Synthesis of monophasic Tl2Ba2Ca2Cu3O10 (Tc=120 K) superconductor

PHYSICAG

Physica C 179 (1991) 353-357 North-Holland

Synthesis of monophasi¢ T12Ba2Ca2Cu3Olo(Tc-- 120 K) superconductor R.S. Liu and P.P. Edwards IRC in Superconductivity, Universityof Cambridge, Madingley Road, Cambridge CB3 0HE, UK Received 3 i May 1991

Bulk high-To ( 120 K ) superconductors in the TI-Ba-Ca-Cu-O system have been synthesised from the stoichiometric compositions Tlz_xBa2Ca:+xCu30r Their structures, chemical compositions and superconducting properties were studied by X-ray powder diffraction, energy dispersive X-ray spectrometry, electrical and magnetic measurements. A monophasic compound, TILsBa2Ca2.2Cu30, had Tc= 120 K and an optimum superconducting (Meissner) volume fraction (58%). The synthesis of single phase TI2Ba2Ca2CuaO~o by this route may be important for physical property measurements and applications.

I. Introduction Following the discovery of superconductivity in the TI-Ba-Ca-Cu-O system up to 120 K [ 1,2 ], Parkin et al. [ 3 ] improved the superconducting properties of the Tl2Ba2Ca2Cu3Olo phase (hereafter referred to as 2223 ) with zero resistance at 125 K and a Meissner superconducting volume fraction of ~ 20% (in a field of 100 Oe) for a material with nominal composition of TIBaCa3CusOy. This formula has been used extensively by other researchers to prepare a nearly single phase of the 2223 superconductor [4-7 ]. It has also been shown that synthesis using a stoichiometric (nominal) composition of Tl2Ba2CaECU30.v leads to a variety of different phases [3,8 ]. Usually, a slight excess of both Ca and Cu results in the formation of the 2223 phase. The high volatility of the Tl at elevated temperatures is an inherent problem in the production of monophasic 2223. Interestingly, Hiraga et al., using computer simulations of high-resolution images of the 2223 phase [ 9 ] supported by experimental results [ l 0 ], found that the thallium-deficiei~t and calcium-rich composition of the 2223 phase is essential for stabilization of the 2223 phase. These observations led us to study a series of compounds having stoichiometry, Tl2_~Ba2CaE.xCu30.v. In this letter, we demonstrate that the stoichiometric composition,

Tl2_xBa2Ca2+xCu30~ with x=0.2, is an effective starting material for the synthesis of high-quality and monophasic 2223 compound.

2. Experimental High-purity powders of T1203, BaO2, CaO and CuO were weighed in the appropriate proportions to form nominal compositions ofTl2_xBa2Ca2+ ~Cu30,. ( x = 0 , 0.1, 0.2, 0.3 and 0.4) and TIBaCa3Cu30 r The powders were then mixed using a mortar and pestle and pressed into a pellet ( 10 mm in diameter and 3 mm in thickness) under a pressure of 5 ton/cm 2. The pellets were sealed in Au foil to prevent loss of thallium at elevated temperatures, they were then sinfeted at 910°C for 3 h in oxygen. After sintering, the furnace was cooled to room temperature at a rate of 5 °C/min. Bar-shaped samples ( 1 . 5 × 2 × 10 mm 3) were cut born the sintered pellets and used for resistivity measurement. X-ray diffraction (XRD) analyses were performed using a Philips PWI710 X-ray diffractometer with a Cu Kct radiation. Chemical compositions of the specimens were examined by energy dispersive X-ray spectrometry (EDS) from a JEM200CX electron microscope operating at 200 kV. Molybdenum specimen grids were used and background spectra were studied to ensure that no copper

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R.S. Liu, P.P. Edwards I Synthesis of monophasic TizBazCa:Cu~O~o(T¢= 120 K) superconductor

354

signals were detected from the sample free area. A standard four point probe method was used for the electrical resistivity measurements. The electrical contacts to the sample were made by fine copper wires with a conductive silver paint; the applied current was ! mA. The temperature was recorded using a calibrated silicon diode sensor located close to the sample. Magnetisation data were obtained using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design ).

2223 phase (marked by (.) in fig. 1(a)) except for a small previously [3,7] unidentified impurity [ marked by ( × ) in fig. 1 (a)). EDS analysis of this phase mixture leads us to propose that the impurity phase (marked by ( X ) in fig. 1 (a)) has the composition Tlo.2Cao.sCuO~; this composition was used to prepare single phase material, which was found to be semiconducting [ 11 ]. The XRD pattern of the TI2Ba2Ca2Cu30.v sample indicated a major phase of Tl2Ba2Ca2Cu30~o (marked by (.) in fig. 1 (b)) and a minor phase of TI2Ba2CaCu2Os (marked by ( . ) in fig. 1(b)), and no evidence of any further impurity phases. For this stoichiometry, it has proved difficult to completely eliminate the 2212 phase even after numerous heat treatments. All the XRD peaks in the TI,.sBa2Ca2.2Cu~O~sample (fig. l(c)) could be indexed to a 2223 phase having a tetragonal unit cell with dimensions 3.859x3.859x35.94 A ~. The chemical compositions of twenty small crystallites from the sample were examined by EDS; the average cation compo-

3. Results and discussion In fig. 1 we show the XPD patterns of samples with nominal composmons of (a) TIBaCa~Cu30,,, (b) TI2Ba2Ca2Cu30,, and (c) Tl..sBa2Ca2.2Cu30~.. The XRD pattern of the TIBaCa~Cu~Oy sample (fig. l(a) ) is similar to the pattern reported previously [3,7 ] using the same nominal starting composition. The major phase can be indexed on the basis of the

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Fig. 1. XRD patterns of samples with nominal compositions of (a) TIBaCa3Cu30~, {b) Tl2Ba2Ca2Cu3Oy and (c) Tin.sBazCaz.2Cu3Oy. (Marked by (.), ( x ) and ( • ) are presented as 2223, Tl02Cao.sCuOy and 22 ! 2 phase, respectively. )

355

R.S. Liu. P.P.Edwards/Synthesis of monophasic Tl2Ba2Ca2Cu30~o(T~=120 K) superconductor

sition was less than 8% deviation from the nominal composition. Importantly, both XRD and EDS results confirmed that the sintered sample of Tl~.sBa2Caa.2CuaOv was found to be homogeneous, both in structure and composition. In fig. 2 we show the temperature dependence of electrical resistivity of samples with nominal compositions of (a) TIBaCa3Cu30, (b) Tl2Ba2Ca2CuaOy and (c) Tl,.sBa2Ca2.2CuaOy. All the samples were characterised by a metallic behaviour in their normal state ( T > Tc), and the magnitude of the room temperature resistivities are 5.58, 3.17 and 3.47 m f l c m for TIBaCa3Cu30, TIEBa2Ca2Cu3Oy and Tl,.sBa2Ca2.2Cu3Oy, respectively. It is worth pointing out that all the samples have a negative intercept in the (extrapolated) normal state resistivity curves. The samples exhibited an onset superconducting transition at around 130 K and zero resistance temperatures of 116 K, l l0 K and 117 K for TIBaCa3Cu3Oy, Tl2Ba2CaECU30.v and Tli.sBa2Ca2.ECUaOy, respectively. In fig. 3 we show the Meissner signals (field-cooled, 10 Oe) of powder samples of (a) TIBaCa3Cu30, (b) Tl2Ba2Ca2Cu3Oy and (c) Tll.8Ba2Ca2.2Cu3Oy as measured by a SQUID magnetometer. The onset of diamagnetism appears at a temperature of 120 K, 115 K and 120 K for TIBaCa3Cu30.v, Tl2Ba2Ca2Cu3Oy and TI,.sBa2Ca2.2Cu30, respectively, which is consistent with the electrical resistivity data. We estimate the superconducting (Meissner) volume frac-

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tion of 29.6%, 48.9% and 58% of - 1/4~ at 5 K for TIBaCa3Cu30 r TI2Ba2Ca2Cu30.,, and Tll.sBa2Ca2.2 Cu30~, respectively. However, the superconducting volume fraction as measured in a low applied magnetic field ( i.e. < 100 Oe) are generally smaller than the true volume fraction of the superconducting phase because of effects such as the specimen size and the penetration deptl~. [l 2]. Therefore, it is reasonable to assure that the observed Meissner fraction is, in fact, a lower limit of true volume fraction of the superconductor. The reason for the lowest zero resistance temperature (110 K) and onset diamagnetic signal ( 115 K) in the TI2Ba2Ca2Cu30.,, sample should be correlated with the coexistence of the high-To ( 120 K) 2223 phase and low Tc ( 110 K) 2212 phase which has been proved by previous XRD result. However, a small amount of semiconducting impurity phase of Tlo.2Cao.sCuOy in the TIBaCa3Cu3Oy sample causes the decrease of the superconducting volume fraction. In fig. 4 we show the XRD patterns of samples with .-.-,----:.-,,! I I ~ I I I i i I ~ L !

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[x= (a) 0.1, (b) 0.2, (c) 0.3 and (d) 0.4]. The XRD pattern of the Tl, 9Ba2Ca2 ,CuaOr sample [as shown in fig. 4 (a)] is similar to that from the nominal composition TIEBaECaaCu30:, sample [as shown in fig. 1 (b) ] having the major phase Tl2BaECaECu3Ot0, and the minor phase T12Ba2CaCu2Os. An increase in the amount of Ca ( x > 0.1 ) in TIE_xBa2Ca2+.,.Cu30,,

R.S. Liv, P.P. Edwards/ Synthesis of monophasic Tl2Ba2Ca2CujO~o(T~=120 K) superconductor

356

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decreases the concentration of the TI2Ba2CaCu2Os phase and results in a single phase of TI2Ba2Ca2Cu3OIo, as shown in figs. 4 ( b - d ) . However, for compositions in which x>~ 0.3 we find that the samples are inhomogeneous, as far as the EDS results are concerned (generally Ca- and Ba-deficient relative to the nominal composition). This suggests that a maximum solubility of Ca substitution onto the TI sites in Tl2Ba2Ca2Cu3Oio is about 20%, which is in agreement with the v~lue ( ~ 15%) derived from the computer simulations [9]. In fig. 5 we show the superconducting volume fraction at 5 K and 10 Oe of a series compounds with nominal ~... .U.i .111JU~tl . . :'" . l.i .~ . O lc ~I, 12 __ x D~,a 2 1 , ,-,_ ,-,U 3 ,-I,i U , a 2 + xl.., t)y (x=0, 0.1, 0.2, 0.3 and 0.4). An increase in the superconducting volume fraction on going from x = 0 to x=0.2 arises because of the reduction in the amount of the 2212 phase and the increase in the 2223 phase. However, for x> 0.2, a decrease in the superconducting volume fraction was observed. We believe that this may be due to the solubility of Ca

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in the TI sites (ca. 20%) in the material. From this study, we conclude that an effective method of synthesising monophasic 2223 is via

R.S. Liu, P.P. Edwards / Synthesis qf .monophasic TlzBa2CazCu30 w (To= 120 K) superconductc:

preparation of material from a nominal stoichiometric composition of TI~ sBa2Ca22Cu30,.

Acknowledgements The authors thank the Fellowship of Engineering, BICC, SERC and BP for support. We are grateful to W.Y. Liang for useful discussions.

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