Synthesis microstructure and superconducting properties of Ce-Doped Ti-Sr-Ca-Cu-O

Physica C 169 (1990) 396-400 North-Holland

SYNTHESIS, MICROSTRUCTURE A N D S U P E R C O N D U C T I N G PROPERTIES OF C e - D O P E D TI-Sr-Ca-Cu-O Z. I Q B A L a, B.L. R A M A K R I S H N A b,c a n d J.C. B A R R Y c,l a Allied-Signal Inc., Research and Technology, Morristown, New Jersey 07962, USA b Department of Chemistry and ~ Center for Solid State Science, Arizona State University, Tempe, Arizona 85287, USA

Received 25 May 1990 Revised manuscript received 8 June 1990

A new Tl~_xCexSr2CaCu207 compound with x= 0.25, which forms a primitive tetragonal structure with a= b= 3.816 A and c= 12.112 A, has been synthesized in nearly pure form. The structure has been confirmed for the heavy atoms at the microstructural level by high resolution lattice imaging techniques. Selected area electron diffraction did not indicate the presence of structural modulations. The presence of Ce has been confirmed by analytical electron microscopic methods and by X-ray photoemission spectroscopy; the latter also indicates that cerium is present as Ce +4 ions. SQUID magnetometry, AC susceptibility and fourprobe resistance measurements indicate superconductivity with a T¢ between 62 and 68 K. Meissner fractions greater than 10% of ideal diamagnetism signal the presence of bulk superconductivity in the samples. The nature of the substitution process, and the reasons for a 20 K drop in T¢ from a value of near 85 K for the isostructural Tlo.sPbo.sSr2CaCuzO7superconductor, are discussed.

T h a l l i u m - b a s e d c o p p e r oxides have p r o v i d e d examples o f the highest Tc superconductors known. Three reasonably well characterized series o f comp o u n d s with Tc's ranging from 0 to 125 K have been synthesized. These are: ( a ) TlzBa2Can_,CunOEn+4 [1], ( b ) T1BaECan_lCunO2~+3 [1], a n d (c) Tlo.sPbo.sSr2Can_ iCunO2n+3 [2,3 ]. Sheng et al. [4 ] have succeeded in stabilizing T 1 - S r - C a - C u - O compounds via doping with Y to Lu (except Ce), to form multiphase materials showing 80 to 90 K superconductivity. In this p a p e r we report the synthesis in nearly single phase form o f a C e - d o p e d T I - S r - C a C u - O c o m p o u n d with two CuO2 layers a n d its detailed microstructural a n d superconducting properties characterization. A series o f synthesis experiments was carried out with n o m i n a l precursor c o m p o s i t i o n s (T1CexSr2Can_lCUnO2n+3 ( m a d e up o f high purity T1203, CeO2, SrCO3, CaCO3, a n d C u O ) in which x ranged from 0.125 to 0.3 and n from 1 to 3. The precursor p o w d e r was pelletized a n d sealed inside a wedged Present address: Electron Microscope Centre, University of Queensland, Brisbane, Australia. 0921-4534/90/$03.50 © Elsevier Science Publishers B.V. ( North-Holland )

gold envelope under 02. The gold envelope was placed inside a quartz tube which was inserted in a tube furnace. The heat t r e a t m e n t was carried out under flowing 02 for times ranging from 3 to 8 h at 925°C. Multiphase samples (as j u d g e d by p o w d e r X-ray diffraction and energy dispersive X-ray analyses) were o b t a i n e d from the n = 1 and 3 precursors a n d for all precursors when the firing time was below 3 h. A nearly single phase sample was o b t a i n e d from precursors containing x = 0.25 a n d n = 2. Energy dispersive X-ray spectra o b t a i n e d from micron-sized crystals in a transmission electron microscope showed that the average c o m p o s i t i o n o f the material is Tlo.75Ceo.25SrzCaCu207. N o t e that the O-content could not be d e t e r m i n e d by titration and thermogravimetry in this c o m p o u n d because o f the presence o f T1. The a p p r o x i m a t i o n value o f 7 quoted above is based on the oxygen occupancy expected in each layer and the formal valences o f the constituent atoms. The p o w d e r X-ray (Cu K s ) diffraction pattern o f the material o b t a i n e d u n d e r o p t i m u m synthesis conditions is shown in fig. 1. Nearly all the peaks can be indexed to a p r i m i t i v e tetragonal (space group P 4 /

Z. lqbal et al. / Synthesis, microstructure and superconducting properties of Ce-doped TI-Sr-Ca-Cu-O

397

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m m m ) unit cell with lattice parameters a = b = 3.816 /k and c = 12.112 A. The impurity peaks are due to small amounts of unreacted CeO2 and SrO, and a T1 (Ce) Sr2CuO5 phase. The structure of the T1-CeS r - C a - C u - O phase obtained is isomorphous with that ofT1Ba2CaCu207 [ 1 ] and Tlo.sPbo.~Sr2CaCu207 [2,3]. The a- and c-lattice parameters in the Cedoped phase are sizably larger than that reported for Tlo.sPbo.sSrzCaCu207 independently by two groups. This may be taken as evidence for Ce substitution. More details regarding the structure of the Cedoped T 1 - S r - C a - C u - O phase has been obtained via selected area electron diffraction and lattice imaging, and Ce core level X-ray photoemission spectroscopy. A lattice image in the (100) setting and its corresponding electron diffraction pattern are shown in fig. 2. The electron diffraction patterns obtained showed no evidence of superstructures which is in contrast to observations in Tlo.sPbo.sSrzCaCu2OT, where partial ordering ofT1 and Pb into separate lay-

ers resulted in the observed weak superstructure in the c-axis direction [ 3 ]. The lattice image obtained is consistent with a c-axis parameter near 12 ~, and a T10-SrO-CuO2Ca-CuOa-SrO-layer sequence. The lattice image qualitatively shows no evidence for the substitution of Ca by the much heavier Ce atoms, although Ce might be expected to go into the fluorite-type Ca layer in the T 1 - S r - C a - C u - O structure. This observation is also supported by the Ce core level photoemission spectra shown in fig. 3 for CeO2, which has a fluorite structure, and for the Ce-doped T 1 - S r - C a - C u - O compound. The core level binding energies for Ca are the same in both compounds, indicating that Ce is present as a + 4 valent ion in Cedoped T1-Sr-Ca-Cu-O. However, the relative intensities of the shake up satellites (labelled S in fig. 3), which reflect the local environment around the Ce ion, show large differences for the two compounds. This suggests that Ce is not present in a fluorite-like environment in Ce-doped T1-Sr-Ca-

398

Z. lqba! et al. / Synthesis, microstructure and superconducting properties of Ce-doped TI-Sr-Ca-Cu-O

Fig. 2. Highresolution lattice imagein the ( 100) settingofa microcrystalof T1-Ce-Sr-Ca-Cu-O. Inset showsthe correspondingelectron diffraction pattern. Cu-O. Ce is therefore likely to be located in the T1 a n d / o r Sr layers. Since the photoemission experiment samples a 10-20/k surface layer of a pellet, the presence of a small amount of unreacted CeO2 in the bulk material is not likely to contribute to the core level spectrum of the T I - C e - S r - C a - C u - O compound. Based on the analytical electron microprobe data, which show an average TI occupancy of 0.75 per formula unit, and the absence of vacancies in the T1 layer (fig. 2), Ce is likely to be located in the T1 layer of the T 1 - C e - S r - C a - C u - O compound. Some Ce may also be present in the Sr layer since a small amount of unreacted SrO is observed in the powder

X-ray diffraction pattern of the T 1 - C e - S r - C a - C u - O material. The superconducting properties of T1-Ce-Sr-CaC u - O preparations were investigated by four-probe DC resistivity, DC SQUID magnetometry and AC susceptibility measurements. The best samples, which were prepared as described above but were rapidly cooled in 02, displayed a metallic temperature dependence of the resistivity and zero resistance near 62 K as shown in fig. 4. Samples that were furnace cooled in 02 or annealed in argon showed semiconducting behavior in the normal state and a reduced zero resistance temperature. Zero field- and field-

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Fig. 4. Normalized resistivityvs. temperature curve for a TI-CeSr-Ca-Cu-O sample. cooled DC SQUID magnetometer data displayed in fig. 5 for a typical sample show Meissner diamagnetic fractions greater than 10%, which indicates bulk superconductivity in the material, and an onset at 68 K consistent with the resistivity data. AC susceptibility data obtained at 100 Hz in three exciting field amplitudes (fig. 6) show an onset at 65 K. At 0.3 Oe

field amplitude the transition is sharp, with shielding in excess of 80% of - 4 n Z ' at low temperatures. In addition, only a single peak in the energy loss 4~Z" curve is evident, which indicates the presence of a single phase contributing to superconductivity. As mentioned above, Tlo.75Ceo.~sSr2CaCu207 has the 1212 structure of T1Ba2CaCu207 and Tlo.sPbo.sSr2CaCu207. This structure is also closely related to that of YBa2Cu307 except that instead of the so-called charge reservoir layer [ 5 ] consisting of C u - O chains, a rock salt charge reservoir layer of T l O occurs and y+3 is replaced by Ca 2+ at the fluorite sites between the two Cu-O2 planes. Except for T1Sr2CaCu2OT, which has not been successfully prepared in either pure form or as a majority fraction, the well characterized 1212 type compounds and YBa2CuaO7 have an optimal value of Tc between 85 and 93 K. Both Tlo.75Ceo.25Sr2CaCu207 and Tlo.sPbo.sSr2CaCu207 involve substitution at the charge reservoir layer (and probably at the SrO layer in the Ce compound) by the + 4 valent Ce and mixed + 2 and + 4 valent Pb ions, which concomitantly stabilizes the TISr2CaCu207 phase backbone in these compounds. This stabilization is likely to be due to

Z. lqbal et al. /Synthesis, microstructure and superconducting properties of Ce-doped TI-Sr-Ca-Cu-O

400

oxygen intercalation, caused by the excess positive charge on the TIO and SrO layers, to give an oxygen count near 7 per formula unit needed for the 1212 structure. The drop in Tc from near 85 K for the 1212 structure compounds to between 62 and 68 K for Tlo.75Ceo.2sSr2CaCu207 is likely to be the result of electron compensation of the holes on the CuO2 planes, causing a drop in hole density and hence in the To. The holes are compensated by electron doping initiated by the presence of Ce ÷4 on the Sr+2-O layers adjacent to the active CuO2 planes. On the other hand, for Tlo.sPbo.sSr2CaCu2OT, Tc remains near 85 K because substitution takes place entirely at the TIO layer which, though linked by the apical oxygen to the CuOz layers, is not directly adjacent to the CuO2 planes (for discussion of this point, see [ 5 ] ). Although Hall effect measurements have yet to be made on the Ce-doped TI-Sr-Ca-Cu-O compound, the observed changes in the superconducting properties with synthesis conditions suggests that the net doping in the material is of the p-type.

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Acknowledgements The authors would like to thank Prof. K.V. Rao and his group at the Royal Institute of Technology in Stockholm, Sweden, for the AC susceptibility measurements on the samples; and F. Reidinger and J. Marti of Allied-Signal Inc. for the X-ray diffraction and analytical electron microscope data.

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References

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