- Email: [email protected]
High-pressure synthesis of the filled skutterudite PrFe4Sb12
ARTICLE IN PRESS
Physica B 378–380 (2006) 213–214 www.elsevier.com/locate/physb
High-pressure synthesis of the filled skutterudite PrFe4Sb12 Kenya Tanaka, Yusuke Kawahito, Daisuke Kikuchi, Hidekazu Aoki, Yuji Aoki, Hideyuki Sato Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
Abstract We have succeeded in synthesizing the filled skutterudite PrFe4Sb12 crystals at high pressures and temperatures. To investigate bulk properties, we measured the magnetic susceptibility down to 2 K and electrical resistivity down to 0.7 K. The magnetic susceptibility exhibits a broad maximum around 10 K and follows the Curie Weiss law above 150 K with the effective magnetic moment of 4:5mB =f:u: The electrical resistivity increases with increasing temperature up to room temperature exhibiting tendencies to saturation at around 10 and 100 K. The relatively large residual resistivity ratio of 24 suggests high quality of the grown sample. r 2006 Elsevier B.V. All rights reserved. PACS: 71.10.Hf; 71.27.+a; 75.30.Mb Keywords: Filled skutterudite; PrFe4Sb12; High-pressure synthesis
Recently, the filled skutterudite compounds, especially Pr-based ones, attracted much attention, since they exhibit a variety of attractive features such as heavy fermion superconductivity in PrOs4Sb12 [1], the field induced heavy fermion state above antiferroquadrupolar ordering in PrFe4P12 [2], associated with its unique crystal structure. In the latter with smaller lattice constant, the strong c–f hybridization is believed to play an important role in the attractive feature of this material. In the former with larger lattice constant, in contrast, the closely located first excited state of crystalline electric field (CEF) split levels is thought to play an essential role in the exotic superconducting state. From such a point of view, PrFe4Sb12 with relatively large lattice constant is a candidate to exhibit some interesting behavior related with the small CEF splitting. However, on the physical properties of PrFe4Sb12, there have been only two reports by two independent groups, on the magnetic ground state below 5 K; ferromagnetic [3] and antiferromagnetic [4]. Such inconsistency may be partly ascribed to the sample quality. In fact, the former reported the sample contained about 10% impurity of FeSb2, and the latter reported the sample contained about 3% impurity of Sb Corresponding author. Tel.: +81 426 77 2487; fax: +81 426 77 2483.
E-mail address: [email protected] (K. Tanaka). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.404
and the filling factor of Pr ions was estimated to be only 73%. Even so, the reported electronic specific heat coefficient of g was 1000 mJ/mol K2 in Ref. [4] suggesting exotic features of this material. In order to clarify the basic feature of this material, it is necessary to prepare higher quality samples. The aim of the present work is to synthesize high quality samples of PrFe4Sb12 with higher Pr-filling factor by high pressure and temperature synthesis and to reveal physical properties. Starting materials of this compound were chips of Pr (99.9 wt%), powder of iron (99.99 wt%) and antimony (99.9999 wt%). The constituent elements in stoichiometric ratio were placed in BN crucible and compressed to 4 GPa in the cubic-anvil 700 ton high-pressure apparatus at room temperature. The sample was heated up to 950 C and kept for 30 min. The chips of Pr are expected to melt at this temperature. Soon after, the sample was quenched to 700 C and cooled to 650 C for 6 h. At this cooling stage, the filled skutterudite is expected to be synthesized. Fig. 1 shows the powder X-ray diffraction pattern of PrFe4Sb12. We confirmed that the filled skutterudite was synthesized and impurities included in the samples, primarily Sb and secondary FeSb2, were estimated to be less than 5%. The secondary impurity of FeSb2 is a semiconductor. It exhibits relatively normal-semiconductor-like temperature
ARTICLE IN PRESS K. Tanaka et al. / Physica B 378–380 (2006) 213–214
214
0.16 (a)
0.12
10000
(emu/mol)
intensity (cps)
PrFe4Sb12 FeSb2
120
PrFe4Sb12
80 0.08
Sb
40
H = 0.5 T
0.04
1/ (mol/emu)
15000
5000 0 600
0
0
20
40
60 2θ (deg.)
80
Fig. 1. Powder X-ray diffraction pattern of PrFe4Sb12.The arrows indicate the impurity peaks of Sb and FeSb2.
(µΩcm)
(b)
PrFe4Sb12
400 80 40
200
0
dependence of the electrical resistivity and small magnetic susceptibility. The lattice constant estimated to be 9.137(2) A˚ agrees with that reported by Bauer et al. [5] within experimental accuracy. To investigate basic features of this sample, we have measured the magnetic susceptibility down to 2 K and electrical resistivity down to 0.7 K. Fig. 2(a) shows temperature dependence of the magnetic susceptibility wðTÞ and inverse susceptibility w1 ðTÞ in this sample. The magnetic susceptibility exhibits a broad maximum at 10 K, which is different from past reports [3–5] and does not seem to be a simple magnetic order. From the Curie–Weiss-like behavior at high temperatures ðTX150 KÞ, the effective magnetic moment is estimated to be 4:5mB =formula unit (f.u.), which is slightly larger than that reported by Bauer et al., 4:19mB =f:u. [4]. The effective magnetic moment larger than the free ion value of Pr3þ indicates the contribution from 3d electrons in iron [3–5]. Simply assume that the magnetic contribution from 3d electrons in our sample and Bauer’s one is basically the same, as the present result indicates the improved filling factor of Pr element in our sample. Preliminary field emission electron microscope measurements confirmed that Pr ion sites are fully occupied [8]. Fig. 2(b) shows the temperature dependence of the electrical resistivity rðTÞ in this sample. rðTÞ increases with increasing temperature, exhibiting tendencies to saturation around 10 and 100 K. The residual resistivity ratio is estimated to be 24, which indicates that the sample quality is highly improved in comparison with samples reported previously [5]. The shoulder around 10 K correlated with the broad maximum in wðTÞ reflects the increasing magnetic conduction-electron scattering asso-
0
0
100
0
10 200
20 300
T (K) Fig. 2. (a) Temperature dependence of magnetic susceptibility wðTÞ and w1 ðTÞ of PrFe4Sb12. (b) Temperature dependence of electrical resistivity rðTÞ of PrFe4Sb12. The inset shows the low temperature behavior in more detail.
ciated with the CEF excitation. Similar temperature dependence of electrical resistivity has been reported in many Pr-based compounds such as PrCu6, PrRu4Sb12 [6,7]. These results indicate that the broad maximum of wðTÞ and the shoulder of rðTÞ around 10 K reflect the existence of the first CEF excited state located 10 K above the ground state. In order to confirm such a scenario, it is necessary to investigate other physical properties such as specific heat measurements, which are now in progress. 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] [2] [3] [4] [5] [6] [7] [8]
Y. Aoki, et al., J. Phys. Soc. Japan 71 (2002) 2098. H. Sugawara, et al., Phys. Rev. B 66 (2002) 134411. M.E. Danebrock, et al., J. Phys. Chem. Solids 57 (1996) 381. E. Bauer, et al., Physica B 312–313 (2002) 840. E. Bauer, et al., Phys. Rev. B 66 (2002) 214421. S. Takayanagi, et al., J. Phys. Soc. Japan 53 (1984) 676. K. Abe, et al., J. Phys.: Condens. Matter 14 (2002) 11757. K. Tanaka, et al., unpublished.
Physica B 378–380 (2006) 213–214 www.elsevier.com/locate/physb
High-pressure synthesis of the filled skutterudite PrFe4Sb12 Kenya Tanaka, Yusuke Kawahito, Daisuke Kikuchi, Hidekazu Aoki, Yuji Aoki, Hideyuki Sato Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
Abstract We have succeeded in synthesizing the filled skutterudite PrFe4Sb12 crystals at high pressures and temperatures. To investigate bulk properties, we measured the magnetic susceptibility down to 2 K and electrical resistivity down to 0.7 K. The magnetic susceptibility exhibits a broad maximum around 10 K and follows the Curie Weiss law above 150 K with the effective magnetic moment of 4:5mB =f:u: The electrical resistivity increases with increasing temperature up to room temperature exhibiting tendencies to saturation at around 10 and 100 K. The relatively large residual resistivity ratio of 24 suggests high quality of the grown sample. r 2006 Elsevier B.V. All rights reserved. PACS: 71.10.Hf; 71.27.+a; 75.30.Mb Keywords: Filled skutterudite; PrFe4Sb12; High-pressure synthesis
Recently, the filled skutterudite compounds, especially Pr-based ones, attracted much attention, since they exhibit a variety of attractive features such as heavy fermion superconductivity in PrOs4Sb12 [1], the field induced heavy fermion state above antiferroquadrupolar ordering in PrFe4P12 [2], associated with its unique crystal structure. In the latter with smaller lattice constant, the strong c–f hybridization is believed to play an important role in the attractive feature of this material. In the former with larger lattice constant, in contrast, the closely located first excited state of crystalline electric field (CEF) split levels is thought to play an essential role in the exotic superconducting state. From such a point of view, PrFe4Sb12 with relatively large lattice constant is a candidate to exhibit some interesting behavior related with the small CEF splitting. However, on the physical properties of PrFe4Sb12, there have been only two reports by two independent groups, on the magnetic ground state below 5 K; ferromagnetic [3] and antiferromagnetic [4]. Such inconsistency may be partly ascribed to the sample quality. In fact, the former reported the sample contained about 10% impurity of FeSb2, and the latter reported the sample contained about 3% impurity of Sb Corresponding author. Tel.: +81 426 77 2487; fax: +81 426 77 2483.
E-mail address: [email protected] (K. Tanaka). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.404
and the filling factor of Pr ions was estimated to be only 73%. Even so, the reported electronic specific heat coefficient of g was 1000 mJ/mol K2 in Ref. [4] suggesting exotic features of this material. In order to clarify the basic feature of this material, it is necessary to prepare higher quality samples. The aim of the present work is to synthesize high quality samples of PrFe4Sb12 with higher Pr-filling factor by high pressure and temperature synthesis and to reveal physical properties. Starting materials of this compound were chips of Pr (99.9 wt%), powder of iron (99.99 wt%) and antimony (99.9999 wt%). The constituent elements in stoichiometric ratio were placed in BN crucible and compressed to 4 GPa in the cubic-anvil 700 ton high-pressure apparatus at room temperature. The sample was heated up to 950 C and kept for 30 min. The chips of Pr are expected to melt at this temperature. Soon after, the sample was quenched to 700 C and cooled to 650 C for 6 h. At this cooling stage, the filled skutterudite is expected to be synthesized. Fig. 1 shows the powder X-ray diffraction pattern of PrFe4Sb12. We confirmed that the filled skutterudite was synthesized and impurities included in the samples, primarily Sb and secondary FeSb2, were estimated to be less than 5%. The secondary impurity of FeSb2 is a semiconductor. It exhibits relatively normal-semiconductor-like temperature
ARTICLE IN PRESS K. Tanaka et al. / Physica B 378–380 (2006) 213–214
214
0.16 (a)
0.12
10000
(emu/mol)
intensity (cps)
PrFe4Sb12 FeSb2
120
PrFe4Sb12
80 0.08
Sb
40
H = 0.5 T
0.04
1/ (mol/emu)
15000
5000 0 600
0
0
20
40
60 2θ (deg.)
80
Fig. 1. Powder X-ray diffraction pattern of PrFe4Sb12.The arrows indicate the impurity peaks of Sb and FeSb2.
(µΩcm)
(b)
PrFe4Sb12
400 80 40
200
0
dependence of the electrical resistivity and small magnetic susceptibility. The lattice constant estimated to be 9.137(2) A˚ agrees with that reported by Bauer et al. [5] within experimental accuracy. To investigate basic features of this sample, we have measured the magnetic susceptibility down to 2 K and electrical resistivity down to 0.7 K. Fig. 2(a) shows temperature dependence of the magnetic susceptibility wðTÞ and inverse susceptibility w1 ðTÞ in this sample. The magnetic susceptibility exhibits a broad maximum at 10 K, which is different from past reports [3–5] and does not seem to be a simple magnetic order. From the Curie–Weiss-like behavior at high temperatures ðTX150 KÞ, the effective magnetic moment is estimated to be 4:5mB =formula unit (f.u.), which is slightly larger than that reported by Bauer et al., 4:19mB =f:u. [4]. The effective magnetic moment larger than the free ion value of Pr3þ indicates the contribution from 3d electrons in iron [3–5]. Simply assume that the magnetic contribution from 3d electrons in our sample and Bauer’s one is basically the same, as the present result indicates the improved filling factor of Pr element in our sample. Preliminary field emission electron microscope measurements confirmed that Pr ion sites are fully occupied [8]. Fig. 2(b) shows the temperature dependence of the electrical resistivity rðTÞ in this sample. rðTÞ increases with increasing temperature, exhibiting tendencies to saturation around 10 and 100 K. The residual resistivity ratio is estimated to be 24, which indicates that the sample quality is highly improved in comparison with samples reported previously [5]. The shoulder around 10 K correlated with the broad maximum in wðTÞ reflects the increasing magnetic conduction-electron scattering asso-
0
0
100
0
10 200
20 300
T (K) Fig. 2. (a) Temperature dependence of magnetic susceptibility wðTÞ and w1 ðTÞ of PrFe4Sb12. (b) Temperature dependence of electrical resistivity rðTÞ of PrFe4Sb12. The inset shows the low temperature behavior in more detail.
ciated with the CEF excitation. Similar temperature dependence of electrical resistivity has been reported in many Pr-based compounds such as PrCu6, PrRu4Sb12 [6,7]. These results indicate that the broad maximum of wðTÞ and the shoulder of rðTÞ around 10 K reflect the existence of the first CEF excited state located 10 K above the ground state. In order to confirm such a scenario, it is necessary to investigate other physical properties such as specific heat measurements, which are now in progress. 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] [2] [3] [4] [5] [6] [7] [8]
Y. Aoki, et al., J. Phys. Soc. Japan 71 (2002) 2098. H. Sugawara, et al., Phys. Rev. B 66 (2002) 134411. M.E. Danebrock, et al., J. Phys. Chem. Solids 57 (1996) 381. E. Bauer, et al., Physica B 312–313 (2002) 840. E. Bauer, et al., Phys. Rev. B 66 (2002) 214421. S. Takayanagi, et al., J. Phys. Soc. Japan 53 (1984) 676. K. Abe, et al., J. Phys.: Condens. Matter 14 (2002) 11757. K. Tanaka, et al., unpublished.