- Email: [email protected]
Effect of rare earth filling on unfilled skutterudite compound CoSb3
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 1715–1717 www.elsevier.com/locate/jmmm
Effect of rare earth filling on unfilled skutterudite compound CoSb3 Kenya Tanakaa,, Yuko Sekiharaa, Yusuke Kawahitoa, Daisuke Kikuchia, Hidekazu Aokia, Keitaro Kuwaharaa, Yuji Aokia, Hitoshi Sugawarab, Hideyuki Satoa b
a Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Faculty of Integrated Arts and Science, The University of Tokushima, Tokushima, 770-8502, Japan
Available online 9 November 2006
Abstract We have succeeded in synthesizing the filled skutterudite compound Prx Co4 Sb12 under high pressures. Pr site filling factor was estimated to be about 5010% by powder X-ray diffraction and chemical composition analysis using field emission electron microscope. Apparent expansion of lattice constant 9:09 A˚ compared to that of CoSb3 9:03 A˚ is also indirect evidence of successful filling of Pr-ions. In the magnetization and specific heat measurements, no anomaly suggesting phase transition has been found down to 2 K. r 2006 Published by Elsevier B.V. PACS: 75.40.Cx; 71.27.þa Keywords: Skutterudite; High pressure; CoSb3
Effect of rare earth elements (RE) filling on unfilled skutterudite is an interesting subject from two view points; i.e., application as thermoelectric materials and basic scientific interest. There are a few reports to attack this subject. To increase the filling factor, some trials on partial replacement of Co site by Fe, Cex ðFe1y Coy Þ4 Sb12 , have been reported, but, 100% filling have been unsuccessful [1]. Since CoSb3 is stable, it is difficult to increase the filling factor of RE in the antimony cage at ambient pressures. In contrast with the REFe4 P12 and RERu4 Sb12 , no successful reports on 100% filling has been made even in REFe4 Sb12 at ambient pressure [2,3]. Recently, we have succeeded in synthesizing high quality sample with almost 100% filling factor of PrFe4 Sb12 by high pressure synthesis, and found the physical properties are quite sensitive to the Pr site filling [4]. The magnetic ground state changes from a ferrimagnet to a highly enhanced paramagnet, which is consistent with the large density of states near the Fermi energy associated with Fe-3d electrons in the band calculation on LaFe4 Sb12 [5]. This fact suggests an important role of 3d electrons in controlling the physical Corresponding author. Tel.: +81 426 77 2487 fax: +81 426 77 2483.
E-mail addresses: [email protected], tanaka-kenya@c. metro-u.ac.jp (K. Tanaka). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.10.544
properties of these systems. To study the effect of 3d electrons on physical properties, we are trying to synthesize PrCo4 Sb12 under high pressures. Starting materials of this compound were ingot of Pr (99.9 wt%) and powder of cobalt (99.99 wt%) and antimony (99.9999 wt%). As a preparatory step, PrSb2 was first synthesized as follows. Pr and Sb were vacuum encapsulated in quartz tube in the atomic ratio 1:12, and heat treated at 800 C for several hours. PrSb2 were separated out from the leftover Sb and reduced to powder. Finally, powder of PrSb2 , Co and Sb were placed in BN crucible in the atomic ratio 1:4:10 and compressed to 4 GPa in the cubic-anvil high-pressure apparatus at room temperature. We have searched for the condition of synthesizing pure single phase sample only by changing the heat treatment process. It was the best condition that the sample was heated up to 750 C and kept for 4 h. If the temperature is lower than 750 C, the ratio of impurity phase increases. If the temperature is elevated higher than 750 C, CoSb2 phase was almost separated out and the skutterudite phase was hardly synthesized. In this best synthesis condition, powder X-ray diffraction reveals the sample is nearly single phase, although there are some amount of Sb, PrSb2 and unknown impurity phases. To evaluate the basic properties of PrCo4 Sb12 , we have tried to
ARTICLE IN PRESS K. Tanaka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1715–1717
1716
further reduce the impurity phase, since they, especially magnetic properties of PrSb2 [6], could lead to fatal error in the experimental results. We have etched the samples in dilute aqua regia ðHCl : HNO3 : H2 O ¼ 3 : 1 : 4Þ for 5 h, and obtained powdered sample. From the powder X-ray diffraction of this powder samples, no impurity phase has been identified and the lattice constant is estimated to be 9:09 A˚ which is larger ˚ It suggests than that of unfilled skutterudite CoSb3 9:03 A. Pr-ions filled into antimony cage increase the lattice constant. In the chemical composition analysis using field emission electron microscope, Pr site filling factor was estimated to be about 50 10%, which is much larger than the previous report of Cex Co4 Sb12 at max 10% synthesized under ambient pressures [1]. To investigate basic features of this sample, we have performed the magnetization and specific heat measurements. In the analysis of physical properties, we assumed 50% as Pr filling factor. Magnetic measurement was performed by Quantum Design SQUID magnetometers in the temperature region of 2 KpTp300 K and applied field up to 7 T. Fig. 1 (a) shows magnetization curve MðHÞ of Prx Co4 Sb12 at 2 and 300 K. MðHÞ exhibits no evidence of any magnetic ordering. Fig. 1(b) shows temperature-
a 0.4 Pr0.5Co4Sb12
M (μB / Pr)
0.3
2K 300 K
0.2 0.1 0
0
2
4
dependent magnetic susceptibility wðTÞ and 1/wðTÞ of Prx Co4 Sb12 . wðTÞ exhibits Curie–Weiss (CW) like behavior in the temperature range of 2–200 K and increases down to 2 K. The deviation of CW law above 200 K may be explained by 3d electron contribution to wðTÞ, which is similar to the magnetic behavior of PrFe4 Sb12 [9]. Specific heat measurement was performed by Quantum Design Physical Properties Measurement System between 2 K and 100 K in zero field. Fig. 2 shows the temperature-dependent specific heat CðTÞ of Prx Co4 Sb12 in the temperature range of 2–70 K. Specific heat exhibits no anomaly of Schottky and magnetic ordering. The inset shows CðTÞ=T vs T 2 plots. From the linear fitting of CðTÞ=T vs T 2 plots, the electric specific heat coefficient g was estimated to be 50 mJ=mol K2 . Next, we will discuss a crystalline electric field (CEF) of Prx Co4 Sb12 . Both the absence of magnetic ordering down to 2 K and the small magnetization at 2 K suggest the nonmagnetic ground state G1 or G23 . In most of the Pr based filled skutterudites, CEF states in T h symmetry are reported to be G1 singlet ground state with the first excited state Gð2Þ 4 triplet [7–9]. Combining this fact, G1 is more probable as a ground state of this compound. The absence of any clear Schottky anomaly in the investigating temperature range indicates the first excited state is situated at high temperature X100 K. In Pr based filled skutterudite with relatively large lattice constant, PrRu4 Sb12 is reported to have similar CEF level scheme. In fact, both the magnetization at 2 K and the specific heat coefficient close to the present results in Prx Co4 Sb12 . It is difficult to discuss the detailed 4f electron state only by present experiment results. In order to develop a more quantitative discussion, it is necessary to synthesize higher quality sample improved filling factor of PrCo4 Sb12 and reference compound LaCo4 Sb12 .
1200
6
0.8 C / T (J / molK2)
H (T)
b 0.1
60 Pr0.5Co4Sb12 H = 0.1 T
0.06
40
0.04
20
0.02 0
0
100
200
0 300
T (K) Fig. 1. (a) Magnetization curve of Pr0:5 Co4 Sb12 at temperature 2 K and 300 K. (b) Temperature-dependent magnetic susceptibility wðTÞ and 1=wðTÞ of Pr0:5 Co4 Sb12 .
C (J/ molK)
0.08
800
1/χ (Pr-mol / emu)
χ (emu / Pr-mol)
b
0.6 0.4 0.2 0 0
5
10 2
15
20
2
T (K )
400
Pr0.5Co4Sb12
0 0
20
40
60
T (K)
Fig. 2. Temperature-dependent specific heat CðTÞ of Pr0:5 Co4 Sb12 . The inset shows CðTÞ=T vs T 2 , solid lines shows the linear fitting line.
ARTICLE IN PRESS K. Tanaka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1715–1717
This work was supported by a Grant-in-Aid for Scientific Research Priority Area Skutterudite (No. 15072206) of the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] D.T. Morelli, et al., Phys. Rev. B 56 (1997) 7376.
[2] [3] [4] [5] [6] [7] [8] [9]
1717
E. Bauer, et al., Phys. Rev. B 66 (2002) 214421. N.P. Butch, et al., Phys. Rev. B 71 (2005) 214417. K. Tanaka, et al., Phys. B 378–380 (2006) 213. K. Takegahara, H. Harima, J. Phys. Soc. Japan 71 (Suppl.) (2002) 240. S.L. Bud’ko, et al., Phys. Rev. B 57 (1998) 13624. N. Takeda, et al., J. Phys. Soc. Japan 69 (2000) 868. Y. Aoki, et al., J. Phys. Soc. Japan 71 (2002) 2098. K. Tanaka, et al., to appear.
Journal of Magnetism and Magnetic Materials 310 (2007) 1715–1717 www.elsevier.com/locate/jmmm
Effect of rare earth filling on unfilled skutterudite compound CoSb3 Kenya Tanakaa,, Yuko Sekiharaa, Yusuke Kawahitoa, Daisuke Kikuchia, Hidekazu Aokia, Keitaro Kuwaharaa, Yuji Aokia, Hitoshi Sugawarab, Hideyuki Satoa b
a Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Faculty of Integrated Arts and Science, The University of Tokushima, Tokushima, 770-8502, Japan
Available online 9 November 2006
Abstract We have succeeded in synthesizing the filled skutterudite compound Prx Co4 Sb12 under high pressures. Pr site filling factor was estimated to be about 5010% by powder X-ray diffraction and chemical composition analysis using field emission electron microscope. Apparent expansion of lattice constant 9:09 A˚ compared to that of CoSb3 9:03 A˚ is also indirect evidence of successful filling of Pr-ions. In the magnetization and specific heat measurements, no anomaly suggesting phase transition has been found down to 2 K. r 2006 Published by Elsevier B.V. PACS: 75.40.Cx; 71.27.þa Keywords: Skutterudite; High pressure; CoSb3
Effect of rare earth elements (RE) filling on unfilled skutterudite is an interesting subject from two view points; i.e., application as thermoelectric materials and basic scientific interest. There are a few reports to attack this subject. To increase the filling factor, some trials on partial replacement of Co site by Fe, Cex ðFe1y Coy Þ4 Sb12 , have been reported, but, 100% filling have been unsuccessful [1]. Since CoSb3 is stable, it is difficult to increase the filling factor of RE in the antimony cage at ambient pressures. In contrast with the REFe4 P12 and RERu4 Sb12 , no successful reports on 100% filling has been made even in REFe4 Sb12 at ambient pressure [2,3]. Recently, we have succeeded in synthesizing high quality sample with almost 100% filling factor of PrFe4 Sb12 by high pressure synthesis, and found the physical properties are quite sensitive to the Pr site filling [4]. The magnetic ground state changes from a ferrimagnet to a highly enhanced paramagnet, which is consistent with the large density of states near the Fermi energy associated with Fe-3d electrons in the band calculation on LaFe4 Sb12 [5]. This fact suggests an important role of 3d electrons in controlling the physical Corresponding author. Tel.: +81 426 77 2487 fax: +81 426 77 2483.
E-mail addresses: [email protected], tanaka-kenya@c. metro-u.ac.jp (K. Tanaka). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.10.544
properties of these systems. To study the effect of 3d electrons on physical properties, we are trying to synthesize PrCo4 Sb12 under high pressures. Starting materials of this compound were ingot of Pr (99.9 wt%) and powder of cobalt (99.99 wt%) and antimony (99.9999 wt%). As a preparatory step, PrSb2 was first synthesized as follows. Pr and Sb were vacuum encapsulated in quartz tube in the atomic ratio 1:12, and heat treated at 800 C for several hours. PrSb2 were separated out from the leftover Sb and reduced to powder. Finally, powder of PrSb2 , Co and Sb were placed in BN crucible in the atomic ratio 1:4:10 and compressed to 4 GPa in the cubic-anvil high-pressure apparatus at room temperature. We have searched for the condition of synthesizing pure single phase sample only by changing the heat treatment process. It was the best condition that the sample was heated up to 750 C and kept for 4 h. If the temperature is lower than 750 C, the ratio of impurity phase increases. If the temperature is elevated higher than 750 C, CoSb2 phase was almost separated out and the skutterudite phase was hardly synthesized. In this best synthesis condition, powder X-ray diffraction reveals the sample is nearly single phase, although there are some amount of Sb, PrSb2 and unknown impurity phases. To evaluate the basic properties of PrCo4 Sb12 , we have tried to
ARTICLE IN PRESS K. Tanaka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1715–1717
1716
further reduce the impurity phase, since they, especially magnetic properties of PrSb2 [6], could lead to fatal error in the experimental results. We have etched the samples in dilute aqua regia ðHCl : HNO3 : H2 O ¼ 3 : 1 : 4Þ for 5 h, and obtained powdered sample. From the powder X-ray diffraction of this powder samples, no impurity phase has been identified and the lattice constant is estimated to be 9:09 A˚ which is larger ˚ It suggests than that of unfilled skutterudite CoSb3 9:03 A. Pr-ions filled into antimony cage increase the lattice constant. In the chemical composition analysis using field emission electron microscope, Pr site filling factor was estimated to be about 50 10%, which is much larger than the previous report of Cex Co4 Sb12 at max 10% synthesized under ambient pressures [1]. To investigate basic features of this sample, we have performed the magnetization and specific heat measurements. In the analysis of physical properties, we assumed 50% as Pr filling factor. Magnetic measurement was performed by Quantum Design SQUID magnetometers in the temperature region of 2 KpTp300 K and applied field up to 7 T. Fig. 1 (a) shows magnetization curve MðHÞ of Prx Co4 Sb12 at 2 and 300 K. MðHÞ exhibits no evidence of any magnetic ordering. Fig. 1(b) shows temperature-
a 0.4 Pr0.5Co4Sb12
M (μB / Pr)
0.3
2K 300 K
0.2 0.1 0
0
2
4
dependent magnetic susceptibility wðTÞ and 1/wðTÞ of Prx Co4 Sb12 . wðTÞ exhibits Curie–Weiss (CW) like behavior in the temperature range of 2–200 K and increases down to 2 K. The deviation of CW law above 200 K may be explained by 3d electron contribution to wðTÞ, which is similar to the magnetic behavior of PrFe4 Sb12 [9]. Specific heat measurement was performed by Quantum Design Physical Properties Measurement System between 2 K and 100 K in zero field. Fig. 2 shows the temperature-dependent specific heat CðTÞ of Prx Co4 Sb12 in the temperature range of 2–70 K. Specific heat exhibits no anomaly of Schottky and magnetic ordering. The inset shows CðTÞ=T vs T 2 plots. From the linear fitting of CðTÞ=T vs T 2 plots, the electric specific heat coefficient g was estimated to be 50 mJ=mol K2 . Next, we will discuss a crystalline electric field (CEF) of Prx Co4 Sb12 . Both the absence of magnetic ordering down to 2 K and the small magnetization at 2 K suggest the nonmagnetic ground state G1 or G23 . In most of the Pr based filled skutterudites, CEF states in T h symmetry are reported to be G1 singlet ground state with the first excited state Gð2Þ 4 triplet [7–9]. Combining this fact, G1 is more probable as a ground state of this compound. The absence of any clear Schottky anomaly in the investigating temperature range indicates the first excited state is situated at high temperature X100 K. In Pr based filled skutterudite with relatively large lattice constant, PrRu4 Sb12 is reported to have similar CEF level scheme. In fact, both the magnetization at 2 K and the specific heat coefficient close to the present results in Prx Co4 Sb12 . It is difficult to discuss the detailed 4f electron state only by present experiment results. In order to develop a more quantitative discussion, it is necessary to synthesize higher quality sample improved filling factor of PrCo4 Sb12 and reference compound LaCo4 Sb12 .
1200
6
0.8 C / T (J / molK2)
H (T)
b 0.1
60 Pr0.5Co4Sb12 H = 0.1 T
0.06
40
0.04
20
0.02 0
0
100
200
0 300
T (K) Fig. 1. (a) Magnetization curve of Pr0:5 Co4 Sb12 at temperature 2 K and 300 K. (b) Temperature-dependent magnetic susceptibility wðTÞ and 1=wðTÞ of Pr0:5 Co4 Sb12 .
C (J/ molK)
0.08
800
1/χ (Pr-mol / emu)
χ (emu / Pr-mol)
b
0.6 0.4 0.2 0 0
5
10 2
15
20
2
T (K )
400
Pr0.5Co4Sb12
0 0
20
40
60
T (K)
Fig. 2. Temperature-dependent specific heat CðTÞ of Pr0:5 Co4 Sb12 . The inset shows CðTÞ=T vs T 2 , solid lines shows the linear fitting line.
ARTICLE IN PRESS K. Tanaka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1715–1717
This work was supported by a Grant-in-Aid for Scientific Research Priority Area Skutterudite (No. 15072206) of the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] D.T. Morelli, et al., Phys. Rev. B 56 (1997) 7376.
[2] [3] [4] [5] [6] [7] [8] [9]
1717
E. Bauer, et al., Phys. Rev. B 66 (2002) 214421. N.P. Butch, et al., Phys. Rev. B 71 (2005) 214417. K. Tanaka, et al., Phys. B 378–380 (2006) 213. K. Takegahara, H. Harima, J. Phys. Soc. Japan 71 (Suppl.) (2002) 240. S.L. Bud’ko, et al., Phys. Rev. B 57 (1998) 13624. N. Takeda, et al., J. Phys. Soc. Japan 69 (2000) 868. Y. Aoki, et al., J. Phys. Soc. Japan 71 (2002) 2098. K. Tanaka, et al., to appear.