- Email: [email protected]
The influence of the oxygen concentration on Tc of TlBa2Ca2Cu3Ox
Physica C 157 (1989) 144-148 North-Holland, Amsterdam
T H E I N F L U E N C E O F T H E O X Y G E N C O N C E N T R A T I O N O N Tc O F TIBa2Ca2Cu30 x A. S C H I L L I N G , H.R. O T T and F. H U L L I G E R Laboratorium fiir Festk6rperphysik, ETHZ, CH-8093 Ziirich, Switzerland Received 10 November 1988
In single-phase Tl2Ba2Ca2Cu30x we observe a similar though less pronounced dependence of the critical temperature Tc on oxygen deficiency as was reported for YBa2Cu307_y. The largest reduction Ax= 0.35 from maximum oxygenation decreases Tc from 117 K to 99 K. c
1. Introduction It is well known that the t r a n s p o r t properties o f YBa2Cu307_y d e p e n d distinctly on the oxygen content which tunes the charge-carrier concentration, or correspondingly the n o m i n a l copper valence. The y d e p e n d e n c e o f the critical t e m p e r a t u r e To, the lattice p a r a m e t e r s and other physical properties for YBaECU307_ e have been r e p o r t e d by various authors [ 1-3 ]. Only few investigations on this subject have been published for b i s m u t h c o m p o u n d s [ 4 - 6 ]. F o r t h a l l i u m c o p p e r oxides [7 ], however, similar work has not yet been reported, p r o b a b l y due to the fact that the oxygen content cannot be easily influenced by annealing the materials in an a p p r o p r i a t e atmosphere. We report on a p r o c e d u r e for obtaining singlephase oxygen-deficient Tl2BaECa2Cu3Ox, the so-called 2223-type s u p e r c o n d u c t o r * with a Tc a r o u n d 120 K [ 7 ], which is easy to oxidize at t e m p e r a t u r e s between 300 a n d 400 ° C. T G A measurements show that this o x i d a t i o n is irreversible (at least in non-reducing a t m o s p h e r e ) ; the v a r i a t i o n o f oxygen content is clearly less easy to achieve than in YBaECU307_y material. As expected, the oxygen content consid* Our notations "Tl2Ba2Ca2CuaOx" and "2223" stand for the compound with the idealized structure given by Torardi and co-workers [ 8 ]. The occupation factors for the metal atoms in the actual material are not ideal, substitutions among the T1 and Ca atoms occur [ 8 ], and the nominal copper valence is still uncertain. Thus we do not specify the value of the oxygen content x which we suppose to be near 10.
Fig. 1. Reaction chamber (cross section). Container: nickel alloy; screws: stainless steel. Volume: about 20 cm 3. erably influences the transition t e m p e r a t u r e T12Ba2Ca2Cu3Ox.
in
2. Sample preparation Samples o f T12Ba2Ca2Cu3Ox were p r e p a r e d from appropriate portions o f T1203, BaO2, CaO (pre-dried at 110 ° C ) and CuO. The reactants were ground a n d pressed into pellets o f 7 m m d i a m e t e r a n d 2 - 1 0 m m thickness. In order to prevent a loss o f thallium in the sintering process we used a tightly closed container o f nickel alloy ( I n c o n e l ) as reaction c h a m b e r (see fig. 1 ). The cap ( C ) is pressed to the container b o d y ( B ) by screws o f stainless steel. In addition, the samples were w r a p p e d into p l a t i n u m foil in o r d e r to prevent any reaction with the container material. The pellets were heated at 850°C (20 m i n ) and 830°C ( 12 h ) , a n d finally furnace cooled to r o o m temper-
0 9 2 1 - 4 5 3 4 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )
145
A. Schilling et al. / Influence o f oxygen variation
ature at a rate of approximately 200°C/h. In spite of oxygen excess from the reactants, we never observed any overpressure upon opening the container after the reaction. The reaction chamber was obviously leaking at high temperatures, most probably due to a difference in thermal expansion of the container and screw materials. The resulting samples were black in colour and essentially single phase. X-ray diffraction patterns were consistent with the tetragonal T12Ba2Ca2Cu30]o structure reported by Torardi et al. [ 8 ]. No addi-
tional lines due to foreign phases were detectable. Although the superconducting transition temperature Tc (here defined by the onset of diamagnetic behaviour) varied between 99 and 106 K from preparation to preparation, each sample showed a sharp transition in the magnetization. Annealing these samples at 300-400°C in flowing oxygen for 1-5 h increased the magnetically-monitored transition to temperatures between 115 and 117 K.
120 T T •
Z Z
. "
I=d I, t J
105
JR 46 II
JR 461
100.0.1
olo
011
012
01.3
0.4
-0.1
olo
oi,
o12
0.3
z~x
LIX
Z Z
:
112
110 :
.=
Z Z
~t
v
:
110
:
KA 51
JG 41
-0.1
0,0
0.1
0.2
Ax
0 3
0.4
1
.05
0 , 0t 0
0 . 0t 5
0. 0
0,15
Ax
Fig. 2. The critical temperature Tcvs. oxygen-contentAx per formula unit for samples JR46I, JR46II, JG41 and KA51, relative to the oxygen-deficientstarting material.
146
A. Schilling et al. / Influence o f oxygen variation
3. Measurements
f.u. in oxygen content x will change the nominal copper valence by +0.067 and the calculated charge carrier concentration by +0.2/f.u..
The transition temperatures were measured in a moving-sample magnetometer at fields smaller than 200 Oe, taking into account that the critical fields Hc~ of these materials are small [9]. The oxygen content of each sample, relative to the oxygen-deficient starting material, was measured by determining the change of weight before and after annealing in flowing oxygen. As it was extremely difficult to control the oxygen absorption process, we chose to perform the measurements on four different samples. Each sample yielded 2-4 data points, depending on the respective oxygen-annealing procedure. The observed values for T~ between 99 and 117 K are shown in fig. 2. Since we have no measure of the absolute values of the oxygen contents, a reasonable procedure to join these data is to shift the T~ vs. oxygen content graphs along the x-axis until the data points fall approximately on one line (see fig. 3 ). Note that a change of + 0.1 per
4. Discussion
Comparing the X-ray data for oxygen-rich and oxygen-deficient materials, we did not find any signs for structural changes across the investigated concentration range. In order to check for possible oxygen ordering, neutron-diffraction experiments are planned. The variation of Tc versus oxygen content is less pronounced than in YBaECuaO7_y (see fig. 4). This is possibly due to a more complicated oxygen chemistry in the case YBa2Cu307_y. In this material, the major change on varying y is the introduction or the removal of oxygen vacancies onto the oxygen 2r sites (of space group P m m m ) in the copper-oxygen chains [ 10], suppressing the orthorhombic distortion with increasing oxygen deficiency and finally
120
115
Tr :: T I
110
+
.1.1, :
+
X. :
0 I--
Z
105
:
I"
:
:
Z
100
95
-0.4
i
I
-0.3
I
I
-0.2
I
I
I
-0.1
[] •
JR 461 JR 4611
I •
JG41 KA51
I
0.0
I
0.1
Relatlve oxygen deflclency z~rJf.u.
Fig. 3. Combination of the Tc vs. oxygen-contentgraphs from fig. 2 (see text). The oxygendeficiencies Ax are counted relative to the oxygenconcentration of the samplewith the highest To.
A. Schilling et al. / Influence o f oxygen variation
147
120 m o o •
[]
•
II
o~
100
f
80
[] • [] • •
40
-0.5
,
n
-0.4
=
I
=
-0.3
Relative
I
m
-0.2
oxygen
I
-0.1
deficiency
i
JR461 JR4611 JG41 KA51 YBCO
I
0.0
i
0.1
~/f.u.
Fig. 4. Comparison of the Tc dependences on oxygen deficiencies of Tl2Ba2Ca2Cu3Ox and YBa2Cu~O7_r The data for YBa2Cu307_y are taken from ref. [ 3 ]. The solid line is to guide the eye.
transforming the structure from orthorhombic to tetragonal. It seems reasonable to assume that in TIEBa2CaECu3Ox the oxygen vacancies are located in the CuO2 planes. We base this conclusion on the fact that for T12BaECaECu2Os(2212), a compound related to the 2223 phase, we did not observe a comparable oxygen-variation effect on Tc as shown in fig. 3. The 2223 structure evolves from the 2212 structure by an intercalation of an additional Cu202 layer, as well as an additional Ca layer (to complete the symmetry). In other words, the Ca layer in the mirror plane of 2212 is substituted in 2223 by a C a CuOE-Ca sandwich, while the other parts of the structure remain virtually identical [ 8,11 ]. The oxygen coordination of the Cu atoms in this additional CuO2 plane is square planar (CN4) compared to square pyramidal (CN5) for the Cu atoms in the CuO2-BaO-T10 blocks that are identical in both phases. Since the oxygen sites in these blocks are nearly identically arranged in both types of c o m -
pounds, we conclude that these particular sites are not involved in the oxygen absorption process. We were not able to synthesize 2223 material with an oxygen deficiency larger than 0.35 per formula unit, relative to our most oxidized state. Trials with an oxygen deficiency in the reactants (e.g. using Cu20 instead of CuO) resulted in multiphase samples. Therefore we suppose that T1EBa2Ca2Cu3Ox is not stable below a critical value of x. We have no experimental indication for the existence of an insulating or a semiconducting version of the 2223 phase, analogous to YBa2Cu306 in the Y - B a - C u - O system. The Tc values reported in the literature for the 2223 phase differ from author to author. According to the present work, different oxygen contents are likely one reason for these discrepancies. Another explanation inferring a decreasing Tc with increasing stackingfault density was offered by Parkin and co-workers [12].
148
A. Schilling et al. / Influence of oxygen variation
5. Conclusion
We have shown that it is possible to prepare oxygen-deficient T12Ba2Ca2Cu30x using a closed reaction chamber. Such samples are easily oxidized in flowing oxygen at temperatures a r o u n d 300°C. The superconducting transition temperature of Tl2BaECa2Cu3Ox depends on the oxygen content of the material in a less p r o n o u n c e d way than in YBaECU307_y, b u t the T o ( x ) variation is still quite appreciable. The oxygen vacancies in T12Ba2Ca2Cu3Ox are most likely located in the CuO2 planes.
Acknowledgements We t h a n k S. Siegrist for his help in sample preparation a n d H. Haug for technical advice. This work was in part financially supported by the Schweizerische N a t i o n a l f o n d s zur F6rderung der wissenschaftlichen Forschung.
References [ 1] M.Tokumoto, H. Ihara, T. Matsubara, M. Hirabayashi, N. Terada, H. Oyanagi,K. Murata and Y. Kimura,Jpn. J. Appl. Phys. 26 (1987) L1565. [2]A. Manthiram, J.S. Swinnea, H. Steinfink and J.B. Goodenough,J. Am. Chem. Soc. 109 (1987) 6667. [3]R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Nature 329 (1987) 423. [4] H. Niu, N. Fukushima and K. Ando, Jpn. J. Appl. Phys. 27 (1987) L1442. [5 ] O. Pefia, A. Perrin and M. Sergent, Physica C 156 (1988) 489. [6] A. Manthiram and J.B. Goodenough, Appl. Phys. Len. 53 ( 1988) 420. [ 7 ] Z.Z. Shengand A.M. Hermann, Nature 332 ( 1988) 138. [8]C.C. Torardi, M.A. Subramanian, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey,T.R. Askew,R.B. Ftippen, U. Chowdhry and A.W. Sleight, Science240 (1988) 631. [9] P. Reith, P. Miiller, C. Allgeier, R. Hoben, J. Heise, J.S. Schillingand K. Andres, Physica C 156 (1988) 319. [ 10] J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm,K.J. Volin, R.L. Hitterman, J.D. Grace and Ivan K. Schuller, Phys. Rev. B 36 (1987) 3608. [11 ] M.A. Subramanian, J.C. Calabrese, C.C. Torardi and J. Gopalakrishnan, Nature 332 ( 1988) 420. [ 12] A.I. Nazzal,T.c. Huang,G. Gorman R. Savoyand R. Beyers, Phys. Rev. Lett. 60 (1988) 2539.
T H E I N F L U E N C E O F T H E O X Y G E N C O N C E N T R A T I O N O N Tc O F TIBa2Ca2Cu30 x A. S C H I L L I N G , H.R. O T T and F. H U L L I G E R Laboratorium fiir Festk6rperphysik, ETHZ, CH-8093 Ziirich, Switzerland Received 10 November 1988
In single-phase Tl2Ba2Ca2Cu30x we observe a similar though less pronounced dependence of the critical temperature Tc on oxygen deficiency as was reported for YBa2Cu307_y. The largest reduction Ax= 0.35 from maximum oxygenation decreases Tc from 117 K to 99 K. c
1. Introduction It is well known that the t r a n s p o r t properties o f YBa2Cu307_y d e p e n d distinctly on the oxygen content which tunes the charge-carrier concentration, or correspondingly the n o m i n a l copper valence. The y d e p e n d e n c e o f the critical t e m p e r a t u r e To, the lattice p a r a m e t e r s and other physical properties for YBaECU307_ e have been r e p o r t e d by various authors [ 1-3 ]. Only few investigations on this subject have been published for b i s m u t h c o m p o u n d s [ 4 - 6 ]. F o r t h a l l i u m c o p p e r oxides [7 ], however, similar work has not yet been reported, p r o b a b l y due to the fact that the oxygen content cannot be easily influenced by annealing the materials in an a p p r o p r i a t e atmosphere. We report on a p r o c e d u r e for obtaining singlephase oxygen-deficient Tl2BaECa2Cu3Ox, the so-called 2223-type s u p e r c o n d u c t o r * with a Tc a r o u n d 120 K [ 7 ], which is easy to oxidize at t e m p e r a t u r e s between 300 a n d 400 ° C. T G A measurements show that this o x i d a t i o n is irreversible (at least in non-reducing a t m o s p h e r e ) ; the v a r i a t i o n o f oxygen content is clearly less easy to achieve than in YBaECU307_y material. As expected, the oxygen content consid* Our notations "Tl2Ba2Ca2CuaOx" and "2223" stand for the compound with the idealized structure given by Torardi and co-workers [ 8 ]. The occupation factors for the metal atoms in the actual material are not ideal, substitutions among the T1 and Ca atoms occur [ 8 ], and the nominal copper valence is still uncertain. Thus we do not specify the value of the oxygen content x which we suppose to be near 10.
Fig. 1. Reaction chamber (cross section). Container: nickel alloy; screws: stainless steel. Volume: about 20 cm 3. erably influences the transition t e m p e r a t u r e T12Ba2Ca2Cu3Ox.
in
2. Sample preparation Samples o f T12Ba2Ca2Cu3Ox were p r e p a r e d from appropriate portions o f T1203, BaO2, CaO (pre-dried at 110 ° C ) and CuO. The reactants were ground a n d pressed into pellets o f 7 m m d i a m e t e r a n d 2 - 1 0 m m thickness. In order to prevent a loss o f thallium in the sintering process we used a tightly closed container o f nickel alloy ( I n c o n e l ) as reaction c h a m b e r (see fig. 1 ). The cap ( C ) is pressed to the container b o d y ( B ) by screws o f stainless steel. In addition, the samples were w r a p p e d into p l a t i n u m foil in o r d e r to prevent any reaction with the container material. The pellets were heated at 850°C (20 m i n ) and 830°C ( 12 h ) , a n d finally furnace cooled to r o o m temper-
0 9 2 1 - 4 5 3 4 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )
145
A. Schilling et al. / Influence o f oxygen variation
ature at a rate of approximately 200°C/h. In spite of oxygen excess from the reactants, we never observed any overpressure upon opening the container after the reaction. The reaction chamber was obviously leaking at high temperatures, most probably due to a difference in thermal expansion of the container and screw materials. The resulting samples were black in colour and essentially single phase. X-ray diffraction patterns were consistent with the tetragonal T12Ba2Ca2Cu30]o structure reported by Torardi et al. [ 8 ]. No addi-
tional lines due to foreign phases were detectable. Although the superconducting transition temperature Tc (here defined by the onset of diamagnetic behaviour) varied between 99 and 106 K from preparation to preparation, each sample showed a sharp transition in the magnetization. Annealing these samples at 300-400°C in flowing oxygen for 1-5 h increased the magnetically-monitored transition to temperatures between 115 and 117 K.
120 T T •
Z Z
. "
I=d I, t J
105
JR 46 II
JR 461
100.0.1
olo
011
012
01.3
0.4
-0.1
olo
oi,
o12
0.3
z~x
LIX
Z Z
:
112
110 :
.=
Z Z
~t
v
:
110
:
KA 51
JG 41
-0.1
0,0
0.1
0.2
Ax
0 3
0.4
1
.05
0 , 0t 0
0 . 0t 5
0. 0
0,15
Ax
Fig. 2. The critical temperature Tcvs. oxygen-contentAx per formula unit for samples JR46I, JR46II, JG41 and KA51, relative to the oxygen-deficientstarting material.
146
A. Schilling et al. / Influence o f oxygen variation
3. Measurements
f.u. in oxygen content x will change the nominal copper valence by +0.067 and the calculated charge carrier concentration by +0.2/f.u..
The transition temperatures were measured in a moving-sample magnetometer at fields smaller than 200 Oe, taking into account that the critical fields Hc~ of these materials are small [9]. The oxygen content of each sample, relative to the oxygen-deficient starting material, was measured by determining the change of weight before and after annealing in flowing oxygen. As it was extremely difficult to control the oxygen absorption process, we chose to perform the measurements on four different samples. Each sample yielded 2-4 data points, depending on the respective oxygen-annealing procedure. The observed values for T~ between 99 and 117 K are shown in fig. 2. Since we have no measure of the absolute values of the oxygen contents, a reasonable procedure to join these data is to shift the T~ vs. oxygen content graphs along the x-axis until the data points fall approximately on one line (see fig. 3 ). Note that a change of + 0.1 per
4. Discussion
Comparing the X-ray data for oxygen-rich and oxygen-deficient materials, we did not find any signs for structural changes across the investigated concentration range. In order to check for possible oxygen ordering, neutron-diffraction experiments are planned. The variation of Tc versus oxygen content is less pronounced than in YBaECuaO7_y (see fig. 4). This is possibly due to a more complicated oxygen chemistry in the case YBa2Cu307_y. In this material, the major change on varying y is the introduction or the removal of oxygen vacancies onto the oxygen 2r sites (of space group P m m m ) in the copper-oxygen chains [ 10], suppressing the orthorhombic distortion with increasing oxygen deficiency and finally
120
115
Tr :: T I
110
+
.1.1, :
+
X. :
0 I--
Z
105
:
I"
:
:
Z
100
95
-0.4
i
I
-0.3
I
I
-0.2
I
I
I
-0.1
[] •
JR 461 JR 4611
I •
JG41 KA51
I
0.0
I
0.1
Relatlve oxygen deflclency z~rJf.u.
Fig. 3. Combination of the Tc vs. oxygen-contentgraphs from fig. 2 (see text). The oxygendeficiencies Ax are counted relative to the oxygenconcentration of the samplewith the highest To.
A. Schilling et al. / Influence o f oxygen variation
147
120 m o o •
[]
•
II
o~
100
f
80
[] • [] • •
40
-0.5
,
n
-0.4
=
I
=
-0.3
Relative
I
m
-0.2
oxygen
I
-0.1
deficiency
i
JR461 JR4611 JG41 KA51 YBCO
I
0.0
i
0.1
~/f.u.
Fig. 4. Comparison of the Tc dependences on oxygen deficiencies of Tl2Ba2Ca2Cu3Ox and YBa2Cu~O7_r The data for YBa2Cu307_y are taken from ref. [ 3 ]. The solid line is to guide the eye.
transforming the structure from orthorhombic to tetragonal. It seems reasonable to assume that in TIEBa2CaECu3Ox the oxygen vacancies are located in the CuO2 planes. We base this conclusion on the fact that for T12BaECaECu2Os(2212), a compound related to the 2223 phase, we did not observe a comparable oxygen-variation effect on Tc as shown in fig. 3. The 2223 structure evolves from the 2212 structure by an intercalation of an additional Cu202 layer, as well as an additional Ca layer (to complete the symmetry). In other words, the Ca layer in the mirror plane of 2212 is substituted in 2223 by a C a CuOE-Ca sandwich, while the other parts of the structure remain virtually identical [ 8,11 ]. The oxygen coordination of the Cu atoms in this additional CuO2 plane is square planar (CN4) compared to square pyramidal (CN5) for the Cu atoms in the CuO2-BaO-T10 blocks that are identical in both phases. Since the oxygen sites in these blocks are nearly identically arranged in both types of c o m -
pounds, we conclude that these particular sites are not involved in the oxygen absorption process. We were not able to synthesize 2223 material with an oxygen deficiency larger than 0.35 per formula unit, relative to our most oxidized state. Trials with an oxygen deficiency in the reactants (e.g. using Cu20 instead of CuO) resulted in multiphase samples. Therefore we suppose that T1EBa2Ca2Cu3Ox is not stable below a critical value of x. We have no experimental indication for the existence of an insulating or a semiconducting version of the 2223 phase, analogous to YBa2Cu306 in the Y - B a - C u - O system. The Tc values reported in the literature for the 2223 phase differ from author to author. According to the present work, different oxygen contents are likely one reason for these discrepancies. Another explanation inferring a decreasing Tc with increasing stackingfault density was offered by Parkin and co-workers [12].
148
A. Schilling et al. / Influence of oxygen variation
5. Conclusion
We have shown that it is possible to prepare oxygen-deficient T12Ba2Ca2Cu30x using a closed reaction chamber. Such samples are easily oxidized in flowing oxygen at temperatures a r o u n d 300°C. The superconducting transition temperature of Tl2BaECa2Cu3Ox depends on the oxygen content of the material in a less p r o n o u n c e d way than in YBaECU307_y, b u t the T o ( x ) variation is still quite appreciable. The oxygen vacancies in T12Ba2Ca2Cu3Ox are most likely located in the CuO2 planes.
Acknowledgements We t h a n k S. Siegrist for his help in sample preparation a n d H. Haug for technical advice. This work was in part financially supported by the Schweizerische N a t i o n a l f o n d s zur F6rderung der wissenschaftlichen Forschung.
References [ 1] M.Tokumoto, H. Ihara, T. Matsubara, M. Hirabayashi, N. Terada, H. Oyanagi,K. Murata and Y. Kimura,Jpn. J. Appl. Phys. 26 (1987) L1565. [2]A. Manthiram, J.S. Swinnea, H. Steinfink and J.B. Goodenough,J. Am. Chem. Soc. 109 (1987) 6667. [3]R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Nature 329 (1987) 423. [4] H. Niu, N. Fukushima and K. Ando, Jpn. J. Appl. Phys. 27 (1987) L1442. [5 ] O. Pefia, A. Perrin and M. Sergent, Physica C 156 (1988) 489. [6] A. Manthiram and J.B. Goodenough, Appl. Phys. Len. 53 ( 1988) 420. [ 7 ] Z.Z. Shengand A.M. Hermann, Nature 332 ( 1988) 138. [8]C.C. Torardi, M.A. Subramanian, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey,T.R. Askew,R.B. Ftippen, U. Chowdhry and A.W. Sleight, Science240 (1988) 631. [9] P. Reith, P. Miiller, C. Allgeier, R. Hoben, J. Heise, J.S. Schillingand K. Andres, Physica C 156 (1988) 319. [ 10] J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm,K.J. Volin, R.L. Hitterman, J.D. Grace and Ivan K. Schuller, Phys. Rev. B 36 (1987) 3608. [11 ] M.A. Subramanian, J.C. Calabrese, C.C. Torardi and J. Gopalakrishnan, Nature 332 ( 1988) 420. [ 12] A.I. Nazzal,T.c. Huang,G. Gorman R. Savoyand R. Beyers, Phys. Rev. Lett. 60 (1988) 2539.