Observation of superconductivity in the Cs–Sr–Bi–Pb–O system

Solid State Communications, Vol. 106, No. 10, pp. 673–676, 1998 䉷 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038–1098/98 $19.00+.00

Pergamon

PII: S0038–1098(98)00125-2

OBSERVATION OF SUPERCONDUCTIVITY IN THE Cs–Sr–Bi–Pb–O SYSTEM T.P. Beales Metal Manufactures Ltd., High Temperature Superconductor Facility, Australian Technology Park, Eveleigh NSW 1430, Australia (Received 13 January 1998; accepted 16 February 1998 by C.N.R. Rao) New phases in the Cs–Ba–Bi–Pb–O and Cs–Sr–Bi–Pb–O systems have been synthesised using conventional solid state reaction techniques. The polycrystalline powders formed were not single phase, but XRD patterns of an unknown phase similar to a distorted perovskite structure were observed. The samples were all highly deliquescent. A nominal (Ba,Cs)(Bi 0.25Pb 0.75)O 3 composition synthesised in air shows a sharp T C onset ¼ 9 K with an observed zero-field-cooled magnetic moment at 5 K ¼ 3 ⫻ 10 ¹ 7 A m ¹2 and a similar nominal (Sr,Cs)(Bi 0.25Pb 0.75)O 3 composition subjected to a post-anneal in N 2 gas showed a T C onset ¼ 7:5 ⫾ 0:5 K. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: A. superconductors, B. chemical synthesis, C. crystal structure and symmetry.

Before the discovery of superconductivity in cuprates with (La 2¹xSr x)CuO 4 by Bednorz and Mu¨ller in 1986 [1], the highest-known superconducting transition temperature (T C) of an oxide material was in the bismuthate Ba(Bi 0.25Pb 0.75)O 3 with TC ¼ 13 K [2]. In many ways this material can be thought of as a progenitor of the high temperature superconducting copper oxide-based superconductors, because both the cuprates and bismuthates have many structural and behavioural similarities. The copper oxide-based high temperature superconducting oxides such as YBa 2Cu 3O 7 do not exhibit the cubic perovskite structures of the bismuthates. The structural difference mainly being the existence of CuO 2 layers in the cuprates which force a reduced dimensionality on their electronic properties. Bi-dimensionality is also found in the K 2NiF 4-structured La 2CuO 4, which is an electrical insulator, but can be made metallic by Sr substitution for La and superconductivity in (La 0.8Sr 0.2)CuO 4 occurs at the metal-insulator phase boundary. A similar situation occurs in the bismuthates, where the degree of metallicity is determined by the position along the BaBiO 3 –BaPbO 3 tie line in the phase diagram; i.e. the y value in Ba(Bi 1¹yPb y)O 3. One end member, BaBiO 3 is an electrical insulator and the other BaPbO 3 exhibits metallic conductivity. Doping Pb on the Bi site in BaBiO 3 changes the conductivity from insulating to semi-metallic and again, superconductivity occurs near

the metal-insulator phase boundary. Cava et al. [3] applied the principle that the T C value in the BaBiO 3 system should be higher in a Bi-rich compound and to achieve this, they and doped onto the Ba site and in 1989, reported that (Ba 1¹zK z)BiO 3 had a TC ¼ 30 K for z ¼ 0:4 i.e. a 2.2-fold increase on the 13 K system. It is still unclear what the definitive mechanism for superconductivity in the bismuthates is. A presumed suppression the formation of a Bi III –Bi V charge density wave is often suggested, but other mechanisms could also contribute. New, substituted bismuthate phases may enable a clearer understanding of superconductivity in layered oxides to evolve, especially the role of reduced dimensionality and non-stoichiometry on T C. This line of research has been very fruitful in the investigation of superconductivity in cuprates. As a preliminary search methodology, polycrystalline bismuthate samples were prepared by decomposing stoichiometric mixtures of 0.1 m aqueous constituent metal-EDTA solutions pipetted from a Zymark robot onto silver disks kept at T ⱖ 100⬚C [4]. The silver disks coated with a partially decomposed EDTA film were then transferred to a muffle furnace and calcined in air at temperatures between 750⬚C ⱕ T ⱕ 900⬚C for 12 h, using a heating rate of 10 K min ¹1. After calcination, the samples were allowed to cool at the natural rate of the furnace (average cool rate 10 K min ¹1) to room

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temperature. X-ray powder diffraction (XRD) was used to determine the phase content and lattice parameter values of the product using a Siemens D5000 powder diffractometer with Cu Ka radiation over the range 5⬚ ⱕ 2v ⱕ 90⬚. The resulting XRD traces were compared to known phases listed in the JCPDS database, making it easy identify any unknown phases. XRD patterns obtained from the EDTA exercise, showed a series of new unidentified peaks, resembling those expected from a distorted perovskite, especially in the samples with higher Cs starting contents. However, all were highly multiphase. This resulted in a new synthesis to fabricate bulk powder samples. This was carried out by mixing stoichiometric quantities of Analar grade Bi 2O 3, PbO, Cs(NO 3) and SrCO 3 powders to make hypothetical Cs xAE(Bi 1¹yPb y)O 3 compositions where AE ¼ Ca, Sr and Ba, 0:5 ⱕ x ⱕ 3:5 and 0 ⱕ y ⱕ 0:75. These were then ground using an agate mortar and pestle, pelletised and calcined at temperatures in the range 650⬚C ⱕ T ⱕ 750⬚C for times up to 48 h in flowing N 2, O 2 or mixed N 2/O 2 gas. The solid state reaction took place in Y 2O 3-stabilised Al 2O 3 crucibles placed inside an impervious SiO 2 tube, heated by a surrounding horizontal tube furnace. At one end of the reaction chamber, an air-tight glass–glass seal allowed a gas inlet pipe through which an ambient reaction atmosphere of flowing gas (either N 2, air or O 2) was supplied directly from a commercial gas cylinder. The gas outlet tube at the other end of the SiO 2 tube led to a bubbler used to dissolve any NO 2 emitted during the chemical decomposition of the nitrates used during the reaction. To compensate for loss of volatile Cs 2O, synthesis in crimp-sealed Ag tubes was also attempted. The XRD results of such a powder synthesis are shown in Fig. 1, suggesting a distorted perovskite-type structure for a powder sample with the nominal stoichiometry (Sr,Cs)(Bi 0.25Pb 0.75)O 3, synthesised in an Ag tube. Indicated in Fig. 1 are the expected (h k l) identification lines from Ba(Bi 0.25Pb 0.75)O 3 (dotted lines). Samples with the nominal composition (Ba,Cs)(Bi 0.25Pb 0.75)O 3 were dark coloured as synthesised, similar to Ba(Bi 0.25Pb 0.75)O 3, however (Sr,Cs)(Bi 0.25Pb 0.75)O 3 samples were more dark grey in colour, with the black coloration mainly located on the surface of the powder. As a consequence of the presence of unreacted Cs in these samples, they were highly deliquescent and this prevented detailed structural examination and the fabrication of pellets for resistivity measurements. The zero-field-cooled magnetic properties of the polycrystalline powders were determined using a Quantum Design MPMS SQUID magnetometer operating in the temperature range 5 K ⱕ T ⱕ 300 K. The powders ( ⱕ 0:2 g) were placed in gelatine capsules, loaded

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Fig. 1. An XRD pattern obtained from a sample with the nominal stoichiometry (Sr,Cs)(Bi 0.25Pb 0.75)O 3 synthesised in an Ag tube at 700⬚C for 8 h. The calculated 2v values from Ba(Bi 0.25Pb 0.75)O 3 are shown (– – –). into the SQUID and cooled to 5 K in the residual magnetic field. Data was then recorded as the sample was warmed to temperatures above T C in an applied magnetic field. Figure 2 shows a plot of the observed magnetisation vs temperature in applied fields of H ¼ 8 ⫻ 103 A m ¹1 (open triangles, solid line) and H ¼ 8 ⫻ 104 A m ¹1 (solid triangles, dashed line), for a bulk sample with nominal (Ba,Cs)(Bi 0.25Pb 0.75)O 3 composition, synthesised in flowing air. This system showed a sharp T C onset ¼ 9 K and a magnetic moment at 5 K ¼ 3 ⫻ 10 ¹ 7 A m ¹2 in an applied field of H ¼ 8 ⫻ 103 A m ¹1. Although secondary phases were present in the XRD patterns, no evidence of a step in the magnetisation at a TC ¼ 12 K (corresponding to

Fig. 2. A plot of the observed zero-field-cooled magnetic moment vs temperature in applied fields of H ¼ 8 ⫻ 103 A m ¹1 (open triangles, solid line) and H ¼ 8 ⫻ 104 A m ¹1 (solid triangles, dashed line), for a nominal (Ba,Cs)(Bi 0.25Pb 0.75)O 3 bulk sample, synthesised in flowing air.

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Ba(Bi 0.25Pb 0.75)O 3 impurity) was seen. A number of Cs stoichiometries was tried and a range of T C values from 6 K to 9 K was seen in the (Ba,Cs)(Bi 0.25Pb 0.75)O 3 system. Due to the highly deliquescent nature of the powders, probably arising from the excess Cs, it was not possible to determine the Cs stoichiometry in these compounds. However, the lack of a T C onset at 12 K and new unidentified XRD lines similar to those expected for a distorted perovskite structure, is strong indirect evidence of Cs substituting into the parent Ba(Bi 0.25Pb 0.75)O 3 phase. For (Sr,Cs)(Bi 0.25Pb 0.75)O 3 samples synthesised as described for (Ba,Cs)(Bi 0.25Pb 0.75)O 3 in flowing air, only a weak paramagnetic signal at temperatures down to 5 K was observed. However, superconductivity was observed in (Sr,Cs)(Bi 0.25Pb 0.75)O 3 samples synthesised using flowing nitrogen during post-calcination cooling. Figure 3 shows a similar magnetic properties plot to Fig. 2, of the observed magnetic moment vs temperature in applied fields of H ¼ 8 ⫻ 102 A m ¹1 (open circles, solid line) and H ¼ 8 ⫻ 103 A m ¹1 (solid circles, dashed line), for a (Sr,Cs)(Bi,Pb)O 3 sample with nominal starting composition of (Sr,Cs)(Bi 0.25Pb 0.75)O 3. A T C onset ¼ 7:5 ⫾ 0:5 K was observed in this system (slightly lower than the (Ba,Cs)(Bi,Pb)O 3 analogue). Also the T C transition was less-sharp in (Sr,Cs)(Bi,Pb)O 3 and the magnitude of the 5 K magnetic moment in an applied field of H ¼ 8 ⫻ 103 A m ¹1 of ca. 6 ⫻ 10 ¹ 8 A m ¹2 was some 50 to 70% lower than that seen in (Ba,Cs)(Bi,Pb)O 3. As for (Ba,Cs)(Bi,Pb)O 3, a range of Cs doping levels in (Sr,Cs)(Bi,Pb)O 3 was tried. This was not as successful in (Sr,Cs)(Bi,Pb)O 3 as in (Ba,Cs)(Bi,Pb)O 3, as some of the (Sr,Cs)(Bi,Pb)O 3

Fig. 3. A plot of the observed zero-field-cooled magnetic moment vs temperature in applied fields of H ¼ 8 ⫻ 102 A m ¹1 (open circles, solid line) and H ¼ 8 ⫻ 103 A m ¹1 (solid circles, dashed line), for a sample with the nominal stoichiometry (Sr,Cs)(Bi 0.25Pb 0.75)O 3.

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samples made were non-superconducting. (This was most likely caused by the loss of volatile Cs during synthesis). In addition, superconductivity could only be induced when samples were either cooled in flowing nitrogen after synthesis or subjected to a post-calcination anneal in flowing nitrogen at 500⬚C for 4 h. Despite the inability to synthesise pure phase material, it is highly likely that Cs is substituting to some degree onto the Sr site in a parent Sr(Bi 0.25Pb 0.75)O 3 phase. Experience in the synthesis of bismuthates so far, has shown that highly-ionic alkali metal cations prefer ‘‘A’’ site substitution in these materials e.g. (Ba 1¹zK z)BiO 3 [5], (Ba 1¹zRb z)BiO 3 [6, 7] and (Ba 1¹xSr x)(Bi1 ¹ y Pb y)O 3 [8], while higher valence and/or more covalent cations prefer ‘‘B’’ site substitution in these materials e.g. BaSn 1¹xSb x [9], BaPb 0.75Sn 0.25 [10] and Tl, La and Te in Ba(Bi 1¹xPb x)O 3 [11]. It is known that Cs will substitute onto the ‘‘A’’ site in bismuthates, albeit with some difficulty. For example, Heinrich et al. [12] have reported on the substitution of Cs in the (Ba 1¹zK z)BiO 3 system. They found that the a lattice parameter increased with increasing Cs levels, with a TC ¼ 9 K for the maximum Cs-containing composition (Ba 0.6K 0.13Cs 0.27)BiO 3. Higher levels of Cs content up to (Ba 0.6Cs 0.4)BiO 3 were not superconducting. The synthetic methods they employed (under Ar gas at 700⬚C for 30 min) were very similar to those used in this experiment. There is also good indirect evidence for the possible existence of a parent Sr(Bi 0.25Pb 0.75)O 3 phase. A new substituted Bi-only phase (SrBiO 3) has recently been reported by Kazakov et al. [13]. They found that when SrBiO 3 was doped with alkali metals (in a similar manner to (Ba 1¹zK z)BiO 3), the authors reported a T C onset ¼ 12 K for Sr 1¹xK xBiO 3 in the doping range 0:45 ⱕ x ⱕ 0:6 and a T C onset ¼ 13 K for Sr 1¹xRb xBiO 3 with x ¼ 0:5. Neither superconductivity or XRD peaks resembling the new structure in the (Ba,Cs)(Bi,Pb)O 3 and the (Sr,Cs)(Bi,Pb)O 3 systems were seen in compositions with nominal starting stoichiometries Cs–Sr–Pb–O, Sr–Cs–Bi–O and (Ca,Cs)(Bi,Pb)O 3. All these samples, when prepared under the conditions used, were highly multi-phase and showed a weak paramagnetic behaviour down to 5 K in all the synthetic conditions used. It’s highly likely that the technique needed to prepare phasepure materials in the (Ba,Cs)(Bi 0.25Pb 0.75)O 3 and (Sr,Cs)(Bi 0.25Pb 0.75)O 3 systems, lies in the use of reaction techniques that would favour the assimilation of the volatile Cs in a stoichiometric manner into the structure e.g. by carrying out the reaction in scaled silica capsules. However, these resources were unfortunately, unavailable to the author. One final point, is that in considering the optimisation of T C values in these systems, the role of oxygen non-stoichiometry must be

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considered, especially given the observation of the effect of post calcination atmosphere on T C in (Sr,Cs)(Bi,Pb)O 3. Acknowledgements—This work was financed by BICC plc and the EC (Contract No. BREU-3092-89). REFERENCES 1. Bednorz, J.G. and Mu¨ller, K.A., Z. Phys., B64, 1986, 189. 2. Sleight, A.W., Gillson, J.L. and Bierstedt, P.E., Solid State Commun., 17, 1975, 27. 3. Cava, R.J., Batlogg, B., Krajewski, J.J., Farrow, R., Rupp, L.W. Jr., White, A.E., Short, K., Peck, W.F. and Kometani, T., Nature, 332, 1988, 814. 4. Hall, S.R. and Harrison, M.R., Chem. Brit., 30, 1994, 739. 5. Tseng, D. and Ruckenstein, E., Mater. Lett., 8, 1989, 69.

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6. Tomeno, I. and Ando, K., Phys. Rev., B40, 1989, 2690. 7. Zhao, L.Z. and Zhang, J.B., Solid State Commun., 90, 1994, 709. 8. Wang, E., Tarascon, J.-M. and Hull, G.W., Solid State Commun., 74, 1990, 471. 9. Cava, R.J., Gammel, P., Batlogg, B., Krajewski, J.J., Peck, W.F. Jr., Rupp, L.W. Jr., Felder, R. and van Dover, R.B., Phys. Rev., B42, 1990, 4815. 10. Cava, R.J., Batlogg, B., Espinosa, G.P., Ramirez, A.P., Krajewski, J.J., Peck, W.F. Jr., Rupp, L.W. Jr. and Cooper, A.S., Nature, 339, 1989, 291. 11. Nagarajan, R., Vasanthacharya, N.Y., Gopalakrishnan, J. and Rao, C.N.R., Solid State Commun., 77, 1991, 373. 12. Heinrich, A. and Urland, W., Solid State Commun., 80, 1991, 519. 13. Kazakov, S.M., Chaillout, C., Bordet, P., Capponi, J.J., Nunez-Reguerio, M., Rysak, A., Tholence, J.L., Radaelli, P.G., Putilin, S.N. and Antipov, E.V., Nature, 390, 1997, 148.