Electrical properties of approximant phases in Al–Pd–(Fe, Ru) systems

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Abstract

X-ray diffraction patterns of structurally ordered 1/0 cubic approximant phases in the Al–Pd–(Fe, Ru) ternary systems are presented. The 1/0 approximant in the Al–Pd–Fe system was found to possess a body-centered lattice with a=15.3Å, whereas that in the Al–Pd–Ru system a face-centered one with a=15.5Å, and the results were compared with the reported structural models. Their electrical resistivities have been investigated and found to be substantially high for the lowest order approximants: ρ300K∼1000μΩcm. In addition, negative temperature coefficients accompanied by a distinctive resistivity maximum in the ρT curves have been observed. The peculiar ρT dependences will be described in terms of the quantum interference effects with strong spin–orbit scattering and it will be shown that the transport properties of 1/0 approximants are quite sensitive to disorder reflecting the flexibility of their structure.

Introduction

Crystalline approximants have been considered to provide an important clue to understand the anomalous transport of the icosahedral (i) phases and their ρT behaviors were classified according to the degree of approximation or the lattice parameters [1]. It has been found that high-order approximants with lattice parameters larger than 20Å always behave similarly to those of i-phases, implying that the electronic transport is essentially determined by the local atomic order of the length scale of 15–20 Å [2]. For such local atomic order, atomic clusters of less than 20Å diameter are considered to be of significant importance for the electronic states of high-order approximants and i-phases.

Generally, the resistivity and magnetoresistance of i-phases have been well described [3], [4], [5], [6], [7], [8], [9], [10] by quantum interference (QI) effects such as weak localization (WL) [11], [12] and electron–electron interaction (EEI) [12] effects except for the insulating Al–Pd–Re i-phase [13], [14], [15], [16]. The success of the theories naturally has led to a further question as to whether it is due to the quasiperiodicity or structural disorder existing in real i-phases. Recent single-crystal X-ray structural analyses on approximants have revealed a distinct feature of disorder phenomena inside atom clusters [17], [18]. Not only chemical disorder as implied by statistical occupancies of certain sites but also presence of considerable amounts of topological disorder have been inferred in the Al–Pd–TM alloys. Since the electronic states of i-phases and approximants seem mostly determined by the nature of certain atom clusters, disorder inside clusters is considered to affect seriously the transport properties of approximants.

In this paper, first, results of powder X-ray diffraction measurements are presented, comparing them with reported structure models available for Al–Pd–(Fe, Ru) systems. Second, we discuss resistivity behaviors of 1/0 cubic approximants in the Al–Pd–(Fe, Ru) systems in view of the approximation degree and of basic atom clusters related. Third, the temperature dependence of the resistivity for samples showing a maximum in the ρT curves is described by the WL theory in strong spin–orbit scattering regime and we discuss a disorder phenomenon in the electrical resistivity of the lowest order approximant phases.

Section snippets

Experimental

Pure Al, Pd, Fe or Ru elements were melted together in an arc furnace under Ar atmosphere. Thin ribbon samples were prepared by melt-spinning and subsequently annealed at 1173–1273 K for 1–24 h in Ar atmosphere. Phases of the samples were probed by powder X-ray diffraction measurement using Cu Kα radiation. Electrical resistivity was measured by the conventional four probe method in the temperature range 12–300 K.

X-ray diffraction measurement

Typical examples of powder X-ray diffraction patterns taken from Al70Pd20Fe10 (Fig. 1a and b) and Al68.5Pd20Ru11.5 (Fig. 1c). For the Al–Pd–Fe system, as shown in Fig. 1a and b, a single i-phase formed in the melt-spun state completely transforms into a 1/0 rational approximant phase upon heating to 1173 K for 1 h, as is evidenced by good agreement between the observed pattern and that of 1/0 cubic approximant phase calculated using the projection method in Fig. 1d. The same situation has

Conclusions

Quite high values of the resistivity up to ∼1000μΩcm and negative TCR as well as maxima in the ρT curves have been observed for well-ordered 1/0 approximants in the Al–Pd–(Fe, Ru) systems, which were annealed at high temperatures of 1173–1273 K for 1–24 h. Such large resistivities manifest peculiar electronic states of the approximants due to cluster aggregation and/or d-orbital resonances. The resistivity maxima in the ρT curves are attributed to the WL effect with strong spin–orbit

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