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Oxygen nonstoichiometry of the (Nd, Ce)2CuO4−δ system
Physica C 166 (1990) North-Holland
357-360
OXYGEN NONSTOICHIOMETRY K. SUZUKI,
OF THE (Nd, Ce)2CuOq-6 SYSTEM
K. KISHIO, T. HASEGAWA and K. KITAZAWA
Department of Industrial Chemistry University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received
25 December
1989
Oxygen nonstoichiometry of the electron-doped superconductor Nd1.85Ce,,,SCu04_-6 and nondoped Nd2Cu04_-6 was determined using thermogravimetry and chemical titration analysis. The oxygen deficiency of Nd#ZuO,_, at 900°C and PO, = IO-’ atm was approximately 0.04, while that of Nd,.85Ce0,,5 CUO.,_~ was about 0.008, both being substantially smaller than that of BazYCusOr_+ In this system, however, a small amount of oxygen defect seems to play an essential role in the manifestations of superconductivity.
1. Introduction Recently, the electron-doped superconductor (Nd, Ce)2Cu04 has been discovered by Tokura et al. [ 1,2]. It has a so-called T’-phase structure with no apical oxygen around the Cu ions. As-sintered samples in air show semiconducting behavior, while superconductivity with T,=24 K appears only when they are annealed in a reducing atmosphere. Therefore, it seems to be quite important to study the oxygen deficiency of this system in terms of a comparison of the electronic structures between holedoped and electron-doped superconductors. Although the oxygen deficiency of (Nd, Ce)2C~04 system has been discussed by various groups [ 3-51, these investigations have been limited to a very narrow or a rather rough range of temperatures and oxygen partial pressures. The purpose of the present investigation was to obtain detailed values of oxygen nonstoichiometry through thermogravimetry and chemical analysis.
2. Experimental Powder specimens used in the present study were prepared by solid state reaction from starting mixtures of Ndz03, Ce02 and CuO. As for the Ce-free sample preparation, the mixed powder was calcined at 950°C for 18 h and further fired at 1050°C for 20 0921-4534/90/$03.50 (North-Holland )
0 Elsevier Science Publishers
B.V.
h. The powder specimen of the Ce-doped sample was calcined at 950°C for 18 h and fired at 1100°C for 20 h. All heat treatments were performed in air. Powder specimens of and Nd&uOd--6 NdI,ssCe0.1sCu04_s prepared as above were subjected to X-ray diffraction analysis. SEM observation was performed on the pellet specimens sintered at 1000°C for 20 h. From both X-ray and SEM analyses, it was confirmed that the specimens used in the present study consisted of a single phase of (Nd, Ce)zCuO,_+ Oxygen nonstoichiometry was measured using a thermogravimetric microbalance apparatus (Shimadzu TG-31H) connected to a gas mixture system. A mixed gas of Ar/O* was employed throughout this study. The buoyancy correction of the weight was appropriately made for all experimental points. Powders were accurately weighed and placed in an A1203 basket. The nonstoichiometry was determined in a temperature range of 300 to 900°C under an oxygen partial pressure (Po, ) of 0.4 to 10e4 atm, where the reaction of the specimen with the Al,O, basket was negligible. The absolute values of the oxygen deficiency were chemically determined by a standard iodometric titration technique.
3. Results and discussion Figure
1 shows the oxygen nonstoichiometry
of
K. Suzuki et al. / Oxygen nonstoichiometry of (Nd, Ce),CuO,_,
358
NdZCuOq-6
I
I
-4
-3
-2 log(Po2/atm)
-1
0
3.96’
’
-4
-3
-2 log(Po2/atm)
-1
I 0
Fig. 1. Measured weight change of the NdZCuOq_d specimen as a function of oxygen partial pressure and temperature.
Fig. 2. Oxygen nonstoichiometry of NdzCu04_d corrected for the CuO-Cu20 phase transition.
Nd0t04_-6 vs. oxygen partial pressure at various temperatures. Chemical analysis was performed for the specimen which had been slowly cooled from 400’ C to room temperature. No weight gain was detected during the cooling process. The determined oxygen composition was 3.99kO.02, so the maximum oxygen composition was assumed to be 4.00 in the present study. It is notable in the figure that the 6 vs. log Po, curve indicated a discontinuity above 800°C. This is possibly because the specimen investigated included a small amount of CuO, which causes the following phase transition depending on temperature and Po, [ 6 ] :
sure, the partial molar enthalpy and entropy of solution of oxygen in Nd2Cu04_-6 for a given composition can be calculated from the thermodynamic relationship alo@,, /a( 1 /T). Based on a linear fit, the slope yields floo2, and the intercept, A$,,. The results are shown in figs. 3 (a) and 3 (b), respectively. For comparison, the values of Al& and Aso in BYCO were in the range of - 180 to -250 kJ mol-‘, and - 150 to -300 KJ-’ mol-‘, respectively [ 71. Both the absolute values of A&,, and A,.?&, in NdzCuOd_s are much larger than those in This implies that oxygen in Ba2YCu307_-b NdzCuOd_a is less easily released. Figure 4 shows the observed weight change of Nd,.ssCeo.,5Cu04_-6 vs. oxygen partial pressure at various temperatures. As was the case for NdzCuOd_B, the weight loss due to the CuO-Cu20 phase transition was clearly observed above 800°C. The CuO content estimated from the weight change of this specimen was approximately 0.2 wt%. The nonstoichiometry of Nd,,ssCeo.1SCu04_-6 corrected for this contribution is shown in fig. 5. The oxygen content of the specimen slowly cooled from 400°C to room temperature was determined to be
2Cu0=Cu~O+fo~. From the discontinuity in fig. 1, the sample is estimated to contain 0.15 wt% CuO, which is hardly detectable from X-ray analysis and SEM observation. The nonstoichiometry of Nd2Cu04_-6 corrected for the CuO-CuzO phase transition is illustrated in fig. 2. It is remarkable that the nonstoichiometry of this system is much smaller than that of Ba2YCu207_s (BYCO) system [7]. From the temperature dependence of partial pres-
359
K. Suzuki et al. /Oxygen nonstoichiometry of (Nd, Ce),CuO,_, Nd2Cu04_8
-200/
0.
rg
-3oo-
0.
I :: 2
-4oo-
0.
I! -5oo-
Fig. 4. Measured weight change of the Nd,,ssCeo.,sCu04_d specimen as a function of oxygen partial pressure and temperature.
Nd1.85Ce0.15Cuoy
I ’
I
4.
3.
3.
Nd2CuO4_8 b
3.
-2ool-----250-
4 !_ -3oo4
Is
3.990’
? !a -360,z 4
-4oo-
-460-
!I
-2
-1
0
log(Pog/atm) Fig. 5. Oxygen nonstoichiometry of Nd1.85Ce0.LSCu04_-6 corrected for the CuO-Cu20 phase transition.
P
I 0.01
I -3
P
-5001
0.00
’
-4
0.02 6
0.03
0.04
Fig. 3. Partial molar enthalpy (a) and partial molar entropy (b) of oxygen in Nd2Cu04_-d.
3.99 f 0.02 by titration analysis: This value was calculated using the fact that cerium exists as Ce3+ in solution [ 8 1. So the maximum oxygen composition of the Ce-doped sample is assumed to be 4.0 as well for Nd2Cu0,_G If cerium was assumed to exist as Ce4+ in the solution, we might mistakenly determine the oxygen content as being 4.08. Moran et al. [ 41 had reported that even a reduced sample of Nd2Cu04 had an oxygen content larger
360
K. Suzuki et al. /Oxygen nonstoichiometry of (Nd, Ce)zCuO,_,
than 4.0. As described above, however, we consider the maximum oxygen content to be 4.0. We have also measured the electrical conductivity of Nd2Cu04_-6 in the temperature range from 400 to 900°C. Since the conductivity increased with a decrease of the oxygen partial pressure from 1 to lo-* atm, the major conduction carriers are considered to be electrons and, thus, the oxygen content should be under 4.0 in this temperature range. If the oxygen content was above 4.0, the average valence of copper would become more than 2.0 and the conduction carriers would be holes. This result is consistent with that of our chemical titration analysis. It is found in figs. 2 and 5 that oxygen nonstoichiometry of Nd1.8~Ce,,,5Cu04_-6 is much smaller than that of Nd2Cu0,_+ It is thought that oxygen in the Ce-doped sample is less easily released since the higher valent element Ce was doped. However, superconductivity with T, (onset) = 24 K appeared by reducing the specimen in pure Ar gas. Therefore, a small amount of oxygen deficiency is suggested to play an important role in the manifestation of superconductivity. In summary, we have determined the equilibrium value of the oxygen nonstoichiometry of the (Nd, Ce),CuOd_d system through thermogravimetric measurement in conjunction with chemical analysis. The range of 6 in the Ce-doped system was found to
be quite small; nevertheless the oxygen composition must be essential to realizing superconductivity.
Acknowledgement The present study was financially supported by a Grant-in-Aid for Scientific Research on the Chemistry of New Superconductors from the Ministry of Education, Science and Culture of Japan.
References [ 1 ] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1989) 345. [2] H. Takagi, S. Uchida and Y. Tokura, Phys. Rev. Lett. 62 (1989) 1197. [ 31 E. Takayama-Muromachi, F. Izumi, Y. Uchida, K. Kato and H. Asano, Physica C 159 (1989) 634. [4] E. Moran, A.I. Nazzal, T.C. Huang and J.B. Torrance, Physica C 160 (1989) 30. [ 51J.M. Tarascon, E. Wang, L.H. Greene, R. Ramesh, B.C. Bagley, G.W. Hull, P.F. Miceli, Z.Z. Wang, D. Brawner and N.P. Ong, to be published in Physica C. [6] J.P. Neumann, T. Zhong and Y.A. Chang, Bull. Alloy Phase Diagrams 5 ( 1984) 136. [ 7 ] K. Kishio, K. Suzuki, T. Hasegawa, T. Yamamoto, K. Kitazawa and K. Fueki, J. Solid State Chem. 82 ( 1989) 192. [8] N. de Zoubov and J.V. Muylder, in: Atlas d’Equilibres Electrochimiques, ed. M. Pourbaix (Gauthier-Villars. Paris, 1963) p. 194.
357-360
OXYGEN NONSTOICHIOMETRY K. SUZUKI,
OF THE (Nd, Ce)2CuOq-6 SYSTEM
K. KISHIO, T. HASEGAWA and K. KITAZAWA
Department of Industrial Chemistry University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received
25 December
1989
Oxygen nonstoichiometry of the electron-doped superconductor Nd1.85Ce,,,SCu04_-6 and nondoped Nd2Cu04_-6 was determined using thermogravimetry and chemical titration analysis. The oxygen deficiency of Nd#ZuO,_, at 900°C and PO, = IO-’ atm was approximately 0.04, while that of Nd,.85Ce0,,5 CUO.,_~ was about 0.008, both being substantially smaller than that of BazYCusOr_+ In this system, however, a small amount of oxygen defect seems to play an essential role in the manifestations of superconductivity.
1. Introduction Recently, the electron-doped superconductor (Nd, Ce)2Cu04 has been discovered by Tokura et al. [ 1,2]. It has a so-called T’-phase structure with no apical oxygen around the Cu ions. As-sintered samples in air show semiconducting behavior, while superconductivity with T,=24 K appears only when they are annealed in a reducing atmosphere. Therefore, it seems to be quite important to study the oxygen deficiency of this system in terms of a comparison of the electronic structures between holedoped and electron-doped superconductors. Although the oxygen deficiency of (Nd, Ce)2C~04 system has been discussed by various groups [ 3-51, these investigations have been limited to a very narrow or a rather rough range of temperatures and oxygen partial pressures. The purpose of the present investigation was to obtain detailed values of oxygen nonstoichiometry through thermogravimetry and chemical analysis.
2. Experimental Powder specimens used in the present study were prepared by solid state reaction from starting mixtures of Ndz03, Ce02 and CuO. As for the Ce-free sample preparation, the mixed powder was calcined at 950°C for 18 h and further fired at 1050°C for 20 0921-4534/90/$03.50 (North-Holland )
0 Elsevier Science Publishers
B.V.
h. The powder specimen of the Ce-doped sample was calcined at 950°C for 18 h and fired at 1100°C for 20 h. All heat treatments were performed in air. Powder specimens of and Nd&uOd--6 NdI,ssCe0.1sCu04_s prepared as above were subjected to X-ray diffraction analysis. SEM observation was performed on the pellet specimens sintered at 1000°C for 20 h. From both X-ray and SEM analyses, it was confirmed that the specimens used in the present study consisted of a single phase of (Nd, Ce)zCuO,_+ Oxygen nonstoichiometry was measured using a thermogravimetric microbalance apparatus (Shimadzu TG-31H) connected to a gas mixture system. A mixed gas of Ar/O* was employed throughout this study. The buoyancy correction of the weight was appropriately made for all experimental points. Powders were accurately weighed and placed in an A1203 basket. The nonstoichiometry was determined in a temperature range of 300 to 900°C under an oxygen partial pressure (Po, ) of 0.4 to 10e4 atm, where the reaction of the specimen with the Al,O, basket was negligible. The absolute values of the oxygen deficiency were chemically determined by a standard iodometric titration technique.
3. Results and discussion Figure
1 shows the oxygen nonstoichiometry
of
K. Suzuki et al. / Oxygen nonstoichiometry of (Nd, Ce),CuO,_,
358
NdZCuOq-6
I
I
-4
-3
-2 log(Po2/atm)
-1
0
3.96’
’
-4
-3
-2 log(Po2/atm)
-1
I 0
Fig. 1. Measured weight change of the NdZCuOq_d specimen as a function of oxygen partial pressure and temperature.
Fig. 2. Oxygen nonstoichiometry of NdzCu04_d corrected for the CuO-Cu20 phase transition.
Nd0t04_-6 vs. oxygen partial pressure at various temperatures. Chemical analysis was performed for the specimen which had been slowly cooled from 400’ C to room temperature. No weight gain was detected during the cooling process. The determined oxygen composition was 3.99kO.02, so the maximum oxygen composition was assumed to be 4.00 in the present study. It is notable in the figure that the 6 vs. log Po, curve indicated a discontinuity above 800°C. This is possibly because the specimen investigated included a small amount of CuO, which causes the following phase transition depending on temperature and Po, [ 6 ] :
sure, the partial molar enthalpy and entropy of solution of oxygen in Nd2Cu04_-6 for a given composition can be calculated from the thermodynamic relationship alo@,, /a( 1 /T). Based on a linear fit, the slope yields floo2, and the intercept, A$,,. The results are shown in figs. 3 (a) and 3 (b), respectively. For comparison, the values of Al& and Aso in BYCO were in the range of - 180 to -250 kJ mol-‘, and - 150 to -300 KJ-’ mol-‘, respectively [ 71. Both the absolute values of A&,, and A,.?&, in NdzCuOd_s are much larger than those in This implies that oxygen in Ba2YCu307_-b NdzCuOd_a is less easily released. Figure 4 shows the observed weight change of Nd,.ssCeo.,5Cu04_-6 vs. oxygen partial pressure at various temperatures. As was the case for NdzCuOd_B, the weight loss due to the CuO-Cu20 phase transition was clearly observed above 800°C. The CuO content estimated from the weight change of this specimen was approximately 0.2 wt%. The nonstoichiometry of Nd,,ssCeo.1SCu04_-6 corrected for this contribution is shown in fig. 5. The oxygen content of the specimen slowly cooled from 400°C to room temperature was determined to be
2Cu0=Cu~O+fo~. From the discontinuity in fig. 1, the sample is estimated to contain 0.15 wt% CuO, which is hardly detectable from X-ray analysis and SEM observation. The nonstoichiometry of Nd2Cu04_-6 corrected for the CuO-CuzO phase transition is illustrated in fig. 2. It is remarkable that the nonstoichiometry of this system is much smaller than that of Ba2YCu207_s (BYCO) system [7]. From the temperature dependence of partial pres-
359
K. Suzuki et al. /Oxygen nonstoichiometry of (Nd, Ce),CuO,_, Nd2Cu04_8
-200/
0.
rg
-3oo-
0.
I :: 2
-4oo-
0.
I! -5oo-
Fig. 4. Measured weight change of the Nd,,ssCeo.,sCu04_d specimen as a function of oxygen partial pressure and temperature.
Nd1.85Ce0.15Cuoy
I ’
I
4.
3.
3.
Nd2CuO4_8 b
3.
-2ool-----250-
4 !_ -3oo4
Is
3.990’
? !a -360,z 4
-4oo-
-460-
!I
-2
-1
0
log(Pog/atm) Fig. 5. Oxygen nonstoichiometry of Nd1.85Ce0.LSCu04_-6 corrected for the CuO-Cu20 phase transition.
P
I 0.01
I -3
P
-5001
0.00
’
-4
0.02 6
0.03
0.04
Fig. 3. Partial molar enthalpy (a) and partial molar entropy (b) of oxygen in Nd2Cu04_-d.
3.99 f 0.02 by titration analysis: This value was calculated using the fact that cerium exists as Ce3+ in solution [ 8 1. So the maximum oxygen composition of the Ce-doped sample is assumed to be 4.0 as well for Nd2Cu0,_G If cerium was assumed to exist as Ce4+ in the solution, we might mistakenly determine the oxygen content as being 4.08. Moran et al. [ 41 had reported that even a reduced sample of Nd2Cu04 had an oxygen content larger
360
K. Suzuki et al. /Oxygen nonstoichiometry of (Nd, Ce)zCuO,_,
than 4.0. As described above, however, we consider the maximum oxygen content to be 4.0. We have also measured the electrical conductivity of Nd2Cu04_-6 in the temperature range from 400 to 900°C. Since the conductivity increased with a decrease of the oxygen partial pressure from 1 to lo-* atm, the major conduction carriers are considered to be electrons and, thus, the oxygen content should be under 4.0 in this temperature range. If the oxygen content was above 4.0, the average valence of copper would become more than 2.0 and the conduction carriers would be holes. This result is consistent with that of our chemical titration analysis. It is found in figs. 2 and 5 that oxygen nonstoichiometry of Nd1.8~Ce,,,5Cu04_-6 is much smaller than that of Nd2Cu0,_+ It is thought that oxygen in the Ce-doped sample is less easily released since the higher valent element Ce was doped. However, superconductivity with T, (onset) = 24 K appeared by reducing the specimen in pure Ar gas. Therefore, a small amount of oxygen deficiency is suggested to play an important role in the manifestation of superconductivity. In summary, we have determined the equilibrium value of the oxygen nonstoichiometry of the (Nd, Ce),CuOd_d system through thermogravimetric measurement in conjunction with chemical analysis. The range of 6 in the Ce-doped system was found to
be quite small; nevertheless the oxygen composition must be essential to realizing superconductivity.
Acknowledgement The present study was financially supported by a Grant-in-Aid for Scientific Research on the Chemistry of New Superconductors from the Ministry of Education, Science and Culture of Japan.
References [ 1 ] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1989) 345. [2] H. Takagi, S. Uchida and Y. Tokura, Phys. Rev. Lett. 62 (1989) 1197. [ 31 E. Takayama-Muromachi, F. Izumi, Y. Uchida, K. Kato and H. Asano, Physica C 159 (1989) 634. [4] E. Moran, A.I. Nazzal, T.C. Huang and J.B. Torrance, Physica C 160 (1989) 30. [ 51J.M. Tarascon, E. Wang, L.H. Greene, R. Ramesh, B.C. Bagley, G.W. Hull, P.F. Miceli, Z.Z. Wang, D. Brawner and N.P. Ong, to be published in Physica C. [6] J.P. Neumann, T. Zhong and Y.A. Chang, Bull. Alloy Phase Diagrams 5 ( 1984) 136. [ 7 ] K. Kishio, K. Suzuki, T. Hasegawa, T. Yamamoto, K. Kitazawa and K. Fueki, J. Solid State Chem. 82 ( 1989) 192. [8] N. de Zoubov and J.V. Muylder, in: Atlas d’Equilibres Electrochimiques, ed. M. Pourbaix (Gauthier-Villars. Paris, 1963) p. 194.