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May 6, 2010 - [22] G. Xiao, G. X. Rebello, Physica C 1993, 211, 433. [23] B. Fisher, J. Genssar, C.G. Kuper, L. Patlagan, G.M. Reisner,. A. Knizhnik, Phys. Rev.
Research Article Received: 13 August 2009

Revised: 14 March 2010

Accepted: 15 March 2010

Published online in Wiley Interscience: 6 May 2010

(www.interscience.wiley.com) DOI 10.1002/sia.3466

The effect of Ca co-substitution in (Y1−x Cax )Ba2 (Cu0.98Co0.02)3O7−δ (0 ≤ x ≤ 0.35) ceramic† E. H. Boudjema,a∗ M. Mahtali,a S. Chamekha and A. Taoufikb YBa2 Cu3 O7−δ cuprate is a suitable system for the study of impurity effect on superconducting properties because carrier concentration can be widely changed without introducing significant disorder. Thus, the effect of replacing Cu by Co atoms, and Y by Ca atoms is reported to provide useful information about the mechanism of high-Tc superconductivity. In this paper, we have investigated the effect of Ca and Co simultaneous substitution on the structural and physical properties for a series of (Y1−x Cax )Ba2 (Cu0.98 Co0.02 )3 O7−δ (0 ≤ x ≤ 0.35) ceramic. Doped Y123 samples are prepared by conventional solid-state reaction method and characterized by X-ray diffraction (XRD), SEM, and resistivity measurements. We found that the obtained crystalline structure is mainly orthorhombic. No parasite phase is observed. The orthorhombicity increases with Ca content. The analysis of the superconducting behavior of the samples showed that co-substitution of Ca does not counteract the adverse c 2010 John Wiley & Sons, Ltd. influence of Co within the single-phase range. Copyright  Keywords: ceramics; substitution; superconductivity; YBaCuO; electrical resistivity

Introduction

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Since the discovery of superconductivity above 90 K in the oxygen-deficient cuprate YBa2 Cu3 O7−δ (YBCO) in 1987[1,2] several works have been reported on this highly critical temperature superconductor. However, properties of the superconducting oxide co-doped by Ca and Co have not attracted considerable interest yet, especially in its polycrystalline state. The substitution of Co for copper in the YBa2 Cu3 O7−δ oxide has a significant effect on the structure and on the transition-critical temperature Tc decrease.[3 – 6] It is known that the Cu atoms in YBCO are located at two different sites in the unit cell. One of the Cu sites is in CuO4 in the basal plane (Cu(1)) which forms the Cu–O chains; the other is called Cu(2), is in pyramids slightly displaced from the basal plane, and forms Cu–O planes. It has been reported that Co atoms preferentially occupy Cu(1).[7] While Liu et al.[8] confirm that the Co atoms locate primarily at the Cu(1) sites at low doping level, upon increasing the Co concentration, some of the Cu(2) sites become occupied. The clear effect of Co on superconductivity may be due to the carrier concentration modification which depends generally from the oxygenation. The exact mechanism of the suppression of Tc upon Co incorporation remains subtle. The brutal diminution of Tc upon substitution of Co for Cu may be attributed to the fact that carriers produced by this process of doping are supplied to the Cu–O planes from Cu–O chains[9,10] which form a charge reservoir. If all the oxygen is removed from the Cu–O chain layer, YBCO becomes tetragonal and does not exhibit superconductivity. It has been demonstrated that the transition temperature of YBa2 (Cu1−x Cox )3 O7−δ drastically decreases upon Co substitution, and that superconductivity is lost at 6% Co doping.[11 – 13] Besides, the carrier concentration can also be changed by cation doping[14 – 18] the case of Ca doping on the Y site has been found to be interesting. Since the oxidation states of Y(+3) and Ca(+2) are different, the calcium substitution will introduce excess holes into CuO2 planes of Y123 independently from the oxygenation of the Cu–O chains.[19,20] Hole doping by Ca has been reported by many authors.[21 – 27] For instance,

Surf. Interface Anal. 2010, 42, 996–999

replacement of about 20% of Y by Ca in tetragonal YBa2 Cu5 O6 induces superconductivity at temperature as high as 44 K.[20,22] The PrBa2 Cu3 Oy , which is a semiconductor, becomes superconducting if the Ca is introduced at the Pr site.[25] That means the hole doping by Ca compensates the suppression of superconductivity by Pr atoms. Furthermore, the substitution of Ca at the Y site in YBa2 Cu2.55 Co0.45 Oy is known to enhance Tc from 0 to 52 K.[3] This implies the holes supplied by Ca counteract the effect of Co3+ . In this work, we present a study of Ca and Co simultaneous substitution effects on the properties of YBa2 Cu3 O7−δ high-Tc superconducting ceramic, in order to measure the impact of Ca on the unfavorable effects resulting from Co incorporation.

Experimental The samples of Ca co-doped (Y1−x Cax )Ba2 (Cu0.98 Co0.02 )3 O7−δ ceramics with the composition range (0 ≤ x ≤ 0.35) were prepared by solid-state reaction method under identical conditions. Stoichiometric quantities of high-purity powders of Y2 O3 , BaCO3 , CaCO3 , CuO and CoO were mixed, ground and calcined at 940 ◦ C in air for 24 h. The precursors were reground, pellet shaped and then sintered in similar conditions of the calcinations treatments. The synthesized samples are characterized by SEM for



Correspondence to: E. H. Boudjema, Thin Films and Interfaces Laboratory, Department of Physics, Faculty of Science, University Mentouri of Constantine, Campus Chaab-Erassas, 25000 Constantine, Algeria. E-mail: eh [email protected]

† Paper published as part of the ECASIA 2009 special issue. a Thin Films and Interfaces Laboratory, Department of Physics, Faculty of Science, University Mentouri of Constantine, Campus Chaab-Erassas, 25000 Constantine, Algeria b Equipe des Mat´eriaux Supraconducteurs a` Haute Tc. Universit´e IBN ZOHR, Facult´e des Sciences, BP 8106, 8000, Agadir. Maroc

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The effect of Ca co-substitution in a ceramic variety

0.0084

(b-a)/(b+a)

0.0082 0.0080 0.0078 0.0076 0.0074 -0.05 0.00

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Figure 1. XRD spectra of 0.02 Co and 0; 0.05; 0.1 5; 0.25 and 0.35 Ca co-doped YBa2 Cu3 O7−δ sintered samples.

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a b c

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3.86 3.85

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c(Å)

3.87 a(Å), b(Å)

Figure 3. Variation of the orthorhombicity (b − a)/(b + a) as a function of Ca content in (Y1−x Cax )Ba2 (Cu0,98 Co0,02 )3 O7−δ .

3.83 11.65 3.82 0.05

0.10

0.15

0.20 0.25 Ca content

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Figure 2. Lattice parameters a, b and c as a function of doping level x in (Y1−x Cax )Ba2 (Cu0,98 Co0,02 )3 O7−δ HTSC.

microstructure, X-ray diffractometer (XRD) for phase identification and the standard four-probe method for resistivity.

Results and Discussion

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Figure 4. (a) Variation in the intensity and position of (013), (103) reflections; and (b) (020), (200) Reflections for (Y1−x Cax )Ba2 (Cu0,98 Co0,02 )3 O7−δ superconductors.

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The XRD patterns shown in Fig. 1 indicate that all studied samples exhibit the polycrystalline superconducting phase with more intensity of diffraction lines for undoped sample. Specifically, the peak intensity of the doped samples decreases regularly as Ca content is increased from x = 0 to 0.35, while other peaks as (003) and (010) disappear completely. The XRD analysis allows to confirm that the crystalline structure of the superconducting phase is orthorhombic for the used compositions. Further, no secondary phase containing Ca or any other cation is detected even up to x = 0.35. The absence of secondary phases support that Ca is incorporated into the crystalline structure of Y123 superconductor. The majority of peaks disappear with increasing Ca content. For x = 0.35 practically, only one peak subsists (013). We can then conclude that the strong addition of Ca in (Y,Ca)Ba(Cu,Co)O leads to texturing. The variation of lattice parameters a, b and c with Ca concentration in (Y1−x Cax )Ba2 (Cu0.98 Co0.02 )3 O7−δ is shown in

E. H. Boudjema et al.

Figure 5. Micrographies of (Y1−x Cax )Ba2 (Cu0,98 Co0,02 )3 O7−δ sintered samples, (x = 0; 0.05; 0.15; 0.25 and 0.35).

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Fig. 2. It may be noticed that the lattice constant, a, remains roughly constant, while lattice constant, b, increases with Ca addition until x = 0.25. The c lattice parameter also increases with Ca doping level. This implies that the addition of Ca at Y site in YBaCu(Co)O increases somewhat the difference between a and b parameters, and thus increases the orthorhombicity (a − b)/(a + b) of the superconducting phase (as shown in Fig. 3). In contrast, when used separately in doping of Y123 phase, each Ca and Co ion leads undeniably to a decrease in the orthorhombicity, as reported elsewhere.[9] This augmentation in orthorhombicity is validated by the characteristic orthorhombic alteration of several peaks. Indeed, separation of (013), (103) and (020), (200) reflections with increasing Ca content, as shown in Fig. 4(a) and (b), respectively, indicates that orthorhombicity of the system increases with addition of Ca. The SEM micrographs taken at the same magnification (×2000) of the surfaces of free and Ca doped (Y1−x Cax )Ba2 (Cu0.98 Co0.02 )3 O7−δ samples are shown in Fig. 5. The structural morphology of the samples varies with increase in x value. The x = 0 sample shows clear and characteristic round grains, with a size of about 4–10 µm. The general aspect is typical of highly porous ceramic. But, as x value increases from x = 0.05 the shape changes from round to plate form. The grains become more joined. The porosity of the samples decreases slightly due to the reduction of intergranular space. In x = 0.35 sample we note a coalescence of the porosity. The observation confirms that all samples are polycrystalline single phase. The grain boundaries are exempt from intergranular no-superconducting phase. Compaction of thin grains visible on micrographs leads to grain alignment of samples. We can legitimately conclude to a texturing that is due to the massive addition of Ca. These observations are in perfect agreement with results obtained by XRD analysis. The temperature dependence of the electrical resistivity of (Y1−x Cax )Ba2 (Cu0.98 Co0.02 )3 O7−δ (0 ≤ x ≤ 0.35) is depicted in Fig. 6. Resistivity measurements are made on regularly shaped samples using the standard four-probe method. The electrical resistivity vanishes at 79, 80, 79 and 78 K, respectively, for x = 0.05; 0.15; 0.25; and 0.35. The samples show a roughly linear decrease in resistivity during cooling from room temperature to onset temperature of superconducting transition, thus indicating a

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Figure 6. Resistivity versus Co0,02 )3 O7−δ compounds.

temperature

of

(Y1−x Cax )Ba2 (Cu0,98

metallic behaviour for such compound types. The critical transition temperature of the reference sample (0.02 Co doped) is Tc = 79 K. The incorporation of Ca in addition to Co does not affect largely the critical transition temperature. Despite massive addition of Ca the Tc values increase by one degree and then decrease continuously from 80 K to 78 K upon increasing the value of x from 0.15 to 0.35 probably due to the overdoping and the disorder induced in such doped samples.

Conclusion In the present study, the effect of co-substitution of Ca for Y and Co for Cu on the structural and superconducting properties of (Y1−x Cax )Ba2 (Cu0.98 Co0.02 )3 O7−δ are investigated. All studied samples are superconductors and present an orthorhombic polycrystalline single phase. XRD results reveal that the orthorhombicity of the system increases with increasing Ca content. The porosity

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Surf. Interface Anal. 2010, 42, 996–999

The effect of Ca co-substitution in a ceramic variety is to some extent disapproved by the incorporation of Ca. The observed reduction of Tc visibly attests that within the phase limit co-doping of Ca does not counter balance the unfavorable effects of Co and every substituent ion practically acts separately and applies its own effect on Tc.

References [1] M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. U. Wang, C. W. Chu, Phys. Rev. Lett. 1987, 58, 908. [2] R. J. Cava, B. Batlogg, R. B. Van Dover, D. W. Murphy, S. Sunshine, T. Siegrist, J. P. Remeika, E. A. Rietman, S. Zahurak, A. P. Espinosa, Phy. Rev. Lett. 1987, 58, 1676. [3] V. N. Narozhnyi, V. N. Kochetkov, Phy. Rev. B 1996, 53, 5856. [4] V. E. Gqusumyants. S. A. Kasmin, V. I. Kaidanov, E. V. Vladimirskaya, Supercond. Phys. Chem. Technol. 1992, 5, 674. [5] K. Westerholt, H. J. Wuller, H. Bach, P. Stauche, Phys. Rev. B 1989, 39, 11680. [6] J. M. Tarascon, P. Barboux, P. F. Miceli, L. H. Greene, G. W. Hull, M. Eibcshulz, S. A. Sunshine, Phys. Rev. B 1988, 37, 7458. [7] M. Murugesan, H. Obara, A. Sawa, S. Kosaka, Y. Nakagawa, J. C. Nie, H. Yamasaki, Physica C, 2003, 400, 65. [8] L. Liu, C. Dong, J. Zhang, J. Li, Physica C, 2002, 377, 348. [9] Y. Tokura, J. B. Torrance, T. C. Huang, A. I. Nazzal, Phys. Rev. B 1988, 38, 7156. [10] J. D. Jorjensen, B. W. Veal, A. Paulicas, L. J. Nowicki, G. W. Crabtree, H. Claus, W. K. Kwok, Phys. Rev. B 1990, 41, 1863.

[11] Y. Maeno, T. Tomita, M. Kyogoku, A. Awaji, Y. Aoki, K. Hoshino, A. Minami, T. Fujita, Nature 1987, 328, 512. [12] J. M. Tarascon, P. Barboux, P. F. Miceli, L. H. Greene, G. W. Hull, M. Eibschutz, S. A. Sunshine, Phys. Rev. B 1988, 37(13), 7458. [13] F. Bridges, J. B. Boyce, T. Claeson, T. H. Geballe, J. M. Tarascon, Phys. Rev. B 1989, 39(16), 11603. [14] X. S. Wu, J. Gao, Physica C 1999, 315, 215. [15] J. L. Peng, P. Klavins, R. N. Shelton, H. B. Radousky, P. A. Hahn, L. Bernardez, Phys. Rev. B 1989, 40, 4517. [16] Y. Sun, G. Strasser, E. Gornik, Physica C 1993, 206, 29. [17] E. Suard, A. Maignan, V. Caignaert. B. Raveau, Physica C 1992, 200, 43. [18] R. G. Buckley, D. M. Poke, J. L. Talon, M. R. Presland, N. E. Flower, M. P. Staines, H. L. Johnson, M. Meylan, G. V. M. Williams, M. Bowden, Physica C 1991, 174, 383. [19] G. Xiao, N. S. Rebello, Physica C 1993, 211, 433. [20] R. S. Liu, J. R. Cooper, J. W. Loram, W. Zhou, W. Lo, P. P. Edwards, W. Y. Liang, Solid State Commun. 1990, 76, 679. [21] X. S Wu, S. S Jiang, J. Lin, J. S. Liu. W. M. Chen, X. Jin, Physica C 1998, 309, 25. [22] G. Xiao, G. X. Rebello, Physica C 1993, 211, 433. [23] B. Fisher, J. Genssar, C. G. Kuper, L. Patlagan, G. M. Reisner, A. Knizhnik, Phys. Rev. B 1993, 47, 6054. [24] A. Manthiram, S. J. Lee, J. B. Goodenough, J. Solid State Chem. 1988, 73, 278. [25] A. Manthiram, J. B. Goodnough, Physica C 1989, 159, 760. [26] C. Legros-Gledel, J. F. Marucco, E. Vincent, D. Favrot, B. Poumellec, B. Touzelin, M. Gupta, H. Alloul, Physica C 1991, 175, 279. [27] C. Gledel, J. F. Marucco, B. Touzelin, Physica C 1990, 165, 437.

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