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Correlation between Cu valence, hole density and Tc of Bi2201, Bi2212 and Bi2223 superconductors
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Applied Superconductivity Vol. 5, Nos 1±6, pp. 53±60, 1997 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0964-1807(98)00034-9 0964-1807/98 $19.00 + 0.00
CORRELATION BETWEEN Cu VALENCE, HOLE DENSITY AND Tc OF Bi2201, Bi2212 AND Bi2223 SUPERCONDUCTORS SHIRO KAMBE*, KENICHIRO ABE$, KIYOSHI KOIKE*, YUICHIRO TAKASUGI*, OSAMU ISHII*, TOSHIO FURUSAWA*{, TSUYOSHI SHIOMI$} and SHIGETOSHI OHSHIMA$ *Graduate School of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992, Japan $Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992, Japan AbstractÐFor the Bi2201 phase, by comparing the correlation between them with the Mott±Hubbard split model, it was found that while the correlation deviates from the Mott±Hubbard model for air-sintered samples, the correlation agree well with the Mott±Hubbard model for Ar-annealed samples. The deviation is believed to come from the destruction of the Mott±Hubbard character. For the airannealed samples of Bi2212 phase, hole concentration, p scales very well with v, and the p±v line passes through the origin of the coordinates. However, the proportional constant is about two. The simplest explanation of the double proportional constant is to assume the coexistence of a hole of CuO2 character and BiO character. For the Bi2223 phase, only an Ar-annealed sample lied on the p = v line, suggesting that it has a Mott±Hubbard like electronic structure. However, the air-sintered and oxygenannealed samples deviated unexpectedly far from the p = vline, indicating charge balance in the sample is drastically changed in the system. We made an accurate Tc vs v and Tc vs p diagrams for the Bi-based superconductors. While the Tc vs v diagram for the Bi2201 and 2212 superconductors showed a bell-shape, the Tc vs p diagram for those superconductors had a wide plateau on the top. The puzzling shape of Tc vs p diagrams is also explained by the two hole model. Around the plateau, mainly the number of hole of the BiO character is changed. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Since the discovery of (La,Sr)2CuO4 by Bednorz and MuÈller [1], a lot of high-temperature superconductors based on cuprate have been discovered. These superconductors contain the square network of the planes composed of copper and oxygen. It is well-known that the superconducting transition temperature, Tc is determined by a number of CuO2 plane, m [2]. The Bi(Pb)±Sr±Ca±Cu±O, Tl±Ba±Ca±Cu±O and Hg±Ba±Ca±Cu±O system form a homologous series of compounds. The Tc of the Bi(Pb)±Sr±Ca±Cu±O system is 10 K for m = 1, 80 K for m = 2, and 107 K for m = 3, respectively. For the Tl±Ba±Ca±Cu±O and Hg±Ba±Ca±Cu±O system, the Tc increases with increasing number of CuO2 plane to m = 3. Thus, the maximum Tc is ®rstly determined by a number of CuO2 plane, m. Even if a high temperature superconductor have the same number of CuO2 plane in structure, the Tc is strongly dependent on hole concentration [3]. In the (La,Sr)2CuO4 system, which has a single CuO2 square network, the system becomes superconducting from the antiferromagnetic insulator with increasing hole concentration. With increasing hole concentration further, it becomes similar to a normal metal to disappear superconductivity [4]. In the YBa2Cu3Oy [5] and Bi2Sr2Ca1 ÿ xYxCu2Oy [6±8] system, which have a double layer of the CuO2 square network, the similar change in Tc was found although the disappearance of superconductivity in the region of high hole concentration has not yet been found. Most of the hole concentration has so far been measured either by the Hall coecient measurement or by the iodometric measurement, and few works which include both the Hall coecient and the iodometry have been reported. One of such work is reported by Maeda et al. [8]. They measured both the Hall coecient and average Cu valence from iodometry of the {Present address: Stanley Electric Co. Ltd, R&D Laboratory, 1-3-1 Eda Nishi, Aoba-ku, Yokohama 225, Japan. }Present address: Alpine Electronics, Inc., 20-1 Yoshima Kogyou-danchi, Iwaki 970-11, Japan. 53
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Bi(Pb)±Sr±Ca±Cu±O system (m = 1,2,3). They revealed that in m = 1 materials, the Hall coecient deviates strongly from the inverse of hole concentration (v) measured from average Cu valence with increasing hole concentration (v). On the other hand, in the m = 2 materials, the Hall coecient roughly scales with the inverse of hole concentration (v) even for the samples exhibiting superconductivity above liquid-nitrogen temperature. In the previous work [9, 10], we reported that by annealing the Bi1.6Pb0.4Sr2CaCu2Oy and Bi1.6Pb0.4Sr1.6La0.4Cu2Oy in the absence of oxygen, the high-Tc of 96.5 and 39.0 K was achieved, and that the formation of this superconductor with an unexpectedly high Tc value for the m = 1 and 2 phase of the Bi(Pb)±Sr±(Ca)±Cu±O is dicult to explain simply by consideration of the optimum hole concentration. The ®rst purpose of this work is to elucidate the correlation between hole concentration (p) measured by the Hall coecient experiment and that (v) meaured by the iodometry for the Bibased superconductors (m = 1, 2 and 3), and to make sure in which region the hole doping model into the Mott±Hubbard split band is appropriate. The second purpose is to make an accurate Tc vs v and Tc vs p diagrams for the Bibased superconductors (m = 1, 2 and 3). In the Bi-based superconductors, many authors reported that the control of the hole concentration is possible by substitution of cation [8, 11±18] or the control of oxygen content [19±27]. Most of the reports show that while the hole concentration of the Bi2Sr2CuOy and Bi2Sr2CaCu2Oy superconductors are overdoped and the highest Tc is achieved by decreasing the hole concentration, that of the Bi2223 is in the underdoped region and the highest Tc is achieved by increasing the hole concentration. From the Ts vs v and Tc vs p diagram, we discuss the hole-doping mechanism of Bi-based superconductors. Part of the result have already been published in the form of short communication [28, 29]. EXPERIMENTAL
Preparation of Bi2Sr2 ÿ xLaxCuOy (0.3 Ri x R1.0) and Bi2Sr2Ca1ÿxYxCu2Oy (0 R x R0.5) Samples of Bi2Sr2 ÿ xLaxCuOy (0.3 R xR 1.0) and Bi2Sr2Ca1 ÿ xYxCu2Oy (0 R x R0.5) were prepared by a conventional solid-state reaction method. Powders of Bi2O3, SrCO3, CaCO3, Y2O3, La2O3 and CuO were mixed in an agate mortar, calcinated at 8008C for 12 h in air. On the other hand, samples of Bi-2223 were prepared by a nitrate solution mixing method. Starting powders of Bi2(NO3)5 H2O, Pb(NO3)2, Sr(NO3)2, Ca(NO3)2 and Cu(NO3)22.5 H2O were dissolved in water, dried by evaporation, mixed in an agate mortar, and calcinated at 7708C for 10 h in air. The prepared m = 1, 2 and 3 powders were mixed, pressed and sintered at 770± 8648C for 50±160 h in air. Control of hole concentration The hole concentration of the samples were changed by substitution of La for Sr (m = 1), Y for Ca (m = 2), and change in the oxygen content (m = 1,2,3). Some of the samples were annealed in Ar at 7008C for 20 h, or O2 at 525±7008C for 20 h for changing hole density. Con®rmation of chemical composition and crystal structure Chemical composition of the samples was determined by induction-coupled plasma atomic emission spectroscopy (ICP-AES) (ICPS-50A, Shimadzu Co.). Atomic ratio of the Bi2Sr2CaCu2Oy samples were Bi1.99±2.12Sr1.98±2.14Ca1.01±1.10Cu2 Oy, which were consistent with the nominal ones within 10%. There was no dierence in cationic concentrations before and after annealing in N2. Cell parameters were determined by powder X-ray diraction (XRD) using Cu Ka (RADIIIB, Rigaku denki). The peaks were assigned according to those indexed by Onoda et al. [30]. Hole concentration (p) was estimated from oxygen content. Determination of hole concentration by chemical measurement Hole concentration, v was determined by an ordinary iodometric titration methods [9]. We determined oxygen content, y, Cu valence and ®nally hole concentration, v, which is calculated
Correlation between Cu valence, hole density and Tc
55
by (Cu valence)-2. We had already reported that Bi (+Pb) valence determined by the permanganate titration method corresponds to the O±Bi±O length rather than the Bi (+Pb) valence [31]. Consequently, on calculating Cu valence from oxygen content, y, we assumed Bi and Pb valence to be trivalent and divalent, respectively. We assumed that holes are distributed with equal probability in two and three CuO2 planes, respectively. Determination of hole concentration by physical measurement We determined hole concentration also from Hall coecient. We made a cross-shaped sample by a computer aided modeling machine (Roland: CAMM-2). The sample made contact with leads by the silver paste. The current was passed in the sample and 215 kG of magnetic ®eld were applied to the sample. The Hall eect experiment was performed at a room temperature (0300 K). To compare the Hall coecient of the Bi2201, Bi2212 and Bi2223 phase, hole concentration was expressed not by hole density, nH (cmÿ3), but by hole number per a Cu site, p. The correlation between n and p is expressed by n p : x Here, x is given by x
s ; V
where s and V represent a number of Cu atom per a unit cell and volume of a unit cell, respectively. It should be noted that the hole concentration (p) is measured by a polycrystalline sample, whose Hall coecient re¯ects the hole concentration along the ab plane of the sample. RESULTS AND DISCUSSION
Chemical composition of Bi2Sr2Cam ÿ 1CumOy (m = 1, 2, 3), hole concentrations (v and p) and critical temperature (Tc) In Tables 1±3, chemical composition of Bi2Sr2Cam ÿ 1CumOy (m = 1, 2, 3), hole concentrations (v and p) and a critical temperature (Tc) are listed, respectively. It is con®rmed that substitutions of La for Sr (m = 1) or Y for Ca (m = 2), and change in oxygen content (m = 1, 2 and 3) changes hole concentration (v and p) and Tc drastically.
Table 1. Hole concentrations, v and p and Tc of Bi2(Sr2\- ÿ i xLax)2CuOy La content (x) 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Hole concentration v (per a Cu)
Hole concentration p (per a Cu)
Sintered at 830±8608C for 40±100 h 0.24 0.893 0.21 0.714 0.19 0.486 0.16 0.479 0.16 0.393 0.10 0.227 0.09 0.141 0.03 0.052 Argon annealed at 7008C for 20 h after sintering 0.17 0.191 0.14 0.216 0.11 0.132 0.9 0.089 0.07 0.100 0.04 0.046 0.03 0.036 0 0.023
Critical temperature Tc [K] 17.9 25.7 25 0 0 0 0 0 21.5 8.6 0 0 0 0 0 0
S. KAMBE et al.
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Table 2. Hole concentrations, v and p and Tc of Bi2Sr2(Ca1 ÿ xYx)CuOy Y content (x)
Hole concentration v (per a Cu)
Hole concentration p (per a Cu)
Critical temperature Tc [K]
Sintered at 830±8608C for 40±100 h 0.17 0.439 0.13 0.409 0.10 0.314 0.08 0.240 0.04 0.143 0.03 0.076 Oxygen annealed at 7008C for 20 h after sintering 0.21 0.489 0.15 0.479 0.12 0.477 0.12 0.241 0.07 0.143 0.05 0.071 Argon annealed at 7008C for 20 h after sintering 0.09 0.211 0.10 0.125 0.06 0.112 0.06 0.074 0.03 0.078 0.01 0.011
0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5
87.3 81.8 95.0 65.0 43.0 0 62.5 83.7 88.2 70.1 43.5 0 83.8 85.4 84.9 72.2 20.5 0
Bi2201 phase In Fig. 1(a), the correlation between v and p, is shown. If we focus on the Ar-annealed samples represented by solid circles, it is found that p is proportional to v with a proportional constant of 1.0. The correlation is extrapolated to p = 0 at v = 0. This is consistent with the picture of the existence of only one kind of hole doped in a Mott±Hubbard split band. In other word, the correlation between v and p of the Ar-annealed Bi-2201 samples agree very well with that of the Mott±Hubbard model, indicating that extra charge caused by the excess oxygen is all doped to the CuO2 plane as a hole. However, for air-annealed samples represented by hatched circles, the correlation between p and v is dierent from that for the Ar-annealed samples. The correlation between p and v is almost the same at v = 2.03±2.09 for the air-annealed samples. On the other hand, the trend of p vs v is changed at 2.09 R vR 2.24. Its proportional constant is 4.0±5.0, which is 4±5 times larger than that of the Mott±Hubbard model. Similar deviation has already been reported for Bi2201 and La214 superconductors [19, 32] and it is believed that the deviation comes from the destruction of the Mott±Hubbard character. A. Maeda et al. [8] ®rst revealed that with increasing hole concentration of the Bi2201 phase, the correlation between p and v deviates from the line of p = v and superconductivity occurs in the concentration range (v) where the deviation becomes clear. However, for Ar-annealed samples in our experiment, superconductivity occurs in the concentration range of 0.14 R v R0.18 where the deviation is not observed, in other words, where the Mott±Hubbard character of the electronic structure still survives. Therefore, it is likely that the appearance of the high-Tc superconductivity of the Bi2201 phase does not correlate directly with the destruction of Mott±Hubbard character of electronic structure. Table 3. Hole concentrations, v and p and Tc of Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06Oy Oxygen content (y) 9.95 9.98 10.01 10.01 9.96
Hole concentration v (per a Cu)
Hole concentration p (per a Cu)
Critical temperature Tc [K]
Sintered at 864±8658C for 40±100 h 0.034 0.287 0.056 0.240 Oxygen annealed at 525±5458C for 20 H after sintering 0.074 0.171 0.074 0.159 Argon annealed at 7008C for 20 h after sintering 0.037 0.040
107 104.9 99 105 84.3
Correlation between Cu valence, hole density and Tc
57
Fig. 1. Correlation between hole concentrations, p measured by iodometry and v measured by Hall eect. The dashed line represents the theoretical line based on the Mott±Hubbard split band model.
Bi-2212 phase Next, we move to the m = 2 phase. For the air-annealed samples, it is found that hole concentration, p scales very well with v, and that the p±v line passes through the origin of the coordinates by extrapolating the line. However, otherwise the Bi2201 phase, the proportional constant, dp/dv is about two, which is in good agreement with the previous report [8]. The simplest explanation of the proportional constant ``two'' for m = 2 phase is to assume the coexistence of two kind of hole. Band structure calculations for the m = 2 samples show the existence of two bands crossing the Fermi level: one has CuO2 character and the other has BiO character. From the STM/STS study of fully O2-annealed Bi2Sr2CaCu2Oy superconductor, the BiO layer are found to be metallic [35]. For the Ar-annealed samples represented by solid triangles, which should lose hole of the BiO character by loss of the excess oxygen in the BiO layer, the proportional constant, the dp/dv is 1.0±1.5, which is smaller than 2.0 of the air-annealed samples. In particular, in the region of pR 0.1, the solid triangles roughly lie near the dashed line, suggesting that the Ar-annealed samples in this region have Mott±Hubbard type electronic structure.zzz Interestingly, hole concentration, p, was found to be saturated at 0.5 in the range of 0.12 R v R0.20 for the oxygen-annealed samples represented by open triangles, suggesting that hole concentration (p) is stabilized at p = 0.5. It would be emphasized that the saturation at p = 0.5 is also observed for a Bi2212 thin ®lm [36]. However, such saturation is not observed for Y123 sample [37] in spite of having double CuO2 plane in it, suggesting that the stabilization is correlated with the interaction between holes of the CuO2 character and the BiO character.
S. KAMBE et al.
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Fig. 2. Superconducting transition temperature, Tc as a function of the hole concentration (v) measured by iodometry.
Bi-2223 phase Only an argon-annealed sample, represented by a solid square in Fig. 1(b), lied on the p = v line, suggesting that it has a Mott±Hubbard like electronic structure. However, the air-sintered samples, represented by hatched squares, unexpectedly deviated far from the p = v line represented by broken line, indicating charge balance in the sample is drastically changed in the system. More insertion of oxygen by O2-annealing, which corresponds to v = 0.074, decreased the p value below 0.2 again, suggesting that a hole is not doped, but eliminated with an increase in oxygen content. This puzzling behavior may be caused by localization of hole at a Pb site in the BiO layer. We examined correlation between v and p, for Pb-doped Bi2212 phase, revealing that a Pb ion localizes hole at the Pb site. For the Pb-doped Bi2223 phase, similar localization of hole may occur. Another possible explanation is decomposition of the sample by O2-annealing. Although we could not ®nd any impurity from the XRD peaks, the metastable Bi2223 sample might be decomposed by O2-annealing. Further experiment will be needed to elucidate the p±v shape in more detail for the Bi2223 phase. Critical temperature vs hole concentration In Figs 2 and 3, Tc's are plotted against hole concentration, v and p, respectively. The result of Fig. 2 is consistent with the bell-shaped v±Tc line reported previously [6±9]. It is to be noted
Fig. 3. Superconducting transition temperature, Tc as a function of the hole concentration (p) measured by Hall eect.
Correlation between Cu valence, hole density and Tc
59
that the optimum hole concentration, v becomes smaller from 0.2 to 0.05 with increase in the number of the CuO2 plane, m from 1 to 3. As shown in Fig. 3, the trend of Tc vs p is similar to that of Tc vs v in Fig. 2 except that the bell-shape of m = 1 and 2 has a wide plateau on the top. The mysterious shape probably comes from hole of the Bi±O character. As described in the previous chapters, electronic structure of the BiO plane is known to be drastically changed from metal to insulator by removing the excess oxygen in the BiO layer. Thus, the number of hole of BiO character will be drastically changed by O2/Ar annealing. In this case, the number of CuO2 character does not change so much as that of BiO character does. Since the Tc is mainly determined by hole concentration of the CuO2 character, Tc does not change so much even if the hole number of BiO character is changed. This would be the origin of the plateau shape of the Tc vs p diagrams, suggesting that the plateau is originated from the change in the number of hole of the BiO character. CONCLUSION
We examined correlation between hole concentrations, p measured by Hall eect experiment, v measured by iodometry and superconducting transition temperature, Tc for the Bi2201, 2212 and 2223 superconductors. For Bi2201 phase, by comparing the correlation between them with the Mott±Hubbard split model, it was found that while the correlation deviates from the Mott±Hubbard model for airsintered samples, the correlation agree well with the Mott±Hubbard model for Ar-annealed samples. The deviation can be explained by the change in the electronic structure from insulator to metal, that is, the destruction of Mott±Hubbard character. For the air-annealed samples of Bi2212 phase, hole concentration, p scales very well with v, and the p±v line passes through the origin of the coordinates. However, the proportional constant is about two. The simplest explanation of the double proportional constant is to assume the coexistence of a hole of CuO2 character and BiO character. We found that the hole concentration, 0.5 is very stable for the oxygen-annealed Bi2212 samples. Considering that hole of these samples has both characters of CuO2 and BiO, and that the stabilization does not occur for the Y123 superconductor, the stabilization is probably strongly correlated with the interaction between holes of the CuO2 character and the BiO character. For Bi2223 phase, only an Ar-annealed sample lied on the p = v line, suggesting that it has a Mott±Hubbard like electronic structure. However, the air-sintered samples deviated unexpectedly far from the p = v line represented by broken line, indicating charge balance in the sample is drastically changed in the system. More insertion of oxygen by O2-annealing, which corresponds to v = 0.074, decreased the p value below 0.2 again, suggesting that a hole is not doped but eliminated with further increase in oxygen content. We made accurate Tc vs v and Tc vs p diagrams for the Bi-based superconductors. While the Tc vs v diagram for the Bi2201 and 2212 superconductors showed a bell-shape, the Tc vs p diagram for those superconductors had a wide plateau on the top. The puzzling shape of Tc vs p diagrams is also explained by the two hole model. Around the plateau, mainly the number of the BiO character is changed, where the Tc is not changed so much. AcknowledgementsÐWe are thankful to Professor Iye in ISSP for Hall coecient measurement. A part of this work supported by the Grant of Nissan Foundation and the Ministry of Education.
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Applied Superconductivity Vol. 5, Nos 1±6, pp. 53±60, 1997 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0964-1807(98)00034-9 0964-1807/98 $19.00 + 0.00
CORRELATION BETWEEN Cu VALENCE, HOLE DENSITY AND Tc OF Bi2201, Bi2212 AND Bi2223 SUPERCONDUCTORS SHIRO KAMBE*, KENICHIRO ABE$, KIYOSHI KOIKE*, YUICHIRO TAKASUGI*, OSAMU ISHII*, TOSHIO FURUSAWA*{, TSUYOSHI SHIOMI$} and SHIGETOSHI OHSHIMA$ *Graduate School of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992, Japan $Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992, Japan AbstractÐFor the Bi2201 phase, by comparing the correlation between them with the Mott±Hubbard split model, it was found that while the correlation deviates from the Mott±Hubbard model for air-sintered samples, the correlation agree well with the Mott±Hubbard model for Ar-annealed samples. The deviation is believed to come from the destruction of the Mott±Hubbard character. For the airannealed samples of Bi2212 phase, hole concentration, p scales very well with v, and the p±v line passes through the origin of the coordinates. However, the proportional constant is about two. The simplest explanation of the double proportional constant is to assume the coexistence of a hole of CuO2 character and BiO character. For the Bi2223 phase, only an Ar-annealed sample lied on the p = v line, suggesting that it has a Mott±Hubbard like electronic structure. However, the air-sintered and oxygenannealed samples deviated unexpectedly far from the p = vline, indicating charge balance in the sample is drastically changed in the system. We made an accurate Tc vs v and Tc vs p diagrams for the Bi-based superconductors. While the Tc vs v diagram for the Bi2201 and 2212 superconductors showed a bell-shape, the Tc vs p diagram for those superconductors had a wide plateau on the top. The puzzling shape of Tc vs p diagrams is also explained by the two hole model. Around the plateau, mainly the number of hole of the BiO character is changed. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Since the discovery of (La,Sr)2CuO4 by Bednorz and MuÈller [1], a lot of high-temperature superconductors based on cuprate have been discovered. These superconductors contain the square network of the planes composed of copper and oxygen. It is well-known that the superconducting transition temperature, Tc is determined by a number of CuO2 plane, m [2]. The Bi(Pb)±Sr±Ca±Cu±O, Tl±Ba±Ca±Cu±O and Hg±Ba±Ca±Cu±O system form a homologous series of compounds. The Tc of the Bi(Pb)±Sr±Ca±Cu±O system is 10 K for m = 1, 80 K for m = 2, and 107 K for m = 3, respectively. For the Tl±Ba±Ca±Cu±O and Hg±Ba±Ca±Cu±O system, the Tc increases with increasing number of CuO2 plane to m = 3. Thus, the maximum Tc is ®rstly determined by a number of CuO2 plane, m. Even if a high temperature superconductor have the same number of CuO2 plane in structure, the Tc is strongly dependent on hole concentration [3]. In the (La,Sr)2CuO4 system, which has a single CuO2 square network, the system becomes superconducting from the antiferromagnetic insulator with increasing hole concentration. With increasing hole concentration further, it becomes similar to a normal metal to disappear superconductivity [4]. In the YBa2Cu3Oy [5] and Bi2Sr2Ca1 ÿ xYxCu2Oy [6±8] system, which have a double layer of the CuO2 square network, the similar change in Tc was found although the disappearance of superconductivity in the region of high hole concentration has not yet been found. Most of the hole concentration has so far been measured either by the Hall coecient measurement or by the iodometric measurement, and few works which include both the Hall coecient and the iodometry have been reported. One of such work is reported by Maeda et al. [8]. They measured both the Hall coecient and average Cu valence from iodometry of the {Present address: Stanley Electric Co. Ltd, R&D Laboratory, 1-3-1 Eda Nishi, Aoba-ku, Yokohama 225, Japan. }Present address: Alpine Electronics, Inc., 20-1 Yoshima Kogyou-danchi, Iwaki 970-11, Japan. 53
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Bi(Pb)±Sr±Ca±Cu±O system (m = 1,2,3). They revealed that in m = 1 materials, the Hall coecient deviates strongly from the inverse of hole concentration (v) measured from average Cu valence with increasing hole concentration (v). On the other hand, in the m = 2 materials, the Hall coecient roughly scales with the inverse of hole concentration (v) even for the samples exhibiting superconductivity above liquid-nitrogen temperature. In the previous work [9, 10], we reported that by annealing the Bi1.6Pb0.4Sr2CaCu2Oy and Bi1.6Pb0.4Sr1.6La0.4Cu2Oy in the absence of oxygen, the high-Tc of 96.5 and 39.0 K was achieved, and that the formation of this superconductor with an unexpectedly high Tc value for the m = 1 and 2 phase of the Bi(Pb)±Sr±(Ca)±Cu±O is dicult to explain simply by consideration of the optimum hole concentration. The ®rst purpose of this work is to elucidate the correlation between hole concentration (p) measured by the Hall coecient experiment and that (v) meaured by the iodometry for the Bibased superconductors (m = 1, 2 and 3), and to make sure in which region the hole doping model into the Mott±Hubbard split band is appropriate. The second purpose is to make an accurate Tc vs v and Tc vs p diagrams for the Bibased superconductors (m = 1, 2 and 3). In the Bi-based superconductors, many authors reported that the control of the hole concentration is possible by substitution of cation [8, 11±18] or the control of oxygen content [19±27]. Most of the reports show that while the hole concentration of the Bi2Sr2CuOy and Bi2Sr2CaCu2Oy superconductors are overdoped and the highest Tc is achieved by decreasing the hole concentration, that of the Bi2223 is in the underdoped region and the highest Tc is achieved by increasing the hole concentration. From the Ts vs v and Tc vs p diagram, we discuss the hole-doping mechanism of Bi-based superconductors. Part of the result have already been published in the form of short communication [28, 29]. EXPERIMENTAL
Preparation of Bi2Sr2 ÿ xLaxCuOy (0.3 Ri x R1.0) and Bi2Sr2Ca1ÿxYxCu2Oy (0 R x R0.5) Samples of Bi2Sr2 ÿ xLaxCuOy (0.3 R xR 1.0) and Bi2Sr2Ca1 ÿ xYxCu2Oy (0 R x R0.5) were prepared by a conventional solid-state reaction method. Powders of Bi2O3, SrCO3, CaCO3, Y2O3, La2O3 and CuO were mixed in an agate mortar, calcinated at 8008C for 12 h in air. On the other hand, samples of Bi-2223 were prepared by a nitrate solution mixing method. Starting powders of Bi2(NO3)5 H2O, Pb(NO3)2, Sr(NO3)2, Ca(NO3)2 and Cu(NO3)22.5 H2O were dissolved in water, dried by evaporation, mixed in an agate mortar, and calcinated at 7708C for 10 h in air. The prepared m = 1, 2 and 3 powders were mixed, pressed and sintered at 770± 8648C for 50±160 h in air. Control of hole concentration The hole concentration of the samples were changed by substitution of La for Sr (m = 1), Y for Ca (m = 2), and change in the oxygen content (m = 1,2,3). Some of the samples were annealed in Ar at 7008C for 20 h, or O2 at 525±7008C for 20 h for changing hole density. Con®rmation of chemical composition and crystal structure Chemical composition of the samples was determined by induction-coupled plasma atomic emission spectroscopy (ICP-AES) (ICPS-50A, Shimadzu Co.). Atomic ratio of the Bi2Sr2CaCu2Oy samples were Bi1.99±2.12Sr1.98±2.14Ca1.01±1.10Cu2 Oy, which were consistent with the nominal ones within 10%. There was no dierence in cationic concentrations before and after annealing in N2. Cell parameters were determined by powder X-ray diraction (XRD) using Cu Ka (RADIIIB, Rigaku denki). The peaks were assigned according to those indexed by Onoda et al. [30]. Hole concentration (p) was estimated from oxygen content. Determination of hole concentration by chemical measurement Hole concentration, v was determined by an ordinary iodometric titration methods [9]. We determined oxygen content, y, Cu valence and ®nally hole concentration, v, which is calculated
Correlation between Cu valence, hole density and Tc
55
by (Cu valence)-2. We had already reported that Bi (+Pb) valence determined by the permanganate titration method corresponds to the O±Bi±O length rather than the Bi (+Pb) valence [31]. Consequently, on calculating Cu valence from oxygen content, y, we assumed Bi and Pb valence to be trivalent and divalent, respectively. We assumed that holes are distributed with equal probability in two and three CuO2 planes, respectively. Determination of hole concentration by physical measurement We determined hole concentration also from Hall coecient. We made a cross-shaped sample by a computer aided modeling machine (Roland: CAMM-2). The sample made contact with leads by the silver paste. The current was passed in the sample and 215 kG of magnetic ®eld were applied to the sample. The Hall eect experiment was performed at a room temperature (0300 K). To compare the Hall coecient of the Bi2201, Bi2212 and Bi2223 phase, hole concentration was expressed not by hole density, nH (cmÿ3), but by hole number per a Cu site, p. The correlation between n and p is expressed by n p : x Here, x is given by x
s ; V
where s and V represent a number of Cu atom per a unit cell and volume of a unit cell, respectively. It should be noted that the hole concentration (p) is measured by a polycrystalline sample, whose Hall coecient re¯ects the hole concentration along the ab plane of the sample. RESULTS AND DISCUSSION
Chemical composition of Bi2Sr2Cam ÿ 1CumOy (m = 1, 2, 3), hole concentrations (v and p) and critical temperature (Tc) In Tables 1±3, chemical composition of Bi2Sr2Cam ÿ 1CumOy (m = 1, 2, 3), hole concentrations (v and p) and a critical temperature (Tc) are listed, respectively. It is con®rmed that substitutions of La for Sr (m = 1) or Y for Ca (m = 2), and change in oxygen content (m = 1, 2 and 3) changes hole concentration (v and p) and Tc drastically.
Table 1. Hole concentrations, v and p and Tc of Bi2(Sr2\- ÿ i xLax)2CuOy La content (x) 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Hole concentration v (per a Cu)
Hole concentration p (per a Cu)
Sintered at 830±8608C for 40±100 h 0.24 0.893 0.21 0.714 0.19 0.486 0.16 0.479 0.16 0.393 0.10 0.227 0.09 0.141 0.03 0.052 Argon annealed at 7008C for 20 h after sintering 0.17 0.191 0.14 0.216 0.11 0.132 0.9 0.089 0.07 0.100 0.04 0.046 0.03 0.036 0 0.023
Critical temperature Tc [K] 17.9 25.7 25 0 0 0 0 0 21.5 8.6 0 0 0 0 0 0
S. KAMBE et al.
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Table 2. Hole concentrations, v and p and Tc of Bi2Sr2(Ca1 ÿ xYx)CuOy Y content (x)
Hole concentration v (per a Cu)
Hole concentration p (per a Cu)
Critical temperature Tc [K]
Sintered at 830±8608C for 40±100 h 0.17 0.439 0.13 0.409 0.10 0.314 0.08 0.240 0.04 0.143 0.03 0.076 Oxygen annealed at 7008C for 20 h after sintering 0.21 0.489 0.15 0.479 0.12 0.477 0.12 0.241 0.07 0.143 0.05 0.071 Argon annealed at 7008C for 20 h after sintering 0.09 0.211 0.10 0.125 0.06 0.112 0.06 0.074 0.03 0.078 0.01 0.011
0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5
87.3 81.8 95.0 65.0 43.0 0 62.5 83.7 88.2 70.1 43.5 0 83.8 85.4 84.9 72.2 20.5 0
Bi2201 phase In Fig. 1(a), the correlation between v and p, is shown. If we focus on the Ar-annealed samples represented by solid circles, it is found that p is proportional to v with a proportional constant of 1.0. The correlation is extrapolated to p = 0 at v = 0. This is consistent with the picture of the existence of only one kind of hole doped in a Mott±Hubbard split band. In other word, the correlation between v and p of the Ar-annealed Bi-2201 samples agree very well with that of the Mott±Hubbard model, indicating that extra charge caused by the excess oxygen is all doped to the CuO2 plane as a hole. However, for air-annealed samples represented by hatched circles, the correlation between p and v is dierent from that for the Ar-annealed samples. The correlation between p and v is almost the same at v = 2.03±2.09 for the air-annealed samples. On the other hand, the trend of p vs v is changed at 2.09 R vR 2.24. Its proportional constant is 4.0±5.0, which is 4±5 times larger than that of the Mott±Hubbard model. Similar deviation has already been reported for Bi2201 and La214 superconductors [19, 32] and it is believed that the deviation comes from the destruction of the Mott±Hubbard character. A. Maeda et al. [8] ®rst revealed that with increasing hole concentration of the Bi2201 phase, the correlation between p and v deviates from the line of p = v and superconductivity occurs in the concentration range (v) where the deviation becomes clear. However, for Ar-annealed samples in our experiment, superconductivity occurs in the concentration range of 0.14 R v R0.18 where the deviation is not observed, in other words, where the Mott±Hubbard character of the electronic structure still survives. Therefore, it is likely that the appearance of the high-Tc superconductivity of the Bi2201 phase does not correlate directly with the destruction of Mott±Hubbard character of electronic structure. Table 3. Hole concentrations, v and p and Tc of Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06Oy Oxygen content (y) 9.95 9.98 10.01 10.01 9.96
Hole concentration v (per a Cu)
Hole concentration p (per a Cu)
Critical temperature Tc [K]
Sintered at 864±8658C for 40±100 h 0.034 0.287 0.056 0.240 Oxygen annealed at 525±5458C for 20 H after sintering 0.074 0.171 0.074 0.159 Argon annealed at 7008C for 20 h after sintering 0.037 0.040
107 104.9 99 105 84.3
Correlation between Cu valence, hole density and Tc
57
Fig. 1. Correlation between hole concentrations, p measured by iodometry and v measured by Hall eect. The dashed line represents the theoretical line based on the Mott±Hubbard split band model.
Bi-2212 phase Next, we move to the m = 2 phase. For the air-annealed samples, it is found that hole concentration, p scales very well with v, and that the p±v line passes through the origin of the coordinates by extrapolating the line. However, otherwise the Bi2201 phase, the proportional constant, dp/dv is about two, which is in good agreement with the previous report [8]. The simplest explanation of the proportional constant ``two'' for m = 2 phase is to assume the coexistence of two kind of hole. Band structure calculations for the m = 2 samples show the existence of two bands crossing the Fermi level: one has CuO2 character and the other has BiO character. From the STM/STS study of fully O2-annealed Bi2Sr2CaCu2Oy superconductor, the BiO layer are found to be metallic [35]. For the Ar-annealed samples represented by solid triangles, which should lose hole of the BiO character by loss of the excess oxygen in the BiO layer, the proportional constant, the dp/dv is 1.0±1.5, which is smaller than 2.0 of the air-annealed samples. In particular, in the region of pR 0.1, the solid triangles roughly lie near the dashed line, suggesting that the Ar-annealed samples in this region have Mott±Hubbard type electronic structure.zzz Interestingly, hole concentration, p, was found to be saturated at 0.5 in the range of 0.12 R v R0.20 for the oxygen-annealed samples represented by open triangles, suggesting that hole concentration (p) is stabilized at p = 0.5. It would be emphasized that the saturation at p = 0.5 is also observed for a Bi2212 thin ®lm [36]. However, such saturation is not observed for Y123 sample [37] in spite of having double CuO2 plane in it, suggesting that the stabilization is correlated with the interaction between holes of the CuO2 character and the BiO character.
S. KAMBE et al.
58
Fig. 2. Superconducting transition temperature, Tc as a function of the hole concentration (v) measured by iodometry.
Bi-2223 phase Only an argon-annealed sample, represented by a solid square in Fig. 1(b), lied on the p = v line, suggesting that it has a Mott±Hubbard like electronic structure. However, the air-sintered samples, represented by hatched squares, unexpectedly deviated far from the p = v line represented by broken line, indicating charge balance in the sample is drastically changed in the system. More insertion of oxygen by O2-annealing, which corresponds to v = 0.074, decreased the p value below 0.2 again, suggesting that a hole is not doped, but eliminated with an increase in oxygen content. This puzzling behavior may be caused by localization of hole at a Pb site in the BiO layer. We examined correlation between v and p, for Pb-doped Bi2212 phase, revealing that a Pb ion localizes hole at the Pb site. For the Pb-doped Bi2223 phase, similar localization of hole may occur. Another possible explanation is decomposition of the sample by O2-annealing. Although we could not ®nd any impurity from the XRD peaks, the metastable Bi2223 sample might be decomposed by O2-annealing. Further experiment will be needed to elucidate the p±v shape in more detail for the Bi2223 phase. Critical temperature vs hole concentration In Figs 2 and 3, Tc's are plotted against hole concentration, v and p, respectively. The result of Fig. 2 is consistent with the bell-shaped v±Tc line reported previously [6±9]. It is to be noted
Fig. 3. Superconducting transition temperature, Tc as a function of the hole concentration (p) measured by Hall eect.
Correlation between Cu valence, hole density and Tc
59
that the optimum hole concentration, v becomes smaller from 0.2 to 0.05 with increase in the number of the CuO2 plane, m from 1 to 3. As shown in Fig. 3, the trend of Tc vs p is similar to that of Tc vs v in Fig. 2 except that the bell-shape of m = 1 and 2 has a wide plateau on the top. The mysterious shape probably comes from hole of the Bi±O character. As described in the previous chapters, electronic structure of the BiO plane is known to be drastically changed from metal to insulator by removing the excess oxygen in the BiO layer. Thus, the number of hole of BiO character will be drastically changed by O2/Ar annealing. In this case, the number of CuO2 character does not change so much as that of BiO character does. Since the Tc is mainly determined by hole concentration of the CuO2 character, Tc does not change so much even if the hole number of BiO character is changed. This would be the origin of the plateau shape of the Tc vs p diagrams, suggesting that the plateau is originated from the change in the number of hole of the BiO character. CONCLUSION
We examined correlation between hole concentrations, p measured by Hall eect experiment, v measured by iodometry and superconducting transition temperature, Tc for the Bi2201, 2212 and 2223 superconductors. For Bi2201 phase, by comparing the correlation between them with the Mott±Hubbard split model, it was found that while the correlation deviates from the Mott±Hubbard model for airsintered samples, the correlation agree well with the Mott±Hubbard model for Ar-annealed samples. The deviation can be explained by the change in the electronic structure from insulator to metal, that is, the destruction of Mott±Hubbard character. For the air-annealed samples of Bi2212 phase, hole concentration, p scales very well with v, and the p±v line passes through the origin of the coordinates. However, the proportional constant is about two. The simplest explanation of the double proportional constant is to assume the coexistence of a hole of CuO2 character and BiO character. We found that the hole concentration, 0.5 is very stable for the oxygen-annealed Bi2212 samples. Considering that hole of these samples has both characters of CuO2 and BiO, and that the stabilization does not occur for the Y123 superconductor, the stabilization is probably strongly correlated with the interaction between holes of the CuO2 character and the BiO character. For Bi2223 phase, only an Ar-annealed sample lied on the p = v line, suggesting that it has a Mott±Hubbard like electronic structure. However, the air-sintered samples deviated unexpectedly far from the p = v line represented by broken line, indicating charge balance in the sample is drastically changed in the system. More insertion of oxygen by O2-annealing, which corresponds to v = 0.074, decreased the p value below 0.2 again, suggesting that a hole is not doped but eliminated with further increase in oxygen content. We made accurate Tc vs v and Tc vs p diagrams for the Bi-based superconductors. While the Tc vs v diagram for the Bi2201 and 2212 superconductors showed a bell-shape, the Tc vs p diagram for those superconductors had a wide plateau on the top. The puzzling shape of Tc vs p diagrams is also explained by the two hole model. Around the plateau, mainly the number of the BiO character is changed, where the Tc is not changed so much. AcknowledgementsÐWe are thankful to Professor Iye in ISSP for Hall coecient measurement. A part of this work supported by the Grant of Nissan Foundation and the Ministry of Education.
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