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Effects of Y substitution and oxygen deficiency on the superconducting transitions in YxGd1−xBa2Cu3O7−δ
Physica C 378–381 (2002) 344–348 www.elsevier.com/locate/physc
Effects of Y substitution and oxygen deficiency on the superconducting transitions in YxGd1x Ba2Cu3O7d Chihiro Taka a, Shigeto Teshima a, Akihiko Nishida a
a,b,*
Department of Applied Physics, Faculty of Science, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan b Advanced Materials Institute, Fukuoka University, Fukuoka 814-0180, Japan
Abstract A series of ceramic samples of Yx Gd1x Ba2 Cu3 O7d with various Y substitution x and oxygen deficiency d is prepared and investigated with scanning electron microprobe, X-ray diffraction (XRD) and electrical resistivity. The normal resistivity is almost independent of Y substitution except for the x ¼ 0:3 sample which indicated poor crystallinity. Although oxygen deficiency results in higher normal resistivity and lower superconducting transition temperature Tc , higher Tc is achieved in the Y-substituted sample. XRD analyses show that the increase in the crystal c-axis due to oxygen deficiency is suppressed by the Y substitution. The origin of improved Tc is discussed in relation to the suppression of the c-axis stretching and the suppression of decrease in the hole concentration in the CuO2 plane. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.62.)c; 74.62.Bf; 74.62.Dh; 74.72.)h Keywords: Gd123 high-Tc superconductor; Y substitution; Oxygen deficiency; Tc improvement
1. Introduction GdBa2 Cu3 O7d superconductor consists of two different Cu sites; Cu1 in the CuO chain along the crystal b-axis and Cu2 on the CuO2 plane which superconducts. With increase of the oxygen deficiency d oxygen atoms O1 in the CuO chain are depleted and distance among O1, Ba and Cu2 is increased with decreased interaction among them, resulting in increased c-axis unit length. This causes decrease in the hole concentration on the CuO2 plane and lower the superconducting transition temperature Tc .
*
Corresponding author. Tel.: +81-92-871-6631x6182; fax: +81-92-865-6030. E-mail address: [email protected] (A. Nishida).
It is generally known that the Gd site can be replaced by other rare-earth elements such as R ¼ Y, Nd, Sm, Eu, Dy, Ho, Er, etc. with the identical crystal structure. However, the difference in the ionic radii of R ions possibly introduces difference in the electronic state in the CuO2 plane and affects the transition temperature Tc . In fact, the range of oxygen content where superconductivity is realized becomes narrower with the in), crease of the ionic radii in the order of Y (0.99 A Gd (1.05 A) and Nd (1.10 A) as reported by Veal et al. [1]. Especially, Tc in Gd123 and Nd123 decreases rapidly with only little oxygen deficiency from fully oxygenated state, while in Y123 highest Tc is retained even with certain oxygen deficiency (up to d 0:2). This may be one of the reasons why the Gd123 material is not yet considered for various applications as widely as Y123.
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 4 4 1 - 7
C. Taka et al. / Physica C 378–381 (2002) 344–348
In order to investigate possible effects of different R ions on high-Tc superconductivity and their origins, we have prepared Y-substituted Gd123 ceramics and evaluated their morphology, structure and superconductivity.
2. Experimental Total of 20 samples of Yx Gd1x Ba2 Cu3 O7d have been prepared with different Y substitution x ¼ 0, 0.1, 0.2, 0.3 and various oxygen content 7 d. The stoichiometric mixtures of Y2 O3 , Gd2 O3 , BaCO3 and CuO were thoroughly ground and sintered at 935 °C in air for 24 h repeatedly, pressed into a pellet, and then baked at 950 °C in air for 24 h. During the final cooling down, the pellet was kept at 550 °C for 10 h for sufficient oxygenation. Oxygen contents of fully oxygenated pellets for respective Y substitution have been estimated by thermogravimetric analyses based on hydrogen reduction method with 6.15% H2 þ 93:85% N2 composite gas at 300 ml/min flow rate. The oxygen content 7 d is evaluated from the weight loss by evaporating ð7 2dÞ H2 O molecules as in the formula
345
found only slight tendency of decreasing grain size along with Y substitution, except for drastic degradation of crystal quality at 30% (nominal) substitution. Such degradation probably comes from discrepancy in crystal parameters, especially the for Y123 vs. 11.72 A for c-axis length: 11.68 A Gd123 (typical). Energy dispersion spectroscopic analyses indicated that the analyzed (actually substituted) percentage of Y increases linearly with the nominal value up to 20%, although substituted amount is about 15% lower than the nominal value. Thus, in the following discussion, we examine experimental results with the Y substitution up to 20% nominal. We first pay attention to the normal resistivity. Fig. 1 shows variation of the resistivity at 200 K (R200 ) as a function of oxygen content with various Y substitution. It is clearly seen that all experimental curves coincide and R200 does not depend on the Y substitution. At first sight, this seems to be in good agreement with the SEM observations that the grain size does not much vary with Y. We next examine variations of Tc as a function of oxygen content with different Y substitution, which is represented in Fig. 2. For the pure (Y 0%) Gd123, Tc decreases rapidly with little oxygen
2Yx Gd1x Ba2 Cu3 O7d þ ð7 2dÞH2 ! xY2 O3 þ ð1 xÞGd2 O3 þ 4BaO þ 6Cu þ ð7 2dÞH2 O: Samples with different oxygen contents for respective Y substitution have been obtained by annealing the fully oxygenated pellets in argon atmosphere [2,3] at different temperatures of 330, 360, 390 and 420 °C. The transition temperature Tc is determined at the temperature of zero resistivity, and the transition width DTc is determined as the width between temperatures of 90% and 10% of the onset (normal) resistivity.
3. Results and discussion From surface morphology observations with scanning electron microscope (SEM) we have
Fig. 1. Normal resistivity at 200 K (R200 ) as a function of oxygen content with various Y substitution. The normal resistivity does not depend on the substitution, being consistent with the SEM observations.
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Fig. 2. Variations of Tc as a function of oxygen content with different Y substitution. Rapid decrease of Tc in pure Gd123 with oxygen deficiency is suppressed by the substitution, yielding Tc improvement more than 15 K.
deficiency (as also mentioned in Section 1). On the contrary, for the Y-substituted samples, we observe more mild decrease in Tc as in Fig. 2. Especially, just above 6.8 of the oxygen content we find Tc is increased more than 15 K by the substitution. Such Tc improvement is very stimulating with the possibility of broader industrial applications of Gd123 superconductor whose little allowance for oxygen concentration for good superconductivity might have been some obstacle to wide applicability. We have obtained another improvement as shown in Fig. 3, where the transition width DTc is plotted as a function of oxygen content. We find that at lower oxygen content DTc becomes narrower by the Y substitution getting better quality of ceramic superconductivity. In order to investigate the origin of these improvement effects, we now present and examine variations of the c-axis unit length against oxygen content with different Y substitution in Fig. 4 as analyzed by XRD. We note interesting tendency that rapid increase of the c-axis with oxygen deficiency is suppressed by the substitution, approaching towards dependence of the pure Y123 (dotted line). The suppression of rapid c-axis increase by the Y substitution seems to be along with
Fig. 3. Variations of the transition width DTc as a function of oxygen content with different Y substitution. Severe increase of the width at lower oxygen content is relieved by the substitution.
Fig. 4. Plots of c-axis unit length against oxygen content with different Y substitution. Rapid increase of the c-axis at lower oxygen content is suppressed by the Y substitution, inferring suppression of c-axis increase as the major origin for Tc improvement at lower oxygen content.
the suppression of the rapid decrease in Tc presented in Fig. 2. As mentioned in Section 1, oxygen depletion yields increased c-axis unit length and results in
C. Taka et al. / Physica C 378–381 (2002) 344–348
decreased hole concentration followed by Tc decrease. According to this schema and our observations above, it is simply conceived that the suppression of rapid decrease in Tc due to the Y substitution in Gd123 is originated from the suppression of the rapid increase of the c-axis. Suppression of c-axis stretching is considered to be caused by the smaller ionic radius of Y3þ compared with Gd3þ . We should note that the c-axis for the fully oxygenated sample which is the starting material for oxygen deficient samples is unchanged by the small amount of Y substitution (up to 20%) as seen in Fig. 4, while substitution of 30% Y resulted in severely degraded crystal. This indicates that the Y substitution does not linearly contract the size of the pristine unit cell according to Y amount, but introduces contracting stress into the crystal. Then poor crystallinity is suddenly revealed for the 30% substituted sample due to excessive stress beyond tolerance. Such contracting stress is considered to suppress c-axis stretching during the process of oxygen depletion (Ar annealing) from the oxygenated samples as observed in Fig. 4. Suppression of the c-axis stretching can also explain narrower DTc in Fig. 3, since crystal stretching generally tends to result in increased inhomogeneity of the sample. Improved hole concentration may also contribute to narrower transition width. Improved hole concentration due to Y substitution in oxygen deficient samples is expected to also improve normal resistivity, which however is not actually observed as in Fig. 1. This can be explained by the slight tendency of decreasing grain size along with the Y substitution. Smaller grain size results in lower hole mobility and may cancel the effect of increased hole concentration. One-to-one correspondence between the transition temperature Tc and the c-axis parameters can be checked by plotting Tc as a function of the c length for various Y substitution, as shown in Fig. 5. If all experimental curves coincide to a single universal curve, such correspondence will be confirmed. As can be seen from the figure, experimental curves coincide for the c-axis below 11.715 . On the other hand, we note slight discrepancy A . Thus, we conamong the curves above 11.715 A clude that the origin of the Tc improvement in our Y-substituted Gd123 is mainly the suppression of
347
Fig. 5. Variations of Tc as a function of c-axis unit length with different Y substitution. Coincidence of experimental curves confirms that the suppression of c-axis below 11.715 A stretching is the major origin for Tc improvement. Slight dif suggests somewhat ference among the curves above 11.715 A different origins, such as stability of the so-called stripe structure.
the c-axis increase at lower oxygen contents, while there may be some additional origins. One possibility of additional origins might be related to the so-called 1/8 anomaly, where Tc in La214 superconductor is strongly suppressed due to formation of static charge (hole) stripes around the hole concentration of 1/8 per Cu. It is recently pointed out that some dynamical charge stripes may exist also in Y123 and Bi2212. Such dynamical stripes, if exist, can be pinned by magnetic impurities and might suppress Tc drastically. Considering that the so-called 60 K plateau in Y123 is said to be possibly related to such dynamical stripes, strong Gd spins in the pure Gd123 system could pin such dynamical stripes and result in steeper decrease in Tc with oxygen deficiency. Then, substitution of Y might weaken or dilute the Gd magnetism, and thus the stripes become dynamical again, increasing Tc at lower oxygen concentration. Of course, this kind of mechanism is rather speculative and requires further investigations such as Gd ESR or susceptibility measurements. In summary, we have observed that the Y substitution to oxygen deficient samples improves
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Tc towards higher values; more than 15 K with narrower DTc . The major origin of such improvements is attributed to the suppression of the c-axis stretching at lower oxygen content and the suppression of decrease in the hole concentration in the CuO2 plane. Our results of improved superconductivity at lower oxygen contents are expected to contribute to extended industrial applicability of Gd123 ceramics. Acknowledgements This work has been financially supported by the Ministry of Education, Culture, Sports, Science and Technology (High-Tech Research Center
Program), and supported by the Promotion and Mutual Aid Corporation for Private School of Japan (Distinguished Education and Research Program).
References [1] B.W. Veal, A.P. Paulikas, J.W. Downey, H. Claus, K. Vandervoort, G. Tomlins, H. Shi, M. Jensen, L. Morss, Physica C 162–164 (1989) 97. [2] W.K. Kwok, G.W. Crabtree, A. Umezawa, B.W. Veal, J.D. Jorgensen, S.K. Malik, L.J. Nowicki, A.P. Paulikas, L. Nunez, Phys. Rev. B 37 (1988) 106. [3] G. Cannelli, R. Cantelli, F. Cordero, M. Ferretti, Solid State Commun. 68 (1988) 323.
Effects of Y substitution and oxygen deficiency on the superconducting transitions in YxGd1x Ba2Cu3O7d Chihiro Taka a, Shigeto Teshima a, Akihiko Nishida a
a,b,*
Department of Applied Physics, Faculty of Science, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan b Advanced Materials Institute, Fukuoka University, Fukuoka 814-0180, Japan
Abstract A series of ceramic samples of Yx Gd1x Ba2 Cu3 O7d with various Y substitution x and oxygen deficiency d is prepared and investigated with scanning electron microprobe, X-ray diffraction (XRD) and electrical resistivity. The normal resistivity is almost independent of Y substitution except for the x ¼ 0:3 sample which indicated poor crystallinity. Although oxygen deficiency results in higher normal resistivity and lower superconducting transition temperature Tc , higher Tc is achieved in the Y-substituted sample. XRD analyses show that the increase in the crystal c-axis due to oxygen deficiency is suppressed by the Y substitution. The origin of improved Tc is discussed in relation to the suppression of the c-axis stretching and the suppression of decrease in the hole concentration in the CuO2 plane. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.62.)c; 74.62.Bf; 74.62.Dh; 74.72.)h Keywords: Gd123 high-Tc superconductor; Y substitution; Oxygen deficiency; Tc improvement
1. Introduction GdBa2 Cu3 O7d superconductor consists of two different Cu sites; Cu1 in the CuO chain along the crystal b-axis and Cu2 on the CuO2 plane which superconducts. With increase of the oxygen deficiency d oxygen atoms O1 in the CuO chain are depleted and distance among O1, Ba and Cu2 is increased with decreased interaction among them, resulting in increased c-axis unit length. This causes decrease in the hole concentration on the CuO2 plane and lower the superconducting transition temperature Tc .
*
Corresponding author. Tel.: +81-92-871-6631x6182; fax: +81-92-865-6030. E-mail address: [email protected] (A. Nishida).
It is generally known that the Gd site can be replaced by other rare-earth elements such as R ¼ Y, Nd, Sm, Eu, Dy, Ho, Er, etc. with the identical crystal structure. However, the difference in the ionic radii of R ions possibly introduces difference in the electronic state in the CuO2 plane and affects the transition temperature Tc . In fact, the range of oxygen content where superconductivity is realized becomes narrower with the in), crease of the ionic radii in the order of Y (0.99 A Gd (1.05 A) and Nd (1.10 A) as reported by Veal et al. [1]. Especially, Tc in Gd123 and Nd123 decreases rapidly with only little oxygen deficiency from fully oxygenated state, while in Y123 highest Tc is retained even with certain oxygen deficiency (up to d 0:2). This may be one of the reasons why the Gd123 material is not yet considered for various applications as widely as Y123.
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 4 4 1 - 7
C. Taka et al. / Physica C 378–381 (2002) 344–348
In order to investigate possible effects of different R ions on high-Tc superconductivity and their origins, we have prepared Y-substituted Gd123 ceramics and evaluated their morphology, structure and superconductivity.
2. Experimental Total of 20 samples of Yx Gd1x Ba2 Cu3 O7d have been prepared with different Y substitution x ¼ 0, 0.1, 0.2, 0.3 and various oxygen content 7 d. The stoichiometric mixtures of Y2 O3 , Gd2 O3 , BaCO3 and CuO were thoroughly ground and sintered at 935 °C in air for 24 h repeatedly, pressed into a pellet, and then baked at 950 °C in air for 24 h. During the final cooling down, the pellet was kept at 550 °C for 10 h for sufficient oxygenation. Oxygen contents of fully oxygenated pellets for respective Y substitution have been estimated by thermogravimetric analyses based on hydrogen reduction method with 6.15% H2 þ 93:85% N2 composite gas at 300 ml/min flow rate. The oxygen content 7 d is evaluated from the weight loss by evaporating ð7 2dÞ H2 O molecules as in the formula
345
found only slight tendency of decreasing grain size along with Y substitution, except for drastic degradation of crystal quality at 30% (nominal) substitution. Such degradation probably comes from discrepancy in crystal parameters, especially the for Y123 vs. 11.72 A for c-axis length: 11.68 A Gd123 (typical). Energy dispersion spectroscopic analyses indicated that the analyzed (actually substituted) percentage of Y increases linearly with the nominal value up to 20%, although substituted amount is about 15% lower than the nominal value. Thus, in the following discussion, we examine experimental results with the Y substitution up to 20% nominal. We first pay attention to the normal resistivity. Fig. 1 shows variation of the resistivity at 200 K (R200 ) as a function of oxygen content with various Y substitution. It is clearly seen that all experimental curves coincide and R200 does not depend on the Y substitution. At first sight, this seems to be in good agreement with the SEM observations that the grain size does not much vary with Y. We next examine variations of Tc as a function of oxygen content with different Y substitution, which is represented in Fig. 2. For the pure (Y 0%) Gd123, Tc decreases rapidly with little oxygen
2Yx Gd1x Ba2 Cu3 O7d þ ð7 2dÞH2 ! xY2 O3 þ ð1 xÞGd2 O3 þ 4BaO þ 6Cu þ ð7 2dÞH2 O: Samples with different oxygen contents for respective Y substitution have been obtained by annealing the fully oxygenated pellets in argon atmosphere [2,3] at different temperatures of 330, 360, 390 and 420 °C. The transition temperature Tc is determined at the temperature of zero resistivity, and the transition width DTc is determined as the width between temperatures of 90% and 10% of the onset (normal) resistivity.
3. Results and discussion From surface morphology observations with scanning electron microscope (SEM) we have
Fig. 1. Normal resistivity at 200 K (R200 ) as a function of oxygen content with various Y substitution. The normal resistivity does not depend on the substitution, being consistent with the SEM observations.
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Fig. 2. Variations of Tc as a function of oxygen content with different Y substitution. Rapid decrease of Tc in pure Gd123 with oxygen deficiency is suppressed by the substitution, yielding Tc improvement more than 15 K.
deficiency (as also mentioned in Section 1). On the contrary, for the Y-substituted samples, we observe more mild decrease in Tc as in Fig. 2. Especially, just above 6.8 of the oxygen content we find Tc is increased more than 15 K by the substitution. Such Tc improvement is very stimulating with the possibility of broader industrial applications of Gd123 superconductor whose little allowance for oxygen concentration for good superconductivity might have been some obstacle to wide applicability. We have obtained another improvement as shown in Fig. 3, where the transition width DTc is plotted as a function of oxygen content. We find that at lower oxygen content DTc becomes narrower by the Y substitution getting better quality of ceramic superconductivity. In order to investigate the origin of these improvement effects, we now present and examine variations of the c-axis unit length against oxygen content with different Y substitution in Fig. 4 as analyzed by XRD. We note interesting tendency that rapid increase of the c-axis with oxygen deficiency is suppressed by the substitution, approaching towards dependence of the pure Y123 (dotted line). The suppression of rapid c-axis increase by the Y substitution seems to be along with
Fig. 3. Variations of the transition width DTc as a function of oxygen content with different Y substitution. Severe increase of the width at lower oxygen content is relieved by the substitution.
Fig. 4. Plots of c-axis unit length against oxygen content with different Y substitution. Rapid increase of the c-axis at lower oxygen content is suppressed by the Y substitution, inferring suppression of c-axis increase as the major origin for Tc improvement at lower oxygen content.
the suppression of the rapid decrease in Tc presented in Fig. 2. As mentioned in Section 1, oxygen depletion yields increased c-axis unit length and results in
C. Taka et al. / Physica C 378–381 (2002) 344–348
decreased hole concentration followed by Tc decrease. According to this schema and our observations above, it is simply conceived that the suppression of rapid decrease in Tc due to the Y substitution in Gd123 is originated from the suppression of the rapid increase of the c-axis. Suppression of c-axis stretching is considered to be caused by the smaller ionic radius of Y3þ compared with Gd3þ . We should note that the c-axis for the fully oxygenated sample which is the starting material for oxygen deficient samples is unchanged by the small amount of Y substitution (up to 20%) as seen in Fig. 4, while substitution of 30% Y resulted in severely degraded crystal. This indicates that the Y substitution does not linearly contract the size of the pristine unit cell according to Y amount, but introduces contracting stress into the crystal. Then poor crystallinity is suddenly revealed for the 30% substituted sample due to excessive stress beyond tolerance. Such contracting stress is considered to suppress c-axis stretching during the process of oxygen depletion (Ar annealing) from the oxygenated samples as observed in Fig. 4. Suppression of the c-axis stretching can also explain narrower DTc in Fig. 3, since crystal stretching generally tends to result in increased inhomogeneity of the sample. Improved hole concentration may also contribute to narrower transition width. Improved hole concentration due to Y substitution in oxygen deficient samples is expected to also improve normal resistivity, which however is not actually observed as in Fig. 1. This can be explained by the slight tendency of decreasing grain size along with the Y substitution. Smaller grain size results in lower hole mobility and may cancel the effect of increased hole concentration. One-to-one correspondence between the transition temperature Tc and the c-axis parameters can be checked by plotting Tc as a function of the c length for various Y substitution, as shown in Fig. 5. If all experimental curves coincide to a single universal curve, such correspondence will be confirmed. As can be seen from the figure, experimental curves coincide for the c-axis below 11.715 . On the other hand, we note slight discrepancy A . Thus, we conamong the curves above 11.715 A clude that the origin of the Tc improvement in our Y-substituted Gd123 is mainly the suppression of
347
Fig. 5. Variations of Tc as a function of c-axis unit length with different Y substitution. Coincidence of experimental curves confirms that the suppression of c-axis below 11.715 A stretching is the major origin for Tc improvement. Slight dif suggests somewhat ference among the curves above 11.715 A different origins, such as stability of the so-called stripe structure.
the c-axis increase at lower oxygen contents, while there may be some additional origins. One possibility of additional origins might be related to the so-called 1/8 anomaly, where Tc in La214 superconductor is strongly suppressed due to formation of static charge (hole) stripes around the hole concentration of 1/8 per Cu. It is recently pointed out that some dynamical charge stripes may exist also in Y123 and Bi2212. Such dynamical stripes, if exist, can be pinned by magnetic impurities and might suppress Tc drastically. Considering that the so-called 60 K plateau in Y123 is said to be possibly related to such dynamical stripes, strong Gd spins in the pure Gd123 system could pin such dynamical stripes and result in steeper decrease in Tc with oxygen deficiency. Then, substitution of Y might weaken or dilute the Gd magnetism, and thus the stripes become dynamical again, increasing Tc at lower oxygen concentration. Of course, this kind of mechanism is rather speculative and requires further investigations such as Gd ESR or susceptibility measurements. In summary, we have observed that the Y substitution to oxygen deficient samples improves
348
C. Taka et al. / Physica C 378–381 (2002) 344–348
Tc towards higher values; more than 15 K with narrower DTc . The major origin of such improvements is attributed to the suppression of the c-axis stretching at lower oxygen content and the suppression of decrease in the hole concentration in the CuO2 plane. Our results of improved superconductivity at lower oxygen contents are expected to contribute to extended industrial applicability of Gd123 ceramics. Acknowledgements This work has been financially supported by the Ministry of Education, Culture, Sports, Science and Technology (High-Tech Research Center
Program), and supported by the Promotion and Mutual Aid Corporation for Private School of Japan (Distinguished Education and Research Program).
References [1] B.W. Veal, A.P. Paulikas, J.W. Downey, H. Claus, K. Vandervoort, G. Tomlins, H. Shi, M. Jensen, L. Morss, Physica C 162–164 (1989) 97. [2] W.K. Kwok, G.W. Crabtree, A. Umezawa, B.W. Veal, J.D. Jorgensen, S.K. Malik, L.J. Nowicki, A.P. Paulikas, L. Nunez, Phys. Rev. B 37 (1988) 106. [3] G. Cannelli, R. Cantelli, F. Cordero, M. Ferretti, Solid State Commun. 68 (1988) 323.