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Effects of Re substitution on the structure and superconductivity of Cu1−xRexBa2YCu2Ow
Physica C 355 (2001) 267±277
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Eects of Re substitution on the structure and superconductivity of Cu1 x Rex Ba2YCu2Ow F. Licci a,*, A. Gauzzi a, M. Marezio a, Q. Huang b,c, A. Santoro b, R. Masini d, C. Bougerol-Chaillout e, P. Bordet e a
Istituto Materiali Speciali per Elettronica e Magnetismo del Consiglio Nazionale delle Ricerche, Parco Area delle Scienze 37A, 43010 Fontanini-Parma, Italy b Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA c Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742, USA d Istituto per la Tecnologia dei Materiali e Processi Energetici del Consiglio Nazionale delle Ricerche, Via R.Cozzi, 53, 20125 Milano, Italy e Laboratoire de Cristallographie, Centre National de la Recherche Scienti®que-Universit e Joseph Fourier, B.P. 166, 38042 Grenoble Cedex 9, France Received 14 November 2000; accepted 15 January 2001
Abstract Samples of Cu1 x Rex Ba2 YCu2 Ow , with nominal x 0, 0.05, 0.1 and 0.15, and 6:91 6 w 6 7:17 were prepared and characterized. The aim was to evaluate the possibility of improving the transport and magnetic properties of CuBa2 YCu2 Ow (YBCO) by substituting Cu in the reservoir blocks (Cu1) with a high-valent element such as Re. X-ray powder diraction and energy dispersive spectroscopy (EDS) studies indicated that Re substitutes for Cu1 up to Such an impurity was identi®ed x 0:1, beyond which it segregates, mostly by forming a cubic phase with a 8:275 A. as YBa2 ReO6 on the basis of the powder x-ray and neutron diraction data and EDS microanalysis. The crystal structure of Cu1 x Rex Ba2 YCu2 Ow was re®ned by the Rietveld analysis based on powder neutron diraction data. It is depending on x and w. The Re cations orthorhombic (Pmmm space group), with a, b and c (3.84, 3.88 and 11.70 A) were found to occupy the (0 0 0) position and to be surrounded by 6 oxygen atoms at the vertices of an octahedron (2O1, 2O4 and 2O5). Such units do not signi®cantly perturb the YBCO structure. No ordering between Cu and Re was observed by electron diraction. The resistive and magnetic measurements indicated that the optimum oxygen doping of the Re-doped YBCO occurs at w > 7. The corresponding Cu average valence is 2:27 1%. Tc of the optimally doped samples is not very sensitive to the Re content and remains as high as 91 K when x 0:1. The residual resistivity extrapolated at T 0 K scales linearly with x, according to the MathiessenÕs rule. SQUID measurements for the x 0:05 and x 0:1 samples indicated that Hirr and jc are lower than those of the undoped phase at any temperature. Ó 2001 Published by Elsevier Science B.V. PACS: 74.25.-q; 74.72.Bk; 74.62.Dh; 61.66.Fn Keywords: Cu-1212 superconductors; Re-substitutions; Structural re®nement; Electrical properties; Magnetic properties
* Correponding author. Tel.: +39-0521-269204; fax: +390521-269206. E-mail address: [email protected] (F. Licci).
0921-4534/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 0 7 8 - 8
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1. Introduction Chemical substitutions in high Tc superconducting cuprates have generated a great interest because they represent an ecient tool for improving the physical properties of these compounds. Even though it has not been always possible to induce an increase of Tc through chemical substitutions, it seems that some of them have a positive eect on other important properties, such as anisotropy, irreversibility ®eld, and critical current density. A review published by Jorgensen et al. [1] singled out the structural features that could improve Tc and jc . They are the shortening and/or the metallization of the reservoir blocks and the presence of suitable defects acting as pinning centers for the ¯ux vortices. A number of groups are presently involved in the search of optimal substituents [2±7]. The Sr-substitution for Ba in HgBa2 Can 1 Cun O2n2d (Hg-12 n 1n) [8,9] and CuBa2 YCu2 Ow (YBCO or 123) [5] systems simulates the eects of the mechanical pressure as it induces the contraction of the unit cell and the decrease of most of the interatomic distances inside the reservoir block. However, it must unsettle other structural parameters responsible for the observed Tc decreases. The halogen substitutions for oxygen in the reservoir blocks improve the metallic character of the Hg or Cu layers in the blocks [4,7]. The substitutions of high-valent elements for Hg or Cu cations increase the metallic character of the blocks themselves [1±3] and generate defects forming clusters of dimensions suitable to act as pinning centers. For example, Cr-substitution for Hg in HgSr2 CuO4 was reported to increase jc [10], together with only a moderate reduction of Tc . Resubstitution for Hg in the Hg-based cuprates was investigated with [2,3,11] or without [12±16] a simultaneous Sr substitution for Ba. The double substitution was expected to improve the superconducting properties by shortening the block thickness and decreasing its resistivity. In the Hg± Ba cuprates Re was reported to reduce the structural anisotropy, to raise the irreversibility ®eld and to improve the ¯ux pinning, at least at temperature lower than 70±80 K. Meng et al. [15] found that the Re substitution for Hg facilitates
the synthesis of the n > 2 members of the Hg12 n 1n homologous series and, furthermore, once formed, the phases are more stable than the undoped ones. These authors also reported that the Re-substitution induces only a moderate decrease of Tc in Hg-1223 (3±4 K). Chmaissem et al. [16] determined the microstructural changes induced by the Re-substitution for Hg in (Hg,Re)Ba2 Can 1 Cun O2n2d , and measured a moderate enhancement of Hirr with Re content at temperatures <70 K. A few reports dealing with the Re substitution in CuSr2 YCu2 O7d (Sr-123) [17±19] indicate that Re stabilizes the phase and makes the synthesis possible under normal pressure, while high-pressure is necessary for obtaining the undoped Sr-123 [20±22]. This suggests that, although the HgBa2 Can 1 Cun O2n2d and CuBa2 YCu2 O7 d systems are quite dierent, they exhibit similar behavior with respect to Sr and high-valent element double substitutions. With the aim of investigating the possibility of improving the transport and magnetic properties of CuBa2 YCu2 Ow without deteriorating Tc , we studied the eect of Re substitution for Cu in the reservoir block of CuBa2 YCu2 Ow (Cu1). We analyzed in detail the phase composition and the structural and physical properties of Cu1 x Rex Ba2 YCu2 Ow at dierent oxygen stoichiometries. We concluded that the Re solubility in YBCO is limited to about 10% of Cu1 and the substitution occurs at random. Tc was found to remain as high as 91 K, but Hirr and jc did not improve. 2. Experimental Cu1 x Rex Ba2 YCu2 Ow samples, with nominal x 0, 0.05, 0.1 and 0.15, were prepared by solidstate reaction of stoichiometric mixtures of 99:99% pure oxides and BaCO3: (Two samples with nominal x 0:15 were prepared in two dierent but identical runs. In the following they will be labelled as x 0:15i and x 0:15ii, respectively). The reagents were mixed and ground under acetone in an agate mortar. The syntheses were carried out in alumina crucibles, in air at 930°C for a total of 80± 100 h, with several intermediate coolings and grindings. The process was stopped when the
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X-ray patterns did not show any dierence after two consecutive treatments. Pellets of 700 mg in weight and 1 cm in diameter were obtained by pressing the powders, sintering in air at 930°C overnight, and cooling down to room temperature at 100°C/ h. We labeled these as-prepared samples as ``R''. For optimizing the oxygen concentration, dierent annealings were carried out at varying oxygen partial pressures. Samples ``A'' were obtained by heating the ``R'' pellets in ¯owing oxygen at 880°C for two hours, cooling down to 480°C at 30°C/h, remaining at this temperature for 50±100 h, and ®nally cooling down to room temperature. Samples ``S'' were obtained from the ``A'' pellets. Two or three of them were put in a quartz ampoule together with a 300 mg pellet of KClO3 . The ampoule (1 cm in diameter, 10 cm long, and 2±3 mm thick) was evacuated to 10 5 torr, sealed and heated up to 480°C for 12 h. The system was then cooled down to room temperature at 100°C/h. The Cu average valence, 2 p, was determined by iodometric titration with amperometric dead-stop end point [5]. The Re content, x, in Cu1 x Rex Ba2 YCu2 Ow was determined by energy dispersive spectroscopy (EDS) analysis, by using a Kevex system connected to a Philips CM microscope operating at 300 kV. Several spurious phases were detected and their stoichiometries were determined. For microstructural investigations (HREM analyses) the samples were crushed in a mortar with alcohol. The suspension was recovered onto an aluminum holey carbon grid. The electron diraction patterns were taken along the main zone axes [0 0 1], [1 1 0], and [1 0 0], in order to check the presence of possible superstructure re¯ections related to a Re/ Cu ordering. The phases were identi®ed by X-ray and neutron diraction. The X-ray patterns were recorded with a D5000 Siemens diractometer in transmission geometry, equipped with CuKa radiation. The impurities were identi®ed by trial and error indexing of those re¯ections which did not belong to the 123 phase. The Rietveld analysis of the Xray patterns was carried out by using the Rietan [23] and Fullprof programs. The powder neutron diraction experiments were performed by using the BT-1 high resolution
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powder diractometer located at the reactor of the NIST Center for Neutron Research. A Cu (311) monochromator was employed to produce a monochromatic neutron beam of 1.5401(1) A wavelength. Collimators with horizontal divergence of 150 , 200 , and 70 of arc were used before and after the monochromator, and after the samples, respectively. The intensities were measured in steps of 0.05° in the 3±168° 2h range at room temperature. The samples were ``R'' and ``S'' x 0:15ii powders (about 1 g in weight) The neutron diffraction experiments on x 0:10 ``A'' sample were carried out at the ILL, Grenoble, using the D2B and diractometer. The wavelength was 1.5938 A the scans were performed between 2h 10° and 162°, with 0.05° steps. The structural re®nements were carried out with the Rietveld pro®le ®tting analysis using the GSAS program [24]. The neutron scattering amplitudes used for the re®nement were 0.775, 0.525, 0.772, 0.920, and 0:581 10 12 cm for Y, Ba, Cu, Re, and O, respectively. The electrical resistivity and magnetic susceptibility were measured as a function of temperature between 300 and 4.2 K. The ac resistivity was determined by a four-point probe, in a closed-cycle helium cryostat, by applying a pulsed current of 0.1±1 mA. The magnetic susceptibility was measured by a commercial Lake Shore Model 7000 ac susceptometer. A proper combination of sample dimension and maximum frequency was chosen in order to eliminate the eects of the normal-state eddy current near Tc , so that v00 represented only the hysteretic losses. The data were collected at a ®xed frequency of 333 Hz on similar size barshaped specimens. This allowed a meaningful comparison of the measurements. The applied magnetic ®eld, H, ranged from 0.05 to 30 Oe (5 lT to 3 mT). The calibration was performed by using a Gd2 (SO4 )3 H2 O standard and the demagnetization eects associated to the sample shape were taken into account. The critical current density, jc , and the irreversibility line were measured in a commercial RF-SQUID magnetometer achieving 10 8 emu resolution. The irreversibility line, Tirr , was evaluated by comparing the ®eld cooling (FC) and zero®eld cooling (ZFC) temperature scans, taken under an applied ®eld varying from 1 to 5 T. The
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critical current density was estimated on the basis of the Bean model and by assuming independent grains. The magnetic hysteresis was measured at 0.1, 1, 3 and 5 T. The average grain size and shape, which are required to analyze the data within this model, were estimated from scanning electron microscopy (SEM) observations. 3. Results and discussion 3.1. Phase analysis and stoichiometry Except for the nominal x 0:1 sample, the Resubstituted compounds were found to contain minor amounts of secondary phases, whose composition and concentration depended on the nominal x. The results relevant to the sample purity are summarized in Table 1. The Cu0:9 Re0:1 Ba2 YCu2 Ow sample was single phase and the stoichiometry measured by EDS coincided with the nominal one. The Re concentration measured for the nominal x 0:05 sample was equal to 0.06(1). This value is within one standard deviation with respect to the nominal one, but it is consistent with the results of the EDS microanalysis and X-ray diraction, which evidenced the presence of Refree impurities. Samples with the nominal x 0:15 composition were found to contain secondary Baand Re-based phases and the concentration of Re in the 123 phase was lower than the nominal. The main impurity (about 15% in weight) was identi®ed as YBa2 ReO6 on the basis of the X-ray diffraction data. To our knowledge such a compound has not been reported previously in the literature nor in the JCPD ®les. Its identi®cation was based
on the data of Baud and Capestani [25] who described an isostructural phase containing Yb, or most of the other rare-earth elements, but not Y. The YBa2 ReO6 stoichiometry was con®rmed by EDS and neutron diraction experiments. A preliminary re®nement of the structure, based on Xray and neutron diraction data was performed and the results are summarized in Table 2. The phase was found to be cubic with the Fm3m space exhibited group. The a lattice parameter (8.27 A) a moderate (<0.1%) dependence on the oxygen partial pressure during annealing. The average Cu valence and the oxygen stoichiometry in Cu1 x Rex Ba2 YCu2 Ow are reported in Table 3 as a function of the cation stoichiometry and the oxygen partial pressure during the postsynthesis annealing. Possible interference by Re in the Cu titration was ruled out by carrying out ``blank'' tests. It should be pointed out that in the presence of impurity phases the measured 2 p valences could be aected by errors larger than the standard deviation typical of the method (<1%). By taking into account the impurities and their compositions it was estimated that the eective 2 p valence could dier up to a maximum of 2% from the measured values. The oxygen stoichiometry, wj , was calculated from the average copper valence and by assuming the Re valence to be 6, which is typical for 6-coordinated Re cations [1]. For a few samples the oxygen stoichiometry, wn , was also determined from the structural re®nements based on neutron diraction data. It was assumed that wn is equal to 6 nO4 nO5 ), where nO4 and nO5 are the occupancies of the 0; 1=2; 0 and 1=2; 0; 0 site, respectively. The wj and wn values corresponding to the same
Table 1 Phase analysis and stoichiometry of Cu1 x Rex Ba2 YCu2 Ow Nominal x
0.05 0.10 0.15i 0.15ii 0.15ii a
Number of EDS examined grains
Number of homogeneous 123 grains
Average measured x in 123 phase
Secondary phases (number of grains) YBa2 ReO6
Y2 BaCuO5
BaCuO2
CuO
12 10 10 10 ±a
9 10 8 6 76%
0.06(1) 0.10(1) 0.11(2) 0.09(2)
Absent Absent 2 2 15%
1 Absent Absent 1 6%
1 Absent Absent 1 2%
1 Absent Absent Absent Absent
Values shown in this row were obtained by X-ray diraction data re®nement (see text).
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Table 2 Structure and composition of the main impurity YBa2 ReO6 Fm3m 2583.116 566.5 (average) 7.573
Nominal composition Space group Unit cell formula weight 3 ) Volume (A Density (g cm 3 )
a Parameter (A) Atom position and thermal 2 ) parameter, B (A
Ba (1/4,1/4,1/4) Y (1/2,1/2,1/2) Re (0,0,0) O (1/4,0,0)
``R'' sample
``S'' sample
8.2740(4)
8.2756(4)
1.13(17) 0.59(11) 0.59(11) 2.19(12)
0.81(18) 0.48(13) 0.48(13) 2.25(14)
Table 3 Average copper valence (2 p), oxygen stoichiometry (wj and wn ), Tc , DTc , resistivity and % diamagnetic shielding of Cu1 x Rex Ba2 YCu2 Ow . xnom represents the nominal Re concentration xnom
Annealing
2p
wj
0
R A S R A S R A S R A S R S
2.27 2.31 2.31 2.23 2.28 2.30 2.20 2.26 2.27 2.19 2.23 2.26 2.17 2.23
6.91 6.96 6.96 6.94 7.02 7.04 7.00 7.08 7.09 7.07 7.13 7.17 7.04 7.13
0.05 0.10 0.15i 0.15ii
wn
7.05
7.00 7.13
Tc (R) (K)
DTc (K)
q(0) (mX)
Tc (Dm) (K)
% Shielding (77 K, 0.05 Oe)
93 92.5 91 86 93 92.5 70.5 89 91 60 85 85 85b 91.5b
2 4 5 10 2 3 20 5 2 35 12 10
1.6 0 0.9 3.0 1.6 3.5 3.1 3.1 6.3 33.3 6.9 9.4
93 92 92 88 92 92 91.5 90 91 91.5 90 91
100 97 98 78 96 99 0.6a 84 89 0.3a 66 82
wj was calculated by assuming the Re valence 6. wn values were re®ned from the neutron diraction data. Tc (R) represents the resistive oset (T at R 0). DTc and q(0) represent the FWHM of the ®rst derivative and the extrapolated value at T 0 K of the q(T) curves, respectively. Tc (Dm) is the temperature at the diamagnetic onset. Letters in the third column indicate dierent annealing, as described in the text. a Measured under an applied ®eld of 1 Oe. b T at 50% of the resistive transition.
samples were found to be consistent (within 0:5%), which proves that the 6 valence assumed for Re is correct and the titration error is small. The average Cu valence in the samples of the same series (``R'', ``A'' or ``S'') decreases by increasing Re concentration. It increases, instead, by increasing the oxygen partial pressure at constant Re concentration (``S''>``A''>``R''). The overall oxygen stoichiometry increases by increasing either the Re content and/or the oxygen partial pressure. Inde-
pendently of x and w, the average Cu valence corresponding to the maximum Tc , is practically constant at 2:27 1%. 3.2. Structural re®nements The neutron diraction data of Cu0:9 Re0:1 Ba2 YCu2 Ow (x 0:10 ``A'' sample) and of Cu0:85 Re0:15 Ba2 YCu2 Ow (x 0:15ii ``R'' and ``S'' samples) were used to re®ne the structures. The
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results are summarized in Tables 4 and 5. The positional, thermal and occupancy parameters are reported in Table 4 and compared with those of undoped CuBa2 YCu2 O6:93 [26]. The unit cell parameters and selected interatomic distances are reported in Table 5. The re®nements showed that Re occupies the (0 0 0) site, substituting for Cu, and is surrounded by 6 oxygen atoms in an octahedral con®guration, similar to that observed in the Re-substituted Hg-12 n 1n compounds [2,16]. The vertices of the octahedra are the two apical O1 oxygen atoms while two oxygen atoms (O4) at 0; 1=2; 0 and two (O5) at 1=2; 0; 0 form
the base. Dierently from what observed in the Hg-cuprates, the Re cations do not induce any appreciable distortion of the 123 structure. No displacements of the oxygen atoms from the original positions were detected. In the x 0:15ii ``R'' sample the O5 occupancy corresponds to approximately twice that of Re (as deduced from EDS analysis, Table 1), that is 0.19(2) vs 0.09, respectively. After annealing in oxygen (x 0:15ii ``S'' sample) nO5 remains constant within the experimental uncertainty (i.e. 0.23(2)), while w and nO4 increase (from 7.04 to 7.13 and from 0.81(2) to 0.90(2), respectively). These data are consistent
Table 4 Position, thermal and occupancy parameters of Cu0:9 Re0:1 Ba2 YCu2 Ow (x 0:10 ``A'' sample) and Cu0:85 Re0:15 Ba2 YCu2 Ow (x 0:15ii ``R'' and ``S'' samples), compared with those of YB2 Cu3 O6:93 (YBCO, from Ref. [26]) Atom Y Ba Cu1/Re
Cu2 O1
O2 O3 O4
O5
YBCO 2 ) B (A z 2 ) B (A 2 ) B (A U11 (A) U22 (A) U33 (A) z 2 ) B (A z 2 ) B (A U11 (A) U22 (A) U33 (A)
z 2 ) B (A z 2 ) B (A 2 ) B (A U11 (A) U22 (A) U33 (A) n 2 ) B (A U11 (A) U22 (A) U33 (A) n Rp Rwp v2
0.28(3) 0.1843(2) 0.44(3) 0.41(3)
0.3556(1) 0.20(2) 0.1590(2) 0.009(1) 0.007(1) 0.010(1) 0.3779(2) 0.51(4) 0.3790(2) 0.35(3) 0.022(3) 0.001(2) 0.019(2) 0.9
0.03(1) 3.33 5.96
x 0:10 ``A''
x 0:15ii ``R''
x 0:15ii ``S''
0.39(4) 0.18522(24) 0.53(6) 0.66 0.015 0.003 0.007 0.35684(15) 0.38 0.15824(23) 1.02
0.71(7) 0.18734(27) 0.48(7) 0.82(7)
0.77(8) 0.18584(30) 0.50(8) 0.67(8)
0.35795(18) 0.32(5) 0.15678(30) 1.89(7)
0.35694(19) 0.25(5) 0.15743(32) 1.29(7)
0.37949(33) 0.51(5) 0.3780(3) 0.51(5) 5.92(30)
0.3787(4) 0.46(5) 0.3785(4) 0.46(5) 6.35(22)
0.809(23) 5.92(30)
0.902(24) 6.35(22)
0.190(17) 5.4 6.88 2.866
0.233(19) 6.07 7.4 2.295
0.37838(25) 0.62(3) 0.37793(26) 0.62(3) 1.14 0.027 0.005 0.011 0.804(8) 0.053 0.027 0.011 0.245(11) 5.45 7.03 3.01
Rietveld re®nements were done in the orthorhombic Pmmm space group. Atom positions are Y(1/2,1/2,1/2), Ba(1/2,1/2,z), Cu1/ Re(0,0,0), Cu2(0,0,z), O1(0,0,z), O2(1/2,0,z), O3(0,1/2,z), O4(0,1/2,0), and O5(1/2,0,0). Numbers in parentheses are statistical standard deviations of the last signi®cant digit. When not indicated the occupancy factor, n, was equal to 1. The data relevant to Cu0:9 Re0:1 Ba2 YCu2 Ow were obtained from the neutron diraction experiments carried out at D2B facility at ILL, Grenoble.
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Table 5 of Cu1 x Rex Ba2 YCu2 Ow , compared with those of YBa2 Cu3 O6:93 (YBCO, Unit cell parameters and selected interatomic distances (A) from Ref. [26]) a b c 2 b a= b a 103 V Ba±O1 Ba±O2 Ba±O3 Ba±O4 Ba±O5 Y±O2 Y±O3 Cu1/Re±O1 Cu1/Re±O4 Cu1/Re±O5 Cu2±O1 Cu2±O2 Cu2±O3 Cu2±Cu2
YBCO
x 0:10 ``A''
x 0:15ii ``R''
x 0:15ii ``S''
3.8227(1) 3.8872(2) 11.6802(2) 16.7 173.56 2.7418 2.9803(4) 2.9707(2) 2.8787(4) 2.9002(4) 2.4107(2) 2.3771(2) 1.8571 1.9436(2) 1.91135(10) 2.29632 1.9290 1.9627 3.3732
3.84192(5) 3.87655(5) 11.69134(19) 9.0 174.12 2.7471 2.9764 2.9607 2.8945 2.9060 2.4037 2.3933 1.8496 1.9382 1.9209 2.3219 1.9375 1.9539 3.3475(1)
3.84602(12) 3.87911(12) 11.73153(38) 8.6 175.024(13) 2.7547(6) 2.974(4) 2.950(4) 2.9203(24) 2.9312(24) 2.4001(23) 2.3972(24) 1.839(4) 1.93956(6) 1.92301(6) 2.360(4) 1.9395(6) 1.9538(6) 3.3329(2)
3.84411(13) 3.87984(14) 11.70437(41) 9.2 174.565(14) 2.7510(6) 2.976(4) 2.963(5) 2.9026(27) 2.9145(26) 2.4042(25) 2.3907(26) 1.843(4) 1.93992(7) 1.92205(7) 2.335(4) 1.9388(6) 1.9563(6) 3.3488(2)
The data relevant to the x 0:10 ``A'' sample were collected on the D2B facility at ILL.
with the assumption that the additional oxygen goes to the O4 positions around the (0 0 0) sites occupied by the Cu cations. The O4 sites are preferentially depleted at low oxygen concentration because the Re±O bonds are stronger than the Cu1±O bonds. In the structure of YBCO there are two empty O5 sites per unit formula, thus all Cu atoms in the reservoir block could in principle be replaced by Re with full occupation of the O5 sites. However, the samples with more than 10% Re could not be synthesized as a single phase. The low solubility of Re in the 123 structure may be explained by the fact that the structure can not sustain a large O5 occupancy. The increase of this occupancy induces an increase of the Ba layer strain. A tentative evaluation of the bond valence sum of Ba in the x 0:15ii ``S'' sample nO5 0:23 2 gives a value of 2.19, which suggests an overstrained arrangement [27]. This hypothesis is corroborated by the fact that Re is more soluble in Sr-123 than in Ba123 [17,28]. Single-phase Cu1 x Rex Sr2 YCu2 Ow samples were obtained with x as high as 0.3. The bond valence sum of the Sr sites in Cu0:85 Re0:15 Sr2 YCu2 O7:44 , calculated from the re®ned neutron
data, was found to be 1.78 [28]. In this case the increase of the O5 occupancy seems to play a positive role by reducing the 1=2; 1=2; z site dimension, which is too large for Sr [22]. When compared with the undoped phase [26], the Re-doped YBCO does not exhibit signi®cant dierences as far as the structural features are concerned. These dierences can be summarized as follows. (1) As a consequence of the occupation of the O5 sites the Re-substituted compounds result in being less orthorhombic than the undoped one. The orthorhombicity parameter, S 2 b a= b a, for Cu1 x Rex Ba2 YCu2 Ow are quoted in Table 5 at dierent x and w values. (2) Qualitatively the c axis increases in the Redoped phases. A quantitative calculation of the increasing rate is not straightforward, due to the fact the oxygen concentration is not constant in the examined samples. When normalized to the c axis value, the Cu2±O1 distance increases and the Cu1±O1 decreases in the Re-doped compounds. This means that the incorporation of Re displaces the apical O1 away from the superconducting CuO2 plane. According to Jorgensen et al. [1], such
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a displacement generally reduces the tilting angle of the CuO2 squares in the superconducting layers and increases Tc , provided that the oxygen doping remains at optimum level. However these features were not observed in the Re-doped123 phase. (3) The Ba cation moves toward the Cu2±O layer and away from O4 and O5, as indicated by the increase of the zBa parameter. This is probably a consequence of the fact that O4 and O5 are strongly bonded to Re and would become overbonded with short Ba±O4 and Ba±O5 distances. (4) The thickness of the superconducting block, taken as the Cu2±Cu2 distance, is smaller in the Re-doped phases than in the undoped compound. At constant Re concentration this distance moderately increases with increasing oxygen content, as it does in undoped YBCO. As pointed out for the Sr-substituted 123 phase [5], the variation of this parameter does not scale with Tc in this type of superconductors. 3.3. Cation ordering The electron diraction studies did not reveal any superlattice spot or diuse scattering for all the explored zones. This indicates that the Re distribution occurs at random, at least within the This correlation length of the technique (300 A). disorder is plausible in view of the low Re content ( 6 10%). High resolution images taken along the [1 0 0] zone axis show a cationic arrangement similar to that observed for the unsubstituted samples. Twinning was detected along the [0 0 1] zone axis by the splitting of all re¯ections except for those along the [1 1 0] row. 3.4. Electronic and transport properties Table 3 shows the resistive and diamagnetic characteristics of the samples as a function of x and w. The critical temperature, Tc , is reported either, as the zero-resistance temperature, Tc (R), or as the diamagnetic onset temperature under an applied ®eld of 0.05 Oe, Tc (Dm). The value q(0) represents the extrapolated value at T 0 K of the linear portion of the high temperature resistivity vs temperature curves, q(T). The transition width, DTc , is de®ned as the full width at half maximum
(FWHM) of the ®rst derivative of the q(T) curves, dq/dT. The % diamagnetic shielding is measured at 77 K and 0.05 Oe. All these data are consistent with the results described in the previous sections. By increasing the Re content the quantity of oxygen necessary for obtaining the maximum Tc increases, even though the corresponding average Cu valence remains constant, within the titration accuracy. This is consistent with the neutron diffraction results, which indicate that most of the extra oxygen introduced by annealing increases the Cu1 coordination. On the other hand, the small quantities of Re which can enter the phase do not signi®cantly perturb the structure, thus Tc of the optimally doped samples remains as high as P 91 K for x 6 0:1. With respect to the extrinsic characteristics of the samples, we can make the following comments: the samples with maximum Tc (at constant x 6 0:1) also exhibit the minimum DTc (2±3 K) and the maximum diamagnetic shielding ( P 89%). In this case the values of Tc (R) and Tc (Dm) are obviously close to each other. Significant dierence between the two Tc values are instead observed when the resistive transitions are broad (large DTc ). The x 0:15 and x 0:10 samples of the three ``R'', ``A'' and ``S'' series, have identical Tc (Dm) and dierent Tc (R). We attribute this to the fact that the nominal x 0:15 sample is not a single phase. The eective composition of the superconducting 123-phase, responsible for the diamagnetic onset, is dierent from the nominal one and quite close to that of the x 0:1 sample (Table 1). The q vs T curves are shown in Fig. 1. All samples exhibit a metallic q vs T behaviour at T > Tc , with the exception of the poorly oxygenated x 0:10 and x 0:15i samples (``R'' curves in Fig.1c and d, respectively). The q(T) curve for the x 0:1 sample moderately ¯attens out and that of x 0:15i sample has a negative curvature before the transition. These features indicate the proximity to an insulating localized state. The q(0) value increases with increasing x, according to the MathiessenÕs rule. The q(0) dependence on x is linear within one series of samples and the dq(0)/ dx rate increases with increasing w (``R''<``A''< ``S''). Consistently with the MathiessenÕs rule, the ``A''-type, x 0 sample, which has no disorder
F. Licci et al. / Physica C 355 (2001) 267±277
275
Fig. 1. Resistivity vs temperature of Cu1 x Rex Ba2 YCu2 Ow as a function of x and the annealing conditions.
at the cation sites and the optimum oxygen doping, has q 0 0 within the experimental uncertainty. At ®xed x values, q(0) is minimum in the ``A'' series and increases on going to ``R'' and ``S'' samples. The increase of q(0) in the ``R'' samples is consistent with the degradation of the metallic character and of the diamagnetic properties and can be understood in terms of non-optimum oxygen doping (Table 3). The ``S'' samples, instead, have the same or slightly higher oxygen content and comparable or better Tc and diamagnetic properties than the corresponding ``A'' materials (Table 3). We hypothesize that the increase of q(0) in this case is due to an oxygen disorder occurring in a length scale (of the order of the mean free dierent from that aecting path, > = 100 A) Tc (of the order of coherence length, n, 10 A). Indeed, the ``S'' samples were treated at 480°C, under 30 atm oxygen, for 10±12 h. These conditions favor the oxygen uptaking, but the annealing period, 10 times shorter than that used for the ``A'' samples, seems to be not sucient to induce a complete oxygen ordering. Due to the limited mechanical resistance of the ampoule, attempts to prolong the annealing time were not successful. Fig. 2 shows typical ac susceptibility vs T curves for Cu0:95 Re0:05 Ba2 YCu2 O7:02 . (x 0:05 ``A'' sam-
ple). The shape analysis of the dissipative part for the v vs T curves at dierent H indicates that the grain coupling deteriorates for increasing Re concentration according to what can be deduced from the DTc and Tc (R) oset data. Typically, the v0 and v00 curves exhibit double transitions, which become more evident with increasing the magnetic ®eld values. The ®rst transition occurring at higher temperature corresponds to the intra-grain properties and is almost independent of H. In Fig. 3 the temperatures corresponding to the maximum of the v00 curves (Tp ) are plotted against H, for both the intra-granular and the inter-granular transitions. Tp decreases linearly with increasing H. This is consistent with what one would expect on the basis of a critical state model [29]. Tp has the same dependence on H as jc , even though the absolute values are aected by many extrinsic factors, such as the sample dimensions and granulometry. In our experiments, however, the specimen size was kept constant and the grain shape and dimension, as estimated from the SEM micrographs, were found to be similar (10 lm). It seems reasonable to deduce that, independently from the secondary phases and the intergrain coupling, the jc value of the Re-doped phases is intrinsically lower than that of the undoped YBCO, and decreases with increasing Re concentration.
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Fig. 3. Temperature at the maximum of the v00 peak of Cu1 x Rex Ba2 YCu2 Ow vs the applied ®eld. a and b refer to intragrain and inter-grain characteristics, respectively. The lines are guides for the eyes.
Cu in YBCO is similar to that of other high-valent elements [30]. 4. Conclusions
Fig. 2. ac susceptibility vs temperature of ``A''-type Cu0:95 Re0:05 Ba2 YCu2 Ow as a function of the applied ®eld: a and b refer to the real, v0 , and imaginary, v00 , components, respectively.
The Cu cations in the reservoir blocks of CuBa2 YCu2 Ow can be substituted by Re up to 10%. The neutron diraction data indicate that Re enters the 123 structure as Re6 and is coordinated to six oxygen atoms (2O1, 2O4, and 2O5) located at the vertices of an octahedron. The occupancy of both the available oxygen sites in the Cu1 plane induces an additional strain in the Ba layers. This is probably responsible for the phase unstability when the Re concentration exceeds 10%. The main structural modi®cations induced by the Re doping are the reduction of the orthorhombicity and
SQUID-measured Tirr and jc values of selected undoped and Re-doped samples are reported in Table 6. Both quantities result to be depressed by the presence of Re. Under an applied ®eld of 5 T, jc for the x 0:1 sample is approximately 30% of jc of the undoped sample at 5 K, and about 6% at 50 K. The irreversibility temperature under comparable ®eld also decreases with the presence of Re. In this respect the eect of the Re substitution for Table 6 Tirr and jc of dierent samples Sample
Tirr (1 T) (K)
Tirr (5 T) (K)
jc (5 T, 5 K) (A/cm2 )
jc (5 T, 50 K) (A/cm2 )
CuBa2 YCu2 O6:91 (x 0 ``R'') Cu0:95 Re0:05 Ba2 YCu2 O6:94 (x 0:05 ``R'') Cu0:9 Re0:1 Ba2 YCu2 O7:08 (x 0:10 ``A'')
85 77 77
70 42 41
61 500
1200
23 300
68
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a moderate increase of the c axis and cell volume. No Re/Cu ordering was observed by electron diraction. Tc remains as high as 91 K at the optimum oxygen doping, while q(0), linearly increases with increasing x, according to the MathiessenÕs rule. Hirr and jc for Re-doped samples are lower than those of the undoped compound. Such changes are comparable to those observed in the Re-substituted Hg cuprates at temperatures higher than 70 and 80 K [14,16]. The improvement of Hirr , observed in the latter phases at lower temperature, is not observed for the Redoped 123 superconductors. This dierence can be explained by the low solubility of Re in the 123 structure, which is not sucient to induce any signi®cant microstructural changes and cation ordering. Acknowledgements The authors thank T. Besagni and P. Ferro for technical assistance, and F. Bolzoni and G. Guaglio for SQUID measurements. The neutron diffraction experiments at ILL were carried out with the help of T. Hansen. This work is sponsored by the 5% Government Project ``Applicazioni della Superconduttivit a ad alta Tc ''. References [1] J.D. Jorgensen, D.G. Hinks, O. Chmaissem, D.N. Argyriou, J.F. Mitchell, B. Dabrowski, in: J. Klamut, B.W. Veal, B.M. Dabrowski, P.W. Klamut, M. Kazimierski (Eds.), Recent developments in high temperature superconductivity, Springer, Berlin, 1996, p. 1. [2] O. Chmaissen, J.D. Jorgensen, K. Yamaura, Z. Hiroi, M. Takano, J. Shimoyama, K. Kishio, Phys. Rev. B 53 (1996) 14647. [3] J.L. Tallon, C. Bernhard, Ch. Niedermayer, J. Shimoyama, S. Hahakura, K. Yamaura, Z. Hiroi, M. Takano, K. Kishio, J. Low Temp. Phys. 105 (1996) 1379. [4] A.M. Abakumov, V.L. Aksenov, V.A. Alyoshin, E.V. Antipov, A.M. Balagurov, D.A. Mikhailova, S.N. Putilin, M.G. Rozova, Appl. Phys. Lett. 80 (1998) 385. [5] F. Licci, A. Gauzzi, M. Marezio, P. Radaelli, R. Masini, C. Chaillout-Bougerol, Phys. Rev. B 58 (1998) 15208.
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www.elsevier.nl/locate/physc
Eects of Re substitution on the structure and superconductivity of Cu1 x Rex Ba2YCu2Ow F. Licci a,*, A. Gauzzi a, M. Marezio a, Q. Huang b,c, A. Santoro b, R. Masini d, C. Bougerol-Chaillout e, P. Bordet e a
Istituto Materiali Speciali per Elettronica e Magnetismo del Consiglio Nazionale delle Ricerche, Parco Area delle Scienze 37A, 43010 Fontanini-Parma, Italy b Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA c Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742, USA d Istituto per la Tecnologia dei Materiali e Processi Energetici del Consiglio Nazionale delle Ricerche, Via R.Cozzi, 53, 20125 Milano, Italy e Laboratoire de Cristallographie, Centre National de la Recherche Scienti®que-Universit e Joseph Fourier, B.P. 166, 38042 Grenoble Cedex 9, France Received 14 November 2000; accepted 15 January 2001
Abstract Samples of Cu1 x Rex Ba2 YCu2 Ow , with nominal x 0, 0.05, 0.1 and 0.15, and 6:91 6 w 6 7:17 were prepared and characterized. The aim was to evaluate the possibility of improving the transport and magnetic properties of CuBa2 YCu2 Ow (YBCO) by substituting Cu in the reservoir blocks (Cu1) with a high-valent element such as Re. X-ray powder diraction and energy dispersive spectroscopy (EDS) studies indicated that Re substitutes for Cu1 up to Such an impurity was identi®ed x 0:1, beyond which it segregates, mostly by forming a cubic phase with a 8:275 A. as YBa2 ReO6 on the basis of the powder x-ray and neutron diraction data and EDS microanalysis. The crystal structure of Cu1 x Rex Ba2 YCu2 Ow was re®ned by the Rietveld analysis based on powder neutron diraction data. It is depending on x and w. The Re cations orthorhombic (Pmmm space group), with a, b and c (3.84, 3.88 and 11.70 A) were found to occupy the (0 0 0) position and to be surrounded by 6 oxygen atoms at the vertices of an octahedron (2O1, 2O4 and 2O5). Such units do not signi®cantly perturb the YBCO structure. No ordering between Cu and Re was observed by electron diraction. The resistive and magnetic measurements indicated that the optimum oxygen doping of the Re-doped YBCO occurs at w > 7. The corresponding Cu average valence is 2:27 1%. Tc of the optimally doped samples is not very sensitive to the Re content and remains as high as 91 K when x 0:1. The residual resistivity extrapolated at T 0 K scales linearly with x, according to the MathiessenÕs rule. SQUID measurements for the x 0:05 and x 0:1 samples indicated that Hirr and jc are lower than those of the undoped phase at any temperature. Ó 2001 Published by Elsevier Science B.V. PACS: 74.25.-q; 74.72.Bk; 74.62.Dh; 61.66.Fn Keywords: Cu-1212 superconductors; Re-substitutions; Structural re®nement; Electrical properties; Magnetic properties
* Correponding author. Tel.: +39-0521-269204; fax: +390521-269206. E-mail address: [email protected] (F. Licci).
0921-4534/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 0 7 8 - 8
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1. Introduction Chemical substitutions in high Tc superconducting cuprates have generated a great interest because they represent an ecient tool for improving the physical properties of these compounds. Even though it has not been always possible to induce an increase of Tc through chemical substitutions, it seems that some of them have a positive eect on other important properties, such as anisotropy, irreversibility ®eld, and critical current density. A review published by Jorgensen et al. [1] singled out the structural features that could improve Tc and jc . They are the shortening and/or the metallization of the reservoir blocks and the presence of suitable defects acting as pinning centers for the ¯ux vortices. A number of groups are presently involved in the search of optimal substituents [2±7]. The Sr-substitution for Ba in HgBa2 Can 1 Cun O2n2d (Hg-12 n 1n) [8,9] and CuBa2 YCu2 Ow (YBCO or 123) [5] systems simulates the eects of the mechanical pressure as it induces the contraction of the unit cell and the decrease of most of the interatomic distances inside the reservoir block. However, it must unsettle other structural parameters responsible for the observed Tc decreases. The halogen substitutions for oxygen in the reservoir blocks improve the metallic character of the Hg or Cu layers in the blocks [4,7]. The substitutions of high-valent elements for Hg or Cu cations increase the metallic character of the blocks themselves [1±3] and generate defects forming clusters of dimensions suitable to act as pinning centers. For example, Cr-substitution for Hg in HgSr2 CuO4 was reported to increase jc [10], together with only a moderate reduction of Tc . Resubstitution for Hg in the Hg-based cuprates was investigated with [2,3,11] or without [12±16] a simultaneous Sr substitution for Ba. The double substitution was expected to improve the superconducting properties by shortening the block thickness and decreasing its resistivity. In the Hg± Ba cuprates Re was reported to reduce the structural anisotropy, to raise the irreversibility ®eld and to improve the ¯ux pinning, at least at temperature lower than 70±80 K. Meng et al. [15] found that the Re substitution for Hg facilitates
the synthesis of the n > 2 members of the Hg12 n 1n homologous series and, furthermore, once formed, the phases are more stable than the undoped ones. These authors also reported that the Re-substitution induces only a moderate decrease of Tc in Hg-1223 (3±4 K). Chmaissem et al. [16] determined the microstructural changes induced by the Re-substitution for Hg in (Hg,Re)Ba2 Can 1 Cun O2n2d , and measured a moderate enhancement of Hirr with Re content at temperatures <70 K. A few reports dealing with the Re substitution in CuSr2 YCu2 O7d (Sr-123) [17±19] indicate that Re stabilizes the phase and makes the synthesis possible under normal pressure, while high-pressure is necessary for obtaining the undoped Sr-123 [20±22]. This suggests that, although the HgBa2 Can 1 Cun O2n2d and CuBa2 YCu2 O7 d systems are quite dierent, they exhibit similar behavior with respect to Sr and high-valent element double substitutions. With the aim of investigating the possibility of improving the transport and magnetic properties of CuBa2 YCu2 Ow without deteriorating Tc , we studied the eect of Re substitution for Cu in the reservoir block of CuBa2 YCu2 Ow (Cu1). We analyzed in detail the phase composition and the structural and physical properties of Cu1 x Rex Ba2 YCu2 Ow at dierent oxygen stoichiometries. We concluded that the Re solubility in YBCO is limited to about 10% of Cu1 and the substitution occurs at random. Tc was found to remain as high as 91 K, but Hirr and jc did not improve. 2. Experimental Cu1 x Rex Ba2 YCu2 Ow samples, with nominal x 0, 0.05, 0.1 and 0.15, were prepared by solidstate reaction of stoichiometric mixtures of 99:99% pure oxides and BaCO3: (Two samples with nominal x 0:15 were prepared in two dierent but identical runs. In the following they will be labelled as x 0:15i and x 0:15ii, respectively). The reagents were mixed and ground under acetone in an agate mortar. The syntheses were carried out in alumina crucibles, in air at 930°C for a total of 80± 100 h, with several intermediate coolings and grindings. The process was stopped when the
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X-ray patterns did not show any dierence after two consecutive treatments. Pellets of 700 mg in weight and 1 cm in diameter were obtained by pressing the powders, sintering in air at 930°C overnight, and cooling down to room temperature at 100°C/ h. We labeled these as-prepared samples as ``R''. For optimizing the oxygen concentration, dierent annealings were carried out at varying oxygen partial pressures. Samples ``A'' were obtained by heating the ``R'' pellets in ¯owing oxygen at 880°C for two hours, cooling down to 480°C at 30°C/h, remaining at this temperature for 50±100 h, and ®nally cooling down to room temperature. Samples ``S'' were obtained from the ``A'' pellets. Two or three of them were put in a quartz ampoule together with a 300 mg pellet of KClO3 . The ampoule (1 cm in diameter, 10 cm long, and 2±3 mm thick) was evacuated to 10 5 torr, sealed and heated up to 480°C for 12 h. The system was then cooled down to room temperature at 100°C/h. The Cu average valence, 2 p, was determined by iodometric titration with amperometric dead-stop end point [5]. The Re content, x, in Cu1 x Rex Ba2 YCu2 Ow was determined by energy dispersive spectroscopy (EDS) analysis, by using a Kevex system connected to a Philips CM microscope operating at 300 kV. Several spurious phases were detected and their stoichiometries were determined. For microstructural investigations (HREM analyses) the samples were crushed in a mortar with alcohol. The suspension was recovered onto an aluminum holey carbon grid. The electron diraction patterns were taken along the main zone axes [0 0 1], [1 1 0], and [1 0 0], in order to check the presence of possible superstructure re¯ections related to a Re/ Cu ordering. The phases were identi®ed by X-ray and neutron diraction. The X-ray patterns were recorded with a D5000 Siemens diractometer in transmission geometry, equipped with CuKa radiation. The impurities were identi®ed by trial and error indexing of those re¯ections which did not belong to the 123 phase. The Rietveld analysis of the Xray patterns was carried out by using the Rietan [23] and Fullprof programs. The powder neutron diraction experiments were performed by using the BT-1 high resolution
269
powder diractometer located at the reactor of the NIST Center for Neutron Research. A Cu (311) monochromator was employed to produce a monochromatic neutron beam of 1.5401(1) A wavelength. Collimators with horizontal divergence of 150 , 200 , and 70 of arc were used before and after the monochromator, and after the samples, respectively. The intensities were measured in steps of 0.05° in the 3±168° 2h range at room temperature. The samples were ``R'' and ``S'' x 0:15ii powders (about 1 g in weight) The neutron diffraction experiments on x 0:10 ``A'' sample were carried out at the ILL, Grenoble, using the D2B and diractometer. The wavelength was 1.5938 A the scans were performed between 2h 10° and 162°, with 0.05° steps. The structural re®nements were carried out with the Rietveld pro®le ®tting analysis using the GSAS program [24]. The neutron scattering amplitudes used for the re®nement were 0.775, 0.525, 0.772, 0.920, and 0:581 10 12 cm for Y, Ba, Cu, Re, and O, respectively. The electrical resistivity and magnetic susceptibility were measured as a function of temperature between 300 and 4.2 K. The ac resistivity was determined by a four-point probe, in a closed-cycle helium cryostat, by applying a pulsed current of 0.1±1 mA. The magnetic susceptibility was measured by a commercial Lake Shore Model 7000 ac susceptometer. A proper combination of sample dimension and maximum frequency was chosen in order to eliminate the eects of the normal-state eddy current near Tc , so that v00 represented only the hysteretic losses. The data were collected at a ®xed frequency of 333 Hz on similar size barshaped specimens. This allowed a meaningful comparison of the measurements. The applied magnetic ®eld, H, ranged from 0.05 to 30 Oe (5 lT to 3 mT). The calibration was performed by using a Gd2 (SO4 )3 H2 O standard and the demagnetization eects associated to the sample shape were taken into account. The critical current density, jc , and the irreversibility line were measured in a commercial RF-SQUID magnetometer achieving 10 8 emu resolution. The irreversibility line, Tirr , was evaluated by comparing the ®eld cooling (FC) and zero®eld cooling (ZFC) temperature scans, taken under an applied ®eld varying from 1 to 5 T. The
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critical current density was estimated on the basis of the Bean model and by assuming independent grains. The magnetic hysteresis was measured at 0.1, 1, 3 and 5 T. The average grain size and shape, which are required to analyze the data within this model, were estimated from scanning electron microscopy (SEM) observations. 3. Results and discussion 3.1. Phase analysis and stoichiometry Except for the nominal x 0:1 sample, the Resubstituted compounds were found to contain minor amounts of secondary phases, whose composition and concentration depended on the nominal x. The results relevant to the sample purity are summarized in Table 1. The Cu0:9 Re0:1 Ba2 YCu2 Ow sample was single phase and the stoichiometry measured by EDS coincided with the nominal one. The Re concentration measured for the nominal x 0:05 sample was equal to 0.06(1). This value is within one standard deviation with respect to the nominal one, but it is consistent with the results of the EDS microanalysis and X-ray diraction, which evidenced the presence of Refree impurities. Samples with the nominal x 0:15 composition were found to contain secondary Baand Re-based phases and the concentration of Re in the 123 phase was lower than the nominal. The main impurity (about 15% in weight) was identi®ed as YBa2 ReO6 on the basis of the X-ray diffraction data. To our knowledge such a compound has not been reported previously in the literature nor in the JCPD ®les. Its identi®cation was based
on the data of Baud and Capestani [25] who described an isostructural phase containing Yb, or most of the other rare-earth elements, but not Y. The YBa2 ReO6 stoichiometry was con®rmed by EDS and neutron diraction experiments. A preliminary re®nement of the structure, based on Xray and neutron diraction data was performed and the results are summarized in Table 2. The phase was found to be cubic with the Fm3m space exhibited group. The a lattice parameter (8.27 A) a moderate (<0.1%) dependence on the oxygen partial pressure during annealing. The average Cu valence and the oxygen stoichiometry in Cu1 x Rex Ba2 YCu2 Ow are reported in Table 3 as a function of the cation stoichiometry and the oxygen partial pressure during the postsynthesis annealing. Possible interference by Re in the Cu titration was ruled out by carrying out ``blank'' tests. It should be pointed out that in the presence of impurity phases the measured 2 p valences could be aected by errors larger than the standard deviation typical of the method (<1%). By taking into account the impurities and their compositions it was estimated that the eective 2 p valence could dier up to a maximum of 2% from the measured values. The oxygen stoichiometry, wj , was calculated from the average copper valence and by assuming the Re valence to be 6, which is typical for 6-coordinated Re cations [1]. For a few samples the oxygen stoichiometry, wn , was also determined from the structural re®nements based on neutron diraction data. It was assumed that wn is equal to 6 nO4 nO5 ), where nO4 and nO5 are the occupancies of the 0; 1=2; 0 and 1=2; 0; 0 site, respectively. The wj and wn values corresponding to the same
Table 1 Phase analysis and stoichiometry of Cu1 x Rex Ba2 YCu2 Ow Nominal x
0.05 0.10 0.15i 0.15ii 0.15ii a
Number of EDS examined grains
Number of homogeneous 123 grains
Average measured x in 123 phase
Secondary phases (number of grains) YBa2 ReO6
Y2 BaCuO5
BaCuO2
CuO
12 10 10 10 ±a
9 10 8 6 76%
0.06(1) 0.10(1) 0.11(2) 0.09(2)
Absent Absent 2 2 15%
1 Absent Absent 1 6%
1 Absent Absent 1 2%
1 Absent Absent Absent Absent
Values shown in this row were obtained by X-ray diraction data re®nement (see text).
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Table 2 Structure and composition of the main impurity YBa2 ReO6 Fm3m 2583.116 566.5 (average) 7.573
Nominal composition Space group Unit cell formula weight 3 ) Volume (A Density (g cm 3 )
a Parameter (A) Atom position and thermal 2 ) parameter, B (A
Ba (1/4,1/4,1/4) Y (1/2,1/2,1/2) Re (0,0,0) O (1/4,0,0)
``R'' sample
``S'' sample
8.2740(4)
8.2756(4)
1.13(17) 0.59(11) 0.59(11) 2.19(12)
0.81(18) 0.48(13) 0.48(13) 2.25(14)
Table 3 Average copper valence (2 p), oxygen stoichiometry (wj and wn ), Tc , DTc , resistivity and % diamagnetic shielding of Cu1 x Rex Ba2 YCu2 Ow . xnom represents the nominal Re concentration xnom
Annealing
2p
wj
0
R A S R A S R A S R A S R S
2.27 2.31 2.31 2.23 2.28 2.30 2.20 2.26 2.27 2.19 2.23 2.26 2.17 2.23
6.91 6.96 6.96 6.94 7.02 7.04 7.00 7.08 7.09 7.07 7.13 7.17 7.04 7.13
0.05 0.10 0.15i 0.15ii
wn
7.05
7.00 7.13
Tc (R) (K)
DTc (K)
q(0) (mX)
Tc (Dm) (K)
% Shielding (77 K, 0.05 Oe)
93 92.5 91 86 93 92.5 70.5 89 91 60 85 85 85b 91.5b
2 4 5 10 2 3 20 5 2 35 12 10
1.6 0 0.9 3.0 1.6 3.5 3.1 3.1 6.3 33.3 6.9 9.4
93 92 92 88 92 92 91.5 90 91 91.5 90 91
100 97 98 78 96 99 0.6a 84 89 0.3a 66 82
wj was calculated by assuming the Re valence 6. wn values were re®ned from the neutron diraction data. Tc (R) represents the resistive oset (T at R 0). DTc and q(0) represent the FWHM of the ®rst derivative and the extrapolated value at T 0 K of the q(T) curves, respectively. Tc (Dm) is the temperature at the diamagnetic onset. Letters in the third column indicate dierent annealing, as described in the text. a Measured under an applied ®eld of 1 Oe. b T at 50% of the resistive transition.
samples were found to be consistent (within 0:5%), which proves that the 6 valence assumed for Re is correct and the titration error is small. The average Cu valence in the samples of the same series (``R'', ``A'' or ``S'') decreases by increasing Re concentration. It increases, instead, by increasing the oxygen partial pressure at constant Re concentration (``S''>``A''>``R''). The overall oxygen stoichiometry increases by increasing either the Re content and/or the oxygen partial pressure. Inde-
pendently of x and w, the average Cu valence corresponding to the maximum Tc , is practically constant at 2:27 1%. 3.2. Structural re®nements The neutron diraction data of Cu0:9 Re0:1 Ba2 YCu2 Ow (x 0:10 ``A'' sample) and of Cu0:85 Re0:15 Ba2 YCu2 Ow (x 0:15ii ``R'' and ``S'' samples) were used to re®ne the structures. The
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results are summarized in Tables 4 and 5. The positional, thermal and occupancy parameters are reported in Table 4 and compared with those of undoped CuBa2 YCu2 O6:93 [26]. The unit cell parameters and selected interatomic distances are reported in Table 5. The re®nements showed that Re occupies the (0 0 0) site, substituting for Cu, and is surrounded by 6 oxygen atoms in an octahedral con®guration, similar to that observed in the Re-substituted Hg-12 n 1n compounds [2,16]. The vertices of the octahedra are the two apical O1 oxygen atoms while two oxygen atoms (O4) at 0; 1=2; 0 and two (O5) at 1=2; 0; 0 form
the base. Dierently from what observed in the Hg-cuprates, the Re cations do not induce any appreciable distortion of the 123 structure. No displacements of the oxygen atoms from the original positions were detected. In the x 0:15ii ``R'' sample the O5 occupancy corresponds to approximately twice that of Re (as deduced from EDS analysis, Table 1), that is 0.19(2) vs 0.09, respectively. After annealing in oxygen (x 0:15ii ``S'' sample) nO5 remains constant within the experimental uncertainty (i.e. 0.23(2)), while w and nO4 increase (from 7.04 to 7.13 and from 0.81(2) to 0.90(2), respectively). These data are consistent
Table 4 Position, thermal and occupancy parameters of Cu0:9 Re0:1 Ba2 YCu2 Ow (x 0:10 ``A'' sample) and Cu0:85 Re0:15 Ba2 YCu2 Ow (x 0:15ii ``R'' and ``S'' samples), compared with those of YB2 Cu3 O6:93 (YBCO, from Ref. [26]) Atom Y Ba Cu1/Re
Cu2 O1
O2 O3 O4
O5
YBCO 2 ) B (A z 2 ) B (A 2 ) B (A U11 (A) U22 (A) U33 (A) z 2 ) B (A z 2 ) B (A U11 (A) U22 (A) U33 (A)
z 2 ) B (A z 2 ) B (A 2 ) B (A U11 (A) U22 (A) U33 (A) n 2 ) B (A U11 (A) U22 (A) U33 (A) n Rp Rwp v2
0.28(3) 0.1843(2) 0.44(3) 0.41(3)
0.3556(1) 0.20(2) 0.1590(2) 0.009(1) 0.007(1) 0.010(1) 0.3779(2) 0.51(4) 0.3790(2) 0.35(3) 0.022(3) 0.001(2) 0.019(2) 0.9
0.03(1) 3.33 5.96
x 0:10 ``A''
x 0:15ii ``R''
x 0:15ii ``S''
0.39(4) 0.18522(24) 0.53(6) 0.66 0.015 0.003 0.007 0.35684(15) 0.38 0.15824(23) 1.02
0.71(7) 0.18734(27) 0.48(7) 0.82(7)
0.77(8) 0.18584(30) 0.50(8) 0.67(8)
0.35795(18) 0.32(5) 0.15678(30) 1.89(7)
0.35694(19) 0.25(5) 0.15743(32) 1.29(7)
0.37949(33) 0.51(5) 0.3780(3) 0.51(5) 5.92(30)
0.3787(4) 0.46(5) 0.3785(4) 0.46(5) 6.35(22)
0.809(23) 5.92(30)
0.902(24) 6.35(22)
0.190(17) 5.4 6.88 2.866
0.233(19) 6.07 7.4 2.295
0.37838(25) 0.62(3) 0.37793(26) 0.62(3) 1.14 0.027 0.005 0.011 0.804(8) 0.053 0.027 0.011 0.245(11) 5.45 7.03 3.01
Rietveld re®nements were done in the orthorhombic Pmmm space group. Atom positions are Y(1/2,1/2,1/2), Ba(1/2,1/2,z), Cu1/ Re(0,0,0), Cu2(0,0,z), O1(0,0,z), O2(1/2,0,z), O3(0,1/2,z), O4(0,1/2,0), and O5(1/2,0,0). Numbers in parentheses are statistical standard deviations of the last signi®cant digit. When not indicated the occupancy factor, n, was equal to 1. The data relevant to Cu0:9 Re0:1 Ba2 YCu2 Ow were obtained from the neutron diraction experiments carried out at D2B facility at ILL, Grenoble.
F. Licci et al. / Physica C 355 (2001) 267±277
273
Table 5 of Cu1 x Rex Ba2 YCu2 Ow , compared with those of YBa2 Cu3 O6:93 (YBCO, Unit cell parameters and selected interatomic distances (A) from Ref. [26]) a b c 2 b a= b a 103 V Ba±O1 Ba±O2 Ba±O3 Ba±O4 Ba±O5 Y±O2 Y±O3 Cu1/Re±O1 Cu1/Re±O4 Cu1/Re±O5 Cu2±O1 Cu2±O2 Cu2±O3 Cu2±Cu2
YBCO
x 0:10 ``A''
x 0:15ii ``R''
x 0:15ii ``S''
3.8227(1) 3.8872(2) 11.6802(2) 16.7 173.56 2.7418 2.9803(4) 2.9707(2) 2.8787(4) 2.9002(4) 2.4107(2) 2.3771(2) 1.8571 1.9436(2) 1.91135(10) 2.29632 1.9290 1.9627 3.3732
3.84192(5) 3.87655(5) 11.69134(19) 9.0 174.12 2.7471 2.9764 2.9607 2.8945 2.9060 2.4037 2.3933 1.8496 1.9382 1.9209 2.3219 1.9375 1.9539 3.3475(1)
3.84602(12) 3.87911(12) 11.73153(38) 8.6 175.024(13) 2.7547(6) 2.974(4) 2.950(4) 2.9203(24) 2.9312(24) 2.4001(23) 2.3972(24) 1.839(4) 1.93956(6) 1.92301(6) 2.360(4) 1.9395(6) 1.9538(6) 3.3329(2)
3.84411(13) 3.87984(14) 11.70437(41) 9.2 174.565(14) 2.7510(6) 2.976(4) 2.963(5) 2.9026(27) 2.9145(26) 2.4042(25) 2.3907(26) 1.843(4) 1.93992(7) 1.92205(7) 2.335(4) 1.9388(6) 1.9563(6) 3.3488(2)
The data relevant to the x 0:10 ``A'' sample were collected on the D2B facility at ILL.
with the assumption that the additional oxygen goes to the O4 positions around the (0 0 0) sites occupied by the Cu cations. The O4 sites are preferentially depleted at low oxygen concentration because the Re±O bonds are stronger than the Cu1±O bonds. In the structure of YBCO there are two empty O5 sites per unit formula, thus all Cu atoms in the reservoir block could in principle be replaced by Re with full occupation of the O5 sites. However, the samples with more than 10% Re could not be synthesized as a single phase. The low solubility of Re in the 123 structure may be explained by the fact that the structure can not sustain a large O5 occupancy. The increase of this occupancy induces an increase of the Ba layer strain. A tentative evaluation of the bond valence sum of Ba in the x 0:15ii ``S'' sample nO5 0:23 2 gives a value of 2.19, which suggests an overstrained arrangement [27]. This hypothesis is corroborated by the fact that Re is more soluble in Sr-123 than in Ba123 [17,28]. Single-phase Cu1 x Rex Sr2 YCu2 Ow samples were obtained with x as high as 0.3. The bond valence sum of the Sr sites in Cu0:85 Re0:15 Sr2 YCu2 O7:44 , calculated from the re®ned neutron
data, was found to be 1.78 [28]. In this case the increase of the O5 occupancy seems to play a positive role by reducing the 1=2; 1=2; z site dimension, which is too large for Sr [22]. When compared with the undoped phase [26], the Re-doped YBCO does not exhibit signi®cant dierences as far as the structural features are concerned. These dierences can be summarized as follows. (1) As a consequence of the occupation of the O5 sites the Re-substituted compounds result in being less orthorhombic than the undoped one. The orthorhombicity parameter, S 2 b a= b a, for Cu1 x Rex Ba2 YCu2 Ow are quoted in Table 5 at dierent x and w values. (2) Qualitatively the c axis increases in the Redoped phases. A quantitative calculation of the increasing rate is not straightforward, due to the fact the oxygen concentration is not constant in the examined samples. When normalized to the c axis value, the Cu2±O1 distance increases and the Cu1±O1 decreases in the Re-doped compounds. This means that the incorporation of Re displaces the apical O1 away from the superconducting CuO2 plane. According to Jorgensen et al. [1], such
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a displacement generally reduces the tilting angle of the CuO2 squares in the superconducting layers and increases Tc , provided that the oxygen doping remains at optimum level. However these features were not observed in the Re-doped123 phase. (3) The Ba cation moves toward the Cu2±O layer and away from O4 and O5, as indicated by the increase of the zBa parameter. This is probably a consequence of the fact that O4 and O5 are strongly bonded to Re and would become overbonded with short Ba±O4 and Ba±O5 distances. (4) The thickness of the superconducting block, taken as the Cu2±Cu2 distance, is smaller in the Re-doped phases than in the undoped compound. At constant Re concentration this distance moderately increases with increasing oxygen content, as it does in undoped YBCO. As pointed out for the Sr-substituted 123 phase [5], the variation of this parameter does not scale with Tc in this type of superconductors. 3.3. Cation ordering The electron diraction studies did not reveal any superlattice spot or diuse scattering for all the explored zones. This indicates that the Re distribution occurs at random, at least within the This correlation length of the technique (300 A). disorder is plausible in view of the low Re content ( 6 10%). High resolution images taken along the [1 0 0] zone axis show a cationic arrangement similar to that observed for the unsubstituted samples. Twinning was detected along the [0 0 1] zone axis by the splitting of all re¯ections except for those along the [1 1 0] row. 3.4. Electronic and transport properties Table 3 shows the resistive and diamagnetic characteristics of the samples as a function of x and w. The critical temperature, Tc , is reported either, as the zero-resistance temperature, Tc (R), or as the diamagnetic onset temperature under an applied ®eld of 0.05 Oe, Tc (Dm). The value q(0) represents the extrapolated value at T 0 K of the linear portion of the high temperature resistivity vs temperature curves, q(T). The transition width, DTc , is de®ned as the full width at half maximum
(FWHM) of the ®rst derivative of the q(T) curves, dq/dT. The % diamagnetic shielding is measured at 77 K and 0.05 Oe. All these data are consistent with the results described in the previous sections. By increasing the Re content the quantity of oxygen necessary for obtaining the maximum Tc increases, even though the corresponding average Cu valence remains constant, within the titration accuracy. This is consistent with the neutron diffraction results, which indicate that most of the extra oxygen introduced by annealing increases the Cu1 coordination. On the other hand, the small quantities of Re which can enter the phase do not signi®cantly perturb the structure, thus Tc of the optimally doped samples remains as high as P 91 K for x 6 0:1. With respect to the extrinsic characteristics of the samples, we can make the following comments: the samples with maximum Tc (at constant x 6 0:1) also exhibit the minimum DTc (2±3 K) and the maximum diamagnetic shielding ( P 89%). In this case the values of Tc (R) and Tc (Dm) are obviously close to each other. Significant dierence between the two Tc values are instead observed when the resistive transitions are broad (large DTc ). The x 0:15 and x 0:10 samples of the three ``R'', ``A'' and ``S'' series, have identical Tc (Dm) and dierent Tc (R). We attribute this to the fact that the nominal x 0:15 sample is not a single phase. The eective composition of the superconducting 123-phase, responsible for the diamagnetic onset, is dierent from the nominal one and quite close to that of the x 0:1 sample (Table 1). The q vs T curves are shown in Fig. 1. All samples exhibit a metallic q vs T behaviour at T > Tc , with the exception of the poorly oxygenated x 0:10 and x 0:15i samples (``R'' curves in Fig.1c and d, respectively). The q(T) curve for the x 0:1 sample moderately ¯attens out and that of x 0:15i sample has a negative curvature before the transition. These features indicate the proximity to an insulating localized state. The q(0) value increases with increasing x, according to the MathiessenÕs rule. The q(0) dependence on x is linear within one series of samples and the dq(0)/ dx rate increases with increasing w (``R''<``A''< ``S''). Consistently with the MathiessenÕs rule, the ``A''-type, x 0 sample, which has no disorder
F. Licci et al. / Physica C 355 (2001) 267±277
275
Fig. 1. Resistivity vs temperature of Cu1 x Rex Ba2 YCu2 Ow as a function of x and the annealing conditions.
at the cation sites and the optimum oxygen doping, has q 0 0 within the experimental uncertainty. At ®xed x values, q(0) is minimum in the ``A'' series and increases on going to ``R'' and ``S'' samples. The increase of q(0) in the ``R'' samples is consistent with the degradation of the metallic character and of the diamagnetic properties and can be understood in terms of non-optimum oxygen doping (Table 3). The ``S'' samples, instead, have the same or slightly higher oxygen content and comparable or better Tc and diamagnetic properties than the corresponding ``A'' materials (Table 3). We hypothesize that the increase of q(0) in this case is due to an oxygen disorder occurring in a length scale (of the order of the mean free dierent from that aecting path, > = 100 A) Tc (of the order of coherence length, n, 10 A). Indeed, the ``S'' samples were treated at 480°C, under 30 atm oxygen, for 10±12 h. These conditions favor the oxygen uptaking, but the annealing period, 10 times shorter than that used for the ``A'' samples, seems to be not sucient to induce a complete oxygen ordering. Due to the limited mechanical resistance of the ampoule, attempts to prolong the annealing time were not successful. Fig. 2 shows typical ac susceptibility vs T curves for Cu0:95 Re0:05 Ba2 YCu2 O7:02 . (x 0:05 ``A'' sam-
ple). The shape analysis of the dissipative part for the v vs T curves at dierent H indicates that the grain coupling deteriorates for increasing Re concentration according to what can be deduced from the DTc and Tc (R) oset data. Typically, the v0 and v00 curves exhibit double transitions, which become more evident with increasing the magnetic ®eld values. The ®rst transition occurring at higher temperature corresponds to the intra-grain properties and is almost independent of H. In Fig. 3 the temperatures corresponding to the maximum of the v00 curves (Tp ) are plotted against H, for both the intra-granular and the inter-granular transitions. Tp decreases linearly with increasing H. This is consistent with what one would expect on the basis of a critical state model [29]. Tp has the same dependence on H as jc , even though the absolute values are aected by many extrinsic factors, such as the sample dimensions and granulometry. In our experiments, however, the specimen size was kept constant and the grain shape and dimension, as estimated from the SEM micrographs, were found to be similar (10 lm). It seems reasonable to deduce that, independently from the secondary phases and the intergrain coupling, the jc value of the Re-doped phases is intrinsically lower than that of the undoped YBCO, and decreases with increasing Re concentration.
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F. Licci et al. / Physica C 355 (2001) 267±277
Fig. 3. Temperature at the maximum of the v00 peak of Cu1 x Rex Ba2 YCu2 Ow vs the applied ®eld. a and b refer to intragrain and inter-grain characteristics, respectively. The lines are guides for the eyes.
Cu in YBCO is similar to that of other high-valent elements [30]. 4. Conclusions
Fig. 2. ac susceptibility vs temperature of ``A''-type Cu0:95 Re0:05 Ba2 YCu2 Ow as a function of the applied ®eld: a and b refer to the real, v0 , and imaginary, v00 , components, respectively.
The Cu cations in the reservoir blocks of CuBa2 YCu2 Ow can be substituted by Re up to 10%. The neutron diraction data indicate that Re enters the 123 structure as Re6 and is coordinated to six oxygen atoms (2O1, 2O4, and 2O5) located at the vertices of an octahedron. The occupancy of both the available oxygen sites in the Cu1 plane induces an additional strain in the Ba layers. This is probably responsible for the phase unstability when the Re concentration exceeds 10%. The main structural modi®cations induced by the Re doping are the reduction of the orthorhombicity and
SQUID-measured Tirr and jc values of selected undoped and Re-doped samples are reported in Table 6. Both quantities result to be depressed by the presence of Re. Under an applied ®eld of 5 T, jc for the x 0:1 sample is approximately 30% of jc of the undoped sample at 5 K, and about 6% at 50 K. The irreversibility temperature under comparable ®eld also decreases with the presence of Re. In this respect the eect of the Re substitution for Table 6 Tirr and jc of dierent samples Sample
Tirr (1 T) (K)
Tirr (5 T) (K)
jc (5 T, 5 K) (A/cm2 )
jc (5 T, 50 K) (A/cm2 )
CuBa2 YCu2 O6:91 (x 0 ``R'') Cu0:95 Re0:05 Ba2 YCu2 O6:94 (x 0:05 ``R'') Cu0:9 Re0:1 Ba2 YCu2 O7:08 (x 0:10 ``A'')
85 77 77
70 42 41
61 500
1200
23 300
68
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a moderate increase of the c axis and cell volume. No Re/Cu ordering was observed by electron diraction. Tc remains as high as 91 K at the optimum oxygen doping, while q(0), linearly increases with increasing x, according to the MathiessenÕs rule. Hirr and jc for Re-doped samples are lower than those of the undoped compound. Such changes are comparable to those observed in the Re-substituted Hg cuprates at temperatures higher than 70 and 80 K [14,16]. The improvement of Hirr , observed in the latter phases at lower temperature, is not observed for the Redoped 123 superconductors. This dierence can be explained by the low solubility of Re in the 123 structure, which is not sucient to induce any signi®cant microstructural changes and cation ordering. Acknowledgements The authors thank T. Besagni and P. Ferro for technical assistance, and F. Bolzoni and G. Guaglio for SQUID measurements. The neutron diffraction experiments at ILL were carried out with the help of T. Hansen. This work is sponsored by the 5% Government Project ``Applicazioni della Superconduttivit a ad alta Tc ''. References [1] J.D. Jorgensen, D.G. Hinks, O. Chmaissem, D.N. Argyriou, J.F. Mitchell, B. Dabrowski, in: J. Klamut, B.W. Veal, B.M. Dabrowski, P.W. Klamut, M. Kazimierski (Eds.), Recent developments in high temperature superconductivity, Springer, Berlin, 1996, p. 1. [2] O. Chmaissen, J.D. Jorgensen, K. Yamaura, Z. Hiroi, M. Takano, J. Shimoyama, K. Kishio, Phys. Rev. B 53 (1996) 14647. [3] J.L. Tallon, C. Bernhard, Ch. Niedermayer, J. Shimoyama, S. Hahakura, K. Yamaura, Z. Hiroi, M. Takano, K. Kishio, J. Low Temp. Phys. 105 (1996) 1379. [4] A.M. Abakumov, V.L. Aksenov, V.A. Alyoshin, E.V. Antipov, A.M. Balagurov, D.A. Mikhailova, S.N. Putilin, M.G. Rozova, Appl. Phys. Lett. 80 (1998) 385. [5] F. Licci, A. Gauzzi, M. Marezio, P. Radaelli, R. Masini, C. Chaillout-Bougerol, Phys. Rev. B 58 (1998) 15208.
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