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Oxygen concentration dependence of superconductivity in Pr-substituted Ln1−xPrxBa2Cu3O7−y (Ln = Y, Nd, Gd)
PHYSICA ELSEVIER
Physica C 263 (1996) 336-339
Oxygen concentration dependence of superconductivity in Pr-substituted Lnl_xPrxBa2Cu307_y(Ln = Y, Nd, Gd) Kuniyuki Koyama *, Takashi Tange, Yasuyuki Kitamura, Takahito Saito, Kiyoshi Mizuno Faculty of Integrated Arts and Sciences. The University of Tokushima, Minamijosanjima, Tokushima 770, Japan
Abstract The Pr content x dependence of the superconducting transition temperature Tc is examined for various oxygen concentrations by quenching the samples of the Pr-substituted Ln~_ ~PrxBa2Cu307_y, where Ln is Y, Nd and Gd. In the case of Ln = Y, the initial rate of decrease of T~ by Pr substitution ( - d T c / d x ) decreases with decreasing oxygen concentration for 6.6 < 7 - y < 7.0, then becomes constant for 7 - y < 6.6. In contrast, the effect of Tc-suppression by Pr in the case of Ln = Nd does not depend upon the oxygen concentration, and is much stronger than that in the case of Ln = Y. In the case of Ln = Gd, intermediate behavior between those of Y and Nd is observed, corresponding to the ionic size of Ln.
1. Introduction Superconductivity was studied in LnBa2Cu307_y (Ln: rare earth atom) isomorphic to Y B a z C u 3 0 7 _ y and found to be insensitive to the magnetic moments of the rare earth ions. The only exception is PrBa2CusO7_y which is neither metallic nor a superconductor in spite of being of the same orthorhombic structure [1]. Three main pictures are proposed for the mechanism of this suppression o f superconductivity by Pr doping [2]. One is that extra electrons fill the holes in the CuO 2 plane because Pr is substituted as a tetravalent or more than trivalent ion. The second is that the magnetic moment of a trivalent Pr ion suppresses superconductivity through the pairbreaking mechanism [3]. The other is that the hole is
* Corresponding author. Fax: + 81 886 56 7298; e-mail: [email protected].
localized through the hybridization between the local states of the Pr ion and the conduction band states of the CuO 2 planes [4]. The rare earth ionic size effect on the depression of superconductivity by Pr substitution has been studied in the fully oxidized L n B a z C u 3 0 7 _ e series by many researchers [5]. The larger the ionic size difference between Ln and Pr ions, the less Tc decreases and hence the larger the critical Pr content for the destruction of superconductivity. The antiferromagnetic ordering temperature o f Ln ions was conversely related to the Tc [6]. However, the oxygen concentration dependence of the suppression effect of superconductivity by Pr [7] has not yet been examined systematically. The oxygen concentration dependence of Tc was already examined in Yj_xPrxBa2Cu3OT_y [8]. The T~ decreases with decreasing oxygen concentration 7-y in not only Ortho-I (6.8 < 7 - y < 7.0) but also the region between Ortho-II and Tetra (7 - y <
0921-4534/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00752-0
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K. Koyama et al. / Physica C 263 (1996) 336-339
6.6) for the Pr contents of 0 < x < 0.6. However, the Tc conversely increases with decrease of 7 - y for 0.3 £ x < 0.6 in the narrow Ortho-II region (6.6 < 7 - y < 6.8). The Pr content dependence of Tc for Ortho-II is quite different from that for Ortho-I, and is preferably similar to that in stoichiometric Y1- xPrxBa2Cu408 [9]. In this paper, the oxygen concentration dependences of Tc are investigated in Lnl_xPrxBa2Cu 3 O7_y , where Ln is Y, Nd and Gd. The ionic radius of Pr 3÷ is nearly the same as that of Nd 3÷, but larger than those of y3+ and Gd 3+. The effects of ionic size of Ln 3÷ on the suppression of superconductivity by Pr are examined by changing the oxygen concentration.
(a) Yl.xPrxBa2CuzO7.y 100
80
~,~
.
7-y
6O
¢ 4O
20
0.1
0.2
0.3
0.4
0.5
0.7
0.6
Pr content x
(b) Ndl.xPrxBa2Cu3OT.y .
100
2. Experimental
The Lnl_xPrxBa2Cu307_y (Ln = Y, Nd and Gd) samples were prepared through a solid-state reaction between Ln203, Pr6Oj t, BaCO 3 and CuO. The mixed powders were calcined four times at 950°C for 24 h in air, then pressed into pellets and sintered at 930°C (Nd)-950°C (Y, Gd) for 12 h in flowing oxygen, followed by an additional annealing at 350°C for 12 h in order to fully oxidize the samples. The quenching was made as follows: eacii sample was heated up to some quenching temperature Tq (450°C ~ Tq < 750°C) in air and kept for 2 h, after which it was quenched into liquid nitrogen. Oxygen concentration decreases continuously with increasing Tq. The oxygen concentration 7 - y of the singlephase oxides was determined by thermogravimetry, up to about 950°C in 20% hydrogen with argon atmosphere [8]. The reduced powder consisted of Ln203, Pr203, BaO and Cu metal. The oxygen concentration was determined from the weight loss upon reduction. The oxygen concentration was also estimated from the decrease in weight of samples after quenching treatments. 3. Results and discussion All the samples are characterized by X-ray powder diffraction to be single phase of orthorhombic or tetragonal LnBa2Cu307_ r structure. Lattice con-
,
,
,
.
,
.
,
.
7-y
80
~"
,
so
I - ' - 6"65 I --':'--6.60
40
2O
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Pr content x Fig. 1. Pr content x dependence of T¢ in Lnl_xPrxBa2CU3OT_y for various oxygen concentrations: (a) Ln = Y and (b) Ln = Nd.
stants a, b and c increase linearly with increasing Pr content x in the case of Ln = Y and Gd, but are almost constant with change of x in the case of L n = Nd. The ionic radius of Pr 3÷ is nearly the same as that of Nd 3÷, but 12% larger than that of y3+, and 6% larger than that of Gd 3÷. The effect of Pr substitution on lattice constants corresponds to the difference in ionic radii of the Ln ions. The value of T~ is determined as the transition midpoint temperature by the DC four-probe resistivity measurement. The values of midpoint Tc's are plotted against Pr content x for various oxygen concentration ( 7 - y) samples in Fig. 1. Fig. l(a) shows the Pr content x dependence of T~ for the Ln = Y case, and l(b) for the Ln = Nd case. In the case of Ln = Y, the initial rate of decrease of Tc
338
K. Koyama et al. / Physica C 263 (1996) 336-339
with Pr substitution ( - d T c / d x : gradient in Fig. 1) for 0 < x < 0.3 decreases monotonically with decreasing oxygen concentration for 6.6 < 7 - y < 7.0, except for the saturation of T~ above 80 K for 7 - y = 7.0 [8]. The saturation tendency above 80 K is considered as the nonlinear relation between Tc and the hole concentration in the CuO 2 plane. - dTc/d x is almost constant for 7 - y < 6.6. Such a complicated change of T~ is also confirmed by the susceptibility measurement (Meissner effect). In contrast, in the case of Ln = Nd, as shown in Fig. l(b), - d T J d x , which is the measure of the suppression effect of superconductivity by Pr, is almost constant for all the oxygen concentrations as far as superconductivity is observed. The value of - d T c / d x for Ln = Nd is larger than that for Ln = Y. The region where the superconductivity is observed in the case of Ln = Nd is narrower than in the case of Ln = Y, not only as for Pr content [10] but also for oxygen concentration. The gradient of Tc against Pr content in the case of Ln = Y increases abruptly in the range of x > 0.4. This tendency is not observed in the case of Ln = Nd. The two step x-dependence of Tc for Ln = Y is similar to that for the stoichiometric oxide superconductor Yi-xPr~Ba2Cu408 [9]. The radical drop of Tc in the heavily Pr-doped region may be caused by pair breaking by the Pr magnetic moment, while the initial gradient of Tc against x is regarded as hole localization by the hybridization between the local states of the Pr ion and the conduction band states of the CuO 2 planes. Intermediate behavior between those of Y and Nd is observed qualitatively in the case of Ln = Gd. The change of - d T c / d x with the difference in oxygen concentration is also observed in Ln = Gd; however, it is not so remarkable as in the case of Ln = Y. The anomalous increases of Tc with decrease of oxygen concentration in the limited range of oxygen concentration is also observed in the case of Ln = Gd as in the case of Ln = Y. The values of - d T c / d x are plotted in Fig. 2 against oxygen concentration in the cases of Ln = Y, Nd and Gd. The effect of T~-suppression by Pr in the case of Ln = Nd does not depend upon the oxygen concentration, and is much stronger than those in the case of Ln = Y and Ln = Gd. Intermediate behavior between those of Y and Nd is observed qualitatively
300
,
,
~
Ndl.xPrxBazCu3Or.y 250 g
~D
D u IJ C " - -
0O I--°
Gd 1 xPr
150
-
-
100
50
Yl.xPrxBa2Cu3OT.y 0 6.0
6.2
6.4
6.6
6.8
7.0
Oxygen c~ncentration 7-y Fig. 2. Oxygen concentration dependence of - d T c / d x Ln I xPrxBa2Cu307_y , where Ln is Y, Nd and Gd.
in
in the case of Ln = G d . The effect of Tc-suppression by Pr becomes weaker as the ion size of the rare earth ion decreases (Nd ~ Gd ~ Y), or as the difference of rare earth ion size from Pr increases. Furthermore, the effect of To-suppression by Pr becomes greatly dependent on the oxygen concentration with decreasing the rare earth ion size (Nd ~ Gd ~ Y). In the unique case of Ln = Nd, the ion size of which is nearly the same as that of Pr 3÷, the suppression of superconductivity by Pr appears in its simplest form. In the case of the other rare earth ions, the effect of Pr is considered to be rather complicated because of the discrepancy of ion size between Pr and the rare earth ion.
Acknowledgement This work was supported in part by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture.
References [1] A. Kebede, C.S. Jee, J. Schwegler, J.E. Crow, T. Mihalisin, G.H. Myer, M.V. Kurie, S.H. Bloom and R.P. Guertin, Phys. Rev. B 40 (1989) 4453. [2] J.J. Neumeier and M.B. Maple, Physica C 191 (1992) 158.; J.J. Neumeier, T. Bjcmholm, M.B. Maple and I.K. Sehuller, Phys. Rev. Lett. 63 (1989) 2516.
K. Koyama et al./ Physica C 263 (1996) 336-339 [3] J.L. Peng, P. Klavins, R.N. Shelton, H.B. Radousky, P.A. Hahn and L. Bemardez, Phys. Rev. B 40 (1989) 4517. [4] R. Fehrenbacher and T.M. Rice, Phys. Rev. Lett. 70 (1993) 3471. [5] See, for example, W.J. Zhu, P. Liu and Z.X. Zhao, Physica C 199 (1992) 285; E. Orgaz and E. Ruiz-Trejo, Physica C 194 (1992) 76. [6] W.Y. Guan, Y.C. Dhen, J.Y.T. Wei, Y.H. Xu and M.K. Wu, Physica C 209 (1993) 19. [7] D. Wagener, M. Buchgeister, P. Herzog, S.M. Hosseini, P.
339
Seidel, G. Mertler and K. Kopitzki, in: R. Kosowsky et al. (eds.), Physics and Materials Science of High Temperature Superconductors II (Kluwer, Dorhrecht, 1992) p. 247. [8] K. Koyama, Y. Kitamura, T. Saito and K. Mizuno, Physica C 235-240 (1994) 1469. [9] K. Koyama, S. Taga and S. Noguchi, Physica C 185-189 (1991) 771; S. Adachi, N. Watanabe, S. Seiji, N. Koshizuka and H. Yamanchi, Physica C 207 (1993) 127. [10] K. Koyama, Y. Suzuki and S. Noguchi, Jpn. J. Appl. Phys. 27 (1988) 1862.
Physica C 263 (1996) 336-339
Oxygen concentration dependence of superconductivity in Pr-substituted Lnl_xPrxBa2Cu307_y(Ln = Y, Nd, Gd) Kuniyuki Koyama *, Takashi Tange, Yasuyuki Kitamura, Takahito Saito, Kiyoshi Mizuno Faculty of Integrated Arts and Sciences. The University of Tokushima, Minamijosanjima, Tokushima 770, Japan
Abstract The Pr content x dependence of the superconducting transition temperature Tc is examined for various oxygen concentrations by quenching the samples of the Pr-substituted Ln~_ ~PrxBa2Cu307_y, where Ln is Y, Nd and Gd. In the case of Ln = Y, the initial rate of decrease of T~ by Pr substitution ( - d T c / d x ) decreases with decreasing oxygen concentration for 6.6 < 7 - y < 7.0, then becomes constant for 7 - y < 6.6. In contrast, the effect of Tc-suppression by Pr in the case of Ln = Nd does not depend upon the oxygen concentration, and is much stronger than that in the case of Ln = Y. In the case of Ln = Gd, intermediate behavior between those of Y and Nd is observed, corresponding to the ionic size of Ln.
1. Introduction Superconductivity was studied in LnBa2Cu307_y (Ln: rare earth atom) isomorphic to Y B a z C u 3 0 7 _ y and found to be insensitive to the magnetic moments of the rare earth ions. The only exception is PrBa2CusO7_y which is neither metallic nor a superconductor in spite of being of the same orthorhombic structure [1]. Three main pictures are proposed for the mechanism of this suppression o f superconductivity by Pr doping [2]. One is that extra electrons fill the holes in the CuO 2 plane because Pr is substituted as a tetravalent or more than trivalent ion. The second is that the magnetic moment of a trivalent Pr ion suppresses superconductivity through the pairbreaking mechanism [3]. The other is that the hole is
* Corresponding author. Fax: + 81 886 56 7298; e-mail: [email protected].
localized through the hybridization between the local states of the Pr ion and the conduction band states of the CuO 2 planes [4]. The rare earth ionic size effect on the depression of superconductivity by Pr substitution has been studied in the fully oxidized L n B a z C u 3 0 7 _ e series by many researchers [5]. The larger the ionic size difference between Ln and Pr ions, the less Tc decreases and hence the larger the critical Pr content for the destruction of superconductivity. The antiferromagnetic ordering temperature o f Ln ions was conversely related to the Tc [6]. However, the oxygen concentration dependence of the suppression effect of superconductivity by Pr [7] has not yet been examined systematically. The oxygen concentration dependence of Tc was already examined in Yj_xPrxBa2Cu3OT_y [8]. The T~ decreases with decreasing oxygen concentration 7-y in not only Ortho-I (6.8 < 7 - y < 7.0) but also the region between Ortho-II and Tetra (7 - y <
0921-4534/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00752-0
337
K. Koyama et al. / Physica C 263 (1996) 336-339
6.6) for the Pr contents of 0 < x < 0.6. However, the Tc conversely increases with decrease of 7 - y for 0.3 £ x < 0.6 in the narrow Ortho-II region (6.6 < 7 - y < 6.8). The Pr content dependence of Tc for Ortho-II is quite different from that for Ortho-I, and is preferably similar to that in stoichiometric Y1- xPrxBa2Cu408 [9]. In this paper, the oxygen concentration dependences of Tc are investigated in Lnl_xPrxBa2Cu 3 O7_y , where Ln is Y, Nd and Gd. The ionic radius of Pr 3÷ is nearly the same as that of Nd 3÷, but larger than those of y3+ and Gd 3+. The effects of ionic size of Ln 3÷ on the suppression of superconductivity by Pr are examined by changing the oxygen concentration.
(a) Yl.xPrxBa2CuzO7.y 100
80
~,~
.
7-y
6O
¢ 4O
20
0.1
0.2
0.3
0.4
0.5
0.7
0.6
Pr content x
(b) Ndl.xPrxBa2Cu3OT.y .
100
2. Experimental
The Lnl_xPrxBa2Cu307_y (Ln = Y, Nd and Gd) samples were prepared through a solid-state reaction between Ln203, Pr6Oj t, BaCO 3 and CuO. The mixed powders were calcined four times at 950°C for 24 h in air, then pressed into pellets and sintered at 930°C (Nd)-950°C (Y, Gd) for 12 h in flowing oxygen, followed by an additional annealing at 350°C for 12 h in order to fully oxidize the samples. The quenching was made as follows: eacii sample was heated up to some quenching temperature Tq (450°C ~ Tq < 750°C) in air and kept for 2 h, after which it was quenched into liquid nitrogen. Oxygen concentration decreases continuously with increasing Tq. The oxygen concentration 7 - y of the singlephase oxides was determined by thermogravimetry, up to about 950°C in 20% hydrogen with argon atmosphere [8]. The reduced powder consisted of Ln203, Pr203, BaO and Cu metal. The oxygen concentration was determined from the weight loss upon reduction. The oxygen concentration was also estimated from the decrease in weight of samples after quenching treatments. 3. Results and discussion All the samples are characterized by X-ray powder diffraction to be single phase of orthorhombic or tetragonal LnBa2Cu307_ r structure. Lattice con-
,
,
,
.
,
.
,
.
7-y
80
~"
,
so
I - ' - 6"65 I --':'--6.60
40
2O
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Pr content x Fig. 1. Pr content x dependence of T¢ in Lnl_xPrxBa2CU3OT_y for various oxygen concentrations: (a) Ln = Y and (b) Ln = Nd.
stants a, b and c increase linearly with increasing Pr content x in the case of Ln = Y and Gd, but are almost constant with change of x in the case of L n = Nd. The ionic radius of Pr 3÷ is nearly the same as that of Nd 3÷, but 12% larger than that of y3+, and 6% larger than that of Gd 3÷. The effect of Pr substitution on lattice constants corresponds to the difference in ionic radii of the Ln ions. The value of T~ is determined as the transition midpoint temperature by the DC four-probe resistivity measurement. The values of midpoint Tc's are plotted against Pr content x for various oxygen concentration ( 7 - y) samples in Fig. 1. Fig. l(a) shows the Pr content x dependence of T~ for the Ln = Y case, and l(b) for the Ln = Nd case. In the case of Ln = Y, the initial rate of decrease of Tc
338
K. Koyama et al. / Physica C 263 (1996) 336-339
with Pr substitution ( - d T c / d x : gradient in Fig. 1) for 0 < x < 0.3 decreases monotonically with decreasing oxygen concentration for 6.6 < 7 - y < 7.0, except for the saturation of T~ above 80 K for 7 - y = 7.0 [8]. The saturation tendency above 80 K is considered as the nonlinear relation between Tc and the hole concentration in the CuO 2 plane. - dTc/d x is almost constant for 7 - y < 6.6. Such a complicated change of T~ is also confirmed by the susceptibility measurement (Meissner effect). In contrast, in the case of Ln = Nd, as shown in Fig. l(b), - d T J d x , which is the measure of the suppression effect of superconductivity by Pr, is almost constant for all the oxygen concentrations as far as superconductivity is observed. The value of - d T c / d x for Ln = Nd is larger than that for Ln = Y. The region where the superconductivity is observed in the case of Ln = Nd is narrower than in the case of Ln = Y, not only as for Pr content [10] but also for oxygen concentration. The gradient of Tc against Pr content in the case of Ln = Y increases abruptly in the range of x > 0.4. This tendency is not observed in the case of Ln = Nd. The two step x-dependence of Tc for Ln = Y is similar to that for the stoichiometric oxide superconductor Yi-xPr~Ba2Cu408 [9]. The radical drop of Tc in the heavily Pr-doped region may be caused by pair breaking by the Pr magnetic moment, while the initial gradient of Tc against x is regarded as hole localization by the hybridization between the local states of the Pr ion and the conduction band states of the CuO 2 planes. Intermediate behavior between those of Y and Nd is observed qualitatively in the case of Ln = Gd. The change of - d T c / d x with the difference in oxygen concentration is also observed in Ln = Gd; however, it is not so remarkable as in the case of Ln = Y. The anomalous increases of Tc with decrease of oxygen concentration in the limited range of oxygen concentration is also observed in the case of Ln = Gd as in the case of Ln = Y. The values of - d T c / d x are plotted in Fig. 2 against oxygen concentration in the cases of Ln = Y, Nd and Gd. The effect of T~-suppression by Pr in the case of Ln = Nd does not depend upon the oxygen concentration, and is much stronger than those in the case of Ln = Y and Ln = Gd. Intermediate behavior between those of Y and Nd is observed qualitatively
300
,
,
~
Ndl.xPrxBazCu3Or.y 250 g
~D
D u IJ C " - -
0O I--°
Gd 1 xPr
150
-
-
100
50
Yl.xPrxBa2Cu3OT.y 0 6.0
6.2
6.4
6.6
6.8
7.0
Oxygen c~ncentration 7-y Fig. 2. Oxygen concentration dependence of - d T c / d x Ln I xPrxBa2Cu307_y , where Ln is Y, Nd and Gd.
in
in the case of Ln = G d . The effect of Tc-suppression by Pr becomes weaker as the ion size of the rare earth ion decreases (Nd ~ Gd ~ Y), or as the difference of rare earth ion size from Pr increases. Furthermore, the effect of To-suppression by Pr becomes greatly dependent on the oxygen concentration with decreasing the rare earth ion size (Nd ~ Gd ~ Y). In the unique case of Ln = Nd, the ion size of which is nearly the same as that of Pr 3÷, the suppression of superconductivity by Pr appears in its simplest form. In the case of the other rare earth ions, the effect of Pr is considered to be rather complicated because of the discrepancy of ion size between Pr and the rare earth ion.
Acknowledgement This work was supported in part by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture.
References [1] A. Kebede, C.S. Jee, J. Schwegler, J.E. Crow, T. Mihalisin, G.H. Myer, M.V. Kurie, S.H. Bloom and R.P. Guertin, Phys. Rev. B 40 (1989) 4453. [2] J.J. Neumeier and M.B. Maple, Physica C 191 (1992) 158.; J.J. Neumeier, T. Bjcmholm, M.B. Maple and I.K. Sehuller, Phys. Rev. Lett. 63 (1989) 2516.
K. Koyama et al./ Physica C 263 (1996) 336-339 [3] J.L. Peng, P. Klavins, R.N. Shelton, H.B. Radousky, P.A. Hahn and L. Bemardez, Phys. Rev. B 40 (1989) 4517. [4] R. Fehrenbacher and T.M. Rice, Phys. Rev. Lett. 70 (1993) 3471. [5] See, for example, W.J. Zhu, P. Liu and Z.X. Zhao, Physica C 199 (1992) 285; E. Orgaz and E. Ruiz-Trejo, Physica C 194 (1992) 76. [6] W.Y. Guan, Y.C. Dhen, J.Y.T. Wei, Y.H. Xu and M.K. Wu, Physica C 209 (1993) 19. [7] D. Wagener, M. Buchgeister, P. Herzog, S.M. Hosseini, P.
339
Seidel, G. Mertler and K. Kopitzki, in: R. Kosowsky et al. (eds.), Physics and Materials Science of High Temperature Superconductors II (Kluwer, Dorhrecht, 1992) p. 247. [8] K. Koyama, Y. Kitamura, T. Saito and K. Mizuno, Physica C 235-240 (1994) 1469. [9] K. Koyama, S. Taga and S. Noguchi, Physica C 185-189 (1991) 771; S. Adachi, N. Watanabe, S. Seiji, N. Koshizuka and H. Yamanchi, Physica C 207 (1993) 127. [10] K. Koyama, Y. Suzuki and S. Noguchi, Jpn. J. Appl. Phys. 27 (1988) 1862.