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Reducing and oxidizing annealings of bismuth high-Tc superconductors
Physica C 162-164 (1989) 1215-1216 North-Holland
REDUCING AND OXIDIZING ANNEALINGS OF BISMUTH HIGH-To SUPERCONDUCTORS * Octavio PENA, Aziz D1NIA, Christiane PERRIN, Andr~ PERRIN, Marcel SERGENT Laboratoire de Chimie Min6rale B, Unit6 Associ6e au CNRS n°254, Universit6 de Rennes I, Avenue du G6n~ral Leclerc, 35042 Rennes C6dex (France) E f f e c t s of annealings under various a t m o s p h e r e s on the s u p e r c o n d u c t i v i t y of Bi-Sr-Ca-Cu--O m a t e r i a l s are p r e s e n t e d . Samples of nominal compositions 1112, 2212 and 2202 were annealed under vacuum or nitrogen gas, then r e a c t e d under oxygen or NF 3 gas flows. Fully reversible and opposite e f f e c t s on Tc are observed when the 110-K and 85-K phases are annealed under the s a m e conditions. In particular in the 110-K phase, a d e c r e a s e of Tc is obtained under reducing conditions, and then r e s t o r e d by short annealings under oxygen flow at low t e m p e r a t u r e s . Similar trends are r e p o r t e d by r e a c t i o n under diluted NF 3. Negligible weight variations were observed, suggesting t h a t only a very small amount of oxygen (or fluorine) might be eventually inserted, but the main mechanism must be r e l a t e d to internal conversion b e t w e e n oxidation s t a t e s of both bismuth and copper, controled by t h e r m o d y n a m i c a l conditions.
1.
INTRODUCTION Superconductivity of c o p p e r - b a s e d oxides is clearly r e l a t e d to the oxygen c o n t e n t : in YBa2Cu3Ox, for instance, the superconducting p r o p e r t i e s strongly depend on the filling of oxygen vacancies. In c o n t r a s t , no d r a m a t i c dependences, if at aLl, have been r e p o r t e d in the b i s m u t h - c o p p e r - o x i d e family: a small reversible oxygen evolution was shown by TGA e x p e r i m e n t s 1 in the 2212 superconductor (Tc=85 K), but no clear c o r r e l a t i o n with the resistive behaviour was found. Synthesis conditions ( t e m p e r a t u r e and t i m e of annealing) are even more c r i t i c a l in the bismuth family than in Y-Ba-Cu-O, although cooling speed is less i m p o r t a n t (no need of annealing s t e p s to impose s t r u c t u r a l s y m m e t r y or oxygen content). In this r e p o r t , we show t h a t a d r a m a t i c d e p e n d e n c e of superconducting p r o p e r t i e s is also observed in the bismuth family as a function of annealing conditions and surrounding a t m o s p h e r e s , which may be r e l a t e d to small changes of the oxygen c o n t e n t and/or to the formal copper and bismuth oxidation s t a t e s . 2.
RESULTS 2.1. The 110-K phase Samples of nominal composition 1112 show two transitions at 85 and 110 K, a s s o c i a t e d r e s p e c t i v e l y to the 2212 and 2223 phases 2-4. Oxidizing annealings (either oxygen or diluted NF 3 gas) have no e f f e c t e i t h e r on Tc or on the amplitude of the inductive transition a t 110 K. Reducing conditions by annealing under nitrogen or dynamic vacuum in the range 250
0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
0..................
I
i
1
i
i
J
~
I
~
80 90 TEMPERATURE (K)
~
i
LI
,,,,
1(30
110
FIGURE 1 E f f e c t of annealing under oxygen flow, for two hours, of a v a c u u m - a n n e a l e d sample of composition 1112.
and showed t h a t the l l0-K transition is almost fully r e c o v e r e d a f t e r 17 hours of oxygenation a t 220°C'L Fluorination shows the s a m e tendency, t h a t is, the Tc onset increases from 90 upto 106 K a f t e r two hours of r e a c t i o n at 230 °C, but longer t i m e s or higher t e m p e r a t u r e s lead to samples'decomposition5. 2.2. The 2212 phase As-grown pellets show a transition a t Tc onset of 85 K, followed by another sharp d e c r e a s e of xa.c. a t about 58 K, which would suggest the e x i s t e n c e of another superconducting phase in this sample. Similar anomalies were also r e p o r t e d in ref.6, and we think t h a t it might be a g e n e r a l f e a t u r e observed in c e r a m i c samples due to the i n t e r g r o w t h or stackin~g faults, and easily suppressed in single crystals J. In those cases, full transitions are obtained above 65-70 K, and confirm t h a t such anomalies are not intrinsic to the 2212 phase. Figure 2(a) shows the upper transition when annealed under vacuum for 1 hour: the Tc onset
O. Pe~a et al. / A nnealings of bismuth high-Tc superconductors
1216
Vacuum ann~aled
higher temperatures (T> 450 °C) a clear decrease of the amplitude and the appearance of a "bump" at 4.5 K were observed, suggesting a bi-phased compound, although X-Ray patterns remain unchanged.
0.1
as grown I IO0"C 2 220"C 0
3.
(a)
3 240"C
]
I Ozygen annealeO
5 5~C'C 1
8-9 ?
5 , ~
7
:
5: Vacuuma n ~ c
6: 2~'C 300"C
(b)
TEMP~RAIURE ~K)
FIGURE 2 Annealing effects at given temperatures for the 2212 composition. (a) as-grown samples annealed under vacuum for one hour; (b) vacuum-annealed samples reannealed under oxygen flow, for one hour.
increases from 85 to 95 K at rather low annealing temperatures (220 °C ST.< 460 °C). At higher temperatures (T=580 °C), a decrease in amplitude is obtained, although the onset remains at 95 K. The vacuum-annealed sample (curves 5, figure 2) was reoxidized: at low annealing temperatures the transition's amplitude improves (curve 6); then, a complete reversibility of the process is observed, and seems to saturate at about 400 °C. It should be noted that the final curves go beyond the one observed in the as-grown pellet, suggesting that the "85-K" phase is in fact a metastable state which greatly depends on the preparation conditions. A non-monotonic variation was also reported in ref.7, as a function of oxygen pressures. Their annealings were performed at high temperatures (above 600 °C) and led to large reductions of the superconducting signal7. Other authors found a complete loss of superconductivity at high temperatures and high oxygen pressures l . We show, in our case, t h a t such a reversibility of e f f e c t s may be obtained under milder conditions. 2.3. The 2202 phase As-grown pellets show a sharp transition at 6K, ATc= 1.5 K, with a full amplitude at about 3K. No variation is observed when annealing under vacuum below 400 °C. At 450 °C, the amplitude decreases and the Tc onset goes down to 5-5.5 K, while no impurity phases are detected. Oxygen annealing was performed on this latter sample, showing complete restoration of the superconducting properties when annealed at 250 °C. At
DISCUSSION In previous works we evidenced important effects of the annealing on the II 0-K phase 2,4,~5. Surprisingly, no significant weight losses during vacuum annealing, and absolutely no weight increase during the oxygenation process were noticed. A different situation was observed after NF3 treatment, where a detectable, although very slight, weight increase was observed. Basically the same features are seen in the 2212 and 2202 phases. Such weight variations, negligible compared to those observed in Y - B a - C u - O , provoke reversible effects largely comparable. One fundamental difference between the yttrium and bismuth systems is that in the latter, two multiple-valence systems coexist together, Bi(lll)/Bi(V) and Cu(lIl)/Cu(ll) instead of just one in the Y - B a - C u - O phase. An internal conversion between the oxidation states of bismuth and copper is thus possible, which must be largely controlled by thermodynamical conditions (temperature, time, environment, pressure, etc). Such internal mechanisms are consistent with the role played by lead-doping in the stabilization of the respective structures, probably modifying the electrochemical equilibria. It must be noted that similar annealings give opposite effects on the transitions of the 85 and If0 K phases: Tc decreases under reducing conditions in one of them, and under oxidizing atmospheres in the other one.Both phases, differing by just the number of CuO2 planes, are so interrelated that the possible diffusion of very small quantities of oxygen in one of them may act oppositely in the other. In the 2202 phase, only the amplitude of the Meissner effect seems affected. It is possible that in this case, a slightly different mechanism or different occupation of sites takes place, leading to a reduction of the Meissner amplitude with no change of the Tc onset, as we have also observed in YBa2Cu306.7Fy samples 8.
REFERENCES *. Research under MRT contract n°508819/Thomson CSF, and by Rhone-Poulenc contract. I. J.M.Tarascon,Y. Le Page,P.Barboux,B.Bagley, L.Greene, W.McKinnon,G.Hull,M.Giroud, D.Hwang, Phys.Rev.B, 37 (1988) 9382. 2. O. Peha,A.Perrin,M.Sergent,PhysicaC,156(I 988)489 3. A.Dinia et al, to be published. 4. A.Perrin, A.Dinia, O.Peha, C.Perrin, M.Sergent, Materials Letters, 8(1989)165. 5. O. Pefla,C.Perrin,A. Dinia,M.Sergent, P.Christensen, G.Fonteneau, J.Lucas, PhysicaC,158(1989)443. 6. X. Wang,K.Donnelly,T.Bakas,J.Coey,l.Rosenman, C.Simon, Solid State Comm.,69(1989) 829. 7. D. Morris,C. Hultgren,A.Markelz,J. Wei,N.Asmar, J.Nickel, preprint(1988). 8. C.Perrin,O.Pefla,A.Dinia, M.Sergent,G. Fonteneau, P.Christensen,J.Lucas, this Conference.
REDUCING AND OXIDIZING ANNEALINGS OF BISMUTH HIGH-To SUPERCONDUCTORS * Octavio PENA, Aziz D1NIA, Christiane PERRIN, Andr~ PERRIN, Marcel SERGENT Laboratoire de Chimie Min6rale B, Unit6 Associ6e au CNRS n°254, Universit6 de Rennes I, Avenue du G6n~ral Leclerc, 35042 Rennes C6dex (France) E f f e c t s of annealings under various a t m o s p h e r e s on the s u p e r c o n d u c t i v i t y of Bi-Sr-Ca-Cu--O m a t e r i a l s are p r e s e n t e d . Samples of nominal compositions 1112, 2212 and 2202 were annealed under vacuum or nitrogen gas, then r e a c t e d under oxygen or NF 3 gas flows. Fully reversible and opposite e f f e c t s on Tc are observed when the 110-K and 85-K phases are annealed under the s a m e conditions. In particular in the 110-K phase, a d e c r e a s e of Tc is obtained under reducing conditions, and then r e s t o r e d by short annealings under oxygen flow at low t e m p e r a t u r e s . Similar trends are r e p o r t e d by r e a c t i o n under diluted NF 3. Negligible weight variations were observed, suggesting t h a t only a very small amount of oxygen (or fluorine) might be eventually inserted, but the main mechanism must be r e l a t e d to internal conversion b e t w e e n oxidation s t a t e s of both bismuth and copper, controled by t h e r m o d y n a m i c a l conditions.
1.
INTRODUCTION Superconductivity of c o p p e r - b a s e d oxides is clearly r e l a t e d to the oxygen c o n t e n t : in YBa2Cu3Ox, for instance, the superconducting p r o p e r t i e s strongly depend on the filling of oxygen vacancies. In c o n t r a s t , no d r a m a t i c dependences, if at aLl, have been r e p o r t e d in the b i s m u t h - c o p p e r - o x i d e family: a small reversible oxygen evolution was shown by TGA e x p e r i m e n t s 1 in the 2212 superconductor (Tc=85 K), but no clear c o r r e l a t i o n with the resistive behaviour was found. Synthesis conditions ( t e m p e r a t u r e and t i m e of annealing) are even more c r i t i c a l in the bismuth family than in Y-Ba-Cu-O, although cooling speed is less i m p o r t a n t (no need of annealing s t e p s to impose s t r u c t u r a l s y m m e t r y or oxygen content). In this r e p o r t , we show t h a t a d r a m a t i c d e p e n d e n c e of superconducting p r o p e r t i e s is also observed in the bismuth family as a function of annealing conditions and surrounding a t m o s p h e r e s , which may be r e l a t e d to small changes of the oxygen c o n t e n t and/or to the formal copper and bismuth oxidation s t a t e s . 2.
RESULTS 2.1. The 110-K phase Samples of nominal composition 1112 show two transitions at 85 and 110 K, a s s o c i a t e d r e s p e c t i v e l y to the 2212 and 2223 phases 2-4. Oxidizing annealings (either oxygen or diluted NF 3 gas) have no e f f e c t e i t h e r on Tc or on the amplitude of the inductive transition a t 110 K. Reducing conditions by annealing under nitrogen or dynamic vacuum in the range 250
0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
0..................
I
i
1
i
i
J
~
I
~
80 90 TEMPERATURE (K)
~
i
LI
,,,,
1(30
110
FIGURE 1 E f f e c t of annealing under oxygen flow, for two hours, of a v a c u u m - a n n e a l e d sample of composition 1112.
and showed t h a t the l l0-K transition is almost fully r e c o v e r e d a f t e r 17 hours of oxygenation a t 220°C'L Fluorination shows the s a m e tendency, t h a t is, the Tc onset increases from 90 upto 106 K a f t e r two hours of r e a c t i o n at 230 °C, but longer t i m e s or higher t e m p e r a t u r e s lead to samples'decomposition5. 2.2. The 2212 phase As-grown pellets show a transition a t Tc onset of 85 K, followed by another sharp d e c r e a s e of xa.c. a t about 58 K, which would suggest the e x i s t e n c e of another superconducting phase in this sample. Similar anomalies were also r e p o r t e d in ref.6, and we think t h a t it might be a g e n e r a l f e a t u r e observed in c e r a m i c samples due to the i n t e r g r o w t h or stackin~g faults, and easily suppressed in single crystals J. In those cases, full transitions are obtained above 65-70 K, and confirm t h a t such anomalies are not intrinsic to the 2212 phase. Figure 2(a) shows the upper transition when annealed under vacuum for 1 hour: the Tc onset
O. Pe~a et al. / A nnealings of bismuth high-Tc superconductors
1216
Vacuum ann~aled
higher temperatures (T> 450 °C) a clear decrease of the amplitude and the appearance of a "bump" at 4.5 K were observed, suggesting a bi-phased compound, although X-Ray patterns remain unchanged.
0.1
as grown I IO0"C 2 220"C 0
3.
(a)
3 240"C
]
I Ozygen annealeO
5 5~C'C 1
8-9 ?
5 , ~
7
:
5: Vacuuma n ~ c
6: 2~'C 300"C
(b)
TEMP~RAIURE ~K)
FIGURE 2 Annealing effects at given temperatures for the 2212 composition. (a) as-grown samples annealed under vacuum for one hour; (b) vacuum-annealed samples reannealed under oxygen flow, for one hour.
increases from 85 to 95 K at rather low annealing temperatures (220 °C ST.< 460 °C). At higher temperatures (T=580 °C), a decrease in amplitude is obtained, although the onset remains at 95 K. The vacuum-annealed sample (curves 5, figure 2) was reoxidized: at low annealing temperatures the transition's amplitude improves (curve 6); then, a complete reversibility of the process is observed, and seems to saturate at about 400 °C. It should be noted that the final curves go beyond the one observed in the as-grown pellet, suggesting that the "85-K" phase is in fact a metastable state which greatly depends on the preparation conditions. A non-monotonic variation was also reported in ref.7, as a function of oxygen pressures. Their annealings were performed at high temperatures (above 600 °C) and led to large reductions of the superconducting signal7. Other authors found a complete loss of superconductivity at high temperatures and high oxygen pressures l . We show, in our case, t h a t such a reversibility of e f f e c t s may be obtained under milder conditions. 2.3. The 2202 phase As-grown pellets show a sharp transition at 6K, ATc= 1.5 K, with a full amplitude at about 3K. No variation is observed when annealing under vacuum below 400 °C. At 450 °C, the amplitude decreases and the Tc onset goes down to 5-5.5 K, while no impurity phases are detected. Oxygen annealing was performed on this latter sample, showing complete restoration of the superconducting properties when annealed at 250 °C. At
DISCUSSION In previous works we evidenced important effects of the annealing on the II 0-K phase 2,4,~5. Surprisingly, no significant weight losses during vacuum annealing, and absolutely no weight increase during the oxygenation process were noticed. A different situation was observed after NF3 treatment, where a detectable, although very slight, weight increase was observed. Basically the same features are seen in the 2212 and 2202 phases. Such weight variations, negligible compared to those observed in Y - B a - C u - O , provoke reversible effects largely comparable. One fundamental difference between the yttrium and bismuth systems is that in the latter, two multiple-valence systems coexist together, Bi(lll)/Bi(V) and Cu(lIl)/Cu(ll) instead of just one in the Y - B a - C u - O phase. An internal conversion between the oxidation states of bismuth and copper is thus possible, which must be largely controlled by thermodynamical conditions (temperature, time, environment, pressure, etc). Such internal mechanisms are consistent with the role played by lead-doping in the stabilization of the respective structures, probably modifying the electrochemical equilibria. It must be noted that similar annealings give opposite effects on the transitions of the 85 and If0 K phases: Tc decreases under reducing conditions in one of them, and under oxidizing atmospheres in the other one.Both phases, differing by just the number of CuO2 planes, are so interrelated that the possible diffusion of very small quantities of oxygen in one of them may act oppositely in the other. In the 2202 phase, only the amplitude of the Meissner effect seems affected. It is possible that in this case, a slightly different mechanism or different occupation of sites takes place, leading to a reduction of the Meissner amplitude with no change of the Tc onset, as we have also observed in YBa2Cu306.7Fy samples 8.
REFERENCES *. Research under MRT contract n°508819/Thomson CSF, and by Rhone-Poulenc contract. I. J.M.Tarascon,Y. Le Page,P.Barboux,B.Bagley, L.Greene, W.McKinnon,G.Hull,M.Giroud, D.Hwang, Phys.Rev.B, 37 (1988) 9382. 2. O. Peha,A.Perrin,M.Sergent,PhysicaC,156(I 988)489 3. A.Dinia et al, to be published. 4. A.Perrin, A.Dinia, O.Peha, C.Perrin, M.Sergent, Materials Letters, 8(1989)165. 5. O. Pefla,C.Perrin,A. Dinia,M.Sergent, P.Christensen, G.Fonteneau, J.Lucas, PhysicaC,158(1989)443. 6. X. Wang,K.Donnelly,T.Bakas,J.Coey,l.Rosenman, C.Simon, Solid State Comm.,69(1989) 829. 7. D. Morris,C. Hultgren,A.Markelz,J. Wei,N.Asmar, J.Nickel, preprint(1988). 8. C.Perrin,O.Pefla,A.Dinia, M.Sergent,G. Fonteneau, P.Christensen,J.Lucas, this Conference.