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Electrical properties of layered perovskite-type palladium oxides
Materials Science and Engineering B 148 (2008) 65–68
Electrical properties of layered perovskite-type palladium oxides S. Ayukawa a,b,∗ , M. Kato a,b , T. Noji a,b , Y. Koike a,b a
Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan b JST-CREST, Japan
Received 27 May 2007; received in revised form 4 July 2007; accepted 3 September 2007
Abstract Electron-doping into Nd2 PdO4 and Ba2 PdO2 F2 with the same structure as in the electron-doped high-Tc superconductor T -Nd2−x Cex CuO4 has been tried by introducing oxygen vacancies or intercalating Li+ ions in order to bring about superconductivity. Polycrystalline samples of T Nd2 PdO4 were synthesized by the solid-state reaction method using a NaCl-flux technique. In order to introduce oxygen vacancies, the samples were annealed at 600–700 ◦ C in flowing gas of argon or in vacuum. It has been found that the value of the electrical resistivity of T -Nd2 PdO4−δ decreases down to ∼10 cm through the annealing in vacuum, however, but it still remains semiconductive. Polycrystalline samples of T -Ba2 PdO2 F2 were prepared by the low-temperature fluorination of Ba2 PdO3 using ZnF2 as a fluorinating agent. The intercalation of Li into T -Ba2 PdO2 F2 was tried using an electrochemical technique. However, it has been found from the ICP-AES analysis that Li+ ions are not intercalated into T -Ba2 PdO2 F2 . © 2007 Elsevier B.V. All rights reserved. Keywords: Superconductor; Palladium oxide; Lithium-intercalation
1. Introduction In the high-Tc cuprates, CuO2 planes are believed to play an important role in superconductivity. Carrier-doping into the Cu 3dx2 −y2 –O 2p orbitals in the CuO2 planes is essential for the appearance of the high-Tc superconductivity. It takes great interest to search for new superconductors by carrier-doping into the dx2 −y2 –O 2p orbitals in non-cuprates. Both Nd2 PdO4 and Ba2 PdO2 F2 are isostructural to the well-known electron-doped high-Tc superconductor Nd2−x Cex CuO4 with the so-called T type structure, as shown in Fig. 1. In both T -Nd2 PdO4 and T -Ba2 PdO2 F2 , the valency of Pd is 2+. The electronic configuration of Pd2+ is 4d8 . According to magnetic susceptibility measurements of T -La2 PdO4 , Pd2+ is in the low-spin state and therefore the Pd 4dx2 −y2 band is unoccupied [1]. Accordingly, doped electrons are expected to be introduced into the PdO2 planes consisting of Pd 4dx2 −y2 –O 2p orbitals in T -Nd2 PdO4 and T -Ba2 PdO2 F2 , as in the case of T -Nd2−x Cex CuO4 . In fact, electron-doping into T -Nd2 PdO4 has already been tried
∗ Corresponding author at: Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. Tel.: +81 22 795 7977; fax: +81 22 795 7977. E-mail address: [email protected] (S. Ayukawa).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.09.055
through the substitution of Ce4+ for Nd3+ by Kobayashi et al. [2]. However, electron carriers have never been doped enough for the appearance of superconductivity. Here, we have tried electron-doping into T -Nd2 PdO4 and T -Ba2 PdO2 F2 by introducing oxygen vacancies or intercalating Li+ ions in order to bring about superconductivity. 2. Experimental Polycrystalline samples of T -Nd2 PdO4 were synthesized by the solid-state reaction method using a NaCl-flux technique [1]. Stoichiometric amounts of Nd2 O3 and PdO were mixed. NaCl was then added in the molar ratio of PdO:NaCl = 1:16. The mixture was mixed again, and heated at 800 ◦ C for 24 h in flowing gas of oxygen. The product was finely ground and washed with distilled water to remove NaCl. The obtained powder was dried at 110 ◦ C, pelletized and sintered at 800 ◦ C for 12 h in flowing gas of oxygen. In order to reduce the oxygen content, the pelletized samples were annealed at 600–700 ◦ C in vacuum for 24–120 h or at 600 ◦ C in flowing gas of argon for 48–120 h. For comparison, annealing in flowing gas of oxygen was also carried out at 300 ◦ C in order to incorporate excess oxygen. Polycrystalline samples of T -Ba2 PdO2 F2 were prepared by the low-temperature fluorination of Ba2 PdO3 . ZnF2 was used as
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S. Ayukawa et al. / Materials Science and Engineering B 148 (2008) 65–68
Fig. 1. Crystal structures of (a) T -Nd2 PdO4 and (b) T -Ba2 PdO2 F2 .
a fluorinating agent, instead of CuF2 or poly(vinylidene difluoride) used by Baikie et al. [3], to avoid the formation of any superconducting cuprates as impurity phases. The parent compound of Ba2 PdO3 was prepared by the solid-state reaction method. Mixtures of stoichiometric amounts of BaO and PdO were ground in an argon-filled glove box and heated at 1100 ◦ C for 48 h in flowing gas of O2 20%–Ar 80% to avoid the generation of BaCO3 . The obtained powder of Ba2 PdO3 was mixed with the fluorinating agent of ZnF2 , and heated at 250 ◦ C for 14 h in flowing gas of O2 20%–Ar 80%. The obtained powder was pelletized and sintered at 400 ◦ C for 14 h in flowing gas of O2 20%–Ar 80%. The electron-doping was tried by the intercalation
of Li into T -Ba2 PdO2 F2 using an electrochemical technique. The electrochemical Li-intercalation was carried out at room temperature in an argon-filled glove box. A three-electrode cell was set up as T -Ba2 PdO2 F2 |1.0 M LiClO4 /PC|Li, as shown in Fig. 2. The working electrode (WE) was a pellet of T Ba2 PdO2 F2 sandwiched by Ni meshes. As an electrolyte, 1.0 M LiClO4 dissolved in propylene carbonate (PC) was used. Li sheets were used as the counter electrode (CE) and the reference electrode (RE). The Li-intercalation was performed at a constant current of 0.05 mA using a galvanostat. The total amount of Li intercalated into T -Ba2 PdO2 F2 was estimated according to the simple Faraday law and also from the inductively coupled plasma atomic emission spectrometry (ICP-AES). All products were characterized by the powder X-ray diffraction using Cu K␣ radiation. Since T -Ba2 PdO2 F2 was unstable in air, they were mixed with grease in order to avoid the exposure to the moisture in the atmosphere during the measurements. The electrical resistivity was measured by the four-probe method to detect the superconductivity. 3. Results and discussion
Fig. 2. Schematic diagram of the three-electrode cell for the electrochemical Li-intercalation.
Fig. 3 shows the powder X-ray diffraction patterns of T -Nd2 PdO4−δ obtained on various annealing conditions, indicating that the samples are not decomposed through the annealing. The lattice parameters are invariable through the reductive annealing. This may be due to the low concentration of electrons doped by oxygen vacancies. Moreover, a-axis length may be insensitive to the electron-carrier concentration. This behavior is commonly observed in electron-doped superconducting cuprates, in which a-axis length is mainly dependent on the
S. Ayukawa et al. / Materials Science and Engineering B 148 (2008) 65–68
Fig. 3. Powder X-ray diffraction patterns of T -Nd2 PdO4−δ obtained on various anneal conditions.
sites of blocking layer [4]. On the other hand, large expansion of lattice parameters through the oxygen annealing is due to the incorporation of excess oxygen into the Nd2 O2 blocking layer. Fig. 4 shows the temperature dependence of the electrical resistivity, ρ, of T -Nd2 PdO4−δ obtained on various annealing conditions. The value of ρ slightly decreases through the annealing in flowing gas of argon and markedly decreases from 5 × 103 cm close to 101 cm through the annealing in vacuum. The decrease of ρ seems to be due to an increase of oxygen vacancies operating as electron-carrier dopants. However, the value of ρ tends to increase with increasing annealing-time. The excessive increase of oxygen vacancies may have unfavorable influence on the electronic conduction in the PdO2 plane. That is, it may lead to not only the increase in the electron-carrier concentration but also the decrease in the electron mobility due to the trap of electrons at oxygen vacancies. Therefore, the optimization of the annealing condition for the removal of oxygen not from the PdO2 plane but from the Nd2 O2 layer is required. On the other hand, the increase of ρ through the annealing in flowing gas of oxygen suggests that hole carriers doped by excess oxygen are unlikely to itinerate in T -Nd2 PdO4 as in the case of T -Nd2 CuO4 .
Fig. 4. Temperature dependence of the electrical resistivity ρ of T -Nd2 PdO4-δ obtained on by various anneal conditions: (a) as-prepared, (b) in argon at 600 ◦ C for 24 h, (c) in argon at 600 ◦ C for 48 h, (d) in vacuum at 600 ◦ C for 24 h, (e) in vacuum at 600 ◦ C for 48 h, (f) in vacuum at 600 ◦ C for 120 h, (g) in vacuum at 700 ◦ C for 24 h and (h) in oxygen at 300 ◦ C for 72 h.
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Fig. 5. Powder X-ray diffraction pattern of T -Ba2 PdO2 F2 .
Fig. 5 shows the powder X-ray diffraction pattern of T Ba2 PdO2 F2 . All of the peaks, except for those of ZnO generated as a result of the fluorination using ZnF2 , can be indexed on the ˚ and c = 14.046 A, ˚ basis of the tetragonal lattice with a = 4.140 A which are in good agreement with the earlier work [3]. The electron-doping into T -Ba2 PdO2 F2 has been tried by the Liintercalation, because Li+ ions are expected to be intercalated into the F layer. Fig. 6 shows the cyclic voltammogram of T -Ba2 PdO2 F2 in 1.0 M LiClO4 /PC. For reference, the cyclic voltammogram of T-Sr2 CuO2 Br2 with the K2 NiF4 structure, which shows superconductivity with Tc = 8 K through the Liintercalation, is also shown [5]. The reductive current below 1000 mV for T -Ba2 PdO2 F2 is much smaller than that for TSr2 CuO2 Br2 and may be due to the adsorption of the solvated Li+ ions on the surface of the sample, as can be seen in the Liintercalation into graphite [6]. This indicates that Li+ ions are unlikely to be intercalated into T -Ba2 PdO2 F2 . The value of ρ of T -Ba2 PdO2 F2 still remains more than 106 cm at room temperature through the Li-intercalation at a constant current. From the ICP-AES analysis, it has also been found that Li+ ions are hardly intercalated into T -Ba2 PdO2 F2 . In conclusion, the electron-doping into T -Nd2 PdO4 and T Ba2 PdO2 F2 has been tried on the analogy of the electron-doped high-Tc superconductor T -Nd2−x Cex CuO4 . The electrical resistivity of T -Nd2 PdO4 has been found to decrease through the annealing in vacuum at 600–700 ◦ C, but its behavior still
Fig. 6. Cyclic voltammogram of T -Ba2 PdO2 F2 at a sweep rate of 0.1 mV/s. For reference, the cyclic voltammogram of T-Sr2 CoO2 Br2 is also shown [5].
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remains semiconductive. It has also been found that Li+ ions are hardly intercalated into T -Ba2 PdO2 F2 . The annealing for the removal of oxygen or fluorine not from the PdO2 plane but from the Nd2 O2 or Ba2 F2 blocking layer for T -Nd2 PdO4 and T -Ba2 PdO2 F2 may be required. References [1] S. Shibasaki, I. Terasaki, J. Phys. Soc. Jpn. 75 (2006) 024705-1–4.
[2] K. Kobayashi, Y. Tomita, Y. Takagi, Trans. Mater. Res. Soc. Jpn. 26 (2001) 31–34. [3] T. Baikie, E.L. Dixon, J.F. Rooms, N.A. Young, M.G. Francesconi, Chem. Commun. (2003) 1580–1581. [4] H. Noda, A. Tsukada, H. Yamamoto, M. Naito, Physica C 426–431 (2005) 220–224. [5] T. Kajita, M. Kato, T. Suzuki, T. Itoh, T. Noji, Y. Koike, Physica C 426–431 (2005) 500–504. [6] J.O. Besenhard, M. Winter, J. Yang, W. Biberacher, J. Power Sources 54 (1995) 228–231.
Electrical properties of layered perovskite-type palladium oxides S. Ayukawa a,b,∗ , M. Kato a,b , T. Noji a,b , Y. Koike a,b a
Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan b JST-CREST, Japan
Received 27 May 2007; received in revised form 4 July 2007; accepted 3 September 2007
Abstract Electron-doping into Nd2 PdO4 and Ba2 PdO2 F2 with the same structure as in the electron-doped high-Tc superconductor T -Nd2−x Cex CuO4 has been tried by introducing oxygen vacancies or intercalating Li+ ions in order to bring about superconductivity. Polycrystalline samples of T Nd2 PdO4 were synthesized by the solid-state reaction method using a NaCl-flux technique. In order to introduce oxygen vacancies, the samples were annealed at 600–700 ◦ C in flowing gas of argon or in vacuum. It has been found that the value of the electrical resistivity of T -Nd2 PdO4−δ decreases down to ∼10 cm through the annealing in vacuum, however, but it still remains semiconductive. Polycrystalline samples of T -Ba2 PdO2 F2 were prepared by the low-temperature fluorination of Ba2 PdO3 using ZnF2 as a fluorinating agent. The intercalation of Li into T -Ba2 PdO2 F2 was tried using an electrochemical technique. However, it has been found from the ICP-AES analysis that Li+ ions are not intercalated into T -Ba2 PdO2 F2 . © 2007 Elsevier B.V. All rights reserved. Keywords: Superconductor; Palladium oxide; Lithium-intercalation
1. Introduction In the high-Tc cuprates, CuO2 planes are believed to play an important role in superconductivity. Carrier-doping into the Cu 3dx2 −y2 –O 2p orbitals in the CuO2 planes is essential for the appearance of the high-Tc superconductivity. It takes great interest to search for new superconductors by carrier-doping into the dx2 −y2 –O 2p orbitals in non-cuprates. Both Nd2 PdO4 and Ba2 PdO2 F2 are isostructural to the well-known electron-doped high-Tc superconductor Nd2−x Cex CuO4 with the so-called T type structure, as shown in Fig. 1. In both T -Nd2 PdO4 and T -Ba2 PdO2 F2 , the valency of Pd is 2+. The electronic configuration of Pd2+ is 4d8 . According to magnetic susceptibility measurements of T -La2 PdO4 , Pd2+ is in the low-spin state and therefore the Pd 4dx2 −y2 band is unoccupied [1]. Accordingly, doped electrons are expected to be introduced into the PdO2 planes consisting of Pd 4dx2 −y2 –O 2p orbitals in T -Nd2 PdO4 and T -Ba2 PdO2 F2 , as in the case of T -Nd2−x Cex CuO4 . In fact, electron-doping into T -Nd2 PdO4 has already been tried
∗ Corresponding author at: Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. Tel.: +81 22 795 7977; fax: +81 22 795 7977. E-mail address: [email protected] (S. Ayukawa).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.09.055
through the substitution of Ce4+ for Nd3+ by Kobayashi et al. [2]. However, electron carriers have never been doped enough for the appearance of superconductivity. Here, we have tried electron-doping into T -Nd2 PdO4 and T -Ba2 PdO2 F2 by introducing oxygen vacancies or intercalating Li+ ions in order to bring about superconductivity. 2. Experimental Polycrystalline samples of T -Nd2 PdO4 were synthesized by the solid-state reaction method using a NaCl-flux technique [1]. Stoichiometric amounts of Nd2 O3 and PdO were mixed. NaCl was then added in the molar ratio of PdO:NaCl = 1:16. The mixture was mixed again, and heated at 800 ◦ C for 24 h in flowing gas of oxygen. The product was finely ground and washed with distilled water to remove NaCl. The obtained powder was dried at 110 ◦ C, pelletized and sintered at 800 ◦ C for 12 h in flowing gas of oxygen. In order to reduce the oxygen content, the pelletized samples were annealed at 600–700 ◦ C in vacuum for 24–120 h or at 600 ◦ C in flowing gas of argon for 48–120 h. For comparison, annealing in flowing gas of oxygen was also carried out at 300 ◦ C in order to incorporate excess oxygen. Polycrystalline samples of T -Ba2 PdO2 F2 were prepared by the low-temperature fluorination of Ba2 PdO3 . ZnF2 was used as
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S. Ayukawa et al. / Materials Science and Engineering B 148 (2008) 65–68
Fig. 1. Crystal structures of (a) T -Nd2 PdO4 and (b) T -Ba2 PdO2 F2 .
a fluorinating agent, instead of CuF2 or poly(vinylidene difluoride) used by Baikie et al. [3], to avoid the formation of any superconducting cuprates as impurity phases. The parent compound of Ba2 PdO3 was prepared by the solid-state reaction method. Mixtures of stoichiometric amounts of BaO and PdO were ground in an argon-filled glove box and heated at 1100 ◦ C for 48 h in flowing gas of O2 20%–Ar 80% to avoid the generation of BaCO3 . The obtained powder of Ba2 PdO3 was mixed with the fluorinating agent of ZnF2 , and heated at 250 ◦ C for 14 h in flowing gas of O2 20%–Ar 80%. The obtained powder was pelletized and sintered at 400 ◦ C for 14 h in flowing gas of O2 20%–Ar 80%. The electron-doping was tried by the intercalation
of Li into T -Ba2 PdO2 F2 using an electrochemical technique. The electrochemical Li-intercalation was carried out at room temperature in an argon-filled glove box. A three-electrode cell was set up as T -Ba2 PdO2 F2 |1.0 M LiClO4 /PC|Li, as shown in Fig. 2. The working electrode (WE) was a pellet of T Ba2 PdO2 F2 sandwiched by Ni meshes. As an electrolyte, 1.0 M LiClO4 dissolved in propylene carbonate (PC) was used. Li sheets were used as the counter electrode (CE) and the reference electrode (RE). The Li-intercalation was performed at a constant current of 0.05 mA using a galvanostat. The total amount of Li intercalated into T -Ba2 PdO2 F2 was estimated according to the simple Faraday law and also from the inductively coupled plasma atomic emission spectrometry (ICP-AES). All products were characterized by the powder X-ray diffraction using Cu K␣ radiation. Since T -Ba2 PdO2 F2 was unstable in air, they were mixed with grease in order to avoid the exposure to the moisture in the atmosphere during the measurements. The electrical resistivity was measured by the four-probe method to detect the superconductivity. 3. Results and discussion
Fig. 2. Schematic diagram of the three-electrode cell for the electrochemical Li-intercalation.
Fig. 3 shows the powder X-ray diffraction patterns of T -Nd2 PdO4−δ obtained on various annealing conditions, indicating that the samples are not decomposed through the annealing. The lattice parameters are invariable through the reductive annealing. This may be due to the low concentration of electrons doped by oxygen vacancies. Moreover, a-axis length may be insensitive to the electron-carrier concentration. This behavior is commonly observed in electron-doped superconducting cuprates, in which a-axis length is mainly dependent on the
S. Ayukawa et al. / Materials Science and Engineering B 148 (2008) 65–68
Fig. 3. Powder X-ray diffraction patterns of T -Nd2 PdO4−δ obtained on various anneal conditions.
sites of blocking layer [4]. On the other hand, large expansion of lattice parameters through the oxygen annealing is due to the incorporation of excess oxygen into the Nd2 O2 blocking layer. Fig. 4 shows the temperature dependence of the electrical resistivity, ρ, of T -Nd2 PdO4−δ obtained on various annealing conditions. The value of ρ slightly decreases through the annealing in flowing gas of argon and markedly decreases from 5 × 103 cm close to 101 cm through the annealing in vacuum. The decrease of ρ seems to be due to an increase of oxygen vacancies operating as electron-carrier dopants. However, the value of ρ tends to increase with increasing annealing-time. The excessive increase of oxygen vacancies may have unfavorable influence on the electronic conduction in the PdO2 plane. That is, it may lead to not only the increase in the electron-carrier concentration but also the decrease in the electron mobility due to the trap of electrons at oxygen vacancies. Therefore, the optimization of the annealing condition for the removal of oxygen not from the PdO2 plane but from the Nd2 O2 layer is required. On the other hand, the increase of ρ through the annealing in flowing gas of oxygen suggests that hole carriers doped by excess oxygen are unlikely to itinerate in T -Nd2 PdO4 as in the case of T -Nd2 CuO4 .
Fig. 4. Temperature dependence of the electrical resistivity ρ of T -Nd2 PdO4-δ obtained on by various anneal conditions: (a) as-prepared, (b) in argon at 600 ◦ C for 24 h, (c) in argon at 600 ◦ C for 48 h, (d) in vacuum at 600 ◦ C for 24 h, (e) in vacuum at 600 ◦ C for 48 h, (f) in vacuum at 600 ◦ C for 120 h, (g) in vacuum at 700 ◦ C for 24 h and (h) in oxygen at 300 ◦ C for 72 h.
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Fig. 5. Powder X-ray diffraction pattern of T -Ba2 PdO2 F2 .
Fig. 5 shows the powder X-ray diffraction pattern of T Ba2 PdO2 F2 . All of the peaks, except for those of ZnO generated as a result of the fluorination using ZnF2 , can be indexed on the ˚ and c = 14.046 A, ˚ basis of the tetragonal lattice with a = 4.140 A which are in good agreement with the earlier work [3]. The electron-doping into T -Ba2 PdO2 F2 has been tried by the Liintercalation, because Li+ ions are expected to be intercalated into the F layer. Fig. 6 shows the cyclic voltammogram of T -Ba2 PdO2 F2 in 1.0 M LiClO4 /PC. For reference, the cyclic voltammogram of T-Sr2 CuO2 Br2 with the K2 NiF4 structure, which shows superconductivity with Tc = 8 K through the Liintercalation, is also shown [5]. The reductive current below 1000 mV for T -Ba2 PdO2 F2 is much smaller than that for TSr2 CuO2 Br2 and may be due to the adsorption of the solvated Li+ ions on the surface of the sample, as can be seen in the Liintercalation into graphite [6]. This indicates that Li+ ions are unlikely to be intercalated into T -Ba2 PdO2 F2 . The value of ρ of T -Ba2 PdO2 F2 still remains more than 106 cm at room temperature through the Li-intercalation at a constant current. From the ICP-AES analysis, it has also been found that Li+ ions are hardly intercalated into T -Ba2 PdO2 F2 . In conclusion, the electron-doping into T -Nd2 PdO4 and T Ba2 PdO2 F2 has been tried on the analogy of the electron-doped high-Tc superconductor T -Nd2−x Cex CuO4 . The electrical resistivity of T -Nd2 PdO4 has been found to decrease through the annealing in vacuum at 600–700 ◦ C, but its behavior still
Fig. 6. Cyclic voltammogram of T -Ba2 PdO2 F2 at a sweep rate of 0.1 mV/s. For reference, the cyclic voltammogram of T-Sr2 CoO2 Br2 is also shown [5].
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remains semiconductive. It has also been found that Li+ ions are hardly intercalated into T -Ba2 PdO2 F2 . The annealing for the removal of oxygen or fluorine not from the PdO2 plane but from the Nd2 O2 or Ba2 F2 blocking layer for T -Nd2 PdO4 and T -Ba2 PdO2 F2 may be required. References [1] S. Shibasaki, I. Terasaki, J. Phys. Soc. Jpn. 75 (2006) 024705-1–4.
[2] K. Kobayashi, Y. Tomita, Y. Takagi, Trans. Mater. Res. Soc. Jpn. 26 (2001) 31–34. [3] T. Baikie, E.L. Dixon, J.F. Rooms, N.A. Young, M.G. Francesconi, Chem. Commun. (2003) 1580–1581. [4] H. Noda, A. Tsukada, H. Yamamoto, M. Naito, Physica C 426–431 (2005) 220–224. [5] T. Kajita, M. Kato, T. Suzuki, T. Itoh, T. Noji, Y. Koike, Physica C 426–431 (2005) 500–504. [6] J.O. Besenhard, M. Winter, J. Yang, W. Biberacher, J. Power Sources 54 (1995) 228–231.