Jul 2, 2012 - t IRC in Superconductivity, University of Cambridge, West Cambridge Site,. Madingley Road, Cambridge C63 OHE, UK t Cavendish Laboratory ...
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Superconductivity in the Tb-Tl-Ba-Cu-O system
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1990 Supercond. Sci. Technol. 3 213 (http://iopscience.iop.org/0953-2048/3/4/013) View the table of contents for this issue, or go to the journal homepage for more
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Supercond. Sci. Technol. 3 (1990) 213-216. Printed in the UK
I
SuperconductivityintheTb-TI-Ba-Cu-0 system
R S Liut, W Zhout, N H PengS, D N Zhengt and P P Edwardst t IRC in Superconductivity, University of Cambridge, West Cambridge Site, Madingley Road, Cambridge C63 OHE, UK Cavendish Laboratory, University of Cambridge, West Cambridge Site, Madingley Road, Cambridge C63 OHE, UK
t
Received 7 December 1989
Abstract. Superconductivity with Tctonret)= 100 K, Tc(midpoint) = 84 K and Tc(zero) 72 K in the Tb-TI-6a-Cu-O system has been observed. x-ray diffraction as well as energy dispersive spectrometric studies were carried out to determine the probable composition and crystal structure of the superconducting phase. We propose that Tb-substituted TI,Ba,CuO, is responsible for superconductivity at 84 K. Tb4+ substitution at both TI3+ and 6a2+sites is proposed; these substitutions would effectively reduce the hole concentration, or overdoping, of the parent TI,Ba,CuO, compound, leading to enhanced superconducting properties.
1. introduction
2. Experimental details
The superconducting critical temperature ( T , ) of the compound YBa,Cu,O,-, does not change significantly when yttrium is replaced by a variety of rare earth elements; the exceptional cases seem to be for Ce, Pr and Tb substitution [l-31. It is generally assumed that these substitutions are detrimental to superconductivity because of the replacement of trivalent yttrium by the tetrapositive rare earth ions. This, indeed, might be anticipated if superconductivity in YBa,Cu,07 -, depends strongly on the (nominal) hole concentration. In contrast,a new series of multiphase thallium cuprate superconductors based on the 4 + valence elements (e.g. Ce [4], Zr and Hf [S] substituted into Tl,Ba,CuO,), has beendiscoveredwith superconductivity around 90 K. It has been suggested that these new materials might be n-type superconductors. If thisis indeed the case, similar high-transition-temperature superconductors should be formed from Tb4+ substitution of either T13+ or BaZ in the compound T1,Ba,CU06. In this article, we report a stable and reproducible ?&,idPoin,) of around 84 K for a new superconducting material based on the Tb-TI-Ba-Cu-0system.X-ray powder diffraction and energy-dispersive x-ray spectrometry analyses reveal a multiphase mixture in the Tb-TI-Ba-Cu-0samples. Froman examination of samples prepared from a wide range of stoichiometries and firing conditions, we propose that the superconducting phase has chemical composition m = 0.45-0.55,n = 0.35Tl,-,Ba,-,,Tb,+,,CuO, with 0.45 and m + n N 0.9. Therefore, Tb4+ ions can substitute both for Ba2+ and T13+inTl,Ba,CuO, to give superconductivity at i&idpoint) = 84 K.
Samples with the nominal compositions of TbTl,Ba,Cu,O,,TlBaCu,O, and BaTbO, wereprepared by solid-state reaction from stoichiometric mixture of Tb407, Tl,O,, BaO, andCuO powders (Johnson-Matthey Chemicals). The sample powders were weighed, mixed and pressed into pellets of 10 mm diameter and 2 mm thick using a pressure of 5 ton cm-,. The TbTl,Ba,Cu,O, and TlBaCu,O, pellets were wrapped with a gold foil to alleviate loss of thallium during the heat treatment and heated at 870900°C and 870°C respectively for 10 min, then quenched to room temperature. The BaTbO, pellets were heated in 900°C for 12 h, then with a cooling rate 5 "C min- down to room temperature. All heating was carried out in an oxygen atmosphere. After heat treatment, the T1-contained samples are dense and black in colour. However, the BaTbO, sampleshave a green colour similar to that found for Y,BaCuO, . A standard four-point probe method wasusedfor electrical resistance measurements. Electrical contacts to the samples were made by fine copper wires attached to the sampleswith a conductive silver paint. The measurement temperature wasrecordedwith a calibrated platinum resistor located close to the sample. The resistance measurement system was fully automated for data acquisition. AC magnetic susceptibility was measured by mutual inductive technique with frequency of 823 Hz. X-ray diffraction (m) studies were carried out with Cu Ka radiation with the useof a Spectrolab CPS-120 diffractometer. Variant chemical compositions were examined by energy dispersive x-ray spectrometry (EDS) in a Jeol-200CX electron microscope. Molybdenum specimen grids wereused and background spectra were
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0953.2048/90/040213
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R S Liu et a/
studied to ensure that no copper signals were detected from the sample free area. Freshly prepared specimens, BaCuO,, BaTbO, and a series of T1,~xBaxO,(x = 0.4, 0.67, 1 and 1.33), have been used as standard materials in the EDS analysis.
3. Results and discussion
Figure 1 shows the normalised electricalresistivity versus temperature for samples with nominal composition of TlBaCu,O, prepared at 870°C (curve A), and TbTl,Ba,Cu,O, synthesised at 870 "C (curve B), 880 "C (curve C) and 900°C (curve D). Details of the synthetic conditions as well as the resulting properties of the samples are shown in table 1. The superconducting properties forTbTl,Ba,Cu,O,samples are found to depend strongly on the heating temperatures. It can be seen from figure 1 that increasing the firing temperature for the TbTl,Ba,Cu,O,samplesfrom870 to900°C produces an increase in from 58 to 72 K, respec, T(,,,idpoint). The tively, but with an invarient ~ ( o n s c t and highest superconducting transition temperature, with q(onset) = loo K, q(midpoint) = 84 K and Tf(zero) = 72 was found for a TbTl,Ba,Cu,O, sample (nominal composition, see later). Further, comparison with Tb substituted (figure 1 (curves E D ) ) and pure T1-Ba-Cu-0 samples(figure 1 (curve A)) indicated that the former lead to superior superconducting properties. From this, and the XRD and EDS measurements (see below), we con-
0
40
80
120
160
Temperoture
200
240
clude that the incorporation of Tb4+ ions into the T1Ba-Cu-0 system produces the superconducting phase. Figure 2 shows the temperature dependence of the AC magnetic susceptibility of a samplewith nominal composition TbTl,Ba,Cu,O, prepared at900°C. A drop in the AC susceptibility was found at 70 K and this is consistent with the appearance of superconductivity as gaugedfrom the electricalresistivity measurement (figure 1 (curve D)). The powder x-ray diffraction patterns for the samples with nominal' compositions BaTbO, , TbTl,Ba,Cu,O, and TlBaCu,O, are shown in figure 3. The data in figure 3 (curve A) show that most of lines for the semiconducting BaTbO, sample can be indexed to a cubic unit cell with a = 10.51 A. Figure 3 (curves E D ) shows the XRD patterns for the TbTl,Ba,Cu,O, (nominal) samples at heating temperatures of 870, 880 and900°C. The major lines can beassigned to the semiconducting cubic BaTbO, phase. However, a second phase observed is which derived is from Tl,Ba,CuO,(labelled and discussed in the following section) together with a trace of an unidentified phase. Figure 3 (curve E) shows the XRD pattern from Tl,Ba,CuO, (CuO is a small impurity phase in the sample). This phase is identical with that of Huang et al, reported as Tl,Ba,CuO, [S]. The Tb-doped Tl,Ba,CuO, phase (as shown in figure 3 (curves E D ) can befitted to the parent XRD pattern, labelled Tl,Ba,CuO,, but withsmallerunit-celldimensions, namely a = 3.83 A, c = 23.01 A and V = 337.5 A3. This reduction in cell dimensions is consistent with TbS)
280
l K) Temperoture ( K )
Figure 1. Normalised electrical resistivity versus temperature for samples with nominal composition TIBaCu,O, prepared at 870 "C (A) and TbTlzBa,Cu,O, synthesised at 870 "C (B), 880 "C (C) and 900 "C (D).
Figure 2. The AC magnetic susceptibility versus temperature of a sample with nominal composition TbTI,Ba,Cu,O, prepared at 900 "C.
Table 1. Synthetic conditions and resulting properties of the samples prepared inthe present work. Properties (K)
214
Nominal compositions
Heating temperatures
TIBaCu,O,
870°C x 10 min
TbTI,Ba,Cu,O,
870°C x 10 min 880°C x 10 min 900°C x 10 min
Tcconset,
Tc(midooint)
Tc(zero)
94
59
24
100 100 100
84 84 84
58 67 72
Superconductivity in the Tb-TI-Ba-Cu-O
system
N
N
N
BaLa
I
>
.c _ VI
C
+
Figure 4. EDS pattern of TI, ,,Ba, ,,Tbo,,CuO, from the sample with nominal compositionTbTI,Ba,Cu,O, prepared at 900 "C.
C
c
I
14.58
26.20 28
31.59
48.12
ldeg)
Flgure 3. XRD patterns for the samples with nominal composition BaTbO, (A), TbTI,Ba,Cu,O, prepared at 870 "C (B), 880 "C (C) and 900 "C (D) and TIBaCu,O, (E). (Those labelled belong to Tb-doped TI,Ba,CuO, phase, those labelled by x belong to CuO)
-
substitution of the Tl,Ba,CuO, phase, since Tb4+ ions substituting for TI3+ and BaZ+ ions would produce a contraction in the crystal lattice, as well as a change in the electronic structure. The EDS analysis also shows a phase with the chemical composition of T1, - ,,,Ba, -,Tb,,,+,CuO,, m = 0.450.55, n = 0.35-0.45 and m + n N 0.9 (figure 4). From figure 3 (curve B-D), it is also observed that the concentration of this phase is also increasing. From the XRD and EDS data, we propose a substitution of Tb4+ into the Tl,Ba,CuO, phase. In Tl,Ba,CuO,, a wide range of behaviour occurs; varying between metallic character,butnosuperconductivity, and a T, of 90 K [7-91. Shimakura et a1 [S] indicated that T, depends significantly onthe oxygen stoichiometry and Nakajima et a1 [lo] pointed out that an over-doping state T1,Ba,Cu06 occurs in the metallic et a1 character,butnotsuperconductivity.Shimakura [8] also observed thatLa3+substitution for Ba2+ in T1,Ba,Cu06 leads to an increase in T, by a decrease in hole concentration. It is important to note that oxygen-
annealed Tl,Ba,CuO, is almostcertainlyoverdoped, which cancausenon-superconductivity or broadening of the superconducting transition temperature. In this study, the T1,Ba,Cu06 phase (nominal Tctonset) = 94 K, composition: TlBaCu,O,,) has a T,(midpoino = 54 K and = 24 K, figure l (curve A). Thisbroad superconductingtransition is almost certainly due to oxygen imhomogenities in this sample. In Tb-substituted TI,Ba,CuO, samples, we propose that the tetravalent Tb ions substitute in both T1(3+) and Ba(2+) sites; this has the effect of decreasing the hole concentration in an analogousfashion toLa3+ ion substitution of Ba2+ in the Tl,Ba,CuO, phase. In summary, superconductivity has been observed in the Tb-TI-Ba-Cu-0 system with T, at about 84 K. We propose that the superconducting phase is Tbsubstituted Tl,Ba,CuO, and that Tb4+ ions substitute into both T13+ and BaZ+ sites effectively reducing the hole concentration in theparent T1,Ba,Cu06 compound. Further studies are in progress to determine the precise extent, and location, of T1 and Ba substitution in these materials.
Acknowledgments
We thankSERCand British Petroleum(EMRAand Venture Awards) for support.
References [l] Hor P H, Gao L,Ming R L, Huang Z J, Wang Y Q, Forster K, Vassiliou J, Chu C W, Wu M K, Ashburn J R and Torng C J 1987 Phys. Rev. Lett.58 911 [ 2 ] Tarascon J M, McKinon W R, Greene L H, Hull G W and Vogel E M 1987 Phys. Rev. B 36 226 [3] Markert J T, Dalichaouch Y and Maple M B 1989 Physical Properties of High Temperature Superconductors Vol 1 (Singapore: World Scientific) ch 6 215
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[4] Wang J H, Sheng Z Z , Dong C, Fei X, Sheng L, Hermann A M and Zhao Z X 1989 Physica C 158 507 [S] Sheng Z Z , Meason J M, Dong C, Sheng L, Wang J H and Fei X 1989 Preprint [S] Huang T C, Lee V Y, Karimi R, Beyers R and Parkin S S P 1988 Mater. Res. Bull 23 1307 [7] Parkin S S P, Lee V Y, Nazzal A I, Savoy R, Huang T C, Gorman G and Beyers R 1988 Phys. Rev. B 38 6531
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[8] Shimakura Y, Kubo Y, Manako T, Satoh T, Iijima S, Ichihashi T and Igarashi H 1989 Physica C 157 279 [9] Torardi C C, Subramanian M A, Calabrese J C, Gopalakrishnan J, McCarron E M, Morrissey K J, Askew T R, Flippen R B, Chowdhry U and Sleight A W 1988 Phys. Rev. B 38 225 [lo] Hakajima S, Kikuchi M, Oku T, Kobayashi N, Suzuki T, Nagase K, Hiraga K, Muto Y and Syno Y 1989 Physica C 160 458
