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Preparation, structure and superconductivity of Ru1222 and Ta-doped Ru1212
Physica C 349 (2001) 289±294
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Preparation, structure and superconductivity of Ru1222 and Ta-doped Ru1212 Z. Sun, S.Y. Li, Y.M. Xiong, X.H. Chen * Structure Research Laboratory, Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China Received 19 November 1999; received in revised form 29 June 2000; accepted 28 July 2000
Abstract The samples Ru1ÿx Tax Sr2 GdCu2 Oy (Ru1212) (x 0, 0.05, 0.15) and RuSr2 Gd1:4 Ce0:6 Cu2 Oy (Ru1222) were synthesized by solid-state reaction. X-ray diraction data indicate that all samples are in a single phase. Ta doping apparently suppresses superconductivity for Ru1ÿx Tax Sr2 GdCu2 Oy system. The eect of the annealing procedure on superconductivity is signi®cant for Ru1212 system. Oxygen content is crucial to superconductivity accompanied by an apparent change of lattice parameters for Ru1222, the increase of oxygen content results in a decrease of lattice parameters and an increase of Tc . Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Ru1212; Ru1222; Substitution eects; Synthesis; Annealing; Lattice parameters
1. Introduction Recently, much more attention is paid to ruthenate±cuprate layered compounds, Ru1212 and Ru1222, in which a long-range ferromagnetic order and superconductivity coexist [1±8]. In contrast to ferromagnetic superconductors in which the superconducting transition temperature, Tc , is higher than the magnetic transition temperature, Tm , the Tm of Ru1212 and Ru1222 is higher than Tc , and are called superconducting ferromagnets [3]. The superconductivity in superconducting ferromagnets arises in the state with a well developed magnetic order, contrary to previous studies,
* Corresponding author. Tel.: +86-551-3601654; fax: +86551-3631760. E-mail address: [email protected] (X.H. Chen).
of which ferromagnetism arises in the superconducting state. Both the tetragonal Ru1212 and Ru1222 are derived from LnBa2 Cu3 O7 (LnBCO) structure (Ln: lanthanide), the Ru ions replace the Cu(1), and only one distinct Cu site (corresponding to Cu(2)) exists, with ®vefold pyramidal coordination. For Ru1212, the Cu±O layers are connected by perovskite SrRuO3 layers through the apical oxygen atoms. For Ru1222, the Ln layer in LnBCO is replaced by inserting a ¯uorite type (Ln, Ce)O2 layer, thus shifting alternate perovskite blocks by a b=2. By now, all these Ru1212 and Ru1222 samples are derived from the substitution of lanthanide [9,10], such as Gd, Sm, Ce, Eu. Since Ta-1212 is isostructural with Ru1212 [9,11], we synthesize Ru1ÿx Tax Sr2 GdCu2 Oy (x 0, 0.05, 0.15), aiming to investigate the superconductivity of Ru1212 when a part of Ru is substituted by Ta.
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At the same time, RuSr2 Gd1:4 Ce0:6 Cu2 Oy was also synthesized. The eects of annealing on the superconductivity for these samples are presented in this paper. Suppression of Ta doping on superconductivity has also been investigated in the Ru1ÿx Tax Sr2 GdCu2 Oy system.
2. Experiment Similar to the synthesis of RuSr2 GdCu2 Oy previously reported [1,4,5], Ru1ÿx Tax Sr2 GdCu2 Oy (x 0, 0.05, 0.15) was synthesized by solid-state reaction of stoichiometric powders of RuO2 , Ta2 O3 , SrCO3 , Gd2 O3 and CuO. Required amounts of these materials were ground, preheated at 960°C in air for 10 h, then reground and reacted as pellets at 1010°C in ¯owing nitrogen for 24 h to obtain precursor materials (Sr2 GdRuO6 and Cu2 O) and minimize the formation of SrRuO3 [12]. These resulting samples were pulverized, pressed into pellets and calcined at 1050°C in air for 24 h with an interval grinding. In each reaction, the samples were cooled to room temperature by a furnace. Subsequently, as-prepared samples of Ru1ÿx Tax Sr2 GdCu2 Oy (x 0, 0.05) were divided into three groups and sintered in ¯owing oxygen at 1050°C for 24, 48 and 72 h, respectively. These samples are referred to as Ru1ÿx Tax Sr2 GdCu2 Oy -A (24 h), -B (48 h) and -C (72 h), respectively. Ru0:85 Ta0:15 Sr2 GdCu2 Oy was sintered in ¯owing oxygen at 1050°C for 24 h, then a part of this annealed sample (denoted as Ru0:85 Ta0:15 Sr2 GdCu2 Oy -A) was treated for 24 h at 1050°C under high oxygen pressure at 50 bar. The resulting samples are referred to as Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B. To synthesize RuSr2 Gd1:4 Ce0:6 Cu2 Oy , stoichiometric powders of Ta2 O3 , SrCO3 , Gd2 O3 , CeO2 and CuO was preheated in air and calcined in
¯owing nitrogen similarly to Ru1ÿx Tax Sr2 GdCu2 Oy , then the samples were reground, pressed into pellets and calcined in air. Subsequently, the pellets were divided into two groups, which were annealed in air and in ¯owing oxygen (denoted as Ru1222-A and -O), respectively. Finally, some of Ru1222-A and -O were annealed under high oxygen pressure at 50 bar (denoted as Ru1222-AH and -OH). Except that preheating in air was at 960°C for 10 h, all reactions were performed at 1050°C for 24 h. Powder X-ray diraction (XRD) measurements were carried out in Rigaku D/max-rA X-ray diffractometer with graphite monochromatized Lattice parameCuKa radiation (k 1:5406 A). ters for single-phase materials were re®ned using the Bragg peaks over the h range. Resistivity measurements were performed by the standard four-probe method down to 4.2 K.
3. Results and discussion 3.1. Ta-doped Ru1212 XRD data indicate that all the samples Ru1ÿx Tax Sr2 GdCu2 Oy -A, -B and -C (x 0, 0.05, 0.15) are in a single phase, and have the tetragonal structure (P4/mmm). Ta doping has eects on lattice parameters of Ru1ÿx Tax Sr2 GdCu2 Oy system, and their lattice parameters are listed in Table 1. When content of Ta ions increases, both a and c axes of Ru1ÿx Tax Sr2 GdCu2 Oy system expand. For Ru1212, the average valence of Ru ions is close to 5, and the presence of Ru4 cannot be excluded is greater [5,9]. The ionic radius of Ta5 (0.68 A) 4 than that of Ru (0.64 A) and that of Ru5 (0.565 so the expansion of the lattice can be due to A), greater Ta ions substituted for Ru ions with small radius.
Table 1 Lattice parameters and Tc (onset and zero) of RuSr2 GdCu2 Oy , Ru0:95 Ta0:05 Sr2 GdCu2 Oy and Ru0:85 Ta0:15 Sr2 GdCu2 Oy Sample a (A) c (A) Tc (onset) (K) Tc (zero) (K) RuSr2 GdCu2 Oy Ru0:95 Ta0:05 Sr2 GdCu2 Oy Ru0:85 Ta0:15 Sr2 GdCu2 Oy
3.838(1) 3.839(1) 3.843(1)
11.559(6) 11.565(5) 11.573(2)
45 35 ±
22 10 ±
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Fig. 1. The temperature dependence of resistivity for RuSr2 GdCu2 Oy sintered for dierent duration: (.) 24 h, ( ) 48 h and (m) 72 h.
Fig. 1 shows resistivity as a function of temperature for samples RuSr2 GdCu2 Oy -A, -B and -C. After annealing in ¯owing oxygen for 24 h, RuSr2 GdCu2 Oy -A (down triangle) shows a strong semiconductive behavior above Tc (12 K), and does not show zero resistivity until 4.2 K. RuSr2 GdCu2 Oy -B (square), having been sintered in ¯owing oxygen for 48 h, shows much better superconductivity compared with RuSr2 GdCu2 Oy A. It shows superconducting transition at 25 K with slightly metallic behavior in high temperature and a small kink at 135 K, which is similar to that reported in Ref. [5] and is considered to arise from the magnetic transition. But the resistivity is not zero at 4.2 K yet. After annealing for 72 h, RuSr2 GdCu2 Oy -C (up triangle) shows superconducting transition at 45 K with a ¯at in vicinity of Tc and exhibits zero resistivity at 22 K, metallic behavior with a T-linear dependence in character above 80 K is also observed. According to Ref. [8], the ¯at in vicinity of Tc is due to grain boundary eects. Since the oxygen stoichiometry remains ®xed at about 8 for Ru1212, the annealing mainly in¯uences the granularity [4]. In one word, the annealing prolongation is bene®cial to superconductivity and improvement of sample quality. The temperature dependence of resistivity for Ru0:95 Ta0:05 Sr2 GdCu2 Oy -A, -B and -C shown in Fig. 2 is similar to that of RuSr2 GdCu2 Oy shown in Fig. 1. Ru0:95 Ta0:05 Sr2 GdCu2 Oy -A (down trian-
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Fig. 2. The temperature dependence of resistivity for Ru0:95 Ta0:05 Sr2 GdCu2 Oy sintered for dierent durations: (.) 24 h, ( ) 48 h and (m) 72 h.
gle) shows a semiconductive behavior above superconducting transition, and the onset of superconducting transition (9 K) is lower than that of RuSr2 GdCu2 Oy -A. Superconducting transition of Ru0:95 Ta0:05 Sr2 GdCu2 Oy -B (square) takes place at 24 K and does not exhibit zero resistivity until 4.2 K. Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C (up triangle) shows superconducting transition at 35 K, and exhibits zero resistivity at 10 K. It shows T-linear metallic behavior down to 90 K, below which a slight upturn in resistivity is still observed. The temperature dependence of resistivity for Ru0:85 Ta0:15 Sr2 GdCu2 Oy is shown in Fig. 3. After annealing in ¯owing oxygen for 24 h, Ru0:85 Ta0:15 Sr2 GdCu2 Oy -A (down triangle) shows a strong semiconductive behavior, the ratio of q(4.2)/q(285) is as high as 20. Although annealing under high oxygen pressure made Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B (square) change to a narrow-gap semiconductor with metallic behavior down to 80 K, this sample does not show superconducting transition until 4.2 K. The resistivity±temperature curves of RuSr2 GdCu2 Oy -C, Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C and Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B are all shown in Fig. 4. Comparing the three samples, the temperature dependence of resistivity for RuSr2 GdCu2 Oy -C and Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C is similar to each other in character, but the sample Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C has lower superconducting
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observed in Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B. Although Ta-1212 is isostructural with Ru1212, it is not a superconductor. There are three possibilities to explain how the substitution of Ta for Ru suppresses the superconductivity: (1) the concentration of carrier decreases, which is due to the higher valence of doped Ta ions than that of Ru ions, (2) the presence of Ta ions disorders the arrangement of Ru ions and (3) the expansion of lattice, which is due to the greater radius of Ta ions than that of Ru ions, weakens the coupling between CuO2 layer and RuO2 plane. Fig. 3. The temperature dependence of resistivity for Ru0:85 Ta0:15 Sr2 GdCu2 Oy sintered for dierent procedures: (.) sintered in ¯owing oxygen for 24 h, ( ) sintered in ¯owing oxygen for 24 h, then, under high oxygen pressure for 24 h.
Fig. 4. The temperature dependence of resistivity for RuSr2 GdCu2 Oy -C (.), Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C ( ) and Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B (m).
transition temperature and broader transition region, which is due to Ta doping. The value of resistivity is at the same order for the three samples and they all show apparent metallic behavior above 90 K, but no superconducting sign has been
3.2. RuSr2 Gd1:4 Ce0:6 Cu2 Oy XRD measurements indicate that all these samples are in a single phase, and the XRD patterns can be indexed assuming a tetragonal lattice (I4/mmm). Table 2 shows the lattice parameters of RuSr2 Gd1:4 Ce0:6 Cu2 Oy prepared in dierent annealing procedures. According to the annealing procedures, these samples RuSr2 Gd1:4 Ce0:6 Cu2 Oy have dierent oxygen content. Ru1222-A, which was ®nally sintered in air, should have the lowest oxygen content; the oxygen content in Ru1222OH, which was obtained from Ru1222-O annealed under high oxygen pressure of 50 bar, should be the highest. Table 2 shows that c axis lattice parameter decreases with increasing oxygen content, whereas a axis lattice parameter remains nearly unchanged. The oxygen content dependence of lattice parameters is consistent with that reported by Wada et al. in isostructural Cu-1222 system [13,14]. The temperature dependence of resistivity for the samples RuSr2 Gd1:4 Ce0:6 Cu2 Oy , which were obtained in dierent annealing procedures is shown in Fig. 5. The sample Ru1222-O shows superconducting transition at 30 K and exhibits
Table 2 Lattice parameters and Tc (onset and zero) of RuSr2 Gd1:4 Ce0:6 Cu2 Oy Sample a (A) c (A) RuSr2 Gd1:4 Ce0:6 Cu2 Oy -A RuSr2 Gd1:4 Ce0:6 Cu2 Oy -O RuSr2 Gd1:4 Ce0:6 Cu2 Oy -AH RuSr2 Gd1:4 Ce0:6 Cu2 Oy -OH
3.845(1) 3.844(1) 3.844(1) 3.844(1)
28.661(2) 28.640(5) 28.620(5) 28.615(7)
Tc (K) (onset)
Tc (K) (zero)
± 30 45 45
± 18 35 35
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Fig. 5. The temperature dependence of resistivity for RuSr2 Gd1:4 Ce0:6 Cu2 Oy -A (d), RuSr2 Gd1:4 Ce0:6 Cu2 Oy -O ( ), RuSr2 Gd1:4 Ce0:6 Cu2 Oy -AH (.) and RuSr2 Gd1:4 Ce0:6 Cu2 Oy OH (m).
zero resistivity at 18 K. In the high temperature, a slightly metallic behavior is observed until 220 K. The sample shows a weak semiconductor-like behavior before the superconducting transition. However, Ru1222-A does not show superconducting transition after ®nal annealing in air. The resistivity of Ru1222-A remains nearly unchanged above 200 K, below which, Ru1222-A shows a strong semiconductive behavior. Ru1222-AH and -OH obtained from Ru1222-A and -O annealed under high oxygen pressure of 50 bars show superconducting transition at 45 K with metallic behavior above Tc and exhibit zero resistivity at 35 K. It suggests the annealing under high oxygen pressure apparently changes sample character compared with the samples Ru1222-A and Ru1222-O. However, the dierence between Ru1222-AH and -OH still exist. Ru1222-AH shows under-doped behavior with a pseudogap opening at 210 K, while Ru1222-OH is an optimal-doped sample because a T-linear dependence keeps until the superconducting transition. From the annealing procedures, the origin of this dierence can be found. The optimal-doped Ru1222-OH is obtained from Ru1222-O, while the under-doped sample Ru1222-AH is obtained from Ru1222-A. The unique dierence between them is a preannealing in the dierent atmosphere. It suggests that both annealing in ¯owing oxygen and in high oxygen pressure are important to improve super-
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conductivity. It is because annealing in oxygen atmosphere results in an increase of concentration of carrier. It indicates that the superconductivity for Ru1222 system is sensitive to oxygen content. Our results indicate that Ru1222 system is generally in under-doped region. The data in Table 2 indicate that Tc is closely related to the c axis lattice parameter. The c axis lattice parameter of Ru1222-A, which is not a superconductor, is the biggest among these samples. Ru1222-OH has the highest Tc and the smallest c axis lattice parameter. As the superconducting transition temperature gradually decreases, the c axis lattice parameter increases. 4. Conclusions The structural and electrical properties of Tadoped Ru1212 samples were studied as well as the relation between annealing atmosphere and the sample quality for Ru1ÿx Tax Sr2 GdCu2 Oy (x 0, 0.05, 0.15) and RuSr2 Gd1:4 Ce0:6 Cu2 Oy . Dierent annealing procedures play an important role in improving sample quality for Ru1222. For Ru1212, Ta doping suppresses apparently superconductivity, and the annealing time is an important factor for superconductivity due to improvement of granularity. For RuSr2 Gd1:4 Ce0:6 Cu2 Oy , the increase of oxygen content results in an increase of superconducting transition temperature and a decrease of c axis lattice parameter. Acknowledgements This work was supported by a grant from the Natural Science Foundation of China and by the Ministry of Science and Technology of China (NKBRSF-G19990646).
References [1] C.W. Chu, Y.Y. Xue, R.L. Meng, J. Cmaidalka, L.M. Dezaneti, Y.S. Wang, B. Lorenz, A.K. Heilman, cond-mat/ 9910056. [2] I. Felner, U. Asaf, Y. Levi, O. Millo, Phys. Rev. B 55 (1997) 3374.
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[3] E.B. Sonin, I. Felner, Phys. Rev. B 57 (1998) 14000. [4] D.J. Pringle, J.L. Tallon, B.G. Walker, H.J. Trodahl, Phys. Rev. B 59 (1999) 11679. [5] C. Bernhard, J.L. Tallon, Ch. Niedermayer, Th. Blasius, A. Golnik, E. Br ucher, R.K. Kremer, D.R. Noakes, C.E. Stronach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [6] I. Felner, U. Asaf, S. Reich, Y. Tsabba, Physica C 311 (1999) 163. [7] W.D. Pickett, R. Weht, A.B. Shick, Phys. Rev. Lett. 83 (1999) 3713. [8] J.E. McCrone, J.R. Cooper, J.L. Tallon, cond-mat/ 9909263.
[9] L. Bauernfeind, W. Widder, H.F. Braun, Physica C 254 (1995) 151. [10] T. Kaibin, Q. Yitai, Z. Yadun, Y. Li, C. Zuyao, Z. Yuheng, Physica C 259 (1996) 168. [11] M. Vybornov, W. Perthold, H. Michor, T. Holubar, G. Hilscher, P. Rogl, P. Fischer, M. Divis, Phys. Rev. B 52 (1995) 1389. [12] L. Bauernfeind, W. Widder, H.F. Braun, J Low Temp. Phys. 105 (1996) 1605. [13] T. Wada, A. Ichinose, Y. Yaegashi, H. Yamauchi, S. Tanaka, Phys. Rev. B 41 (1990) 1984. [14] T. Wada, A. Ichinose, Y. Yaegashi, H. Yamauchi, S. Tanaka, Jpn J. Appl. Phys. 29 (1990) L266.
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Preparation, structure and superconductivity of Ru1222 and Ta-doped Ru1212 Z. Sun, S.Y. Li, Y.M. Xiong, X.H. Chen * Structure Research Laboratory, Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China Received 19 November 1999; received in revised form 29 June 2000; accepted 28 July 2000
Abstract The samples Ru1ÿx Tax Sr2 GdCu2 Oy (Ru1212) (x 0, 0.05, 0.15) and RuSr2 Gd1:4 Ce0:6 Cu2 Oy (Ru1222) were synthesized by solid-state reaction. X-ray diraction data indicate that all samples are in a single phase. Ta doping apparently suppresses superconductivity for Ru1ÿx Tax Sr2 GdCu2 Oy system. The eect of the annealing procedure on superconductivity is signi®cant for Ru1212 system. Oxygen content is crucial to superconductivity accompanied by an apparent change of lattice parameters for Ru1222, the increase of oxygen content results in a decrease of lattice parameters and an increase of Tc . Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Ru1212; Ru1222; Substitution eects; Synthesis; Annealing; Lattice parameters
1. Introduction Recently, much more attention is paid to ruthenate±cuprate layered compounds, Ru1212 and Ru1222, in which a long-range ferromagnetic order and superconductivity coexist [1±8]. In contrast to ferromagnetic superconductors in which the superconducting transition temperature, Tc , is higher than the magnetic transition temperature, Tm , the Tm of Ru1212 and Ru1222 is higher than Tc , and are called superconducting ferromagnets [3]. The superconductivity in superconducting ferromagnets arises in the state with a well developed magnetic order, contrary to previous studies,
* Corresponding author. Tel.: +86-551-3601654; fax: +86551-3631760. E-mail address: [email protected] (X.H. Chen).
of which ferromagnetism arises in the superconducting state. Both the tetragonal Ru1212 and Ru1222 are derived from LnBa2 Cu3 O7 (LnBCO) structure (Ln: lanthanide), the Ru ions replace the Cu(1), and only one distinct Cu site (corresponding to Cu(2)) exists, with ®vefold pyramidal coordination. For Ru1212, the Cu±O layers are connected by perovskite SrRuO3 layers through the apical oxygen atoms. For Ru1222, the Ln layer in LnBCO is replaced by inserting a ¯uorite type (Ln, Ce)O2 layer, thus shifting alternate perovskite blocks by a b=2. By now, all these Ru1212 and Ru1222 samples are derived from the substitution of lanthanide [9,10], such as Gd, Sm, Ce, Eu. Since Ta-1212 is isostructural with Ru1212 [9,11], we synthesize Ru1ÿx Tax Sr2 GdCu2 Oy (x 0, 0.05, 0.15), aiming to investigate the superconductivity of Ru1212 when a part of Ru is substituted by Ta.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 5 5 6 - 2
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At the same time, RuSr2 Gd1:4 Ce0:6 Cu2 Oy was also synthesized. The eects of annealing on the superconductivity for these samples are presented in this paper. Suppression of Ta doping on superconductivity has also been investigated in the Ru1ÿx Tax Sr2 GdCu2 Oy system.
2. Experiment Similar to the synthesis of RuSr2 GdCu2 Oy previously reported [1,4,5], Ru1ÿx Tax Sr2 GdCu2 Oy (x 0, 0.05, 0.15) was synthesized by solid-state reaction of stoichiometric powders of RuO2 , Ta2 O3 , SrCO3 , Gd2 O3 and CuO. Required amounts of these materials were ground, preheated at 960°C in air for 10 h, then reground and reacted as pellets at 1010°C in ¯owing nitrogen for 24 h to obtain precursor materials (Sr2 GdRuO6 and Cu2 O) and minimize the formation of SrRuO3 [12]. These resulting samples were pulverized, pressed into pellets and calcined at 1050°C in air for 24 h with an interval grinding. In each reaction, the samples were cooled to room temperature by a furnace. Subsequently, as-prepared samples of Ru1ÿx Tax Sr2 GdCu2 Oy (x 0, 0.05) were divided into three groups and sintered in ¯owing oxygen at 1050°C for 24, 48 and 72 h, respectively. These samples are referred to as Ru1ÿx Tax Sr2 GdCu2 Oy -A (24 h), -B (48 h) and -C (72 h), respectively. Ru0:85 Ta0:15 Sr2 GdCu2 Oy was sintered in ¯owing oxygen at 1050°C for 24 h, then a part of this annealed sample (denoted as Ru0:85 Ta0:15 Sr2 GdCu2 Oy -A) was treated for 24 h at 1050°C under high oxygen pressure at 50 bar. The resulting samples are referred to as Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B. To synthesize RuSr2 Gd1:4 Ce0:6 Cu2 Oy , stoichiometric powders of Ta2 O3 , SrCO3 , Gd2 O3 , CeO2 and CuO was preheated in air and calcined in
¯owing nitrogen similarly to Ru1ÿx Tax Sr2 GdCu2 Oy , then the samples were reground, pressed into pellets and calcined in air. Subsequently, the pellets were divided into two groups, which were annealed in air and in ¯owing oxygen (denoted as Ru1222-A and -O), respectively. Finally, some of Ru1222-A and -O were annealed under high oxygen pressure at 50 bar (denoted as Ru1222-AH and -OH). Except that preheating in air was at 960°C for 10 h, all reactions were performed at 1050°C for 24 h. Powder X-ray diraction (XRD) measurements were carried out in Rigaku D/max-rA X-ray diffractometer with graphite monochromatized Lattice parameCuKa radiation (k 1:5406 A). ters for single-phase materials were re®ned using the Bragg peaks over the h range. Resistivity measurements were performed by the standard four-probe method down to 4.2 K.
3. Results and discussion 3.1. Ta-doped Ru1212 XRD data indicate that all the samples Ru1ÿx Tax Sr2 GdCu2 Oy -A, -B and -C (x 0, 0.05, 0.15) are in a single phase, and have the tetragonal structure (P4/mmm). Ta doping has eects on lattice parameters of Ru1ÿx Tax Sr2 GdCu2 Oy system, and their lattice parameters are listed in Table 1. When content of Ta ions increases, both a and c axes of Ru1ÿx Tax Sr2 GdCu2 Oy system expand. For Ru1212, the average valence of Ru ions is close to 5, and the presence of Ru4 cannot be excluded is greater [5,9]. The ionic radius of Ta5 (0.68 A) 4 than that of Ru (0.64 A) and that of Ru5 (0.565 so the expansion of the lattice can be due to A), greater Ta ions substituted for Ru ions with small radius.
Table 1 Lattice parameters and Tc (onset and zero) of RuSr2 GdCu2 Oy , Ru0:95 Ta0:05 Sr2 GdCu2 Oy and Ru0:85 Ta0:15 Sr2 GdCu2 Oy Sample a (A) c (A) Tc (onset) (K) Tc (zero) (K) RuSr2 GdCu2 Oy Ru0:95 Ta0:05 Sr2 GdCu2 Oy Ru0:85 Ta0:15 Sr2 GdCu2 Oy
3.838(1) 3.839(1) 3.843(1)
11.559(6) 11.565(5) 11.573(2)
45 35 ±
22 10 ±
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Fig. 1. The temperature dependence of resistivity for RuSr2 GdCu2 Oy sintered for dierent duration: (.) 24 h, ( ) 48 h and (m) 72 h.
Fig. 1 shows resistivity as a function of temperature for samples RuSr2 GdCu2 Oy -A, -B and -C. After annealing in ¯owing oxygen for 24 h, RuSr2 GdCu2 Oy -A (down triangle) shows a strong semiconductive behavior above Tc (12 K), and does not show zero resistivity until 4.2 K. RuSr2 GdCu2 Oy -B (square), having been sintered in ¯owing oxygen for 48 h, shows much better superconductivity compared with RuSr2 GdCu2 Oy A. It shows superconducting transition at 25 K with slightly metallic behavior in high temperature and a small kink at 135 K, which is similar to that reported in Ref. [5] and is considered to arise from the magnetic transition. But the resistivity is not zero at 4.2 K yet. After annealing for 72 h, RuSr2 GdCu2 Oy -C (up triangle) shows superconducting transition at 45 K with a ¯at in vicinity of Tc and exhibits zero resistivity at 22 K, metallic behavior with a T-linear dependence in character above 80 K is also observed. According to Ref. [8], the ¯at in vicinity of Tc is due to grain boundary eects. Since the oxygen stoichiometry remains ®xed at about 8 for Ru1212, the annealing mainly in¯uences the granularity [4]. In one word, the annealing prolongation is bene®cial to superconductivity and improvement of sample quality. The temperature dependence of resistivity for Ru0:95 Ta0:05 Sr2 GdCu2 Oy -A, -B and -C shown in Fig. 2 is similar to that of RuSr2 GdCu2 Oy shown in Fig. 1. Ru0:95 Ta0:05 Sr2 GdCu2 Oy -A (down trian-
291
Fig. 2. The temperature dependence of resistivity for Ru0:95 Ta0:05 Sr2 GdCu2 Oy sintered for dierent durations: (.) 24 h, ( ) 48 h and (m) 72 h.
gle) shows a semiconductive behavior above superconducting transition, and the onset of superconducting transition (9 K) is lower than that of RuSr2 GdCu2 Oy -A. Superconducting transition of Ru0:95 Ta0:05 Sr2 GdCu2 Oy -B (square) takes place at 24 K and does not exhibit zero resistivity until 4.2 K. Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C (up triangle) shows superconducting transition at 35 K, and exhibits zero resistivity at 10 K. It shows T-linear metallic behavior down to 90 K, below which a slight upturn in resistivity is still observed. The temperature dependence of resistivity for Ru0:85 Ta0:15 Sr2 GdCu2 Oy is shown in Fig. 3. After annealing in ¯owing oxygen for 24 h, Ru0:85 Ta0:15 Sr2 GdCu2 Oy -A (down triangle) shows a strong semiconductive behavior, the ratio of q(4.2)/q(285) is as high as 20. Although annealing under high oxygen pressure made Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B (square) change to a narrow-gap semiconductor with metallic behavior down to 80 K, this sample does not show superconducting transition until 4.2 K. The resistivity±temperature curves of RuSr2 GdCu2 Oy -C, Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C and Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B are all shown in Fig. 4. Comparing the three samples, the temperature dependence of resistivity for RuSr2 GdCu2 Oy -C and Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C is similar to each other in character, but the sample Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C has lower superconducting
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observed in Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B. Although Ta-1212 is isostructural with Ru1212, it is not a superconductor. There are three possibilities to explain how the substitution of Ta for Ru suppresses the superconductivity: (1) the concentration of carrier decreases, which is due to the higher valence of doped Ta ions than that of Ru ions, (2) the presence of Ta ions disorders the arrangement of Ru ions and (3) the expansion of lattice, which is due to the greater radius of Ta ions than that of Ru ions, weakens the coupling between CuO2 layer and RuO2 plane. Fig. 3. The temperature dependence of resistivity for Ru0:85 Ta0:15 Sr2 GdCu2 Oy sintered for dierent procedures: (.) sintered in ¯owing oxygen for 24 h, ( ) sintered in ¯owing oxygen for 24 h, then, under high oxygen pressure for 24 h.
Fig. 4. The temperature dependence of resistivity for RuSr2 GdCu2 Oy -C (.), Ru0:95 Ta0:05 Sr2 GdCu2 Oy -C ( ) and Ru0:85 Ta0:15 Sr2 GdCu2 Oy -B (m).
transition temperature and broader transition region, which is due to Ta doping. The value of resistivity is at the same order for the three samples and they all show apparent metallic behavior above 90 K, but no superconducting sign has been
3.2. RuSr2 Gd1:4 Ce0:6 Cu2 Oy XRD measurements indicate that all these samples are in a single phase, and the XRD patterns can be indexed assuming a tetragonal lattice (I4/mmm). Table 2 shows the lattice parameters of RuSr2 Gd1:4 Ce0:6 Cu2 Oy prepared in dierent annealing procedures. According to the annealing procedures, these samples RuSr2 Gd1:4 Ce0:6 Cu2 Oy have dierent oxygen content. Ru1222-A, which was ®nally sintered in air, should have the lowest oxygen content; the oxygen content in Ru1222OH, which was obtained from Ru1222-O annealed under high oxygen pressure of 50 bar, should be the highest. Table 2 shows that c axis lattice parameter decreases with increasing oxygen content, whereas a axis lattice parameter remains nearly unchanged. The oxygen content dependence of lattice parameters is consistent with that reported by Wada et al. in isostructural Cu-1222 system [13,14]. The temperature dependence of resistivity for the samples RuSr2 Gd1:4 Ce0:6 Cu2 Oy , which were obtained in dierent annealing procedures is shown in Fig. 5. The sample Ru1222-O shows superconducting transition at 30 K and exhibits
Table 2 Lattice parameters and Tc (onset and zero) of RuSr2 Gd1:4 Ce0:6 Cu2 Oy Sample a (A) c (A) RuSr2 Gd1:4 Ce0:6 Cu2 Oy -A RuSr2 Gd1:4 Ce0:6 Cu2 Oy -O RuSr2 Gd1:4 Ce0:6 Cu2 Oy -AH RuSr2 Gd1:4 Ce0:6 Cu2 Oy -OH
3.845(1) 3.844(1) 3.844(1) 3.844(1)
28.661(2) 28.640(5) 28.620(5) 28.615(7)
Tc (K) (onset)
Tc (K) (zero)
± 30 45 45
± 18 35 35
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Fig. 5. The temperature dependence of resistivity for RuSr2 Gd1:4 Ce0:6 Cu2 Oy -A (d), RuSr2 Gd1:4 Ce0:6 Cu2 Oy -O ( ), RuSr2 Gd1:4 Ce0:6 Cu2 Oy -AH (.) and RuSr2 Gd1:4 Ce0:6 Cu2 Oy OH (m).
zero resistivity at 18 K. In the high temperature, a slightly metallic behavior is observed until 220 K. The sample shows a weak semiconductor-like behavior before the superconducting transition. However, Ru1222-A does not show superconducting transition after ®nal annealing in air. The resistivity of Ru1222-A remains nearly unchanged above 200 K, below which, Ru1222-A shows a strong semiconductive behavior. Ru1222-AH and -OH obtained from Ru1222-A and -O annealed under high oxygen pressure of 50 bars show superconducting transition at 45 K with metallic behavior above Tc and exhibit zero resistivity at 35 K. It suggests the annealing under high oxygen pressure apparently changes sample character compared with the samples Ru1222-A and Ru1222-O. However, the dierence between Ru1222-AH and -OH still exist. Ru1222-AH shows under-doped behavior with a pseudogap opening at 210 K, while Ru1222-OH is an optimal-doped sample because a T-linear dependence keeps until the superconducting transition. From the annealing procedures, the origin of this dierence can be found. The optimal-doped Ru1222-OH is obtained from Ru1222-O, while the under-doped sample Ru1222-AH is obtained from Ru1222-A. The unique dierence between them is a preannealing in the dierent atmosphere. It suggests that both annealing in ¯owing oxygen and in high oxygen pressure are important to improve super-
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conductivity. It is because annealing in oxygen atmosphere results in an increase of concentration of carrier. It indicates that the superconductivity for Ru1222 system is sensitive to oxygen content. Our results indicate that Ru1222 system is generally in under-doped region. The data in Table 2 indicate that Tc is closely related to the c axis lattice parameter. The c axis lattice parameter of Ru1222-A, which is not a superconductor, is the biggest among these samples. Ru1222-OH has the highest Tc and the smallest c axis lattice parameter. As the superconducting transition temperature gradually decreases, the c axis lattice parameter increases. 4. Conclusions The structural and electrical properties of Tadoped Ru1212 samples were studied as well as the relation between annealing atmosphere and the sample quality for Ru1ÿx Tax Sr2 GdCu2 Oy (x 0, 0.05, 0.15) and RuSr2 Gd1:4 Ce0:6 Cu2 Oy . Dierent annealing procedures play an important role in improving sample quality for Ru1222. For Ru1212, Ta doping suppresses apparently superconductivity, and the annealing time is an important factor for superconductivity due to improvement of granularity. For RuSr2 Gd1:4 Ce0:6 Cu2 Oy , the increase of oxygen content results in an increase of superconducting transition temperature and a decrease of c axis lattice parameter. Acknowledgements This work was supported by a grant from the Natural Science Foundation of China and by the Ministry of Science and Technology of China (NKBRSF-G19990646).
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