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Structural analysis of the filled skutterudite PrRu4P12 at high pressure and low temperature
ARTICLE IN PRESS
Physica B 378–380 (2006) 199–200 www.elsevier.com/locate/physb
Structural analysis of the filled skutterudite PrRu4P12 at high pressure and low temperature Atsushi Miyakea,, Yuki Nakamotoa, Tomoko Kagayamaa, Keiki Takedab, Yausuo Ohishic, Katsuya Shimizua, Kunihiro Kihoub, Chihiro Sekineb, Ichimin Shirotanib a
KYOKUGEN, Osaka University, Osaka 560-8531, Japan b SPring-8/JASRI, Hyogo 679-5198, Japan c Faculty of Engineering, Muroran Institute of Technology, Hokkaido 050-8585, Japan
Abstract We performed synchrotron X-ray powder diffraction experiments on the filled skutterudite PrRu4 P12 at high pressure and low temperature to investigate the role of structural changes in the metallization and superconducting transition above 11 GPa. We compared the structural parameters between the insulating and metallic phases. An anomalous enhancement of the P atomic displacement is observed at 17.0 GPa and 10 K away from the Ru8 cubic sublattice are observed. This change may play a key role to the metallization at higher pressure. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 74.62.Fj; 61.50.Ks Keywords: PrRu4 P12 ; Metal–insulator transition; Powder X-ray diffraction experiment; High pressure
The filled skutterudite PrRu4 P12 exhibits a metal–insulator (M–I) transition at T MI ¼ 62 K, accompanied by a structural transition [1–3]. The electrical resistivity shows insulating behavior below T MI . Electron and X-ray diffraction studies revealed a structural transition from Im3¯ to Pm3¯ due to a Fermi-surface nesting instability below T MI , indicating the formation of a CDW in the insulator phase [2]. Band calculations revealed that the displacement of the P atoms makes a gap at E F , inducing an insulating state [4,5]. We performed electrical resistance measurements on PrRu4 P12 under high pressure, and observed metallization and a superconducting transition above 11 GPa [6]. T MI is almost independent of pressure, in contrast to the drastic change of the temperature dependence of the resistance at low temperature. Above 11 GPa, the resistance shows metallic behavior, while the anomaly of the resistance at around 60 K was still observed [6]. These facts may indicate that the structural transition due Corresponding author. Tel.: +81 6 6850 6677; fax: +81 6 6850 6662.
E-mail address: [email protected] (A. Miyake). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.075
to Fermi surface nesting, observed at ambient pressure, still occurs even at high pressure. In order to investigate the metallization mechanism, we performed powder X-ray diffraction experiments under high pressure and at low temperature; we compared the structural detail between insulating (below 11 GPa) and metallic phases (above 11 GPa). An angular dispersive method was used with a flat imaging plate detector using the synchrotron radiation source at SPring-8 BL10XU. The wavelength of the incident beam was 0.4143(1) A˚. We employed a diamond anvil cell as a high-pressure apparatus. The single crystals of PrRu4 P12 were crushed into a fine powder. The powdered sample and ruby chips as the pressure marker were put into the hole of a metal gasket. We used compressed He as a pressure-transmitting medium. The structural parameters of PrRu4 P12 were refined by the Rietveld method using the RIETAN-2000 program [7]. No clear sign of a structural transition was observed up to 17 GPa, which is consistent with the previous report [8]. The expected superlattice reflections at ðh; k; lÞ ðh þ k þ l ¼
ARTICLE IN PRESS A. Miyake et al. / Physica B 378–380 (2006) 199–200
200
20000
Table 1 Crystal structure parameters at T ¼ 10 K in PrRu4 P12 (a) in the insulating phase (2.3 GPa) and (b) in the metallic phase (17.0 GPa), space group ¯ Z¼1 Pm3,
15000
Atom
10000
30000 PrRu4P12 at 17.0 GPa and 10 K
Intensity (a. u.)
25000
5000 0 4
6
8
10
12 14 16 18 20 22 24 2θ / °
26 28
Fig. 1. Diffraction pattern of PrRu4 P12 at 10 K and 17.0 GPa. The observed and calculated data are shown as the symbols and line, respectively. The difference plot is shown below, with short vertical markers denoting the calculated peak positions.
x
y
z
Beq ðA˚ 2 Þ
(a) 2.3 GPaa Pr(1) 1a Pr(2) 1b Ru 8i P(1) 12j P(2) 12k
0 1/2 0.2507(8) 0 1/2
0 1/2 0.2507(8) 0.3565(5) 0.8580(5)
0 1/2 0.2507(8) 0.1438(5) 0.6416(5)
0.65(7) 0.65(7) 0.45(7) 0.19(7) 0.19(7)
(b) 17.0 GPab Pr(1) 1a Pr(2) 1b Ru 8i P(1) 12j P(2) 12k
0 1/2 0.2502(8) 0 1/2
0 1/2 0.2502(8) 0.3572(3) 0.8585(3)
0 1/2 0.2502(8) 0.1510(3) 0.6397(3)
0.69(4) 0.64(4) 0.56(1) 0.52(5) 0.40(4)
a
oddÞ [2,3] were not observed even at low pressure and temperature, which may be due to the powdered sample. We fitted the experimental results at 2.3 and 17.0 GPa below 60 K ðT MI ) using the Pm3¯ structure model reported previously [2,3]. Fig. 1 shows a diffraction profile in the metallic phase at 17.0 GPa and 10 K. We can collect a very fine profile at such high pressure and reproduce the observed data satisfactorily with the Pm3¯ model as shown in Fig. 1. The parameters at 2.3 and 17.0 GPa are listed in Table 1. In the Pm3¯ model, the positions of P atoms are represented as the fractional coordinates of ð0; u þ du ; v þ dv Þ and ð0:5; 0:5 þ u du ; 0:5 þ v dv Þ for the 12j- and 12ksite, respectively [2–4]. The structural parameters, u and v are determined in the high-temperature phase. At 70 K, we determined u and v as 0.3572(2) and 0.1429(2) at 2.2 GPa, 0.3576(2) and 0.1455(2) at 17.0 GPa, respectively. In the insulating phase at 2.3 GPa and 10 K, our results show du 7 104 and dv 8 104 , which are a little larger than the ambient pressure values within the experimental uncertainty: at ambient pressure, du ¼ 3 104 and dv ¼ 6 104 were reported [3]. It is considered that P atoms, which carry the conduction electrons, easily move with increasing pressure, which is consistent with the drastic decrease of resistance below T MI [6,8]. In the metallic phase at 17.0 GPa and 10 K, dv 56 104 is larger by one order of magnitude than in the insulating phase, in contrast to the similar value of dv . With increasing pressure, the size of the Ru8 cubic sublattice, which surrounds the PrP12 icosahedron, decreases along with the lattice parameter, displacing the P atoms. These displacements make the Pr–P distance and the PrP12 icosahedron larger. The Fermi-surface nesting instability may still exist due to reproducibility with Pm3¯ model in the metallic state above 11 GPa. This result supports the gap-like behavior of
Site
˚ RI ¼ 3:84%, RWP ¼ 1:57%. a ¼ 7:99895ð3ÞA, ˚ RI ¼ 2:64%, RWP ¼ 1:17%. a ¼ 7:81316ð3ÞA,
b
the resistance at around 60 K and high pressure [6]. The three-dimensional Fermi surface nesting is considered to characterize the properties of PrRu4 P12 , and induce the superconductivity as well as the CDW transition. These structural changes may be considered a common property in the filled skutterudite compounds. If they are tuned by physical or chemical pressure, anomalous behavior like a superconductivity is expected to be observed. In summary, we observed the one order of magnitude larger P atomic displacements in the metallic phase than one in the insulating phase. This structural change may cause metallization at high pressure in the filled skutterudite compound PrRu4 P12 . This work was partially supported by Grant-in-Aid for 21st Century COE Research Program and Scientific Research Priority Area ‘‘Skutterudite’’(no. 15072204) in Grant-in-Aid for Scientific Research of The MEXT, Japan. A.M. thanks the Mochizuki Foundation for awarding a fellowship.
References [1] [2] [3] [4] [5] [6] [7] [8]
C. Sekine, et al., Phys. Rev. Lett. 79 (1997) 3218. C.H. Lee, et al., Phys. Rev. B 70 (2004) 153105. L. Hao, et al., J. Mag. Mag. Mater. 272–276 (2004) e271. H. Harima, et al., Acta Phys. Pol. B 34 (2003) 1189. S.H. Curnoe, et al., Phys. Rev. B 70 (2004) 245112. A. Miyake, et al., J. Phys. Soc. Japan 73 (2004) 2370. F. Izumi, et al., Mater. Sci. Forum 321–324 (2000) 198. I. Shirotani, et al., Physica B 322 (2002) 408.
Physica B 378–380 (2006) 199–200 www.elsevier.com/locate/physb
Structural analysis of the filled skutterudite PrRu4P12 at high pressure and low temperature Atsushi Miyakea,, Yuki Nakamotoa, Tomoko Kagayamaa, Keiki Takedab, Yausuo Ohishic, Katsuya Shimizua, Kunihiro Kihoub, Chihiro Sekineb, Ichimin Shirotanib a
KYOKUGEN, Osaka University, Osaka 560-8531, Japan b SPring-8/JASRI, Hyogo 679-5198, Japan c Faculty of Engineering, Muroran Institute of Technology, Hokkaido 050-8585, Japan
Abstract We performed synchrotron X-ray powder diffraction experiments on the filled skutterudite PrRu4 P12 at high pressure and low temperature to investigate the role of structural changes in the metallization and superconducting transition above 11 GPa. We compared the structural parameters between the insulating and metallic phases. An anomalous enhancement of the P atomic displacement is observed at 17.0 GPa and 10 K away from the Ru8 cubic sublattice are observed. This change may play a key role to the metallization at higher pressure. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 74.62.Fj; 61.50.Ks Keywords: PrRu4 P12 ; Metal–insulator transition; Powder X-ray diffraction experiment; High pressure
The filled skutterudite PrRu4 P12 exhibits a metal–insulator (M–I) transition at T MI ¼ 62 K, accompanied by a structural transition [1–3]. The electrical resistivity shows insulating behavior below T MI . Electron and X-ray diffraction studies revealed a structural transition from Im3¯ to Pm3¯ due to a Fermi-surface nesting instability below T MI , indicating the formation of a CDW in the insulator phase [2]. Band calculations revealed that the displacement of the P atoms makes a gap at E F , inducing an insulating state [4,5]. We performed electrical resistance measurements on PrRu4 P12 under high pressure, and observed metallization and a superconducting transition above 11 GPa [6]. T MI is almost independent of pressure, in contrast to the drastic change of the temperature dependence of the resistance at low temperature. Above 11 GPa, the resistance shows metallic behavior, while the anomaly of the resistance at around 60 K was still observed [6]. These facts may indicate that the structural transition due Corresponding author. Tel.: +81 6 6850 6677; fax: +81 6 6850 6662.
E-mail address: [email protected] (A. Miyake). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.075
to Fermi surface nesting, observed at ambient pressure, still occurs even at high pressure. In order to investigate the metallization mechanism, we performed powder X-ray diffraction experiments under high pressure and at low temperature; we compared the structural detail between insulating (below 11 GPa) and metallic phases (above 11 GPa). An angular dispersive method was used with a flat imaging plate detector using the synchrotron radiation source at SPring-8 BL10XU. The wavelength of the incident beam was 0.4143(1) A˚. We employed a diamond anvil cell as a high-pressure apparatus. The single crystals of PrRu4 P12 were crushed into a fine powder. The powdered sample and ruby chips as the pressure marker were put into the hole of a metal gasket. We used compressed He as a pressure-transmitting medium. The structural parameters of PrRu4 P12 were refined by the Rietveld method using the RIETAN-2000 program [7]. No clear sign of a structural transition was observed up to 17 GPa, which is consistent with the previous report [8]. The expected superlattice reflections at ðh; k; lÞ ðh þ k þ l ¼
ARTICLE IN PRESS A. Miyake et al. / Physica B 378–380 (2006) 199–200
200
20000
Table 1 Crystal structure parameters at T ¼ 10 K in PrRu4 P12 (a) in the insulating phase (2.3 GPa) and (b) in the metallic phase (17.0 GPa), space group ¯ Z¼1 Pm3,
15000
Atom
10000
30000 PrRu4P12 at 17.0 GPa and 10 K
Intensity (a. u.)
25000
5000 0 4
6
8
10
12 14 16 18 20 22 24 2θ / °
26 28
Fig. 1. Diffraction pattern of PrRu4 P12 at 10 K and 17.0 GPa. The observed and calculated data are shown as the symbols and line, respectively. The difference plot is shown below, with short vertical markers denoting the calculated peak positions.
x
y
z
Beq ðA˚ 2 Þ
(a) 2.3 GPaa Pr(1) 1a Pr(2) 1b Ru 8i P(1) 12j P(2) 12k
0 1/2 0.2507(8) 0 1/2
0 1/2 0.2507(8) 0.3565(5) 0.8580(5)
0 1/2 0.2507(8) 0.1438(5) 0.6416(5)
0.65(7) 0.65(7) 0.45(7) 0.19(7) 0.19(7)
(b) 17.0 GPab Pr(1) 1a Pr(2) 1b Ru 8i P(1) 12j P(2) 12k
0 1/2 0.2502(8) 0 1/2
0 1/2 0.2502(8) 0.3572(3) 0.8585(3)
0 1/2 0.2502(8) 0.1510(3) 0.6397(3)
0.69(4) 0.64(4) 0.56(1) 0.52(5) 0.40(4)
a
oddÞ [2,3] were not observed even at low pressure and temperature, which may be due to the powdered sample. We fitted the experimental results at 2.3 and 17.0 GPa below 60 K ðT MI ) using the Pm3¯ structure model reported previously [2,3]. Fig. 1 shows a diffraction profile in the metallic phase at 17.0 GPa and 10 K. We can collect a very fine profile at such high pressure and reproduce the observed data satisfactorily with the Pm3¯ model as shown in Fig. 1. The parameters at 2.3 and 17.0 GPa are listed in Table 1. In the Pm3¯ model, the positions of P atoms are represented as the fractional coordinates of ð0; u þ du ; v þ dv Þ and ð0:5; 0:5 þ u du ; 0:5 þ v dv Þ for the 12j- and 12ksite, respectively [2–4]. The structural parameters, u and v are determined in the high-temperature phase. At 70 K, we determined u and v as 0.3572(2) and 0.1429(2) at 2.2 GPa, 0.3576(2) and 0.1455(2) at 17.0 GPa, respectively. In the insulating phase at 2.3 GPa and 10 K, our results show du 7 104 and dv 8 104 , which are a little larger than the ambient pressure values within the experimental uncertainty: at ambient pressure, du ¼ 3 104 and dv ¼ 6 104 were reported [3]. It is considered that P atoms, which carry the conduction electrons, easily move with increasing pressure, which is consistent with the drastic decrease of resistance below T MI [6,8]. In the metallic phase at 17.0 GPa and 10 K, dv 56 104 is larger by one order of magnitude than in the insulating phase, in contrast to the similar value of dv . With increasing pressure, the size of the Ru8 cubic sublattice, which surrounds the PrP12 icosahedron, decreases along with the lattice parameter, displacing the P atoms. These displacements make the Pr–P distance and the PrP12 icosahedron larger. The Fermi-surface nesting instability may still exist due to reproducibility with Pm3¯ model in the metallic state above 11 GPa. This result supports the gap-like behavior of
Site
˚ RI ¼ 3:84%, RWP ¼ 1:57%. a ¼ 7:99895ð3ÞA, ˚ RI ¼ 2:64%, RWP ¼ 1:17%. a ¼ 7:81316ð3ÞA,
b
the resistance at around 60 K and high pressure [6]. The three-dimensional Fermi surface nesting is considered to characterize the properties of PrRu4 P12 , and induce the superconductivity as well as the CDW transition. These structural changes may be considered a common property in the filled skutterudite compounds. If they are tuned by physical or chemical pressure, anomalous behavior like a superconductivity is expected to be observed. In summary, we observed the one order of magnitude larger P atomic displacements in the metallic phase than one in the insulating phase. This structural change may cause metallization at high pressure in the filled skutterudite compound PrRu4 P12 . This work was partially supported by Grant-in-Aid for 21st Century COE Research Program and Scientific Research Priority Area ‘‘Skutterudite’’(no. 15072204) in Grant-in-Aid for Scientific Research of The MEXT, Japan. A.M. thanks the Mochizuki Foundation for awarding a fellowship.
References [1] [2] [3] [4] [5] [6] [7] [8]
C. Sekine, et al., Phys. Rev. Lett. 79 (1997) 3218. C.H. Lee, et al., Phys. Rev. B 70 (2004) 153105. L. Hao, et al., J. Mag. Mag. Mater. 272–276 (2004) e271. H. Harima, et al., Acta Phys. Pol. B 34 (2003) 1189. S.H. Curnoe, et al., Phys. Rev. B 70 (2004) 245112. A. Miyake, et al., J. Phys. Soc. Japan 73 (2004) 2370. F. Izumi, et al., Mater. Sci. Forum 321–324 (2000) 198. I. Shirotani, et al., Physica B 322 (2002) 408.