooled in a field of 100 G with a Quantum Design MPMS SQUID magnetometer. X-ray powder diffraction patterns for Tl2Ba2_~SrxCaCu2Os, x=O.O, 0.6, 1.0 and 2,0, are shown in fig. 1. For x = 0.0, all of the diffraction lines having intensities I/Io > 5%, with the exception of a weak line at 20-23.9 °, can be indexed based on a tetragonal unit cell in space group I4/mmm. This peak can be attributed to small amounts of barium carbonate impurity [ 21 ]. The absence of this line in the sample with x = 2 . 0 may be taken as further evidence that the 23.9 ° peak is due to Ba impurity. As x increases, a shift of the diffraction peaks to lower d is observed. Some line broaderdng and impurity peaks are observed for x = 1.0. As x approaches 2.0 more impurity peaks are observed, however, the predominant phase present corresponds to the T]2Ba2CaCu20s structure (fig. 1 ). The major peaks of the mixture with the nominal composition TI2Sr2CaCu2Os could be indexed based on a tetragonal unit cell with a = 3 . 7 9 3 ( 1 ) ~ and c=28.28(2) A. Some of the impurity peaks in the XRD correspond to Sr4Tl207 [22]. Lattice parameters of selected samples of "~12B82_~SrxCaCU2Os ( 0 ~
are summarized in table I. Figure 2 illustrates that both the a and c unit cell parameters decrease monotonically with increasing x. The values of a and c for the composition "Tl2Sr2CaCu2Os" fall neatly on a straight line in fig. 2 and support the formation of the end member Tl2Sr2CaCu2Og. Attempts to prepare pure T12Sr2CaCu2Os by altering the reaction temperature or heating time did not significantly improve sample purity. This situation is comparable to the Bi-Sr-Ca-Cu-O and Y-Sr-Cu-O [23 ] systems where single phase polycrysmUine samples are difficult to synthesize. Plots of the normalized resistivity, p/p( 150 K) for samples with x=0.0, 0.6, 1.0, 1.4 and 2.0 are shown in fig. 3. Tc decreases and the transition width broadens as x increases. The data in table I indicate that T¢(onset) and T~(zero) decrease in a non-linear fashion with increasing x. This is illustrated in f ~ 4. For the sample with nominal composition Tl2Sr2CaCu2Os, T¢(onset) and T¢(zero) are 61 and 28 K, respectively. This is consistent with the results of Nagashima et al. who reported T~(onset) ~ 60 K and To(zero) ~40 K for Tll.sSrCaCu2Ox [20]. The room temperature resistivity of the sample with

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Table I a and c lattice parameters, Tc( onset ), Tc( zero ) and Tc( dia ) for T12Ba2_ xSrxCaCu20 s. All temperatures are in kelvin. x Value

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x = 2 . 0 is ~ 10 - 2 £1-cm w h i l e a v a l u e o f ~ l 0 - 3 L'~c m is o b s e r v e d f o r x - - 0 . 0 . G a n g u l i e t al. h a v e reported high resistance and no superconductivity for

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a phase with starting composition Tl2Sr2CaCu2Os

[24]. Magnetic susceptibility versus temperature data for selected samples of Tl2Ba2_~rxCaCu2Os are presented in fig. 5. The measurements were made on brick shaped specimens of similar size with the field parallel to the longest dimension. No attempt was made to correct for demagnetization. A clear decrease in Tc with increasing x is also indicated by the magnetic susceptibilities. Tc is taken as the temperature at which the strong diamagnetic transition begihs. Tc for the mixed phase sample of nominal composition T12Sr2CaCu208 is 44 IC A summary of T, values determined by susceptibility (and resistivity) measurements is included in table I. The Meissner effect of the samples decreases substantially with increasing ~ thi~ suggests that the amount of the bulk superconducting phase present is diminished with increasing x, although, for small values of x (x < 0.6 ) this is not likely to be due to the presence of impurity phases. The magnetic as well as the resistive transition widths for samples with x = 1.4 and 2.0 are noticeably broad. This could be due to inhomogeniety of sample composition or to the presence of another superconducting phase (s) with lower Tc. Assuming that phases T12Sr2Ca,Cu.+ ]O6+2~ analogous to the Ba series exist, the compound might be the n =0 member TI2Sr2CuO6. While samples of Tl2Ba2_~SrxCaCu2Oa with low values o f x appear single phase by powder X-ray diffraction, small amounts of other compounds such as TI2Ba2Ca2Cu3Om are present. Tl2Ba2Ca2Cu3Oto has been shown to have a T, of 125 K [25] and is detectable in the magnetic susceptibility data of

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T12Ba2CaCu,Os in this study• Figure 6 shows that while the susceptibility drops sharply at 110 K the sample also shows a detectable diamagnetic signal at 116 IC A similar observation has been made in T1Ba2CaCu2Os_y [26]. Details of the superconducting transition of"T12Sr2CaCu2Oa'" are shown in fig. 7. Again, while the sharp drop in susceptibility occurs at 44 K, the sample shows diamagnetic susceptibility at 83 K with an apparent onset at 88 K. The major impurity in this sample, Sr4TI2OT, has no magnetic transition in this temperature range. The drop in the susceptibility at 88 K may be due to the formation of small amounts of the higher order members of this system, e.g. T12Sr2CaeCu30,o. During the preparation of this manuscript, we became aware of the work of Subramanian et al. who have synthesized and characterized the compounds (Tlo.sPbo.s)Sr2CaCu207 and (Tlo.sPbo.s)Sr2Ca2Cu309 [27 ]. These are single T1O layer compounds in the TIBaeCa,Cu,+ IO5+2n se*0"

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Fig. 7. Details of the superconducting transition "TI2Sr2CaCu20." in the high temperature region.

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E.A. Hayri and M. G ~ l a t t / SuperconductivitY~'i~'-~,'~j2~St~CaCu:08 sol~dsolutions ries with Pb substituting for half o f the TI and Sr substituting for all o f the Ba. Their samples show sharp superconducting transitions at ~ 85 and ~ 120 K for (Tlo.sPbo.5)Sr2Ca,Cu,+lOs+2~ with n = 1 and 2, respectively. Subramanian et al. report unsuccessful attempts to prepare T12Sr2CanCUn+lO6+2n and T]Sr2Ca,Cu,+ iO5+2~ without Pb, although they do observe a weak Meissner effect at ~ 20 K in some T I Sr-Ca-Cu--O samples. O u r attempts to prepare (T1Pb)Sr2Ca,Cu,+IO6+2~ yielded mixed phase samples with two distinct drops in resistivity at ~ 85 and ~ 60 K (see fig. 8). Similar results were obtained when we tried to prepare T12Sr2Ca2Cu30~o (fig. 8). The anomaly at 60 K appears to be due to (Tl2_xPbx)Sr2CaCu2Os which is observed in the powder X-ray diffraction pattern. The anomaly at 85 K is probably due to the presence o f (T12_xPb~)Sr2Ca2Cu3Olo or other, possibly higher order, members o f (Tll_xPbx)Sr2CanCun+lOs+2n. Very recently Sheng et al. [28] have reported zero resistivity at 10 K and a resistive anomaly at ~ 60 K in samples of nominal composition T12Sr2Ca2Cu3Oto+x and Tl2SrCa2Cu309+x, respectively, with Meissner effects o f less than 1%. They observe the onset o f diamagnetism at ~ 25 K and a weak microwave signal at 70 K for a sample o f mixed phase T I - S r - C a - C u - O [ 28 ]. In summary, we have prepared superconducting phases in the Tl2Ba2_~SrxCaCu2Os system. The lattice parameters as well as Tc decrease with increasing x. A sample with nominal composition Tl2Sr2CaCu2Os appears to contain Tl2Sr2CaCu2Os as the major phase with T c ~ 4 4 IC Resistivity and 1.o[ ...................................

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779

susceptibility anomalies in "T12Sr2CaCu2Os" suggest that the T12Sr2CanCu,+ iO6+2, analogs o f the Ba series o f superconductors might form.

Acknowledgements The authors would like to acknowledge helpful discussions with Dr. ICV. Ramanujachary and Mr. S. Li. This work received support from the National Science Foundation Solid State Chemistry Grant DMR-87-14072 and the National Science Foundation Materials Research Instrumentation Grant DMR-87-05620.

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E.A. Hayri and M. Greenblatt / Superconductivity in TIzBa~_,~SrxCaCu20s solid solutions

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