- Email: [email protected]
Dependence of superconducting transition temperature of GdBaSrCu3O7−δ on the annealing temperature
August 1999
Materials Letters 40 Ž1999. 151–155 www.elsevier.comrlocatermatlet
Dependence of superconducting transition temperature of GdBaSrCu 3 O 7yd on the annealing temperature R.A. Gunasekaran, B. Hellebrand, P.L. Steger, J.D. Pedarnig
)
Angewandte Physik, Johannes-Kepler UniÕersitat, ¨ A-4040 Linz, Austria Received 4 January 1999; accepted 8 March 1999
Abstract The superconducting transition temperature, Tc , of ceramic GdBaSrCu 3 O 7y d was measured for various annealing temperatures. Sintered samples were either pre-annealed in argon and post-annealed in oxygen or annealed only in oxygen at temperatures within the range 200–10008C. In general, pre-annealing at high temperatures Ž9008C. allows low temperature post-annealing ŽG 3008C. to obtain Tc f 86 K. However, both pre-annealing and post-annealing at 400–5008C are sufficient to get Tc ) 84 K and an oxygen content of 6.96 per formula unit. q 1999 Elsevier Science B.V. All rights reserved. PACS: 74.72.-h; 74.72.Bk; 74.62 Keywords: HTc ceramics; GdBaSrCu 3 O 7y d ; Annealing conditions
1. Introduction High-Tc superconductors ŽHTS. of the type REBa 2y x Sr x Cu 3 O 7y d ŽRE s rare-earth. were studied to further understand the intrinsic effect of Sr on Tc and the modification of the crystal structure induced by heat treatments w1–18x. In REBaSrCu 3 O 7y d substitution of Sr for Ba reduces Tc and the orthorhombic distortion of the lattice, depending on the size of the rare-earth ion w7x. However, pre-annealing in argon and subsequent post-annealing in oxygen was found to increase the orthorhombic distortion and the Tc values w12,13,15–18x. REBaSrCu 3 O 7y d ŽRE s Dy, Eu, Sm, Gd. samples were prepared in
) Corresponding author. Tel.: q43-732-24689246; Fax: q43732-24689242; E-mail: [email protected]
two different forms with different degrees of orthorhombic distortion, by adopting different synthesis methods w12,14–18x. In all of the previous investigations on the pre-annealing of REBaSrCu 3 O 7y d , the temperatures were arbitrarily fixed w12,14–18x. In general, pre-annealing in argon was done at 800– 9008C and post-annealing in oxygen at 350–5008C. The influence of varying both pre-annealing and post-annealing temperatures on Tc has not been systematically investigated so far. Recently, we studied the GdBa 2y x Sr x Cu 3 O 7y d system in detail with respect to the crystal structure and oxygen stoichiometry for both the tetragonal and orthorhombic phase w17,18x. In this paper, we report the dependence of the Tc value of GdBaSrCu 3 O 7y d on annealing at different temperatures in argon and oxygen atmosphere. The annealing temperature range effective to maximize the Tc value is discussed.
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 6 6 - X
152
R.A. Gunasekaran et al.r Materials Letters 40 (1999) 151–155
2. Experimental GdBaSrCu 3 O 7y d ŽGBSCO. samples were prepared by solid state reaction. The details of the preparation procedure were reported previously w17,18x. The samples Žsintered at 9508C in air. were pre-annealed in argon and post-annealed in oxygen at various temperatures in the range 200–10008C for 8 h and cooled to room temperature at a rate of 18Crmin. All samples were characterized by powder X-ray diffraction ŽXRD. and electrical resistance measurements using the four-point method. The oxygen content of the samples was determined by iodometric titration. The lattice parameters were calculated by least-square refinement.
3. Results and discussion Fig. 1 shows the XRD patterns of two series of samples annealed in oxygen at different temperatures in the range 300 F Tox F 10008C, with or without pre-annealing treatment in argon at Tar s 9008C. The orthorhombic distortion is inferred from the splitting of the Ž020.rŽ200. peaks at 2 u f 47.08 ŽFig. 1b..
The splitting is relatively weaker for samples annealed only in oxygen and decreases for both low ŽTox F 3008C. and high ŽTox G 7008C. annealing temperatures. However, for samples pre-annealed in argon at 9008C and post-annealed in oxygen, the splitting is increased indicating enhanced orthorhombicity for post-annealing temperatures up to Tox s 7008C. The lattice parameters are shown for both series in Fig. 2. The lattice constants for all the samples pre-annealed in argon were in the range ˚ b s 3.83–3.86 A˚ and c s 11.49– a s 3.79–3.83 A, ˚ For samples without pre-annealing treat11.55 A. ment, lattice constants were in the range a s 3.81– ˚ b s 3.84–3.85 A˚ and c s 11.50–11.55 A. ˚ 3.83 A, For same annealing temperature in oxygen the a-axis parameter of pre-annealed samples was smaller than that of samples without pre-annealing. For both series of samples the a-axis parameter slightly increased with annealing temperature in oxygen. The b- and c-axis parameters did not change significantly upon pre-annealing, but decreased slightly at higher post-annealing temperatures. The Tc value, oxygen content, and orthorhombicity Ž b–a.ra for both series are shown in Fig. 3. Samples annealed only in oxygen at Tox - 4008C
Fig. 1. Powder XRD patterns of GdBaSrCu 3 O 7y d samples annealed in oxygen at different temperatures, without Ža. and with Žb. pre-annealing in argon at 9008C.
R.A. Gunasekaran et al.r Materials Letters 40 (1999) 151–155
153
decreased at higher temperatures and were comparable at 10008C. The dependence of Tc and oxygen stoichiometry upon annealing was investigated also for different pre-annealing temperatures in the range 3008C– 10008C. Samples with high Tc Žf 85 K. and high oxygen content Ž6.93. were obtained for pre-annealing temperatures Tar G 5008C and post-annealing temperatures as low as Tox s 3008C ŽFig. 4.. Pre-annealing at Tar - 4008C required Tox ) 4008C to achieve high Tc value and high oxygen content. Pre-annealing at Tar s 10008C led to partial decomposition of the sample. For this ‘multi-phase’ sample annealing in oxygen at Tox s 10008C was necessary to restore its original single phase nature and render
Fig. 2. Lattice parameters for two series of GBSCO samples annealed in oxygen at different temperatures, without Ž^. and with Ž'. pre-annealing in argon at 9008C.
showed a lower oxygen content ŽF 6.86 " 0.02. and lower Tc values ŽF 73 K.. The sample annealed at Tox s 2008C had an oxygen content of 6.82 and Tc s 23 K. The oxygen stoichiometry of this sample is possibly due to oxygen incorporated during sintering and cooling stages in air and not due to annealing in oxygen at 2008C. Annealing in oxygen in the range 5008C F Tox F 8008C resulted in Tc f 85 K and an oxygen content of 6.95. At higher temperatures ŽTox G 9008C., the Tc was lowered and the oxygen content was slightly decreased. However, after pre-annealing in argon at 9008C a wide window of post-annealing temperatures, 3008C F Tox F 9008C, did yield samples with Tc ( 86 K. These samples had oxygen contents in the range 6.92–6.98 and showed enhanced orthorhombicity. Post-annealing at Tox s 10008C reduced Tc . Post-annealing at Tox s 2008C rendered the samples non-superconducting showing semiconductor-like behavior. For this material, the oxygen content could not be determined by iodometric titration possibly due to the absence of CuŽIII.. For both series of samples the orthorhombicity values were nearly constant up to Tox s 6008C,
Fig. 3. Comparison of Tc , oxygen content and orthorhombicity Ž b – a.r a of two series of GBSCO samples annealed in oxygen, without ŽI. and with ŽB. pre-annealing in argon at 9008C. Orthorhombicity values shown are wŽ b – a.r ax=10 2 .
R.A. Gunasekaran et al.r Materials Letters 40 (1999) 151–155
154
Fig. 4. Tc value and oxygen content of GBSCO samples pre-annealed in argon at different temperatures and post-annealed in oxygen at 3008C Žv . and 4008C Ž^..
it superconducting. Table 1 shows data of selected samples for which the Tc value, oxygen content and lattice parameters are significantly dependent on annealing temperatures. The influence of annealing in argon and oxygen on Tc of GBSCO may be qualitatively explained considering the oxygen stoichiometry and ordering Table 1 Lattice parameters, oxygen content Ž x . and Tc value of some selected GdBaSrCu 3 O x samples annealed at various temperatures in argon ŽTar . and oxygen ŽTox . Tar Ž8C.
Tox Ž8C.
a ˚. ŽA
b ˚. ŽA
c ˚. ŽA
x
Tc0 ŽK.
300 300 300 300 400 400 400 400 500 500 500 500 700 700 700 700 1000
300 400 900 1000 300 400 900 1000 300 400 900 1000 300 400 900 1000 1000
3.826Ž5. 3.817Ž7. 3.825Ž6. 3.820Ž7. 3.821Ž7. 3.824Ž3. 3.823Ž3. 3.824Ž3. 3.819Ž4. 3.808Ž3. 3.812Ž6. 3.826Ž2. 3.820Ž3. 3.820Ž3. 3.826Ž2. 3.827Ž3. 3.824Ž2.
3.849Ž4. 3.848Ž6. 3.844Ž5. 3.849Ž7. 3.848Ž8. 3.846Ž3. 3.847Ž3. 3.845Ž6. 3.847Ž5. 3.850Ž7. 3.850Ž6. 3.847Ž8. 3.845Ž3. 3.848Ž3. 3.851Ž3. 3.840Ž2. 3.830Ž1.
11.519Ž11. 11.530Ž9. 11.520Ž8. 11.519Ž6. 11.525Ž13. 11.521Ž9. 11.519Ž9. 11.510Ž12. 11.529Ž5. 11.540Ž6. 11.541Ž8. 11.540Ž11. 11.506Ž8. 11.512Ž10. 11.524Ž7. 11.530Ž8. 11.497Ž5.
6.89 6.96 6.96 6.94 6.90 6.96 6.96 6.97 6.92 6.95 6.96 6.94 6.98 6.97 6.96 6.95 6.97
66.7 82.2 84.0 83.7 74.3 84.4 84.8 83.5 84.9 85.2 84.0 83.1 85.0 85.2 83.3 82.5 83.4
in the basal plane which influence the CuŽIII. content and the transition temperature according to the charge-transfer model w19x. In our experiments, preannealing at Tar G 5008C and post-annealing at Tox G 3008C enhance the oxygen content, orthorhombicity and Tc as compared to samples annealed only in oxygen. The improved oxygen incorporation and ordering may be due to a removal of oxygen from the basal plane during pre-annealing at high temperatures. However, pre-annealing at lower temperatures, Tar ) 5008C, will not remove completely the oxygen and may enhance a random distribution of oxygen in the basal plane. A random occupancy of oxygen could be responsible for the lower Tc observed for samples which are pre-annealed at Tar - 5008C and post-annealed at Tox s 3008C. For low-temperature pre-annealed samples, post-annealing at Tox G 4008C is necessary to establish high Tc values. At very high post-annealing temperatures, Tox ) 9008C, Tc and orthorhombicity are both decreased probably due to an increased oxygen disorder in the basal plane.
4. Conclusions The influence of pre-annealing in argon and postannealing in oxygen in the temperature range 200– 10008C on the Tc value, oxygen content, orthorhombicity and phase purity of ceramic GdBaSrCu 3 O 7y d
R.A. Gunasekaran et al.r Materials Letters 40 (1999) 151–155
was investigated. Wide windows of pre-annealing temperatures Ž500–9008C. and post-annealing temperatures Ž300–9008C. were found to produce GBSCO samples with Tc f 86 K, high oxygen content and enhanced orthorhombicity. Samples with Tc ) 84 K and high oxygen content were achieved for preannealing and post-annealing temperatures G 4008C. Both pre-annealing and post-annealing at temperatures T - 3008C and T ) 9008C are not useful for enhancing Tc . Acknowledgements
¨ The authors thank the ‘Jubilaumsfonds der Oster¨ reichischen Nationalbank’ for financial support. References w1x B.W. Veal, W.K. Kwok, A. Umezawa, G.W. Crabtree, J.D. Jorgensen, J.W. Downey, L.J. Nowick, A.W. Mitchell, A.P. Paulikas, C.H. Sowers, Appl. Phys. Lett. 51 Ž1987. 279. w2x T. Wada, S. Adachi, T. Mihara, R. Inaba, Jpn. J. Appl. Phys. 26 Ž1987. L706.
155
w3x D.B. Currie, A.M. Forrest, Solid State Commun. 66 Ž1988. 715. w4x Y. Zhao, H. Zhang, T. Zhang, S.F. Sun, Z.Y. Chen, Q.R. Zhang, Physica C 152 Ž1988. 513. w5x L. Parent, C. Moreau, J. Mater. Sci. 26 Ž1991. 5873. w6x X.Z. Wang, D. Bauerle, Physica C 176 Ž1991. 507. ¨ w7x X.Z. Wang, B. Hellebrand, D. Bauerle, Physica C 200 Ž1992. ¨ 12. w8x J. Golben, M. Vlasse, Supercond. Sci. Tech. 5 Ž1992. 231. w9x V. Badri, U.V. Varadaraju, Mater. Res. Bull. 27 Ž1992. 591. w10x A. Nafidi, R. Suryanarayanan, Physica C 249 Ž1995. 262. w11x A. Nafidi, R. Suryanarayanan, Physica C 225 Ž1994. 105. w12x X.Z. Wang, P.L. Steger, M. Reisser, W. Steiner, Physica C 196 Ž1992. 247. w13x J.M. Rozell, G.W. Book, J. Cunningham, C. Glorioso, M. Vlasse, J.P. Golben, Physica C 204 Ž1993. 384. w14x R. Suryanarayanan, A. Nafidi, E. Chavira, N. Le Nagard, Physica C 235–240 Ž1994. 881. w15x R. Suryanarayanan, A. Nafidi, A. Das, J. Appl. Phys. 76 Ž1994. 598. w16x X.Z. Wang, B. Hellebrand, D. Buerle, M. Strecker, G. Wortmann, W. Lang, Physica C 242 Ž1995. 55. w17x R.A. Gunasekaran, B. Hellebrand, P.L. Steger, Physica C 270 Ž1996. 25. w18x B. Hellebrand, R.A. Gunasekaran, P.L. Steger, D. Bauerle, ¨ Appl. Phys. A 65 Ž1997. 235. w19x G. Uimin, J.R. Mignod, Physica C 199 Ž1992. 251.
Materials Letters 40 Ž1999. 151–155 www.elsevier.comrlocatermatlet
Dependence of superconducting transition temperature of GdBaSrCu 3 O 7yd on the annealing temperature R.A. Gunasekaran, B. Hellebrand, P.L. Steger, J.D. Pedarnig
)
Angewandte Physik, Johannes-Kepler UniÕersitat, ¨ A-4040 Linz, Austria Received 4 January 1999; accepted 8 March 1999
Abstract The superconducting transition temperature, Tc , of ceramic GdBaSrCu 3 O 7y d was measured for various annealing temperatures. Sintered samples were either pre-annealed in argon and post-annealed in oxygen or annealed only in oxygen at temperatures within the range 200–10008C. In general, pre-annealing at high temperatures Ž9008C. allows low temperature post-annealing ŽG 3008C. to obtain Tc f 86 K. However, both pre-annealing and post-annealing at 400–5008C are sufficient to get Tc ) 84 K and an oxygen content of 6.96 per formula unit. q 1999 Elsevier Science B.V. All rights reserved. PACS: 74.72.-h; 74.72.Bk; 74.62 Keywords: HTc ceramics; GdBaSrCu 3 O 7y d ; Annealing conditions
1. Introduction High-Tc superconductors ŽHTS. of the type REBa 2y x Sr x Cu 3 O 7y d ŽRE s rare-earth. were studied to further understand the intrinsic effect of Sr on Tc and the modification of the crystal structure induced by heat treatments w1–18x. In REBaSrCu 3 O 7y d substitution of Sr for Ba reduces Tc and the orthorhombic distortion of the lattice, depending on the size of the rare-earth ion w7x. However, pre-annealing in argon and subsequent post-annealing in oxygen was found to increase the orthorhombic distortion and the Tc values w12,13,15–18x. REBaSrCu 3 O 7y d ŽRE s Dy, Eu, Sm, Gd. samples were prepared in
) Corresponding author. Tel.: q43-732-24689246; Fax: q43732-24689242; E-mail: [email protected]
two different forms with different degrees of orthorhombic distortion, by adopting different synthesis methods w12,14–18x. In all of the previous investigations on the pre-annealing of REBaSrCu 3 O 7y d , the temperatures were arbitrarily fixed w12,14–18x. In general, pre-annealing in argon was done at 800– 9008C and post-annealing in oxygen at 350–5008C. The influence of varying both pre-annealing and post-annealing temperatures on Tc has not been systematically investigated so far. Recently, we studied the GdBa 2y x Sr x Cu 3 O 7y d system in detail with respect to the crystal structure and oxygen stoichiometry for both the tetragonal and orthorhombic phase w17,18x. In this paper, we report the dependence of the Tc value of GdBaSrCu 3 O 7y d on annealing at different temperatures in argon and oxygen atmosphere. The annealing temperature range effective to maximize the Tc value is discussed.
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 6 6 - X
152
R.A. Gunasekaran et al.r Materials Letters 40 (1999) 151–155
2. Experimental GdBaSrCu 3 O 7y d ŽGBSCO. samples were prepared by solid state reaction. The details of the preparation procedure were reported previously w17,18x. The samples Žsintered at 9508C in air. were pre-annealed in argon and post-annealed in oxygen at various temperatures in the range 200–10008C for 8 h and cooled to room temperature at a rate of 18Crmin. All samples were characterized by powder X-ray diffraction ŽXRD. and electrical resistance measurements using the four-point method. The oxygen content of the samples was determined by iodometric titration. The lattice parameters were calculated by least-square refinement.
3. Results and discussion Fig. 1 shows the XRD patterns of two series of samples annealed in oxygen at different temperatures in the range 300 F Tox F 10008C, with or without pre-annealing treatment in argon at Tar s 9008C. The orthorhombic distortion is inferred from the splitting of the Ž020.rŽ200. peaks at 2 u f 47.08 ŽFig. 1b..
The splitting is relatively weaker for samples annealed only in oxygen and decreases for both low ŽTox F 3008C. and high ŽTox G 7008C. annealing temperatures. However, for samples pre-annealed in argon at 9008C and post-annealed in oxygen, the splitting is increased indicating enhanced orthorhombicity for post-annealing temperatures up to Tox s 7008C. The lattice parameters are shown for both series in Fig. 2. The lattice constants for all the samples pre-annealed in argon were in the range ˚ b s 3.83–3.86 A˚ and c s 11.49– a s 3.79–3.83 A, ˚ For samples without pre-annealing treat11.55 A. ment, lattice constants were in the range a s 3.81– ˚ b s 3.84–3.85 A˚ and c s 11.50–11.55 A. ˚ 3.83 A, For same annealing temperature in oxygen the a-axis parameter of pre-annealed samples was smaller than that of samples without pre-annealing. For both series of samples the a-axis parameter slightly increased with annealing temperature in oxygen. The b- and c-axis parameters did not change significantly upon pre-annealing, but decreased slightly at higher post-annealing temperatures. The Tc value, oxygen content, and orthorhombicity Ž b–a.ra for both series are shown in Fig. 3. Samples annealed only in oxygen at Tox - 4008C
Fig. 1. Powder XRD patterns of GdBaSrCu 3 O 7y d samples annealed in oxygen at different temperatures, without Ža. and with Žb. pre-annealing in argon at 9008C.
R.A. Gunasekaran et al.r Materials Letters 40 (1999) 151–155
153
decreased at higher temperatures and were comparable at 10008C. The dependence of Tc and oxygen stoichiometry upon annealing was investigated also for different pre-annealing temperatures in the range 3008C– 10008C. Samples with high Tc Žf 85 K. and high oxygen content Ž6.93. were obtained for pre-annealing temperatures Tar G 5008C and post-annealing temperatures as low as Tox s 3008C ŽFig. 4.. Pre-annealing at Tar - 4008C required Tox ) 4008C to achieve high Tc value and high oxygen content. Pre-annealing at Tar s 10008C led to partial decomposition of the sample. For this ‘multi-phase’ sample annealing in oxygen at Tox s 10008C was necessary to restore its original single phase nature and render
Fig. 2. Lattice parameters for two series of GBSCO samples annealed in oxygen at different temperatures, without Ž^. and with Ž'. pre-annealing in argon at 9008C.
showed a lower oxygen content ŽF 6.86 " 0.02. and lower Tc values ŽF 73 K.. The sample annealed at Tox s 2008C had an oxygen content of 6.82 and Tc s 23 K. The oxygen stoichiometry of this sample is possibly due to oxygen incorporated during sintering and cooling stages in air and not due to annealing in oxygen at 2008C. Annealing in oxygen in the range 5008C F Tox F 8008C resulted in Tc f 85 K and an oxygen content of 6.95. At higher temperatures ŽTox G 9008C., the Tc was lowered and the oxygen content was slightly decreased. However, after pre-annealing in argon at 9008C a wide window of post-annealing temperatures, 3008C F Tox F 9008C, did yield samples with Tc ( 86 K. These samples had oxygen contents in the range 6.92–6.98 and showed enhanced orthorhombicity. Post-annealing at Tox s 10008C reduced Tc . Post-annealing at Tox s 2008C rendered the samples non-superconducting showing semiconductor-like behavior. For this material, the oxygen content could not be determined by iodometric titration possibly due to the absence of CuŽIII.. For both series of samples the orthorhombicity values were nearly constant up to Tox s 6008C,
Fig. 3. Comparison of Tc , oxygen content and orthorhombicity Ž b – a.r a of two series of GBSCO samples annealed in oxygen, without ŽI. and with ŽB. pre-annealing in argon at 9008C. Orthorhombicity values shown are wŽ b – a.r ax=10 2 .
R.A. Gunasekaran et al.r Materials Letters 40 (1999) 151–155
154
Fig. 4. Tc value and oxygen content of GBSCO samples pre-annealed in argon at different temperatures and post-annealed in oxygen at 3008C Žv . and 4008C Ž^..
it superconducting. Table 1 shows data of selected samples for which the Tc value, oxygen content and lattice parameters are significantly dependent on annealing temperatures. The influence of annealing in argon and oxygen on Tc of GBSCO may be qualitatively explained considering the oxygen stoichiometry and ordering Table 1 Lattice parameters, oxygen content Ž x . and Tc value of some selected GdBaSrCu 3 O x samples annealed at various temperatures in argon ŽTar . and oxygen ŽTox . Tar Ž8C.
Tox Ž8C.
a ˚. ŽA
b ˚. ŽA
c ˚. ŽA
x
Tc0 ŽK.
300 300 300 300 400 400 400 400 500 500 500 500 700 700 700 700 1000
300 400 900 1000 300 400 900 1000 300 400 900 1000 300 400 900 1000 1000
3.826Ž5. 3.817Ž7. 3.825Ž6. 3.820Ž7. 3.821Ž7. 3.824Ž3. 3.823Ž3. 3.824Ž3. 3.819Ž4. 3.808Ž3. 3.812Ž6. 3.826Ž2. 3.820Ž3. 3.820Ž3. 3.826Ž2. 3.827Ž3. 3.824Ž2.
3.849Ž4. 3.848Ž6. 3.844Ž5. 3.849Ž7. 3.848Ž8. 3.846Ž3. 3.847Ž3. 3.845Ž6. 3.847Ž5. 3.850Ž7. 3.850Ž6. 3.847Ž8. 3.845Ž3. 3.848Ž3. 3.851Ž3. 3.840Ž2. 3.830Ž1.
11.519Ž11. 11.530Ž9. 11.520Ž8. 11.519Ž6. 11.525Ž13. 11.521Ž9. 11.519Ž9. 11.510Ž12. 11.529Ž5. 11.540Ž6. 11.541Ž8. 11.540Ž11. 11.506Ž8. 11.512Ž10. 11.524Ž7. 11.530Ž8. 11.497Ž5.
6.89 6.96 6.96 6.94 6.90 6.96 6.96 6.97 6.92 6.95 6.96 6.94 6.98 6.97 6.96 6.95 6.97
66.7 82.2 84.0 83.7 74.3 84.4 84.8 83.5 84.9 85.2 84.0 83.1 85.0 85.2 83.3 82.5 83.4
in the basal plane which influence the CuŽIII. content and the transition temperature according to the charge-transfer model w19x. In our experiments, preannealing at Tar G 5008C and post-annealing at Tox G 3008C enhance the oxygen content, orthorhombicity and Tc as compared to samples annealed only in oxygen. The improved oxygen incorporation and ordering may be due to a removal of oxygen from the basal plane during pre-annealing at high temperatures. However, pre-annealing at lower temperatures, Tar ) 5008C, will not remove completely the oxygen and may enhance a random distribution of oxygen in the basal plane. A random occupancy of oxygen could be responsible for the lower Tc observed for samples which are pre-annealed at Tar - 5008C and post-annealed at Tox s 3008C. For low-temperature pre-annealed samples, post-annealing at Tox G 4008C is necessary to establish high Tc values. At very high post-annealing temperatures, Tox ) 9008C, Tc and orthorhombicity are both decreased probably due to an increased oxygen disorder in the basal plane.
4. Conclusions The influence of pre-annealing in argon and postannealing in oxygen in the temperature range 200– 10008C on the Tc value, oxygen content, orthorhombicity and phase purity of ceramic GdBaSrCu 3 O 7y d
R.A. Gunasekaran et al.r Materials Letters 40 (1999) 151–155
was investigated. Wide windows of pre-annealing temperatures Ž500–9008C. and post-annealing temperatures Ž300–9008C. were found to produce GBSCO samples with Tc f 86 K, high oxygen content and enhanced orthorhombicity. Samples with Tc ) 84 K and high oxygen content were achieved for preannealing and post-annealing temperatures G 4008C. Both pre-annealing and post-annealing at temperatures T - 3008C and T ) 9008C are not useful for enhancing Tc . Acknowledgements
¨ The authors thank the ‘Jubilaumsfonds der Oster¨ reichischen Nationalbank’ for financial support. References w1x B.W. Veal, W.K. Kwok, A. Umezawa, G.W. Crabtree, J.D. Jorgensen, J.W. Downey, L.J. Nowick, A.W. Mitchell, A.P. Paulikas, C.H. Sowers, Appl. Phys. Lett. 51 Ž1987. 279. w2x T. Wada, S. Adachi, T. Mihara, R. Inaba, Jpn. J. Appl. Phys. 26 Ž1987. L706.
155
w3x D.B. Currie, A.M. Forrest, Solid State Commun. 66 Ž1988. 715. w4x Y. Zhao, H. Zhang, T. Zhang, S.F. Sun, Z.Y. Chen, Q.R. Zhang, Physica C 152 Ž1988. 513. w5x L. Parent, C. Moreau, J. Mater. Sci. 26 Ž1991. 5873. w6x X.Z. Wang, D. Bauerle, Physica C 176 Ž1991. 507. ¨ w7x X.Z. Wang, B. Hellebrand, D. Bauerle, Physica C 200 Ž1992. ¨ 12. w8x J. Golben, M. Vlasse, Supercond. Sci. Tech. 5 Ž1992. 231. w9x V. Badri, U.V. Varadaraju, Mater. Res. Bull. 27 Ž1992. 591. w10x A. Nafidi, R. Suryanarayanan, Physica C 249 Ž1995. 262. w11x A. Nafidi, R. Suryanarayanan, Physica C 225 Ž1994. 105. w12x X.Z. Wang, P.L. Steger, M. Reisser, W. Steiner, Physica C 196 Ž1992. 247. w13x J.M. Rozell, G.W. Book, J. Cunningham, C. Glorioso, M. Vlasse, J.P. Golben, Physica C 204 Ž1993. 384. w14x R. Suryanarayanan, A. Nafidi, E. Chavira, N. Le Nagard, Physica C 235–240 Ž1994. 881. w15x R. Suryanarayanan, A. Nafidi, A. Das, J. Appl. Phys. 76 Ž1994. 598. w16x X.Z. Wang, B. Hellebrand, D. Buerle, M. Strecker, G. Wortmann, W. Lang, Physica C 242 Ž1995. 55. w17x R.A. Gunasekaran, B. Hellebrand, P.L. Steger, Physica C 270 Ž1996. 25. w18x B. Hellebrand, R.A. Gunasekaran, P.L. Steger, D. Bauerle, ¨ Appl. Phys. A 65 Ž1997. 235. w19x G. Uimin, J.R. Mignod, Physica C 199 Ž1992. 251.