- Email: [email protected]
High temperature thermoelectric properties of CoNb1−xHfxSn1−ySby half-Heusler compounds
Journal of Alloys and Compounds 377 (2004) 312–315
High temperature thermoelectric properties of CoNb1−x Hfx Sn1−y Sby half-Heusler compounds Yoshiyuki Kawaharada, Ken Kurosaki∗ , Hiroaki Muta, Masayoshi Uno, Shinsuke Yamanaka Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Received 26 January 2004; received in revised form 12 February 2004; accepted 12 February 2004
Abstract The series of the half-Heusler compounds CoNb1−x Hfx Sn1−y Sby have been prepared and their high temperature electric properties and thermal properties have been measured. The thermoelectric power (TEP) and electrical resistivity show a strong dependence on the transition metal concentration. The temperature and transition metal concentration dependences of the thermoelectric properties of CoNb1−x Hfx Sn1−y Sby have also been studied. © 2004 Elsevier B.V. All rights reserved. Keywords: Thermoelectric; Half-Heusler compounds; Seebeck coefficient; Thermal conductivity; Electrical resistivity
1. Introduction There has been renewed interest in the field of thermoelectrics driven by the need for the reuse of exhausted heat in these days. So, in our laboratory, we have studied semiconductor and insulator materials to develop a new thermoelectric material or to improve the thermoelectric properties [1–4]. In the present study, we studied half-Heusler compounds. First, we describe structural aspects of the half-Heusler structure, which was first discovered by Jeitschko [5]. The unit cell of the half-Heusler XYZ structure with space group ¯ F 43m consists of four inter-penetrating fcc lattices at offsets A = (0, 0, 0), B = (1/4, 1/4, 1/4), and C = (1/2, 1/2, 1/2), with the site occupancies A = Y, B = X, and C = Z. Many compounds have half-Heusler structure, for instance TiCo(Sn,Sb) [6,7], NiFeSb [8], NiZrSn [9], NiUSn [10–12]. Fig. 1 shows the crystal structure of half-Heusler compounds XYZ. At present, 18-electron half-Heusler compounds, especially NiZrSn, have attracted increasing attention as new thermoelectric materials because of their semiconductor like band structure and their high thermoelectric power (TEP). The conduction band and the valence band of these ∗ Corresponding author. Tel.: +81-6-6879-7905; fax: +81-6-6879-7889. E-mail address: [email protected] (K. Kurosaki).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.02.017
18-electron half-Heusler compounds XYZ consist of X d and Y d orbitals, respectively. Therefore, the effect of transition metal replacement has been studied actively. The performance of thermoelectric materials is evaluated by the so-called dimensionless figure of merit ZT. Here Z = α2 σ/κ (α, TEP; σ, electrical conductivity; and κ, thermal conductivity) is called the figure of merit, and the numerator in the expression for Z is called the power factor. Hence, it is desirable for thermoelectric materials to have low thermal conductivity, high electrical conductivity, high TEP. In the present study, we focus not only on the substitution in the transition metal site, but also in the metalloid site for 17-electron half-Heusler compounds of the type CoNb1−x Hfx Sn1−y Sby . For these compounds, the thermoelectric properties at high temperature were studied. The replacement of the constituent elements may contribute to a decrease of the thermal conductivity.
2. Experimental Rare cobalt (2N8), niobium (3N), hafnium (2N5), tin (3N), and antimony (3N) powders were weighted in stoichiometric proportions, and pressed into pellet under 50 MPa pressure. Then the pellets were sealed in quartz ampoules under vacuum (<0.01 MPa) and annealed at 973 K for 100–450 h to obtain homogenized samples, following furnace quenching. The products were ground into powder
Y. Kawaharada et al. / Journal of Alloys and Compounds 377 (2004) 312–315
313
Table 1 Sample characteristics of CoNb1−x Hfx Sn1−y Sby Substitution ratio x, y
Fig. 1. Crystal structure of the half-Heusler type compounds.
in an agate mortar. After that the powders were ball milled to below 75 m by using a planetary ball mill equipment. All the samples were hot pressed at 1023 K in Ar flow with 75 MPa. Various shape samples were obtained for the different measurements; a disc with 10 mm diameter and about 1 mm thickness for the thermal diffusivity measurement, a square column with 12–15 mm height and 2 mm square for measurements of the electric and mechanical properties, and powder for the X-ray diffraction (XRD) measurement. The heat capacity of each compound was estimated from that of the component materials by using the Neumann–Kopp law. The thermal diffusivity was measured by a laser flash method in the temperature range 300–873 K. The thermal conductivity was calculated by using the measured sample density, the thermal diffusivity, and the estimated heat capacity. The electrical resistivity and thermoelectric power were measured by a standard four-probe dc analysis in the temperature range 300–873 K under helium atmosphere.
3. Results and discussion Fig. 2 shows the lattice parameter of each CoNb1−x Hfx Sn1−y Sby evaluated from the XRD measurement. Literature data [13] is also shown. The lattice parameter increases with increasing substitution fraction. It may be caused by the size of the ionic radii of the transition metal atoms. The sample characteristics are summarized in Table 1.
Lattice parameter a (nm)
Hf
0.00 0.05 0.15
CoNbSn CoNb0.95 Hf0.05 Sn CoNb0.85 Hf0.15 Sn
0.5945 0.5958 0.5979
Hf,Sb
0.10, 0.10
CoNb0.9 Hf0.1 Sn0.9 Sb0.1
0.5953
The temperature dependence of the electrical resistivity of CoNb1−x Hfx Sn1−y Sby is shown in Fig. 3. For hafnium substitution as well as for antimony substitution, the nature of the metal-like temperature dependence is not changed. The value of the resistivity is decreased with increasing substitution fraction. Though the band structure of CoNbSn shows semiconductor-like shape and n-type conduction [13], the temperature dependence of the electrical resistivity shows metallic behavior for both samples annealed at 100 and 450 h. The disagreement between the results of the band structure calculation and that of the present study may arise from the disorder between Co and Nb site, which gives a semi-metallic state with a small DOS at EF mainly due to the Co d contribution. The temperature dependence of the thermoelectric power is shown in Fig. 4. The absolute value of the thermoelectric power decreases with increasing hafnium fraction in CoNb1−x Hfx Sn. On the other hand, the antimony substituted compound has a slightly higher absolute value of the thermoelectric power. From the above results, the power factor of each compound is obtained and shown in Fig. 5. The compounds, which are substituted only on the niobium site, indicate a low power factor. It is caused by the significant decrease of the thermopower. For the antimony substituted compounds, the power factor increases drastically, owing to the reduction of the electrical resistivity and the enhancement of the thermopower. The temperature dependence of the thermal conductivity is shown in Fig. 6. Almost all the samples show a lower thermal conductivity than that of the non-substituted compound. The electronic conduction κel was estimated by using the Wiedemann–Franz law. The κel of the sub-
Lattice parameter, a/nm
0.600
0.598
CoNbSn 0.596
CoNb1-xHfxSn
0.594
Literature data [13]
CoNb1-xHfxSn1-ySby
0.592 0.0
0.1
0.2
Substitution fraction, x/-
Fig. 2. Lattice parameter of CoNb1−x Hfx Sn1−y Sby .
314
Y. Kawaharada et al. / Journal of Alloys and Compounds 377 (2004) 312–315 40
30
20
CoNbSn CoNb0.95Hf0.05Sn
10
CoNb0.85Hf0.15Sn CoNb0.90Hf0.10Sn0.90Sb0.10
0 300
400
500 600 700 Temperature,T/K
800
900
Fig. 3. Temperature dependence of the electrical resistivity of CoNb1−x Hfx Sn1−y Sby .
TEP, / VK
-1
0
-50
CoNbSn
-100
CoNb0.95Hf0.05Sn CoNb0.85Hf0.15Sn -150 300
400
500 600 700 Temperature,T/K
800
900
CoNb0.90Hf0.10Sn0.90Sb0.10
Fig. 4. Temperature dependence of the thermoelectric power of CoNb1−x Hfx Sn1−y Sby .
stituted compounds is higher than that of CoNbSn, because of the low resistivity. The thermal conductivity by phonon conduction κlat was estimated by subtracting κel from κ. The κlat of the substituted compounds is lower than that of CoNbSn, which may originate from the mass defect. Fig. 7 shows the temperature dependence of ZT obtained from all the above results. The CoNb1−x Hfx Sn compounds show low ZT in the whole temperature region. The maxi-
mum value is 0.002 for CoNb0.95 Hf0.05 Sn at 609 K. On the contrary, the antimony-containing compound shows high ZT (0.085 at 873 K), which is four times as high as that of CoNbSn. CoNb0.9 Hf0.1 Sn0.9 Sb0.1 shows the largest value of ZT of all the samples, but the value of ZT is lower than those of other thermoelectric materials. It is caused by the high thermal conductivity and slightly low TEP of CoNb1−x Hfx Sn1−y Sby , in spite of its slightly low electrical resistivity.
-2
Power Factor, P/ WK cm
8
6
4
CoNbSn CoNb0.95Hf0.05Sn
2
CoNb0.85Hf0.15Sn 0 300
CoNb0.90Hf0.10Sn0.90Sb0.10 400
500 600 700 Temperature,T/K
800
900
Fig. 5. Temperature dependence of the power factor of CoNb1−x Hfx Sn1−y Sby .
Y. Kawaharada et al. / Journal of Alloys and Compounds 377 (2004) 312–315
315
-1
Thermal conductivity, /WK m
-1
15
10
CoNbSn CoNb0.95Hf0.05Sn CoNb0.85Hf0.15Sn 5 300
400
500
600
700
800
900
CoNb0.90Hf0.10Sn0.90Sb0.10
Temperature,T/K
Fig. 6. Temperature dependence of the thermal conductivity of CoNb1−x Hfx Sn1−y Sby .
Dimensionless figure of merit, ZT/-
0.10
0.05
CoNbSn CoNb0.95Hf0.05Sn CoNb0.85Hf0.15Sn
0.00 400
500
600
700
800
900
CoNb0.90Hf0.10Sn0.90Sb0.10
Temperature,T/K
Fig. 7. Temperature dependence of ZT of CoNb1−x Hfx Sn1−y Sby .
4. Conclusion The CoNb1−x Hfx Sn1−y Sby half-Heusler compounds were prepared in various concentration ranges, and their thermoelectric properties were evaluated. All the thermoelectric properties are strongly dependent on the substituted fraction and the substituted atom sites. Upon substitution of only hafnium for niobium, by the electrical resistivity and TEP of CoNb1−x Mx Sn is decreased. On the other hand, substitution of antimony for tin leads to compounds showing high thermoelectric performance. CoNb0.9 Hf0.1 Sn0.9 Sb0.1 shows the largest power factor and ZT of all the CoNb1−x Hfx Sn1−y Sby samples. The maximum value of ZT is 0.085 at 873 K. References [1] Y. Kawaharada, K. Kurosaki, M. Uno, S. Yamanaka, J. Alloys Compd. 315 (2001) 193. [2] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Alloys Compd. 334 (2002) 317.
[3] K. Kurosaki, T. Matsuda, M. Uno, S. Kobayashi, S. Yamanaka, J. Alloys Compd. 319 (2001) 271. [4] H. Muta, K. Kurosaki, M. Uno, S. Yamanaka, J. Alloys Compd. 335 (2002) 200. [5] W. Jeitschko, Metall. Trans. A 1 (1970) 3159. [6] L. Degiorgi, A.V. Sologubenko, H.R. Ott, F. Drymiotis, Z. Fisk, Phys. Rev. B 65 (4) (2002) 041101/1-4. [7] A.N. Caruso, C.N. Borca, D. Ristoiu, J.P. Nozieres, P.A. Dowben, Surf. Sci. 525 (2003) L109. [8] M. Zhang, Z. Liu, H. Hu, Y. Cui, G. Liu, J. Chen, G. Wu, Y. Sui, Z. Qian, Z. Li, H. Tao, B. Zhao, H. Wen, Solid State Commun. 128 (2003) 107–111. [9] O.I. Bodak, B.V. Padlyak, Yu.V. Stadnyk, J. Pierre, A.V. Tkachuk, L.P. Romaka, Yu.K. Gorelenko, J. Alloys Compd. 317–318 (2001) 357. [10] J.S. Kang, J.G. Park, K.A. McEwen, C.G. Olson, S.K. Kwon, B.I. Min, Phys. Rev. B 64 (2001) 85101. [11] P.M. Oppeneer, A.N. Yaresko, A.Ya. Prelov, V.N. Antonov, H. Eschrig, Phys. Rev. B 54 (1996) 3706. [12] J.S. Kang, J.G. Parl, K.A. McEwen, Y. Onuki, C.G. Olson, B.I. Min, Physica B 312–313 (2002) 882. [13] J. Tobola, J. Pierre, S. Kaprzyk, R.V. Skolozdra, M.A. Kouacou, J. Phys. Condens. Matter 10 (1998) 1013.
High temperature thermoelectric properties of CoNb1−x Hfx Sn1−y Sby half-Heusler compounds Yoshiyuki Kawaharada, Ken Kurosaki∗ , Hiroaki Muta, Masayoshi Uno, Shinsuke Yamanaka Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Received 26 January 2004; received in revised form 12 February 2004; accepted 12 February 2004
Abstract The series of the half-Heusler compounds CoNb1−x Hfx Sn1−y Sby have been prepared and their high temperature electric properties and thermal properties have been measured. The thermoelectric power (TEP) and electrical resistivity show a strong dependence on the transition metal concentration. The temperature and transition metal concentration dependences of the thermoelectric properties of CoNb1−x Hfx Sn1−y Sby have also been studied. © 2004 Elsevier B.V. All rights reserved. Keywords: Thermoelectric; Half-Heusler compounds; Seebeck coefficient; Thermal conductivity; Electrical resistivity
1. Introduction There has been renewed interest in the field of thermoelectrics driven by the need for the reuse of exhausted heat in these days. So, in our laboratory, we have studied semiconductor and insulator materials to develop a new thermoelectric material or to improve the thermoelectric properties [1–4]. In the present study, we studied half-Heusler compounds. First, we describe structural aspects of the half-Heusler structure, which was first discovered by Jeitschko [5]. The unit cell of the half-Heusler XYZ structure with space group ¯ F 43m consists of four inter-penetrating fcc lattices at offsets A = (0, 0, 0), B = (1/4, 1/4, 1/4), and C = (1/2, 1/2, 1/2), with the site occupancies A = Y, B = X, and C = Z. Many compounds have half-Heusler structure, for instance TiCo(Sn,Sb) [6,7], NiFeSb [8], NiZrSn [9], NiUSn [10–12]. Fig. 1 shows the crystal structure of half-Heusler compounds XYZ. At present, 18-electron half-Heusler compounds, especially NiZrSn, have attracted increasing attention as new thermoelectric materials because of their semiconductor like band structure and their high thermoelectric power (TEP). The conduction band and the valence band of these ∗ Corresponding author. Tel.: +81-6-6879-7905; fax: +81-6-6879-7889. E-mail address: [email protected] (K. Kurosaki).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.02.017
18-electron half-Heusler compounds XYZ consist of X d and Y d orbitals, respectively. Therefore, the effect of transition metal replacement has been studied actively. The performance of thermoelectric materials is evaluated by the so-called dimensionless figure of merit ZT. Here Z = α2 σ/κ (α, TEP; σ, electrical conductivity; and κ, thermal conductivity) is called the figure of merit, and the numerator in the expression for Z is called the power factor. Hence, it is desirable for thermoelectric materials to have low thermal conductivity, high electrical conductivity, high TEP. In the present study, we focus not only on the substitution in the transition metal site, but also in the metalloid site for 17-electron half-Heusler compounds of the type CoNb1−x Hfx Sn1−y Sby . For these compounds, the thermoelectric properties at high temperature were studied. The replacement of the constituent elements may contribute to a decrease of the thermal conductivity.
2. Experimental Rare cobalt (2N8), niobium (3N), hafnium (2N5), tin (3N), and antimony (3N) powders were weighted in stoichiometric proportions, and pressed into pellet under 50 MPa pressure. Then the pellets were sealed in quartz ampoules under vacuum (<0.01 MPa) and annealed at 973 K for 100–450 h to obtain homogenized samples, following furnace quenching. The products were ground into powder
Y. Kawaharada et al. / Journal of Alloys and Compounds 377 (2004) 312–315
313
Table 1 Sample characteristics of CoNb1−x Hfx Sn1−y Sby Substitution ratio x, y
Fig. 1. Crystal structure of the half-Heusler type compounds.
in an agate mortar. After that the powders were ball milled to below 75 m by using a planetary ball mill equipment. All the samples were hot pressed at 1023 K in Ar flow with 75 MPa. Various shape samples were obtained for the different measurements; a disc with 10 mm diameter and about 1 mm thickness for the thermal diffusivity measurement, a square column with 12–15 mm height and 2 mm square for measurements of the electric and mechanical properties, and powder for the X-ray diffraction (XRD) measurement. The heat capacity of each compound was estimated from that of the component materials by using the Neumann–Kopp law. The thermal diffusivity was measured by a laser flash method in the temperature range 300–873 K. The thermal conductivity was calculated by using the measured sample density, the thermal diffusivity, and the estimated heat capacity. The electrical resistivity and thermoelectric power were measured by a standard four-probe dc analysis in the temperature range 300–873 K under helium atmosphere.
3. Results and discussion Fig. 2 shows the lattice parameter of each CoNb1−x Hfx Sn1−y Sby evaluated from the XRD measurement. Literature data [13] is also shown. The lattice parameter increases with increasing substitution fraction. It may be caused by the size of the ionic radii of the transition metal atoms. The sample characteristics are summarized in Table 1.
Lattice parameter a (nm)
Hf
0.00 0.05 0.15
CoNbSn CoNb0.95 Hf0.05 Sn CoNb0.85 Hf0.15 Sn
0.5945 0.5958 0.5979
Hf,Sb
0.10, 0.10
CoNb0.9 Hf0.1 Sn0.9 Sb0.1
0.5953
The temperature dependence of the electrical resistivity of CoNb1−x Hfx Sn1−y Sby is shown in Fig. 3. For hafnium substitution as well as for antimony substitution, the nature of the metal-like temperature dependence is not changed. The value of the resistivity is decreased with increasing substitution fraction. Though the band structure of CoNbSn shows semiconductor-like shape and n-type conduction [13], the temperature dependence of the electrical resistivity shows metallic behavior for both samples annealed at 100 and 450 h. The disagreement between the results of the band structure calculation and that of the present study may arise from the disorder between Co and Nb site, which gives a semi-metallic state with a small DOS at EF mainly due to the Co d contribution. The temperature dependence of the thermoelectric power is shown in Fig. 4. The absolute value of the thermoelectric power decreases with increasing hafnium fraction in CoNb1−x Hfx Sn. On the other hand, the antimony substituted compound has a slightly higher absolute value of the thermoelectric power. From the above results, the power factor of each compound is obtained and shown in Fig. 5. The compounds, which are substituted only on the niobium site, indicate a low power factor. It is caused by the significant decrease of the thermopower. For the antimony substituted compounds, the power factor increases drastically, owing to the reduction of the electrical resistivity and the enhancement of the thermopower. The temperature dependence of the thermal conductivity is shown in Fig. 6. Almost all the samples show a lower thermal conductivity than that of the non-substituted compound. The electronic conduction κel was estimated by using the Wiedemann–Franz law. The κel of the sub-
Lattice parameter, a/nm
0.600
0.598
CoNbSn 0.596
CoNb1-xHfxSn
0.594
Literature data [13]
CoNb1-xHfxSn1-ySby
0.592 0.0
0.1
0.2
Substitution fraction, x/-
Fig. 2. Lattice parameter of CoNb1−x Hfx Sn1−y Sby .
314
Y. Kawaharada et al. / Journal of Alloys and Compounds 377 (2004) 312–315 40
30
20
CoNbSn CoNb0.95Hf0.05Sn
10
CoNb0.85Hf0.15Sn CoNb0.90Hf0.10Sn0.90Sb0.10
0 300
400
500 600 700 Temperature,T/K
800
900
Fig. 3. Temperature dependence of the electrical resistivity of CoNb1−x Hfx Sn1−y Sby .
TEP, / VK
-1
0
-50
CoNbSn
-100
CoNb0.95Hf0.05Sn CoNb0.85Hf0.15Sn -150 300
400
500 600 700 Temperature,T/K
800
900
CoNb0.90Hf0.10Sn0.90Sb0.10
Fig. 4. Temperature dependence of the thermoelectric power of CoNb1−x Hfx Sn1−y Sby .
stituted compounds is higher than that of CoNbSn, because of the low resistivity. The thermal conductivity by phonon conduction κlat was estimated by subtracting κel from κ. The κlat of the substituted compounds is lower than that of CoNbSn, which may originate from the mass defect. Fig. 7 shows the temperature dependence of ZT obtained from all the above results. The CoNb1−x Hfx Sn compounds show low ZT in the whole temperature region. The maxi-
mum value is 0.002 for CoNb0.95 Hf0.05 Sn at 609 K. On the contrary, the antimony-containing compound shows high ZT (0.085 at 873 K), which is four times as high as that of CoNbSn. CoNb0.9 Hf0.1 Sn0.9 Sb0.1 shows the largest value of ZT of all the samples, but the value of ZT is lower than those of other thermoelectric materials. It is caused by the high thermal conductivity and slightly low TEP of CoNb1−x Hfx Sn1−y Sby , in spite of its slightly low electrical resistivity.
-2
Power Factor, P/ WK cm
8
6
4
CoNbSn CoNb0.95Hf0.05Sn
2
CoNb0.85Hf0.15Sn 0 300
CoNb0.90Hf0.10Sn0.90Sb0.10 400
500 600 700 Temperature,T/K
800
900
Fig. 5. Temperature dependence of the power factor of CoNb1−x Hfx Sn1−y Sby .
Y. Kawaharada et al. / Journal of Alloys and Compounds 377 (2004) 312–315
315
-1
Thermal conductivity, /WK m
-1
15
10
CoNbSn CoNb0.95Hf0.05Sn CoNb0.85Hf0.15Sn 5 300
400
500
600
700
800
900
CoNb0.90Hf0.10Sn0.90Sb0.10
Temperature,T/K
Fig. 6. Temperature dependence of the thermal conductivity of CoNb1−x Hfx Sn1−y Sby .
Dimensionless figure of merit, ZT/-
0.10
0.05
CoNbSn CoNb0.95Hf0.05Sn CoNb0.85Hf0.15Sn
0.00 400
500
600
700
800
900
CoNb0.90Hf0.10Sn0.90Sb0.10
Temperature,T/K
Fig. 7. Temperature dependence of ZT of CoNb1−x Hfx Sn1−y Sby .
4. Conclusion The CoNb1−x Hfx Sn1−y Sby half-Heusler compounds were prepared in various concentration ranges, and their thermoelectric properties were evaluated. All the thermoelectric properties are strongly dependent on the substituted fraction and the substituted atom sites. Upon substitution of only hafnium for niobium, by the electrical resistivity and TEP of CoNb1−x Mx Sn is decreased. On the other hand, substitution of antimony for tin leads to compounds showing high thermoelectric performance. CoNb0.9 Hf0.1 Sn0.9 Sb0.1 shows the largest power factor and ZT of all the CoNb1−x Hfx Sn1−y Sby samples. The maximum value of ZT is 0.085 at 873 K. References [1] Y. Kawaharada, K. Kurosaki, M. Uno, S. Yamanaka, J. Alloys Compd. 315 (2001) 193. [2] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Alloys Compd. 334 (2002) 317.
[3] K. Kurosaki, T. Matsuda, M. Uno, S. Kobayashi, S. Yamanaka, J. Alloys Compd. 319 (2001) 271. [4] H. Muta, K. Kurosaki, M. Uno, S. Yamanaka, J. Alloys Compd. 335 (2002) 200. [5] W. Jeitschko, Metall. Trans. A 1 (1970) 3159. [6] L. Degiorgi, A.V. Sologubenko, H.R. Ott, F. Drymiotis, Z. Fisk, Phys. Rev. B 65 (4) (2002) 041101/1-4. [7] A.N. Caruso, C.N. Borca, D. Ristoiu, J.P. Nozieres, P.A. Dowben, Surf. Sci. 525 (2003) L109. [8] M. Zhang, Z. Liu, H. Hu, Y. Cui, G. Liu, J. Chen, G. Wu, Y. Sui, Z. Qian, Z. Li, H. Tao, B. Zhao, H. Wen, Solid State Commun. 128 (2003) 107–111. [9] O.I. Bodak, B.V. Padlyak, Yu.V. Stadnyk, J. Pierre, A.V. Tkachuk, L.P. Romaka, Yu.K. Gorelenko, J. Alloys Compd. 317–318 (2001) 357. [10] J.S. Kang, J.G. Park, K.A. McEwen, C.G. Olson, S.K. Kwon, B.I. Min, Phys. Rev. B 64 (2001) 85101. [11] P.M. Oppeneer, A.N. Yaresko, A.Ya. Prelov, V.N. Antonov, H. Eschrig, Phys. Rev. B 54 (1996) 3706. [12] J.S. Kang, J.G. Parl, K.A. McEwen, Y. Onuki, C.G. Olson, B.I. Min, Physica B 312–313 (2002) 882. [13] J. Tobola, J. Pierre, S. Kaprzyk, R.V. Skolozdra, M.A. Kouacou, J. Phys. Condens. Matter 10 (1998) 1013.