Origin of anisotropic nonmetallic transport in theAl80Cr15Fe5decagonal approximant

Abstract

We present a study of the anisotropic transport properties ͑electrical resistivity, thermoelectric power, Hall coefficient, and thermal conductivity͒ of a single-crystalline Al 80 Cr 15 Fe 5 complex metallic alloy that is an excellent approximant to the decagonal quasicrystal with six atomic layers in one periodic unit. Temperaturedependent electrical resistivity along the b and c crystalline directions shows a nonmetallic behavior with a broad maximum, whereas it shows a metallic positive temperature coefficient along the a direction perpendicular to the ͑b , c͒ atomic planes. Ab initio calculations of the electronic density of states reveal that the nonmetallic transport occurs in the presence of a high density of charge carriers. The very different temperature-dependent electrical resistivities along the three crystalline directions can all be treated within the same physical model of slow charge carriers due to weak dispersion of the electronic bands, where the increased electron-phonon scattering upon raising the temperature induces transition from dominant Boltzmann ͑metallic͒ to dominant non-Boltzmann ͑insulatinglike͒ regime. The temperature dependence of the resistivity is governed predominantly by the temperature dependence of the electronic diffusion constant D and the transition has no resemblance to the Anderson-type metal-to-insulator transition based on the gradual electron localization. Structural considerations of the Al 80 Cr 15 Fe 5 phase show that the anisotropy of the transport properties is a consequence of anisotropic atomic order on the scale of nearest-neighbor atoms, suggesting that the role of quasiperiodicity in the anisotropic transport of decagonal quasicrystals is marginal. We also present a relaxed version of the Al 4 ͑Cr, Fe͒ structural model by Deng et al.

Anisotropic electrical and thermal conductivities of the Al76Co22Ni2 decagonal approximant

“…1 It is interesting to note that the ratios of the conductivities along different crystalline directions vary little over the whole investigated temperature range 300-1.5 K (inset in Fig. 1 [2,5,8] we find that the conductivities of both compounds are about the same along the "periodic" direction with metallic behavior. Conductivity in monoclinic plane is somewhat higher than in quasicrystalline plane, but the overall ratio of in plane to out of plane resistivity is similar.…”

“…This compound is a monoclinic approximant of the decagonal phase with two atomic layers within one periodic unit along the stacking direction and a relatively small unit cell, comprising 32 atoms. Therefore the structure of Al 13Àx (Co 1Ày Ni y ) 4 is relatively simple as compared to the much higher complexity of the Al 4 (Cr, Fe) phase with 306 atoms/unit cell [2]. The measurements of the Al 76 Co 22 Ni 2 thus allow comparison of this simple decagonal approximant to the closely related d-Al--Ni--Co quasicrystal.…”

Anisotropic physical properties of the Taylor-phaseT-Al72.5Mn21.5Fe6.0com

“…The structural and chemical disorders in the Taylor phase due to the fractional and mixed occupancy of the lattice sites imply that the relaxation time between the scattering events is short, so that l = v ͑where v is the electron velocity͒ is small and the short mean-free-path limit is met, hence justifying the use of the quantum transport theory. This theory has recently been successfully applied to the Al 4 ͑Cr, Fe͒ decagonal approximant, 7,8 where the temperature-dependent resistivity exhibited a maximum.…”

“…6͒ decagonal approximants, also belonging to the Al 13 TM 4 class of intermetallics, but comprising four atomic layers within one periodic unit of Ϸ0.8 nm along the stacking direction and a larger unit cell containing 102 atoms. The third was the Al 4 ͑Cr, Fe͒ compound with composition Al 80 Cr 15 Fe 5 ,7,8 belonging to the class of orthorhombic Al 4 TM phases first described by Deng et al, 9 which are approximants to the decagonal phase with six atomic layers in a periodic unit of 1.25 nm and 306 atoms in the giant unit cell. Common to all these phases are strongly anisotropic magnetic, electrical, and thermal transport properties between the stacking and the in-plane directions, where the crystals show the highest conductivity for both the electricity and heat along the stacking direction ͑corresponding to the periodic tenfold direction in d-QCs͒, whereas the in-plane anisotropy is considerably smaller, yet significant.…”

Phonon-enhanced thermoelectric power of Y–Al–Ni–Co decagonal approximant

“…A consequence of the anisotropic structure are anisotropic magnetic and transport properties [1] (electrical resistivity, thermoelectric power, Hall coefficient, thermal conductivity), when measured along different crystalline directions. Here we report an experimental study of the anisotropic electrical resistivity and thermoelectric power of a complex metallic alloy Al 13Àx (Co 1Ày Ni y ) 4 [2], known also as the ypsilon--Al--Ni--Co phase (Y--Al--Ni--Co), which is a monoclinic approximant to the decagonal phase with two atomic layers within one periodic unit along the stacking direction.…”