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Mis-substitution effect in Gd1−xPrxBa2Cu3O7−δ system
Physica B 321 (2002) 301–304
Mis-substitution effect in Gd1xPrxBa2Cu3O7d system M.R. Mohammadizadeh, H. Khosroabadi, M. Akhavan* Magnet Research Laboratory, Department of Physics, Sharif University of Technology, P.O. Box 11365-9161, Tehran, Iran
Abstract On polycrystalline Gd1xPrxBa2Cu3O7d , samples with x ¼ 0:00; 0.05, 0.10, and 0.15, SEM, XRD, and electrical transport experiments have been carried out. Although the samples are homogenous, granular, and show a superconducting transition, the Tc decreases and the transition temperature width increases with increasing doping concentration x: Using Rietveld analysis of the XRD patterns, we have examined the real substitution positions of Gd, Pr, and Ba atoms in these samples. Our results show that the mentioned atoms occupy their own sites, and there is no Gd or Pr at the Ba site and vice versa (i.e., no mis-substitution). Thus, in our samples, Pr atom at rare earth sites suppress the superconductivity, favoring theories based on Pr at the rare earth site for describing the Pr anomaly. r 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 61.10.My; 74.62.Dh Keywords: Pr anomaly; Gd1xPrxBa2Cu3O7d; Rietveld method
1. Introduction One of the interesting questions in the hightemperature superconductors has been the superconductor–insulator (SI) transition in RE1xPrxBa2Cu3O7d (RE=rare earth) systems with increasing amount of Pr doping [1]. The PrBa2Cu3O7d (Pr-123) compound in the orthorhombic phase, contrary to other RE-123, compounds is an insulator [2]. Although many researchers have tried to explain the insulating behavior of Pr-123 [3], different groups have reported the observation of superconductivity in the powder, single crystal, polycrystal, and thin films of this compound [4].
*Corresponding author. Tel.: +98-21-6022733-45; fax: +9821-6012983. E-mail address: [email protected] (M. Akhavan).
Among different theories for explaining the SI transition in REPr-123 such as hole filling [5], pair breaking [6], and hybridization [7], the substitution of Pr at the Ba site instead of the RE site (missubstitution) has been proposed to explain the insulating behavior of Pr-123 [8]. Due to the nearly equivalent positions of RE and Ba sites in the center of the peroveskite structure in RE-123, the mis-substitution theory seems to be a good approach to explain both the insulating and the superconducting behaviors of Pr-123, and perhaps the SI transition as well. The location of Pr has attracted much attention, and it can also indicate if defects may play an important role in suppressing superconductivity in the conventionally prepared Pr doped polycrystalline samples [9]. We will explain the experimental examination of the mis-substitution effect in Gd1xPrxBa2Cu3O7d (GdPr-123) system with the Rietveld analysis of XRD experiment.
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 8 6 6 - 9
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M.R. Mohammadizadeh et al. / Physica B 321 (2002) 301–304
2. Experimental Polycrystalline GdPr-123 samples with x ¼ 0:00; 0.05, 0.10, and 0.15 have been prepared by standard solid state reaction from Gd2O3, Pr6O11, BaCO3, and CuO powders with 99.9% purity. The resistivity of the samples has been measured by a four-probe method from room temperature to below their critical temperature (Tc ). The SEM measurements have been done to determine the grain size and homogeneity of the samples. The XRD measurements have been done by a Philips 1340 powder diffractometer with Co ( and Fe filtered Ka radiation with l ¼ 1:7903 A in room temperature with 0.11 step width and 3 s step time.
3. Results and discussion XRD results have been analyzed with DBW3.2S-PC-9207 package based on the Rietveld method [10]. In the refinements, up to 25 parameters, including scale factor, cell parameters (a; b; and c), atomic coordinates, isotropic displacements (B), site occupation factors (N), and profile shape parameters were allowed to vary. The ( 2 [11]. The B of the oxygen sites was fixed to 1 A background was refined, and a Lorentzian profile function was used for all samples. The refinements were based on diffraction data in the range 201p2yp801 containing 53 reflections from orthorhombic phase. The SEM pictures show a granular homogenous structure with micrometer grain sizes. The resistivity vs. temperature curves for different samples show superconducting transition, and metallic behavior in the normal state. Tonset ; Tc ; and transition width (DT) for different amount of Pr doping are presented in Table 1. A typical observed XRD pattern, a calculated spectrum from the Rietveld method, and their differences are shown in Fig. 1. The (2 0 0) and (0 2 0) peaks near the 2y ¼ 551 are characteristics of the existence of an orthorhombic tri-perovskite phase as in YBa2Cu3O7d system [12]. Comparison of XRD patterns with JCPDS files shows that the structure is orthorhombic with Pmmm sym-
Table 1 Tonset ; Tc ; and DT of Gd1xPrxBa2Cu3O7d x
Tonset (K)
Tc (K)
DT (K)
0.00 0.05 0.10 0.15
92 93.05 92.12 88.06
91.58 90.06 82.74 71.32
2.52 3.41 11.66 19.89
metry and the impurity peaks are very small. The amount of oxygen in the samples is nearly complete. In addition, because of the small atomic scattering of the oxygen atoms with respect to the other heavy elements in GdPr-123 compound [13], we have used seven oxygen atoms in each refinement, and whence, the site occupation factor of the other atoms have been refined. The final results of the Rietveld refinements have been presented in Table 2. The Ri factors (R-pattern (RP ), R-weighted pattern (RWP ), RBragg (RB ), and R-structure factor (RF )) are small (o6.18%), and therefore the results are reliable. The site occupation factors for Gd and Ba atoms are less than their stoichiometry and they decrease with increasing x: The NPr for x ¼ 0:10 and 0.15 is stoichiometric, and for x ¼ 0:05 is zero, which may not be meaningful according to sðNPr Þ ¼ 0:04: Due to the very small impurity peaks, the reduction of NGd and NBa can be caused by missubstitution of Gd at Ba site (GdBa) and Ba at RE site (BaRE), or by formation of imperfect unit cells. Therefore, as a next step in the refinement, we have allowed the Gd, Pr, and Ba atoms to occupy missites (GdBa, PrBa, and BaRE) freely. The final refinements show that none of the mis-sites are occupied, and the other parameters (atomic coordinates, atomic displacement parameters, site occupation factors, lattice parameters, and Ri factors) do not change considerably. Thus, missubstitution does not happen, and therefore, it seems that the formation of imperfect unit cells is responsible for the reduction of NGd and NBa : These imperfect cells do not give impurity peaks in the XRD patterns, while they increase the number of formed unit cells, hence decreasing the site occupation factors. Contrary to RP and RWP ; which reflect the progress of the refinement, the RB
M.R. Mohammadizadeh et al. / Physica B 321 (2002) 301–304
303
Intensity (arb. units)
800 600 400 200 0
20
30
40
50
60
70
80
2θ(deg)
Fig. 1. Refinement of XRD pattern for Gd0.95Pr0.05Ba2Cu3O7d. Circles are calculated values. The line shows the observed intensities. The lower curve is the difference between the observed and the calculated intensities.
Table 2 Lattice parameters (a; b; and c), site occupation factors N of different atoms, and Ri factors for Gd1xPrxBa2Cu3O7d x
( a (A)
( b (A)
( c (A)
N (Gd)
N (Pr)
N (Ba)
RP (%)
RWP (%)
RB (%)
RF (%)
0.00 0.05 0.10 0.15
3.852 3.843 3.846 3.848
3.900 3.894 3.903 3.907
11.667 11.704 11.689 11.672
0.87 0.86 0.73 0.70
— 0.00 0.10 0.15
1.84 1.76 1.63 1.68
3.841 2.939 3.020 3.350
5.307 3.893 3.904 4.200
2.75 3.83 4.96 5.56
3.11 4.99 6.14 6.18
and RF are based not on the actually observed Bragg intensities, but on those deduced with the help of the model. Therefore, they are biased in favor of the model being used [10]. In Table 1, RB and RF systematically increase with increasing x; while RP and RWP do not. This shows that the refinements of all samples are almost with the same accuracy, but the expected structure (orthorhombic phase with Gd and Pr at the RE site and Ba at its own site) is less favorable with increasing x: This means that some kinds of disorders are forming with increasing x: The relatively large DT (Table 1), and its increase with increase of x is another evidence that disorders are forming with increasing x: We have also observed the reduction in XRD peak intensities with increasing x: This also shows the reduction of electronic density in the planes, which means the growth of disorders in the system. All of these are compatible with the formation of imperfect unit cells, and not the missubstitution effects. In conclusion, Pr at RE site suppresses the superconductivity as have been observed in our
samples, and it seems that the theories based on Pr at the RE site are more suitable for describing the Pr anomaly.
Acknowledgements We would like to thank C.U. Segre for useful discussions and lending a new version of the DBWS package to us. This work was supported in part by grants from the Offices of the Vice President for Research and Dean of Graduate Studies at Sharif University of Technology.
References [1] Z. Yamani, M. Akhavan, Phys. Rev. B 56 (1997) 894; M. Luszczek, W. Sadowski, T. limczuk, J. Olchowik, B. Susla, R. Czajka, Physica C 322 (1999) 57. [2] L. Soderholm, K. Zhang, D.G. Hinks, M.A. Beno, J.D. Jorgensen, C.U. Segre, I.K. Schuller, Nature 328 (1987) 604.
304
M.R. Mohammadizadeh et al. / Physica B 321 (2002) 301–304
[3] M. Akhavan, in these Proceedings (MSM 2001), Physica B 321 (2002), and references therein. [4] H.A. Blackstead, J.D. Dow, Solid State Commun. 115 (2000) 137 and references therein. [5] J.L. Peng, P. Klavins, R.N. Shelton, H.B. Radousky, P.A. Hahn, L. Bernardez, Phys. Rev. B 40 (1989) 4517. [6] J.J. Neumeier, T. Bjornholm, M.B. Maple, I.K. Schuller, Phys. Rev. Lett. 63 (1989) 2516. [7] Q. Gao, Li-yuan Zhang, Phys. Rev. B 50 (1994) 10230. [8] H.A. Blackstead, D.B. Chrisey, J.D. Dow, J.S. Horwitz, A.E. Klunzinger, D.B. Pulling, Phys. Lett. A 207 (1995) 109.
[9] Z. Klencsar, E. Kuzmann, Z. Homonnay, A. Vertes, K. Vad, J. Bankuti, T. Racz, M. Bodog, I. Kotsis, Physica C 304 (1998) 124. [10] R.A. Young, in: R.A. Young (Ed.), The Rietveld Method, Oxford University Press, New York, 1993, p. 1. [11] Ch. Bertrand, Ph. Galez, R.E. Gladyshevskii, J.L. Jorda, Physica C 321 (1999) 151. [12] M. Izumi, T. Yabe, T. Wada, A. Maeda, K. Uchinokura, S. Tanaka, H. Asano, Phys. Rev. B 40 (1989) 6771. [13] F. Izumi, H. Asano, T. Ishigaki, E. Takayama-Muromachi, Y. Uchida, N. Watanabe, T. Nishikawa, Jpn. J. Appl. Phys. 26 (1987) L649.
Mis-substitution effect in Gd1xPrxBa2Cu3O7d system M.R. Mohammadizadeh, H. Khosroabadi, M. Akhavan* Magnet Research Laboratory, Department of Physics, Sharif University of Technology, P.O. Box 11365-9161, Tehran, Iran
Abstract On polycrystalline Gd1xPrxBa2Cu3O7d , samples with x ¼ 0:00; 0.05, 0.10, and 0.15, SEM, XRD, and electrical transport experiments have been carried out. Although the samples are homogenous, granular, and show a superconducting transition, the Tc decreases and the transition temperature width increases with increasing doping concentration x: Using Rietveld analysis of the XRD patterns, we have examined the real substitution positions of Gd, Pr, and Ba atoms in these samples. Our results show that the mentioned atoms occupy their own sites, and there is no Gd or Pr at the Ba site and vice versa (i.e., no mis-substitution). Thus, in our samples, Pr atom at rare earth sites suppress the superconductivity, favoring theories based on Pr at the rare earth site for describing the Pr anomaly. r 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 61.10.My; 74.62.Dh Keywords: Pr anomaly; Gd1xPrxBa2Cu3O7d; Rietveld method
1. Introduction One of the interesting questions in the hightemperature superconductors has been the superconductor–insulator (SI) transition in RE1xPrxBa2Cu3O7d (RE=rare earth) systems with increasing amount of Pr doping [1]. The PrBa2Cu3O7d (Pr-123) compound in the orthorhombic phase, contrary to other RE-123, compounds is an insulator [2]. Although many researchers have tried to explain the insulating behavior of Pr-123 [3], different groups have reported the observation of superconductivity in the powder, single crystal, polycrystal, and thin films of this compound [4].
*Corresponding author. Tel.: +98-21-6022733-45; fax: +9821-6012983. E-mail address: [email protected] (M. Akhavan).
Among different theories for explaining the SI transition in REPr-123 such as hole filling [5], pair breaking [6], and hybridization [7], the substitution of Pr at the Ba site instead of the RE site (missubstitution) has been proposed to explain the insulating behavior of Pr-123 [8]. Due to the nearly equivalent positions of RE and Ba sites in the center of the peroveskite structure in RE-123, the mis-substitution theory seems to be a good approach to explain both the insulating and the superconducting behaviors of Pr-123, and perhaps the SI transition as well. The location of Pr has attracted much attention, and it can also indicate if defects may play an important role in suppressing superconductivity in the conventionally prepared Pr doped polycrystalline samples [9]. We will explain the experimental examination of the mis-substitution effect in Gd1xPrxBa2Cu3O7d (GdPr-123) system with the Rietveld analysis of XRD experiment.
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 8 6 6 - 9
302
M.R. Mohammadizadeh et al. / Physica B 321 (2002) 301–304
2. Experimental Polycrystalline GdPr-123 samples with x ¼ 0:00; 0.05, 0.10, and 0.15 have been prepared by standard solid state reaction from Gd2O3, Pr6O11, BaCO3, and CuO powders with 99.9% purity. The resistivity of the samples has been measured by a four-probe method from room temperature to below their critical temperature (Tc ). The SEM measurements have been done to determine the grain size and homogeneity of the samples. The XRD measurements have been done by a Philips 1340 powder diffractometer with Co ( and Fe filtered Ka radiation with l ¼ 1:7903 A in room temperature with 0.11 step width and 3 s step time.
3. Results and discussion XRD results have been analyzed with DBW3.2S-PC-9207 package based on the Rietveld method [10]. In the refinements, up to 25 parameters, including scale factor, cell parameters (a; b; and c), atomic coordinates, isotropic displacements (B), site occupation factors (N), and profile shape parameters were allowed to vary. The ( 2 [11]. The B of the oxygen sites was fixed to 1 A background was refined, and a Lorentzian profile function was used for all samples. The refinements were based on diffraction data in the range 201p2yp801 containing 53 reflections from orthorhombic phase. The SEM pictures show a granular homogenous structure with micrometer grain sizes. The resistivity vs. temperature curves for different samples show superconducting transition, and metallic behavior in the normal state. Tonset ; Tc ; and transition width (DT) for different amount of Pr doping are presented in Table 1. A typical observed XRD pattern, a calculated spectrum from the Rietveld method, and their differences are shown in Fig. 1. The (2 0 0) and (0 2 0) peaks near the 2y ¼ 551 are characteristics of the existence of an orthorhombic tri-perovskite phase as in YBa2Cu3O7d system [12]. Comparison of XRD patterns with JCPDS files shows that the structure is orthorhombic with Pmmm sym-
Table 1 Tonset ; Tc ; and DT of Gd1xPrxBa2Cu3O7d x
Tonset (K)
Tc (K)
DT (K)
0.00 0.05 0.10 0.15
92 93.05 92.12 88.06
91.58 90.06 82.74 71.32
2.52 3.41 11.66 19.89
metry and the impurity peaks are very small. The amount of oxygen in the samples is nearly complete. In addition, because of the small atomic scattering of the oxygen atoms with respect to the other heavy elements in GdPr-123 compound [13], we have used seven oxygen atoms in each refinement, and whence, the site occupation factor of the other atoms have been refined. The final results of the Rietveld refinements have been presented in Table 2. The Ri factors (R-pattern (RP ), R-weighted pattern (RWP ), RBragg (RB ), and R-structure factor (RF )) are small (o6.18%), and therefore the results are reliable. The site occupation factors for Gd and Ba atoms are less than their stoichiometry and they decrease with increasing x: The NPr for x ¼ 0:10 and 0.15 is stoichiometric, and for x ¼ 0:05 is zero, which may not be meaningful according to sðNPr Þ ¼ 0:04: Due to the very small impurity peaks, the reduction of NGd and NBa can be caused by missubstitution of Gd at Ba site (GdBa) and Ba at RE site (BaRE), or by formation of imperfect unit cells. Therefore, as a next step in the refinement, we have allowed the Gd, Pr, and Ba atoms to occupy missites (GdBa, PrBa, and BaRE) freely. The final refinements show that none of the mis-sites are occupied, and the other parameters (atomic coordinates, atomic displacement parameters, site occupation factors, lattice parameters, and Ri factors) do not change considerably. Thus, missubstitution does not happen, and therefore, it seems that the formation of imperfect unit cells is responsible for the reduction of NGd and NBa : These imperfect cells do not give impurity peaks in the XRD patterns, while they increase the number of formed unit cells, hence decreasing the site occupation factors. Contrary to RP and RWP ; which reflect the progress of the refinement, the RB
M.R. Mohammadizadeh et al. / Physica B 321 (2002) 301–304
303
Intensity (arb. units)
800 600 400 200 0
20
30
40
50
60
70
80
2θ(deg)
Fig. 1. Refinement of XRD pattern for Gd0.95Pr0.05Ba2Cu3O7d. Circles are calculated values. The line shows the observed intensities. The lower curve is the difference between the observed and the calculated intensities.
Table 2 Lattice parameters (a; b; and c), site occupation factors N of different atoms, and Ri factors for Gd1xPrxBa2Cu3O7d x
( a (A)
( b (A)
( c (A)
N (Gd)
N (Pr)
N (Ba)
RP (%)
RWP (%)
RB (%)
RF (%)
0.00 0.05 0.10 0.15
3.852 3.843 3.846 3.848
3.900 3.894 3.903 3.907
11.667 11.704 11.689 11.672
0.87 0.86 0.73 0.70
— 0.00 0.10 0.15
1.84 1.76 1.63 1.68
3.841 2.939 3.020 3.350
5.307 3.893 3.904 4.200
2.75 3.83 4.96 5.56
3.11 4.99 6.14 6.18
and RF are based not on the actually observed Bragg intensities, but on those deduced with the help of the model. Therefore, they are biased in favor of the model being used [10]. In Table 1, RB and RF systematically increase with increasing x; while RP and RWP do not. This shows that the refinements of all samples are almost with the same accuracy, but the expected structure (orthorhombic phase with Gd and Pr at the RE site and Ba at its own site) is less favorable with increasing x: This means that some kinds of disorders are forming with increasing x: The relatively large DT (Table 1), and its increase with increase of x is another evidence that disorders are forming with increasing x: We have also observed the reduction in XRD peak intensities with increasing x: This also shows the reduction of electronic density in the planes, which means the growth of disorders in the system. All of these are compatible with the formation of imperfect unit cells, and not the missubstitution effects. In conclusion, Pr at RE site suppresses the superconductivity as have been observed in our
samples, and it seems that the theories based on Pr at the RE site are more suitable for describing the Pr anomaly.
Acknowledgements We would like to thank C.U. Segre for useful discussions and lending a new version of the DBWS package to us. This work was supported in part by grants from the Offices of the Vice President for Research and Dean of Graduate Studies at Sharif University of Technology.
References [1] Z. Yamani, M. Akhavan, Phys. Rev. B 56 (1997) 894; M. Luszczek, W. Sadowski, T. limczuk, J. Olchowik, B. Susla, R. Czajka, Physica C 322 (1999) 57. [2] L. Soderholm, K. Zhang, D.G. Hinks, M.A. Beno, J.D. Jorgensen, C.U. Segre, I.K. Schuller, Nature 328 (1987) 604.
304
M.R. Mohammadizadeh et al. / Physica B 321 (2002) 301–304
[3] M. Akhavan, in these Proceedings (MSM 2001), Physica B 321 (2002), and references therein. [4] H.A. Blackstead, J.D. Dow, Solid State Commun. 115 (2000) 137 and references therein. [5] J.L. Peng, P. Klavins, R.N. Shelton, H.B. Radousky, P.A. Hahn, L. Bernardez, Phys. Rev. B 40 (1989) 4517. [6] J.J. Neumeier, T. Bjornholm, M.B. Maple, I.K. Schuller, Phys. Rev. Lett. 63 (1989) 2516. [7] Q. Gao, Li-yuan Zhang, Phys. Rev. B 50 (1994) 10230. [8] H.A. Blackstead, D.B. Chrisey, J.D. Dow, J.S. Horwitz, A.E. Klunzinger, D.B. Pulling, Phys. Lett. A 207 (1995) 109.
[9] Z. Klencsar, E. Kuzmann, Z. Homonnay, A. Vertes, K. Vad, J. Bankuti, T. Racz, M. Bodog, I. Kotsis, Physica C 304 (1998) 124. [10] R.A. Young, in: R.A. Young (Ed.), The Rietveld Method, Oxford University Press, New York, 1993, p. 1. [11] Ch. Bertrand, Ph. Galez, R.E. Gladyshevskii, J.L. Jorda, Physica C 321 (1999) 151. [12] M. Izumi, T. Yabe, T. Wada, A. Maeda, K. Uchinokura, S. Tanaka, H. Asano, Phys. Rev. B 40 (1989) 6771. [13] F. Izumi, H. Asano, T. Ishigaki, E. Takayama-Muromachi, Y. Uchida, N. Watanabe, T. Nishikawa, Jpn. J. Appl. Phys. 26 (1987) L649.