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Electrical properties of the binary icosahedral quasicrystal and its approximant in the Cd–Yb system
Materials Science and Engineering A 375–377 (2004) 1002–1005
Electrical properties of the binary icosahedral quasicrystal and its approximant in the Cd–Yb system R. Tamura a,∗ , Y. Murao a , S. Kishino a , S. Takeuchi a , K. Tokiwa b , T. Watanabe b a
Department of Materials Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan b Department of Applied Electronics, Tokyo University of Science, Noda, Chiba 278-8510, Japan
Abstract Electrical resistivity, magnetoresistance, and specific heat measurements have been performed for the binary icosahedral Cd5.7 Yb and its cubic approximant Cd6 Yb. The resistivity of both the icosahedral phase (i-phase) and the approximant is found to show a rather metallic behavior with a large positive temperature coefficient of the resistivity (TCR) below 200–300 K followed by leveling-off at low temperatures below 10 K. For the approximant, we observe a stepwise change of the resistivity at 110 K which is due to a phase transition. Giant magnetoresistance as high as 200% at 9 T below 4.2 K is observed for the i-phase and the electronic specific heat coefficient γ is found to be extraordinary large in both alloys, i.e., 2.87 mJ/(mole K2 ) for the i-phase and 7.60 mJ/(mole K2 ) for the approximant. Such large γ values may be attributed to the Yb-derived states at the Fermi level. Furthermore, the Debye temperatures (ΘD ) are very low and almost the same for both alloys, i.e., 142 and 144 K, respectively, which are indeed the lowest values ever reported in i-phases and approximants. The metallic character of the binary i-phase implies that the negative TCR is not necessarily a consequence of the quasiperiodicity. © 2003 Elsevier B.V. All rights reserved. Keywords: Quasicrystal; Approximant; Electrical resistivity; Magnetoresistance; Specific heat; Cadmium–ytterbium
1. Introduction The binary Cd5.7 Yb and Cd17 Ca3 intermetallic compounds [1] were recently found to be P-type icosahedral phases (i-phases) [2,3]. These i-phases are of particular interest since (i) they are binary stoichiometric compounds which are expected to contain less chemical disorder than ternary i-phases and (ii) there exists a cubic crystalline approximant in the very vicinity of the i-phase compositions. With respect to (i), no chemical disorder in the i-phases is also suggested from the structural model of the approximant Cd6 Yb [4], where there exist no sites occupied statistically by the different atomic species. From (ii) it has now become possible to investigate the influence of the long-periodicity, or quasiperiodicity, on the electronic transport in the binary alloy systems since these two phases possess a common local atomic structure. Up to now studies on ternary i-phases have shown that stable i-phases commonly possess a very high resistivity value and a large negative temperature coefficient of the resistiv-
ity (TCR) [5,6], implying that the high resistivity values and the negative TCR’s may be a consequence of the quasiperiodicity. The most striking is the metal-insulator transition observed in i-AlPdRe [7–10], which indicates the electronic states at the Fermi level (EF ) can be localized in a structurally ordered i-phase. However, until now the question as to the origin of such localized states, whether it is due to the long-range quasiperiodicity combined with the peculiar local atomic structure or to some kind of structural disorder such as topological or chemical one, has not been fully answered yet. In this paper, results on the electronic transport of i-Cd5.7 Yb and 1/1-Cd6 Yb including the electrical resistivity, magnetoresistance, and specific heat are reported and various anomalous behaviors of the binary i-phase as well as the binary approximant are clarified. The electronic transport of the quasicrystal is discussed in terms of the role of the chemical disorder in the electronic transport. A part of the results of the electronic transport measurements of i-Cd5.7 Yb was reported elsewhere [11]. 2. Experimental
∗
Corresponding author. Tel.: +81-471-24-1501; fax: +81-471-23-9362. E-mail address: [email protected] (R. Tamura). 0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.205
Pure elements of Cd (99.9999 wt.%) and Yb (99.9 wt.%)/ Ca (99 wt.%) with slightly different nominal compositions
R. Tamura et al. / Materials Science and Engineering A 375–377 (2004) 1002–1005
Cd100−X YbX with X in the range between 14 and 17 and Cd85.7 Ca14.3 , respectively, were wrapped tightly in molybdenum foil and melted in a quartz tube sealed under argon atmosphere. The ingots were subsequently annealed at 673 K for 24 h to obtain a homogeneous, equilibrium phase. The phase identification of the samples was performed by X-ray diffraction measurement with Cu K␣ radiation. The electrical resistivity was measured from 2 to 873 K by a four probe method. The magnetoresistance was measured in the field up to 9 T at temperatures 1.8, 4.2, 10, 20, and 50 K. The specific heat measurement was performed by an adiabatic relaxation method between 1.8 and 200 K.
3. Results and discussion Single i-phases are obtained at the compositions Cd100−X YbX with X in the range between 15.1 and 16.1, and a single approximant phase at Cd85.7 Yb14.3 and at Cd85.7 Ca14.3 , from powder X-ray diffraction measurements. Fig. 1a presents the temperature dependence of the resistivity ρ(T) normalized by ρ300 K from 2 to 300 K for Cd100−X YbX with X = 15.4, 15.7, 15.9, and 16.1, and Fig. 1b shows ρ(T) of the 1/1-Cd85.7 Yb14.3 together with that of i-Cd84.6 Yb15.4 for a comparison. The magnitude of ρ300 K is 560, 350, 240, and 490 cm, respectively, for the i-phases and 300 cm for the approximant. These resistivity values of the i-phases are rather low compared
Fig. 1. Temperature dependence of (a) the normalized resistivity ρ/ρ300 K for i-Cd100−X YbX with X = 15.4, 15.7, 15.9, and 16.1 and (b) temperature dependence of the resistivity ρ for 1/1-Cd85.7 Yb14.3 . In (b), ρ(T) of i-Cd84.6 Yb15.4 is also shown for a comparison.
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with those of Al-TM (transition metal)-based i-phases but are similar to those of ternary rare-earth (RE)-containing i-phases such as i-ZnMgRE (RE = Tb, Dy, Ho, and Er) [12]. We observe three distinct regions in the ρ(T) behavior of i-Cd–Yb with varying temperature. (1) TCR is negative above 200–300 K up to the melting point (not shown), as in the cases of ternary i-phases. (2) ρ(T) decreases sharply with decreasing temperature below 200–300 K exhibiting a broad maximum and the change of the resistivity ρ/ρ300 K is found to be considerably large ranging from 10 to 80 at.%. The variation of the resistivity is quite sensitive to the Yb concentration and large ρ/ρ300 K is observed for Yb-rich compositions, implying that Yb-derived states may play a significant role in the electronic transport. Such a large positive TCR has not been observed in ternary i-phases although a maximum in ρ(T) has been seen in i-AlPdMn [13] and i-AlCuFe [14]. (3) ρ(T) is found to become temperature-independent at lowest temperatures below 10 K as in the cases of normal metals with no indication of the quantum interference effects. Similar metallic behavior of i-Cd5.7 Yb and 1/1-Cd6 Yb at low temperatures has been also reported by Pope et al. [15] and Dhar et al. [16]. We note that a maximum in ρ(T) has been also observed of i-Cd5.7 Yb at around 80 and 50 K, respectively, in these papers. The behaviors (2) and (3) of i-Cd–Yb are quite unique among other stable i-phases and indicate that the negative TCR at low temperatures, often accompanying the quantum interference effects, may not be a consequence of the quasiperiodicity, whereas the negative TCR at high temperatures are confirmed to be a common property of i-phases and high-order approximants. On the other hand, TCR of the approximant is found to be positive in the whole temperature region up to the melting point, exhibiting essentially a metallic nature [17]. However, as seen from the figure the overall feature of ρ(T) of 1/1-Cd85.7 Yb14.3 below 300 K is rather similar to that of the i-phase. The resistivity decreases by 70% with decreasing temperature below 300 K. Thus, the low resistivity values and positive TCR of the i-phase are taken as indication that the binary i-phase is a rather good metal in contrast to other stable i-phases. Such a result is consistent with the sharp Fermi cut-off with considerably large density of states (DOS) at EF observed in the photoemission spectroscopy (PS) measurement [18]. The metallic nature observed in the binary i-phase suggests that the chemical disorder might be the origin for the QC-like behaviors such as large ρ and a negative TCR in some ternary i-phases. In addition, as seen from the figure, we observe a stepwise change of the ρ(T) at 110 K in 1/1-Cd85.7 Yb14.3 with a discontinuous shift of TCR across the step [16], which phenomenon has been ascribed to the occurrence of a phase transition, i.e., an order–disorder transition [22]. Then it is of interest that no sign of such a phase transition has been observed in ρ(T) of the i-phase. Fig. 2a and b present the results of the magnetoresistance for i-Cd84.6 Yb15.4 and i-Cd84.1 Yb15.9 , respectively.
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R. Tamura et al. / Materials Science and Engineering A 375–377 (2004) 1002–1005
Fig. 3. Specific heat data plotted as C/T vs. T2 for i-Cd84.6 Yb15.4 , 1/1Cd85.7 Yb14.3 , and 1/1-Cd85.7 Ca14.3 . Solid lines are fits by C/T = γ +βT 2 .
Fig. 2. Magnetoresistance ρ/ρ vs. magnetic field B at temperatures 1.8, 4.2, 10, 20, and 50 K for (a) i-Cd84.6 Yb15.4 and (b) i-Cd84.1 Yb15.9 .
The observed magnetoresistance is extraordinary large for the i-phase alloys despite their low resistivity values: ρ/ρ at 9 T below 4.2 K reaches about 20% for i-Cd84.6 Yb15.4 and, surprisingly, a giant magnetoresistance reaching as high as 200% is observed for i-Cd84.1 Yb15.9 . ρ/ρ of the present i-Cd84.1 Yb15.9 is even larger than those of the insulating i-AlPdRe [19] by more than a factor of two despite its low resistivity value, violating the empirical rule seen in other i-phases studied so far [6]. We also find that neither the large magnitude of ρ/ρ nor its field dependence ρ(H) fits into the quantum interference theories such as the weak localization [20] and the electron–electron interaction [21] theories, which is consistent with the metallic character of ρ(T) for i-Cd–Yb. Another striking feature concerning the magnetoresistance is the temperature-independent ρ/ρ at lowest temperatures below 4.2 K as clearly seen in the figures. The present result on ρ/ρ may be correlated with the temperature-independent ρ(T) below about 10 K. Fig. 3 presents the result of the specific heat measurement for i-Cd84.6 Yb15.4 , 1/1-Cd85.7 Yb14.3 , and 1/1-Cd85.7 Ca14.3 in the form of C/T versus T2 . Electronic specific heat coefficients γ are determined to be 2.87 and 7.60 mJ/(mole K2 ) for i-Cd84.6 Yb15.4 and 1/1-Cd85.7 Yb14.3 , respectively. Such large γ values for the quasicrystalline materials are indeed unexpected and are much larger that those seen in other stable i-phases, where γ generally falls into the range γ < 1.0 (typically 0.2–0.3) mJ/(mole K2 ).[6] Pope et al. [15] have reported a smaller γ value (1.1 mJ/(mole K2 )) for the
i-Cd5.7 Yb, whereas even larger γ values for i-Cd5.7 Yb and 1/1-Cd6 Yb, i.e., 7.5 and 51 mJ/(mole K2 ), respectively, have been reported by Dhar et al. [16]. The large γ values are normally attributed either to enhanced DOS at EF or to a large thermal mass of electrons. Recently, Ishii and Fujiwara have shown that the band contribution to γ in 1/1-Cd6 Yb is ∼0.8 mJ/(mole K2 ) [23], which is an order of magnitude lower than the experiment. Besides, the PS measurements [18] have clearly shown the existence of a pseudogap at EF in i-Cd–Yb and 1/1-Cd–Yb. Therefore, it is reasonable to consider that the observed large γ values are not due to large DOS at EF but to another factor. In Fig. 3, the specific heat of 1/1-Cd85.7 Ca14.3 , which is isostructural to 1/1-Cd85.7 Ca14.3 , is presented for a comparison. We find that the γ value of 1/1-Cd85.7 Ca14.3 is not as large, i.e., 2.0 mJ/(mole K2 ), and there is an appreciable difference in the values of γ between the two approximant phases of the same atomic structure, indicating that the extraordinary large γ values of the Cd–Yb compounds may be attributed to Yb-derived states at EF . For such Yb-derived states, Yb 4f states may be responsible for the large γ values, which is located in the vicinity of EF , i.e., 0.7 eV below EF [18]. We note that the position of the Yb 4f states is slightly shifted towards higher binding energies in the case of the i-Cd–Yb, which may account for the observed trend that the γ value is smaller in the i-phase than in the approximant. However, further investigation is certainly needed to clarify the origin of the large γ values. The Debye temperatures (ΘD ) are obtained as 142, 144 and 144 K for i-Cd84.6 Yb15.4 , 1/1-Cd85.7 Yb14.3 and 1/1-Cd85.7 Ca14.3 , respectively, from Fig. 3, which are in agreement with the values reported for i-Cd5.7 Yb and 1/1-Cd6 Yb by Dhar et al. [16], i.e., 138 and 144 K, respectively. Here we find that ΘD values are almost the same for these three alloys. We should say that the ΘD value of the i-phase is anomalously small, which is in striking contrast with those seen in other i-phases, where, for instance, ΘD is 500–600 K [24,25] and about 300 K [26] for stable Al-TM-based and Zn–Mg-based i-phases, respectively.
R. Tamura et al. / Materials Science and Engineering A 375–377 (2004) 1002–1005
ΘD ’s of these phases are even smaller than that of Cd metal and the weighted average of the constituent elements. The almost identical ΘD values of the three alloys are understood by their similar local environments for the Cd atoms.
4. Conclusions Anomalous transport properties of the Cd–Yb binary i-phase and its approximant are reported based on the results of the electrical resistivity, magnetoresistance, and specific heat measurements: the temperature dependence of the resistivity of the i-phase is found to show three distinct regions with changing temperature: a negative TCR above 300 K up to the melting point, a rapid decrease of ρ/ρ300 K by 10–80% with decreasing temperature below 200–300 K and the leveling-off at lowest temperatures below 10 K. On the other hand, the resistivity of the approximant is found to possess a positive TCR in the whole temperature region accompanying a stepwise change of ρ at 110 K due to a structural transition. In addition, unexpected giant magnetoresistance reaching 200%, much larger than the highest one obtained in ternary i-phases, is observed at low temperatures and its field dependence is almost linear. Moreover, the electronic specific heat coefficient is extremely large, i.e., 2.87 mJ/(mole K2 ) for the i-phase and 7.60 mJ/(mole K2 ) for the approximant, and the Debye temperatures are as small as 140 K. Finally, the metallic nature observed at low temperatures in i-Cd–Yb indicates that the negative TCR observed in some ternary i-phases might be due to chemical disorder.
Acknowledgements We thank Prof. K. Motoya of Tokyo University of Science for his help in the experiment. This work was supported in part by a Grant-in-Aid for Scientific Research (13750616) from Japan Society for Promotion of Science and also by Japan Science and Technology Corporation (CREST).
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References [1] T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak (eds.), Binary Alloy Phase Diagram second ed., vol. 2, ASM International, Gaithersburg, 1990, p. 996. [2] A.P. Tsai, J.Q. Guo, E. Abe, H. Takakura, T.J. Sato, Nature (London) 408 (2000) 538. [3] J.Q. Guo, E. Abe, A.P. Tsai, Phys. Rev. B 62 (2000) 14605. [4] A. Palenzona, J. Less-Common Met. 25 (1971) 367. [5] S.J. Poon, Adv. Phys. 4 (1992) 303. [6] O. Rapp, Physical Properties of Quasicrystals, in: Z.M. Stadnik (Ed.), Springer, Berlin, 1999, p. 127. [7] H. Akiyama, Y. Honda, T. Hashimoto, K. Edagawa, S. Takeuchi, Jpn. J. Appl. Phys. 32 (1993) L1003. [8] F.S. Pierce, S.J. Poon, Q. Guo, Science 261 (1993) 737. [9] C. Berger, T. Grenet, P. Lindqvist, P. Lanco, J.C. Grieco, G. Fourcaudot, F. Cyrot-Lackmann, Solid State Commun. 87 (1993) 977. [10] J. Delahaye, J.P. Brison, C. Berger, Phys. Rev. Lett. 81 (1999) 4204. [11] R. Tamura, Y. Murao, S. Takeuchi, K. Tokiwa, T. Watanabe, T.J. Sato, A.P. Tsai, Jpn. J. Appl. Phys. 40 (2001) 912. [12] I.R. Fisher, K.O. Cheon, A.F. Panchula, P.C. Canfield, M. Chernikov, H.R. Ott, K. Dennis, Phys. Rev. B 59 (1999) 308. [13] H. Akiyama, T. Hashimoto, T. Shibuya, K. Edagawa, S. Takeuchi, J. Phys. Soc. Jpn. 62 (1993) 639. [14] T. Klein, C. Berger, D. Mayou, F. Cyrot-Lackmann, Phys. Rev. Lett. 66 (1991) 2907. [15] A.L. Pope, T.M. Tritt, R. Gagnon, J. Strom-Olsen, Appl. Phys. Lett. 79 (2001) 2345. [16] S.K. Dhar, A. Palenzona, P. Manfrinetti, S.M. Pattalwar, J. Phys.: Condens. Matter 14 (2002) 517. [17] Y. Muro, T. Sasakawa, T. Suemitsu, T. Takabatake, R. Tamura, S. Takeuchi, Jpn. J. Appl. Phys. 41 (2002) 3787. [18] R. Tamura, Y. Murao, S. Takeuchi, T. Kiss, T. Yokoya, S. Shin, Phys. Rev. B 65 (2002) 224207. [19] M. Ahlgren, M. Rodmar, C. Gignoux, C. Berger, O. Rapp, Mater. Sci. Eng. A226/228 (1997) 981. [20] H. Fukuyama, K. Hoshino, J. Phys. Soc. Jpn. 50 (1981) 2131. [21] P.A. Lee, T.V. Ramakrishnan, Phys. Rev. B 26 (1982) 4009. [22] (a) R. Tamura, Y. Murao, S. Takeuchi, M. Ichihara, M. Isobe, Y. Ueda, Jpn. J. Appl. Phys. 41 (2002) 524; (b) R. Tamura, K. Edagawa, Y. Murao, S. Takeuchi, K. Suzuki, M. Ichihara, M. Isobe, Y. Ueda, J. Non-Cryst. Solids 334–335 (2004) 173. [23] Y. Ishii, T. Fujiwara, J. Alloys Compd. 342 (2002) 343. [24] B.D. Biggs, S.J. Poon, N.R. Munirathnan, Phys. Rev. Lett. 65 (1990) 2700. [25] B.D. Biggs, Y. Li, S.J. Poon, Phys. Rev. Lett. 43 (1991) 8747. [26] A. Inaba, H. Takakura, A.P. Tsai, I.R. Fisher, P.C. Canfield, Mater. Sci. Eng. 294/296 (2000) 723.
Electrical properties of the binary icosahedral quasicrystal and its approximant in the Cd–Yb system R. Tamura a,∗ , Y. Murao a , S. Kishino a , S. Takeuchi a , K. Tokiwa b , T. Watanabe b a
Department of Materials Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan b Department of Applied Electronics, Tokyo University of Science, Noda, Chiba 278-8510, Japan
Abstract Electrical resistivity, magnetoresistance, and specific heat measurements have been performed for the binary icosahedral Cd5.7 Yb and its cubic approximant Cd6 Yb. The resistivity of both the icosahedral phase (i-phase) and the approximant is found to show a rather metallic behavior with a large positive temperature coefficient of the resistivity (TCR) below 200–300 K followed by leveling-off at low temperatures below 10 K. For the approximant, we observe a stepwise change of the resistivity at 110 K which is due to a phase transition. Giant magnetoresistance as high as 200% at 9 T below 4.2 K is observed for the i-phase and the electronic specific heat coefficient γ is found to be extraordinary large in both alloys, i.e., 2.87 mJ/(mole K2 ) for the i-phase and 7.60 mJ/(mole K2 ) for the approximant. Such large γ values may be attributed to the Yb-derived states at the Fermi level. Furthermore, the Debye temperatures (ΘD ) are very low and almost the same for both alloys, i.e., 142 and 144 K, respectively, which are indeed the lowest values ever reported in i-phases and approximants. The metallic character of the binary i-phase implies that the negative TCR is not necessarily a consequence of the quasiperiodicity. © 2003 Elsevier B.V. All rights reserved. Keywords: Quasicrystal; Approximant; Electrical resistivity; Magnetoresistance; Specific heat; Cadmium–ytterbium
1. Introduction The binary Cd5.7 Yb and Cd17 Ca3 intermetallic compounds [1] were recently found to be P-type icosahedral phases (i-phases) [2,3]. These i-phases are of particular interest since (i) they are binary stoichiometric compounds which are expected to contain less chemical disorder than ternary i-phases and (ii) there exists a cubic crystalline approximant in the very vicinity of the i-phase compositions. With respect to (i), no chemical disorder in the i-phases is also suggested from the structural model of the approximant Cd6 Yb [4], where there exist no sites occupied statistically by the different atomic species. From (ii) it has now become possible to investigate the influence of the long-periodicity, or quasiperiodicity, on the electronic transport in the binary alloy systems since these two phases possess a common local atomic structure. Up to now studies on ternary i-phases have shown that stable i-phases commonly possess a very high resistivity value and a large negative temperature coefficient of the resistiv-
ity (TCR) [5,6], implying that the high resistivity values and the negative TCR’s may be a consequence of the quasiperiodicity. The most striking is the metal-insulator transition observed in i-AlPdRe [7–10], which indicates the electronic states at the Fermi level (EF ) can be localized in a structurally ordered i-phase. However, until now the question as to the origin of such localized states, whether it is due to the long-range quasiperiodicity combined with the peculiar local atomic structure or to some kind of structural disorder such as topological or chemical one, has not been fully answered yet. In this paper, results on the electronic transport of i-Cd5.7 Yb and 1/1-Cd6 Yb including the electrical resistivity, magnetoresistance, and specific heat are reported and various anomalous behaviors of the binary i-phase as well as the binary approximant are clarified. The electronic transport of the quasicrystal is discussed in terms of the role of the chemical disorder in the electronic transport. A part of the results of the electronic transport measurements of i-Cd5.7 Yb was reported elsewhere [11]. 2. Experimental
∗
Corresponding author. Tel.: +81-471-24-1501; fax: +81-471-23-9362. E-mail address: [email protected] (R. Tamura). 0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.205
Pure elements of Cd (99.9999 wt.%) and Yb (99.9 wt.%)/ Ca (99 wt.%) with slightly different nominal compositions
R. Tamura et al. / Materials Science and Engineering A 375–377 (2004) 1002–1005
Cd100−X YbX with X in the range between 14 and 17 and Cd85.7 Ca14.3 , respectively, were wrapped tightly in molybdenum foil and melted in a quartz tube sealed under argon atmosphere. The ingots were subsequently annealed at 673 K for 24 h to obtain a homogeneous, equilibrium phase. The phase identification of the samples was performed by X-ray diffraction measurement with Cu K␣ radiation. The electrical resistivity was measured from 2 to 873 K by a four probe method. The magnetoresistance was measured in the field up to 9 T at temperatures 1.8, 4.2, 10, 20, and 50 K. The specific heat measurement was performed by an adiabatic relaxation method between 1.8 and 200 K.
3. Results and discussion Single i-phases are obtained at the compositions Cd100−X YbX with X in the range between 15.1 and 16.1, and a single approximant phase at Cd85.7 Yb14.3 and at Cd85.7 Ca14.3 , from powder X-ray diffraction measurements. Fig. 1a presents the temperature dependence of the resistivity ρ(T) normalized by ρ300 K from 2 to 300 K for Cd100−X YbX with X = 15.4, 15.7, 15.9, and 16.1, and Fig. 1b shows ρ(T) of the 1/1-Cd85.7 Yb14.3 together with that of i-Cd84.6 Yb15.4 for a comparison. The magnitude of ρ300 K is 560, 350, 240, and 490 cm, respectively, for the i-phases and 300 cm for the approximant. These resistivity values of the i-phases are rather low compared
Fig. 1. Temperature dependence of (a) the normalized resistivity ρ/ρ300 K for i-Cd100−X YbX with X = 15.4, 15.7, 15.9, and 16.1 and (b) temperature dependence of the resistivity ρ for 1/1-Cd85.7 Yb14.3 . In (b), ρ(T) of i-Cd84.6 Yb15.4 is also shown for a comparison.
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with those of Al-TM (transition metal)-based i-phases but are similar to those of ternary rare-earth (RE)-containing i-phases such as i-ZnMgRE (RE = Tb, Dy, Ho, and Er) [12]. We observe three distinct regions in the ρ(T) behavior of i-Cd–Yb with varying temperature. (1) TCR is negative above 200–300 K up to the melting point (not shown), as in the cases of ternary i-phases. (2) ρ(T) decreases sharply with decreasing temperature below 200–300 K exhibiting a broad maximum and the change of the resistivity ρ/ρ300 K is found to be considerably large ranging from 10 to 80 at.%. The variation of the resistivity is quite sensitive to the Yb concentration and large ρ/ρ300 K is observed for Yb-rich compositions, implying that Yb-derived states may play a significant role in the electronic transport. Such a large positive TCR has not been observed in ternary i-phases although a maximum in ρ(T) has been seen in i-AlPdMn [13] and i-AlCuFe [14]. (3) ρ(T) is found to become temperature-independent at lowest temperatures below 10 K as in the cases of normal metals with no indication of the quantum interference effects. Similar metallic behavior of i-Cd5.7 Yb and 1/1-Cd6 Yb at low temperatures has been also reported by Pope et al. [15] and Dhar et al. [16]. We note that a maximum in ρ(T) has been also observed of i-Cd5.7 Yb at around 80 and 50 K, respectively, in these papers. The behaviors (2) and (3) of i-Cd–Yb are quite unique among other stable i-phases and indicate that the negative TCR at low temperatures, often accompanying the quantum interference effects, may not be a consequence of the quasiperiodicity, whereas the negative TCR at high temperatures are confirmed to be a common property of i-phases and high-order approximants. On the other hand, TCR of the approximant is found to be positive in the whole temperature region up to the melting point, exhibiting essentially a metallic nature [17]. However, as seen from the figure the overall feature of ρ(T) of 1/1-Cd85.7 Yb14.3 below 300 K is rather similar to that of the i-phase. The resistivity decreases by 70% with decreasing temperature below 300 K. Thus, the low resistivity values and positive TCR of the i-phase are taken as indication that the binary i-phase is a rather good metal in contrast to other stable i-phases. Such a result is consistent with the sharp Fermi cut-off with considerably large density of states (DOS) at EF observed in the photoemission spectroscopy (PS) measurement [18]. The metallic nature observed in the binary i-phase suggests that the chemical disorder might be the origin for the QC-like behaviors such as large ρ and a negative TCR in some ternary i-phases. In addition, as seen from the figure, we observe a stepwise change of the ρ(T) at 110 K in 1/1-Cd85.7 Yb14.3 with a discontinuous shift of TCR across the step [16], which phenomenon has been ascribed to the occurrence of a phase transition, i.e., an order–disorder transition [22]. Then it is of interest that no sign of such a phase transition has been observed in ρ(T) of the i-phase. Fig. 2a and b present the results of the magnetoresistance for i-Cd84.6 Yb15.4 and i-Cd84.1 Yb15.9 , respectively.
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R. Tamura et al. / Materials Science and Engineering A 375–377 (2004) 1002–1005
Fig. 3. Specific heat data plotted as C/T vs. T2 for i-Cd84.6 Yb15.4 , 1/1Cd85.7 Yb14.3 , and 1/1-Cd85.7 Ca14.3 . Solid lines are fits by C/T = γ +βT 2 .
Fig. 2. Magnetoresistance ρ/ρ vs. magnetic field B at temperatures 1.8, 4.2, 10, 20, and 50 K for (a) i-Cd84.6 Yb15.4 and (b) i-Cd84.1 Yb15.9 .
The observed magnetoresistance is extraordinary large for the i-phase alloys despite their low resistivity values: ρ/ρ at 9 T below 4.2 K reaches about 20% for i-Cd84.6 Yb15.4 and, surprisingly, a giant magnetoresistance reaching as high as 200% is observed for i-Cd84.1 Yb15.9 . ρ/ρ of the present i-Cd84.1 Yb15.9 is even larger than those of the insulating i-AlPdRe [19] by more than a factor of two despite its low resistivity value, violating the empirical rule seen in other i-phases studied so far [6]. We also find that neither the large magnitude of ρ/ρ nor its field dependence ρ(H) fits into the quantum interference theories such as the weak localization [20] and the electron–electron interaction [21] theories, which is consistent with the metallic character of ρ(T) for i-Cd–Yb. Another striking feature concerning the magnetoresistance is the temperature-independent ρ/ρ at lowest temperatures below 4.2 K as clearly seen in the figures. The present result on ρ/ρ may be correlated with the temperature-independent ρ(T) below about 10 K. Fig. 3 presents the result of the specific heat measurement for i-Cd84.6 Yb15.4 , 1/1-Cd85.7 Yb14.3 , and 1/1-Cd85.7 Ca14.3 in the form of C/T versus T2 . Electronic specific heat coefficients γ are determined to be 2.87 and 7.60 mJ/(mole K2 ) for i-Cd84.6 Yb15.4 and 1/1-Cd85.7 Yb14.3 , respectively. Such large γ values for the quasicrystalline materials are indeed unexpected and are much larger that those seen in other stable i-phases, where γ generally falls into the range γ < 1.0 (typically 0.2–0.3) mJ/(mole K2 ).[6] Pope et al. [15] have reported a smaller γ value (1.1 mJ/(mole K2 )) for the
i-Cd5.7 Yb, whereas even larger γ values for i-Cd5.7 Yb and 1/1-Cd6 Yb, i.e., 7.5 and 51 mJ/(mole K2 ), respectively, have been reported by Dhar et al. [16]. The large γ values are normally attributed either to enhanced DOS at EF or to a large thermal mass of electrons. Recently, Ishii and Fujiwara have shown that the band contribution to γ in 1/1-Cd6 Yb is ∼0.8 mJ/(mole K2 ) [23], which is an order of magnitude lower than the experiment. Besides, the PS measurements [18] have clearly shown the existence of a pseudogap at EF in i-Cd–Yb and 1/1-Cd–Yb. Therefore, it is reasonable to consider that the observed large γ values are not due to large DOS at EF but to another factor. In Fig. 3, the specific heat of 1/1-Cd85.7 Ca14.3 , which is isostructural to 1/1-Cd85.7 Ca14.3 , is presented for a comparison. We find that the γ value of 1/1-Cd85.7 Ca14.3 is not as large, i.e., 2.0 mJ/(mole K2 ), and there is an appreciable difference in the values of γ between the two approximant phases of the same atomic structure, indicating that the extraordinary large γ values of the Cd–Yb compounds may be attributed to Yb-derived states at EF . For such Yb-derived states, Yb 4f states may be responsible for the large γ values, which is located in the vicinity of EF , i.e., 0.7 eV below EF [18]. We note that the position of the Yb 4f states is slightly shifted towards higher binding energies in the case of the i-Cd–Yb, which may account for the observed trend that the γ value is smaller in the i-phase than in the approximant. However, further investigation is certainly needed to clarify the origin of the large γ values. The Debye temperatures (ΘD ) are obtained as 142, 144 and 144 K for i-Cd84.6 Yb15.4 , 1/1-Cd85.7 Yb14.3 and 1/1-Cd85.7 Ca14.3 , respectively, from Fig. 3, which are in agreement with the values reported for i-Cd5.7 Yb and 1/1-Cd6 Yb by Dhar et al. [16], i.e., 138 and 144 K, respectively. Here we find that ΘD values are almost the same for these three alloys. We should say that the ΘD value of the i-phase is anomalously small, which is in striking contrast with those seen in other i-phases, where, for instance, ΘD is 500–600 K [24,25] and about 300 K [26] for stable Al-TM-based and Zn–Mg-based i-phases, respectively.
R. Tamura et al. / Materials Science and Engineering A 375–377 (2004) 1002–1005
ΘD ’s of these phases are even smaller than that of Cd metal and the weighted average of the constituent elements. The almost identical ΘD values of the three alloys are understood by their similar local environments for the Cd atoms.
4. Conclusions Anomalous transport properties of the Cd–Yb binary i-phase and its approximant are reported based on the results of the electrical resistivity, magnetoresistance, and specific heat measurements: the temperature dependence of the resistivity of the i-phase is found to show three distinct regions with changing temperature: a negative TCR above 300 K up to the melting point, a rapid decrease of ρ/ρ300 K by 10–80% with decreasing temperature below 200–300 K and the leveling-off at lowest temperatures below 10 K. On the other hand, the resistivity of the approximant is found to possess a positive TCR in the whole temperature region accompanying a stepwise change of ρ at 110 K due to a structural transition. In addition, unexpected giant magnetoresistance reaching 200%, much larger than the highest one obtained in ternary i-phases, is observed at low temperatures and its field dependence is almost linear. Moreover, the electronic specific heat coefficient is extremely large, i.e., 2.87 mJ/(mole K2 ) for the i-phase and 7.60 mJ/(mole K2 ) for the approximant, and the Debye temperatures are as small as 140 K. Finally, the metallic nature observed at low temperatures in i-Cd–Yb indicates that the negative TCR observed in some ternary i-phases might be due to chemical disorder.
Acknowledgements We thank Prof. K. Motoya of Tokyo University of Science for his help in the experiment. This work was supported in part by a Grant-in-Aid for Scientific Research (13750616) from Japan Society for Promotion of Science and also by Japan Science and Technology Corporation (CREST).
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