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Ion size effect on Tc in (Pb,V)Sr2(Ca,R)Cu2Oz (R=Sm, Gd, Dy, Er and Lu) systems
Physica C 321 Ž1999. 183–190
Ion size effect on Tc in žPb,V /Sr2 žCa,R /Cu 2 Oz žR s Sm, Gd, Dy, Er and Lu / systems H.K. Lee ) , T.Y. Kim Department of Physics, Kangwon National UniÕersity Chunchon 200-701, South Korea Received 4 January 1999; received in revised form 21 June 1999; accepted 1 July 1999
Abstract The superconducting transition temperature ŽTc . and lattice constants of ŽPb,V.-based 1212 superconducting oxides ŽPb1yxVx .Sr2 ŽCa 1yy R y .Cu 2 Oz ŽR s Sm, Gd, Dy, Er and Lu. which were prepared in an oxidizing atmosphere have been investigated. The as-prepared samples were annealed at 8008C in oxygen and then quenched. The Pb:V ratio have been optimized for the highest Tc . Correlation between the lattice constants and the ionic radii of R is observed in the compositions of ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz with y s 0.35 and 0.40. The Tc decreases with increasing R ion size as observed in ŽPb,Cu.-1212 systems, which suggests that the R ion size dependence of Tc can be a common characteristic of Pb-based 1212 systems. The sample with R s Lu and y s 0.35 exhibits Tc Žonset. s 70 K and Tc Žzero. s 63.8 K. q 1999 Elsevier Science B.V. All rights reserved. PACS: 74.72.Jt; 74.62 y c; 74.62.Dh Keywords: Superconducting transition temperature; Lattice constants; Pb-based 1212 systems
1. Introduction Substitution of rare-earth ions in cupric oxide superconductors has been an important experimental attempt in the study of high-Tc superconductivity. It is now well known that the rare-earth element ŽR. can be substituted for Y in YBa 2 Cu 3 O 7 ŽY-123., YBa 2 Cu 4 O 8 ŽY-124. and Y2 Ba 4 Cu 7 O y ŽY-247. compounds. The RBa 2 Cu 4 O 8 ŽR-124 . w1 x and R 2 Ba 4 Cu 7 O y ŽR-247. w2x compounds with larger rare-earth elements exhibited decreased Tc , but this )
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was not the case in RBa 2 Cu 3 O 7 ŽR-123. w3x compound where Tc is nearly constant. In connection with Y-123 compound, superconductors with Pbbased 1212 structure which has the general chemical formula ŽPb,M.Sr2 ŽCa,Y.Cu 2 Oz are of special interest due to their structural relationship with Y-123 compound. If we start with Y-123 and replace the Y by ŽCa,Y. and the CuO chain by a rock-salt type ŽPb,M.O layer, we have the Pb-1212 structure. Superconducting compounds with M s Cu w4–8x, Sr w9x, Ca w10x, Mg w11x and Cd w12x have been discovered. Considering the remarkable similarity between the Pb-1212 and Y-123 structure, it would be interesting to study the effect of rare-earth doping on
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superconductivity in the Pb-1212 systems. However, despite the great flexibility of rock-salt type layers, there have been very limited reports on the rare-earth doping in the Pb-1212 systems w7,8x. Contrary to the case of the R-123 compounds, the Tc of the rareearth-substituted ŽPb,Cu.-1212 compounds was found to vary significantly with the size of rare-earth element. In order to be sure that the Tc variation caused by the rare-earth doping is a common characteristic of Pb-based 1212 systems, it would be useful to study the R ion size dependence of Tc on other types of Pb-based 1212 systems. Recently, we have successfully synthesized a series of ŽPb,V.-1212 cuprates ŽPb 0.5V0.5 .Sr2 ŽCa 1yyYy .Cu 2 Oz w13x in an oxidizing atmosphere. In this paper, we have investigated the optimum composition of ŽPb1y xVx .Sr2 ŽCa 1yy Er y . Cu 2 Oz with the highest Tc and then studied the correlation between Tc , lattice constants and rareearth ionic size in ŽPb,V.Sr2 ŽCa,R.Cu 2 Oz ŽR s Sm, Gd, Dy, Er and Lu. systems.
2. Experiments Polycrystalline samples of Ž Pb 1y xVx . Sr 2 ŽCa 1y y Er y .Cu 2 Oz ŽR s Sm, Gd, Dy, Er and Lu. were prepared by the standard solid-state reaction method. The powder mixtures were thoroughly ground in an agate mortar and pestle and pressed into pellets. They were first heated at 7908C for 10 h in air and then cooled to room temperature. The pellets were reground, repressed, and sintered at 995–9978C in flowing O 2 for 4 h and then cooled to room temperature in the furnace. The as-sintered pellet samples were cut into rectangular specimen with dimension of about 2 = 3 = 10 mm3 and annealed at 8008C for 12 h in oxygen and then quenched into liquid nitrogen. Preliminary annealing studies indicated that this post heat-treatment resulted in nearly maximum Tc for the rare-earth-substituted compounds w14x. The samples were examined by X-rays at room temperatures using a Rigaku X-ray diffractometer to determine their phase purity and the lattice parameters. High purity Si was used as an internal standard to correct the 2 u position of diffraction peaks. The temperature dependence of electrical resistivity was measured by a conventional four-probe technique. The applied current was 5 mA.
The measurement of the dc magnetic susceptibility was made using a Quantum Design SQUID magnetometer. 3. Results and discussion Based on previous work w13x, a series of samples with nominal composition ŽPb 0.5V0.5 .Sr2 ŽCa 1yy Er y .Cu 2 Oz Ž0.25 F y F 1. was prepared. Fig. 1 shows the temperature dependence of electrical resistivity of the post-treated samples with y s 0.25, 0.3, 0.4, 0.5 and 0.6. The sample with y s 0.6 shows a semiconducting behavior. This semiconducting behavior was also observed in all samples with y ) 0.6. As the erbium content y decreases from 0.6 to 0.3, superconductivity appears, both the superconducting onset temperature ŽTc Žonset.. and the zero-resistivity temperature ŽTc Žzero.. increase, and the normal state resistivity at Tc Žonset. decreases. However, as y decreases further below 0.3, both Tc Žonset. and Tc Žzero. begin to decrease again as is seen for the sample with y s 0.25. The decrease of the Tc Žzero. is also accompanied by the increase of the normal state resistivity. This result indicates that the best quality samples can be obtained for y s 0.3 to 0.4 in the composition of ŽPb 0.5V0.5 .Sr2 ŽCa 1yy Er y .Cu 2 Oz . Following the above experimental result, the erbium content was then fixed at 0.35 and the Pb:V ratio was varied to obtain ŽPb1y xVx .Sr2ŽCa 0.65 Er0.35 .Cu 2 Oz Ž0.3 F x F 0.5.. Fig. 2 shows the temperature dependence of electrical resistivity of these samples obtained by the post heattreatments. We can see that the highest superconducting transition is observed for x s 0.4. We have also carried out similar experiments for ŽPb1y xVx .Sr2 ŽCa 0.7 Er0.3 .Cu 2 Oz Ž0.3 F x F 0.5. and ŽPb1y xVx .Sr2 ŽCa 0.6 Er0.4 .Cu 2 Oz Ž0.3 F x F 0.5. and ŽPb1y xVx .Sr2 ŽCa 0.5 Er0.5 .Cu 2 Oz Ž0.3 F x F 0.5. and shown in Fig. 3 Tc Žzero. vs. V concentration x for each composition. This figure reveals that the best Tc is again observed at x s 0.4 for all cases. The X-ray diffraction Ž XRD . patterns of ŽPb1y xVx .Sr2 ŽCa 0.6 Er0.4 .Cu 2 Oz Ž0.3 F x F 0.5. samples are shown in Fig. 4. As is seen in the figure, the phase purity of the sample with x s 0.5 is nearly the same as that of the sample with x s 0.4, but the impurity content slightly increases with the decrease of x below x s 0.4.
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Fig. 1. Temperature dependence of electrical resistivity for ŽPb 0.5V0.5 .Sr2 ŽCa 1yy Er y .Cu 2 Oz Ž0.25 F y F 0.6. samples.
Considering the result of Fig. 3, two series of samples with nominal compositions of ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz Ž y s 0.35 and 0.40, R s Sm, Gd, Dy, Er and Lu. were prepared to investigate the R ion size dependence of Tc . The as-prepared sam-
ples were post-treated under identical conditions as before. The dependence of lattice constants a and c on the ionic radii of the rare-earth elements at 8-coordination w15x are shown in Fig. 5. One can see that the lattice constants a and c are approximately
Fig. 2. Temperature dependence of electrical resistivity for ŽPb1y xVx .Sr2 ŽCa 0.65 Er0.35 .Cu 2 Oz Ž0.3 F x F 0.5. samples.
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Fig. 3. Tc Žzero. vs. V concentration x for ŽPb1yxVx .Sr2 ŽCa 1yy Er y .Cu 2 Oz samples with y s 0.3, 0.35, 0.4 and 0.5.
linearly dependent on the size of the rare-earth element. This correlation between the lattice constants and ionic radii of the rare-earths suggests that the rare-earth ions could be successfully incorporated in the ŽPb,V.-1212 phase. Fig. 6 shows the temperature dependence of electrical resistivity for samples with five rare-earth elements, Sm, Dy, Gd, Er and Lu which have different ionic radii. We can find a systematic R ion
Fig. 4. Powder XRD patterns of samples with nominal compositions of ŽPb1y xVx .Sr2 ŽCa 0.6 Er0.4 .Cu 2 Oz samples with x s 0.3, 0.4 and 0.5.
size dependence of Tc Žzero. for the ŽPb 0.6V0.4 .Sr2 ŽCa 0.65 R 0.35 .Cu 2 Oz systems. The sample with R s Lu exhibits Tc Žonset. of 70 K and Tc Žzero. of 63.8 K. The dependence of Tc Žzero. on ionic radius of R3q at 8-coordination for ŽPb 0.6V0.4 .Sr2ŽCa 0.65 R 0.35 .Cu 2 Oz and ŽPb 0.6V0.4 .Sr2 ŽCa 0.6 R 0.4 .Cu 2 Oz systems are summarized in Fig. 7. It can be clearly seen that the Tc Žzero. increases when ionic radius decreases for both studied systems. The change of Tc variation is relatively small when the ionic ˚ Žthe ionic radii of radius decreases below 1.004 A Er.. Fig. 8 shows the temperature dependence of the diamagnetic susceptibility for the compounds of ŽPb 0.6V0.4 .Sr2 ŽCa 0.6 R 0.4 .Cu 2 Oz ŽR s Sm, Dy, Er and Lu. measured under field-cooled conditions of 10 Oe. It can be seen that the diamagnetic transition onset temperature decreases with increasing R ion size. The superconducting volume fractions estimated from the susceptibility at 10 K and crystal density are 20.2%, 12.4%, 12.2%, and 9.7% for R s Lu, Er, Dy and Sm, respectively. This result indicates that the observed superconductivity is indeed that of bulk superconductivity. Similar behavior of R ion size dependence of Tc has been also reported for R-124 w1x and R-247 w2x compounds. It is evident from Figs. 5 and 7 that both lattice constants and Tc ’s in ŽPb,V.-1212 systems are correlated by the size of the R ion. The linear correlation
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Fig. 5. The dependence of a and c lattice parameters on ionic radius of the rare-earth elements for ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz Ž y s 0.35 and 0.40, R s Sm, Gd, Dy, Er and Lu. systems.
between lattice constants and the ionic radii of the rare-earth elements has been also observed in ŽPb,Cu.-1212 systems studied by two research groups w7,8x, but the reported dependence of Tc on the size
of rare-earth element is slightly different between the research groups. In the rare-earth-substituted ŽPb 0.5Cu 0.5 .Sr2 ŽCa 0.5 R 0.5 .Cu 2 Oz ŽR s Nd, Sm, Eu, Gd, Ho, Er and Tm. compounds prepared by heat-treat-
Fig. 6. Temperature dependence of electrical resistivity for ŽPb 0.6V0.4 .Sr2 ŽCa 0.65 R 0.35 .Cu 2 Oz ŽR s Sm, Gd, Dy, Er and Lu. systems.
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Fig. 7. The dependence of Tc Žzero. on ionic radius of the rare-earth elements for ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz Ž y s 0.35 and 0.40, R s Sm, Gd, Dy, Er and Lu. systems.
ment in high-pressure Ž100 bars. oxygen, Liu and Morris w7x observed that Tc decreases linearly as the size of the rare-earth ion increases from Tm Ž r s ˚ . to Gd Ž r s 1.053 A˚ . and then remain 0.994 A
constant for larger ions up to Nd. Meanwhile, Adachi et al. w8x reported a monotonic decrease of Tc with increasing rare-earth ionic size in ŽPb 0.75 Cu 0.25 . Sr2 ŽCa 0.6 R 0.4 .Cu 2 Oz ŽR s Nd, Sm, Eu, Gd, Ho, Er,
Fig. 8. Temperature dependence of dc magnetic susceptibility for ŽPb 0.6V0.4 .Sr2 ŽCa 0.6 R 0.4 .Cu 2 Oz ŽR s Sm, Dy, Er and Lu. systems.
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Yb, Lu and Y. compounds prepared in one bar oxygen. Although the reason for the disagreement of the detailed dependence of the Tc on R ion size is not clear, it is a common experimental result that small ionic radius in ŽCa,R. site in ŽPb,Cu.-1212 systems are more favorable for high-Tc superconductivity. Our present work provides further evidence that the substitution of a smaller R ion at the ŽCa,R. site brings about the Tc enhancement in another type of Pb-based 1212 systems, i.e., ŽPb,V.-1212 systems and suggests that R ion size dependence of Tc can be a common characteristic of Pb-based 1212 systems. To explain the R ion size dependence of Tc , two possible mechanisms need to be considered. The first mechanism involves pair breaking by local moments due to the spin-dependent exchange scattering of holes in the CuO 2 planes. According to the pairbreaking theory of Abrikosov and Gor’kov for low impurity concentration x, the depression of Tc with impurity concentration x is given w16x by Tc Ž x . s Tc Ž 0 . y
Ž p 2r4 k B . 2
=N Ž EF . Iex2 Ž g y 1 . J Ž J q 1 . x where N Ž EF . is the density of states at the Fermi level, g is the Lande g factor, J is the total angular momentum of Hund’s rule ground state for the magnetic ion, and Iex is the exchange interaction parameter. When the depression of Tc is primarily caused by the exchange interaction, the depression for a given impurity concentration is essentially scaled by the de Gennes factor Ž g y 1. 2 J Ž J q 1. as far as N Ž E F . and Iex does not change appreciably. Since the de Gennes factor is not varied monotonically with the size of the R ion, the magnetism due to the exchange interaction alone cannot explain the monotonic decrease of Tc with the increase of the R ion size as shown in Fig. 7. However, the larger extension of the R 4f orbitals of the lighter rare-earth elements may cause a hybridization of the 4f electrons with the CuO 2 bands w16–18x. The hybridization could generate an appreciable exchange interaction between the R ion moments and the spin of the mobile holes in the CuO 2 planes and thereby, leads to the depression of Tc through the magnetic pair-
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breaking effect. The second mechanism for the R ion size dependence of Tc involves the nonmagnetic volume effects related to the lanthanide contraction. Since the decrease of the c lattice parameter with decreasing the size of the rare-earth element shown in Fig. 5 implies that the distance between the two CuO 2 layers in Pb-based 1212 structure is decreasing, the substitution of a smaller rare-earth element at ŽCa,R. site is expected to increase the coupling between CuO 2 layers and may help the hole transfer into the CuO 2 layers when the sample contains optimum oxygen content by appropriate heat-treatments w7,19x. If it is possible to find out the depression of Tc due to nonmagnetic R ion size contribution, the magnetic pair-breaking effect could be discerned. 4. Conclusions We have investigated the correlation between Tc , lattice constants and ionic radii of rare-earth elements in ŽPb,V.-based 1212 compounds with the formula ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz Ž y s 0.35 and 0.4, R s Sm, Gd, Dy, Er and Lu.. The Tc decreases with increasing R ion size as observed in R-124 and R-247 compounds and the a and c lattice parameters increase approximately linearly with the size of R ion. The sample with R s Lu and y s 0.35 exhibits Tc Žonset. s 70 K and Tc Žzero. s 63.8 K. The Tc enhancement, observed in this work by the substitution of a smaller rare-earth element in ŽPb,V.-1212 compounds and previously reported by other authors for similar studies in ŽPb,Cu.-1212 compounds, suggests that R ion size dependence of Tc is likely to be a common characteristic of Pb-based 1212 compounds. This characteristic may be useful to obtain a higher Tc for a given Pb-based 1212 system and for testing theories of high-Tc superconductivity. References w1x D.E. Morris, J.H. Nickel, J.Y.T. Wei, N.G. Asmar, J.S. Scott, U.M. Scheven, C.T. Hultgren, A.G. Markelt, J.E. Post, P.J. Heaney, D.R. Veblen, R.M. Hazen, Phys. Rev. B 39 Ž1989. 7347. w2x D.E. Morris, N.G. Asmar, J.Y.T. Wei, J.H. Nickel, R.L. Sid, J.S. Scott, J.E. Post, Phys. Rev. B 40 Ž1989. 11406.
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w3x J.M. Tarascon, W.R. McKinnon, L.H. Greene, G.W. Hull, E.M. Vogel, Phys. Rev. B 36 Ž1987. 226. w4x A. Ono, Uchida, Jpn. J. Appl. Phys. 29 Ž1990. L586. w5x T. Maeda, K. Sakuyama, S. Koriyama, H. Yamauchi, S. Tanaka, Phys. Rev. B 43 Ž1991. 7866. w6x X.X. Tang, D.E. Morris, A.P.B. Sinha, Phys. Rev. B 43 Ž1991. 7936. w7x H.B. Liu, D.E. Morris, Phys. Rev. B 44 Ž1991. 5369. w8x S. Adachi, H. Adachi, K. Setsune, K. Wasa, Jpn. J. Appl. Phys. 30 Ž1991. L1099. w9x T. Rouillon, J. Provost, M. Hervieu, D. Groult, C. Michel, B. Raveau, Physica C 159 Ž1989. 201. w10x T. Rouillon, A. Maignan, M. Hervieu, C. Michel, D. Groult, B. Raveau, Physica C 171 Ž1990. 7.
w11x H.B. Liu, D.E. Morris, A.B.P. Sinha, Physica C 204 Ž1993. 262. w12x T.P. Beales, C. Dineen, W.G. Freeman, D.M. Jacobson, S.R. Hall, M.R. Harrison, S.J. Zammattio, Supercond. Sci. Technol. 5 Ž1992. 47. w13x H.K. Lee, Physica C 308 Ž1998. 289. w14x H.K. Lee, unpublished. w15x R.D. Shannon, Acta Crystallogr. A 32 Ž1976. 751. w16x Y. Xu, W. Guan, Phys. Rev. B 45 Ž1992. 3176. w17x Y. Chen, T.S. Lai, M.K. Wu, W.Y. Guan, J. Appl. Phys. 78 Ž1995. 5212. w18x G.Y. Guo, W.M. Temmerman, Phys. Rev. B 41 Ž1990. 6372. w19x P. Sumana Prabhu, U.V. Varadaraju, Phys. Rev. B 53 Ž1996. 14637.
Ion size effect on Tc in žPb,V /Sr2 žCa,R /Cu 2 Oz žR s Sm, Gd, Dy, Er and Lu / systems H.K. Lee ) , T.Y. Kim Department of Physics, Kangwon National UniÕersity Chunchon 200-701, South Korea Received 4 January 1999; received in revised form 21 June 1999; accepted 1 July 1999
Abstract The superconducting transition temperature ŽTc . and lattice constants of ŽPb,V.-based 1212 superconducting oxides ŽPb1yxVx .Sr2 ŽCa 1yy R y .Cu 2 Oz ŽR s Sm, Gd, Dy, Er and Lu. which were prepared in an oxidizing atmosphere have been investigated. The as-prepared samples were annealed at 8008C in oxygen and then quenched. The Pb:V ratio have been optimized for the highest Tc . Correlation between the lattice constants and the ionic radii of R is observed in the compositions of ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz with y s 0.35 and 0.40. The Tc decreases with increasing R ion size as observed in ŽPb,Cu.-1212 systems, which suggests that the R ion size dependence of Tc can be a common characteristic of Pb-based 1212 systems. The sample with R s Lu and y s 0.35 exhibits Tc Žonset. s 70 K and Tc Žzero. s 63.8 K. q 1999 Elsevier Science B.V. All rights reserved. PACS: 74.72.Jt; 74.62 y c; 74.62.Dh Keywords: Superconducting transition temperature; Lattice constants; Pb-based 1212 systems
1. Introduction Substitution of rare-earth ions in cupric oxide superconductors has been an important experimental attempt in the study of high-Tc superconductivity. It is now well known that the rare-earth element ŽR. can be substituted for Y in YBa 2 Cu 3 O 7 ŽY-123., YBa 2 Cu 4 O 8 ŽY-124. and Y2 Ba 4 Cu 7 O y ŽY-247. compounds. The RBa 2 Cu 4 O 8 ŽR-124 . w1 x and R 2 Ba 4 Cu 7 O y ŽR-247. w2x compounds with larger rare-earth elements exhibited decreased Tc , but this )
Corresponding author. Fax: q82-361-257-9689; E-mail: [email protected]
was not the case in RBa 2 Cu 3 O 7 ŽR-123. w3x compound where Tc is nearly constant. In connection with Y-123 compound, superconductors with Pbbased 1212 structure which has the general chemical formula ŽPb,M.Sr2 ŽCa,Y.Cu 2 Oz are of special interest due to their structural relationship with Y-123 compound. If we start with Y-123 and replace the Y by ŽCa,Y. and the CuO chain by a rock-salt type ŽPb,M.O layer, we have the Pb-1212 structure. Superconducting compounds with M s Cu w4–8x, Sr w9x, Ca w10x, Mg w11x and Cd w12x have been discovered. Considering the remarkable similarity between the Pb-1212 and Y-123 structure, it would be interesting to study the effect of rare-earth doping on
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 3 9 6 - 2
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superconductivity in the Pb-1212 systems. However, despite the great flexibility of rock-salt type layers, there have been very limited reports on the rare-earth doping in the Pb-1212 systems w7,8x. Contrary to the case of the R-123 compounds, the Tc of the rareearth-substituted ŽPb,Cu.-1212 compounds was found to vary significantly with the size of rare-earth element. In order to be sure that the Tc variation caused by the rare-earth doping is a common characteristic of Pb-based 1212 systems, it would be useful to study the R ion size dependence of Tc on other types of Pb-based 1212 systems. Recently, we have successfully synthesized a series of ŽPb,V.-1212 cuprates ŽPb 0.5V0.5 .Sr2 ŽCa 1yyYy .Cu 2 Oz w13x in an oxidizing atmosphere. In this paper, we have investigated the optimum composition of ŽPb1y xVx .Sr2 ŽCa 1yy Er y . Cu 2 Oz with the highest Tc and then studied the correlation between Tc , lattice constants and rareearth ionic size in ŽPb,V.Sr2 ŽCa,R.Cu 2 Oz ŽR s Sm, Gd, Dy, Er and Lu. systems.
2. Experiments Polycrystalline samples of Ž Pb 1y xVx . Sr 2 ŽCa 1y y Er y .Cu 2 Oz ŽR s Sm, Gd, Dy, Er and Lu. were prepared by the standard solid-state reaction method. The powder mixtures were thoroughly ground in an agate mortar and pestle and pressed into pellets. They were first heated at 7908C for 10 h in air and then cooled to room temperature. The pellets were reground, repressed, and sintered at 995–9978C in flowing O 2 for 4 h and then cooled to room temperature in the furnace. The as-sintered pellet samples were cut into rectangular specimen with dimension of about 2 = 3 = 10 mm3 and annealed at 8008C for 12 h in oxygen and then quenched into liquid nitrogen. Preliminary annealing studies indicated that this post heat-treatment resulted in nearly maximum Tc for the rare-earth-substituted compounds w14x. The samples were examined by X-rays at room temperatures using a Rigaku X-ray diffractometer to determine their phase purity and the lattice parameters. High purity Si was used as an internal standard to correct the 2 u position of diffraction peaks. The temperature dependence of electrical resistivity was measured by a conventional four-probe technique. The applied current was 5 mA.
The measurement of the dc magnetic susceptibility was made using a Quantum Design SQUID magnetometer. 3. Results and discussion Based on previous work w13x, a series of samples with nominal composition ŽPb 0.5V0.5 .Sr2 ŽCa 1yy Er y .Cu 2 Oz Ž0.25 F y F 1. was prepared. Fig. 1 shows the temperature dependence of electrical resistivity of the post-treated samples with y s 0.25, 0.3, 0.4, 0.5 and 0.6. The sample with y s 0.6 shows a semiconducting behavior. This semiconducting behavior was also observed in all samples with y ) 0.6. As the erbium content y decreases from 0.6 to 0.3, superconductivity appears, both the superconducting onset temperature ŽTc Žonset.. and the zero-resistivity temperature ŽTc Žzero.. increase, and the normal state resistivity at Tc Žonset. decreases. However, as y decreases further below 0.3, both Tc Žonset. and Tc Žzero. begin to decrease again as is seen for the sample with y s 0.25. The decrease of the Tc Žzero. is also accompanied by the increase of the normal state resistivity. This result indicates that the best quality samples can be obtained for y s 0.3 to 0.4 in the composition of ŽPb 0.5V0.5 .Sr2 ŽCa 1yy Er y .Cu 2 Oz . Following the above experimental result, the erbium content was then fixed at 0.35 and the Pb:V ratio was varied to obtain ŽPb1y xVx .Sr2ŽCa 0.65 Er0.35 .Cu 2 Oz Ž0.3 F x F 0.5.. Fig. 2 shows the temperature dependence of electrical resistivity of these samples obtained by the post heattreatments. We can see that the highest superconducting transition is observed for x s 0.4. We have also carried out similar experiments for ŽPb1y xVx .Sr2 ŽCa 0.7 Er0.3 .Cu 2 Oz Ž0.3 F x F 0.5. and ŽPb1y xVx .Sr2 ŽCa 0.6 Er0.4 .Cu 2 Oz Ž0.3 F x F 0.5. and ŽPb1y xVx .Sr2 ŽCa 0.5 Er0.5 .Cu 2 Oz Ž0.3 F x F 0.5. and shown in Fig. 3 Tc Žzero. vs. V concentration x for each composition. This figure reveals that the best Tc is again observed at x s 0.4 for all cases. The X-ray diffraction Ž XRD . patterns of ŽPb1y xVx .Sr2 ŽCa 0.6 Er0.4 .Cu 2 Oz Ž0.3 F x F 0.5. samples are shown in Fig. 4. As is seen in the figure, the phase purity of the sample with x s 0.5 is nearly the same as that of the sample with x s 0.4, but the impurity content slightly increases with the decrease of x below x s 0.4.
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Fig. 1. Temperature dependence of electrical resistivity for ŽPb 0.5V0.5 .Sr2 ŽCa 1yy Er y .Cu 2 Oz Ž0.25 F y F 0.6. samples.
Considering the result of Fig. 3, two series of samples with nominal compositions of ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz Ž y s 0.35 and 0.40, R s Sm, Gd, Dy, Er and Lu. were prepared to investigate the R ion size dependence of Tc . The as-prepared sam-
ples were post-treated under identical conditions as before. The dependence of lattice constants a and c on the ionic radii of the rare-earth elements at 8-coordination w15x are shown in Fig. 5. One can see that the lattice constants a and c are approximately
Fig. 2. Temperature dependence of electrical resistivity for ŽPb1y xVx .Sr2 ŽCa 0.65 Er0.35 .Cu 2 Oz Ž0.3 F x F 0.5. samples.
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Fig. 3. Tc Žzero. vs. V concentration x for ŽPb1yxVx .Sr2 ŽCa 1yy Er y .Cu 2 Oz samples with y s 0.3, 0.35, 0.4 and 0.5.
linearly dependent on the size of the rare-earth element. This correlation between the lattice constants and ionic radii of the rare-earths suggests that the rare-earth ions could be successfully incorporated in the ŽPb,V.-1212 phase. Fig. 6 shows the temperature dependence of electrical resistivity for samples with five rare-earth elements, Sm, Dy, Gd, Er and Lu which have different ionic radii. We can find a systematic R ion
Fig. 4. Powder XRD patterns of samples with nominal compositions of ŽPb1y xVx .Sr2 ŽCa 0.6 Er0.4 .Cu 2 Oz samples with x s 0.3, 0.4 and 0.5.
size dependence of Tc Žzero. for the ŽPb 0.6V0.4 .Sr2 ŽCa 0.65 R 0.35 .Cu 2 Oz systems. The sample with R s Lu exhibits Tc Žonset. of 70 K and Tc Žzero. of 63.8 K. The dependence of Tc Žzero. on ionic radius of R3q at 8-coordination for ŽPb 0.6V0.4 .Sr2ŽCa 0.65 R 0.35 .Cu 2 Oz and ŽPb 0.6V0.4 .Sr2 ŽCa 0.6 R 0.4 .Cu 2 Oz systems are summarized in Fig. 7. It can be clearly seen that the Tc Žzero. increases when ionic radius decreases for both studied systems. The change of Tc variation is relatively small when the ionic ˚ Žthe ionic radii of radius decreases below 1.004 A Er.. Fig. 8 shows the temperature dependence of the diamagnetic susceptibility for the compounds of ŽPb 0.6V0.4 .Sr2 ŽCa 0.6 R 0.4 .Cu 2 Oz ŽR s Sm, Dy, Er and Lu. measured under field-cooled conditions of 10 Oe. It can be seen that the diamagnetic transition onset temperature decreases with increasing R ion size. The superconducting volume fractions estimated from the susceptibility at 10 K and crystal density are 20.2%, 12.4%, 12.2%, and 9.7% for R s Lu, Er, Dy and Sm, respectively. This result indicates that the observed superconductivity is indeed that of bulk superconductivity. Similar behavior of R ion size dependence of Tc has been also reported for R-124 w1x and R-247 w2x compounds. It is evident from Figs. 5 and 7 that both lattice constants and Tc ’s in ŽPb,V.-1212 systems are correlated by the size of the R ion. The linear correlation
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Fig. 5. The dependence of a and c lattice parameters on ionic radius of the rare-earth elements for ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz Ž y s 0.35 and 0.40, R s Sm, Gd, Dy, Er and Lu. systems.
between lattice constants and the ionic radii of the rare-earth elements has been also observed in ŽPb,Cu.-1212 systems studied by two research groups w7,8x, but the reported dependence of Tc on the size
of rare-earth element is slightly different between the research groups. In the rare-earth-substituted ŽPb 0.5Cu 0.5 .Sr2 ŽCa 0.5 R 0.5 .Cu 2 Oz ŽR s Nd, Sm, Eu, Gd, Ho, Er and Tm. compounds prepared by heat-treat-
Fig. 6. Temperature dependence of electrical resistivity for ŽPb 0.6V0.4 .Sr2 ŽCa 0.65 R 0.35 .Cu 2 Oz ŽR s Sm, Gd, Dy, Er and Lu. systems.
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Fig. 7. The dependence of Tc Žzero. on ionic radius of the rare-earth elements for ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz Ž y s 0.35 and 0.40, R s Sm, Gd, Dy, Er and Lu. systems.
ment in high-pressure Ž100 bars. oxygen, Liu and Morris w7x observed that Tc decreases linearly as the size of the rare-earth ion increases from Tm Ž r s ˚ . to Gd Ž r s 1.053 A˚ . and then remain 0.994 A
constant for larger ions up to Nd. Meanwhile, Adachi et al. w8x reported a monotonic decrease of Tc with increasing rare-earth ionic size in ŽPb 0.75 Cu 0.25 . Sr2 ŽCa 0.6 R 0.4 .Cu 2 Oz ŽR s Nd, Sm, Eu, Gd, Ho, Er,
Fig. 8. Temperature dependence of dc magnetic susceptibility for ŽPb 0.6V0.4 .Sr2 ŽCa 0.6 R 0.4 .Cu 2 Oz ŽR s Sm, Dy, Er and Lu. systems.
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Yb, Lu and Y. compounds prepared in one bar oxygen. Although the reason for the disagreement of the detailed dependence of the Tc on R ion size is not clear, it is a common experimental result that small ionic radius in ŽCa,R. site in ŽPb,Cu.-1212 systems are more favorable for high-Tc superconductivity. Our present work provides further evidence that the substitution of a smaller R ion at the ŽCa,R. site brings about the Tc enhancement in another type of Pb-based 1212 systems, i.e., ŽPb,V.-1212 systems and suggests that R ion size dependence of Tc can be a common characteristic of Pb-based 1212 systems. To explain the R ion size dependence of Tc , two possible mechanisms need to be considered. The first mechanism involves pair breaking by local moments due to the spin-dependent exchange scattering of holes in the CuO 2 planes. According to the pairbreaking theory of Abrikosov and Gor’kov for low impurity concentration x, the depression of Tc with impurity concentration x is given w16x by Tc Ž x . s Tc Ž 0 . y
Ž p 2r4 k B . 2
=N Ž EF . Iex2 Ž g y 1 . J Ž J q 1 . x where N Ž EF . is the density of states at the Fermi level, g is the Lande g factor, J is the total angular momentum of Hund’s rule ground state for the magnetic ion, and Iex is the exchange interaction parameter. When the depression of Tc is primarily caused by the exchange interaction, the depression for a given impurity concentration is essentially scaled by the de Gennes factor Ž g y 1. 2 J Ž J q 1. as far as N Ž E F . and Iex does not change appreciably. Since the de Gennes factor is not varied monotonically with the size of the R ion, the magnetism due to the exchange interaction alone cannot explain the monotonic decrease of Tc with the increase of the R ion size as shown in Fig. 7. However, the larger extension of the R 4f orbitals of the lighter rare-earth elements may cause a hybridization of the 4f electrons with the CuO 2 bands w16–18x. The hybridization could generate an appreciable exchange interaction between the R ion moments and the spin of the mobile holes in the CuO 2 planes and thereby, leads to the depression of Tc through the magnetic pair-
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breaking effect. The second mechanism for the R ion size dependence of Tc involves the nonmagnetic volume effects related to the lanthanide contraction. Since the decrease of the c lattice parameter with decreasing the size of the rare-earth element shown in Fig. 5 implies that the distance between the two CuO 2 layers in Pb-based 1212 structure is decreasing, the substitution of a smaller rare-earth element at ŽCa,R. site is expected to increase the coupling between CuO 2 layers and may help the hole transfer into the CuO 2 layers when the sample contains optimum oxygen content by appropriate heat-treatments w7,19x. If it is possible to find out the depression of Tc due to nonmagnetic R ion size contribution, the magnetic pair-breaking effect could be discerned. 4. Conclusions We have investigated the correlation between Tc , lattice constants and ionic radii of rare-earth elements in ŽPb,V.-based 1212 compounds with the formula ŽPb 0.6V0.4 .Sr2 ŽCa 1yy R y .Cu 2 Oz Ž y s 0.35 and 0.4, R s Sm, Gd, Dy, Er and Lu.. The Tc decreases with increasing R ion size as observed in R-124 and R-247 compounds and the a and c lattice parameters increase approximately linearly with the size of R ion. The sample with R s Lu and y s 0.35 exhibits Tc Žonset. s 70 K and Tc Žzero. s 63.8 K. The Tc enhancement, observed in this work by the substitution of a smaller rare-earth element in ŽPb,V.-1212 compounds and previously reported by other authors for similar studies in ŽPb,Cu.-1212 compounds, suggests that R ion size dependence of Tc is likely to be a common characteristic of Pb-based 1212 compounds. This characteristic may be useful to obtain a higher Tc for a given Pb-based 1212 system and for testing theories of high-Tc superconductivity. References w1x D.E. Morris, J.H. Nickel, J.Y.T. Wei, N.G. Asmar, J.S. Scott, U.M. Scheven, C.T. Hultgren, A.G. Markelt, J.E. Post, P.J. Heaney, D.R. Veblen, R.M. Hazen, Phys. Rev. B 39 Ž1989. 7347. w2x D.E. Morris, N.G. Asmar, J.Y.T. Wei, J.H. Nickel, R.L. Sid, J.S. Scott, J.E. Post, Phys. Rev. B 40 Ž1989. 11406.
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