S. Takeuchi

Toward Insulating Quasicrystalline Alloy in Al-Pd-Re Icosahedral Phase

The resistivities in Al70Pd20Re10 alloy of the icosahedral phase are quite sample dependent, possibly due to slight variation of the alloy composition. In some samples, the resistivity at room temperature is ~0.01 Ωcm and that at 4.2 K is as high as ~0.1 Ωcm; these values are much higher than any values reported so far for quasicrystals. The temperature dependence of the conductivity is almost linear above 4.2 K, and the magnetoconductivity is negative. Such dependence is interpreted by the theory of weak localization which takes into account the spin-orbit scattering.

Electrical Resistivities of Al–Pd–Mn Icosahedral Quasicrystals

The resistivity of Al-Pd-Mn icosahedral quasicrystals increases almost linearly with decreasing temperature, shows a maximum at a temperature (between 10 and 100 K) and then decreases in low Mn alloys ( 9at%Mn) the resistivity increases again steeply at even lower temperatures. The resistivity at helium temperature varies from 1500 to 9500 µΩcm depending on the composition and quasicrystallinity of the sample. The magnetoresistance in single-grained sample of Al 70 Pd 21 Mn 9 is positive and temperature dependent, with Δρ/ρ at 0.5 K at 8 T being as large as 6%. It is concluded that the temperature and magnetic-field dependences are governed dominantly by the weak localization effect for the case of strong spin-orbit scattering, and in high Mn concentration alloys, a Kondo effect superimposes to the weak localization effect.

Icosahedral and approximant phases in a Mg–Ga–Al–Zn alloy and their electrical resistivities

Abstract It has been found in a Mg Ga-Al-Zn alloy that an icosahedral phase produced by melt-spinning transforms to the 1/1 cubic approximant of the icosahedral phase at 653 K and successively to the 2/1 cubic approximant at around 668 K upon heating. Structural relations of the two approximant phases to the icosahedral phase are discussed, in the framework of the projection scheme, on the basis of X-ray diffraction data. Electrical resistivities have been measured from 4·2 K to 280 K for single-phase samples of the icosahedral and approximant phases. The icosahedral phase and the 2/1 approximant one show similar behaviour, in which the resisitivity is ∼ 120 μω2 cm at 4-2 K and the temperature coefficient is negative. In contrast, the 1/1 approximant phase shows a relatively smali resistivity value of ∼ 60 μω cm at 4-2 K and a positive temperature coefficient. Changes in the resistivity accompanied by the phase transformations have been confirmed by measuring the resistivity upon heating the melt-spun sam...

Electrical resistivity of single-grained quasicrystals

Abstract Temperature and magnetic-field dependences of electrical resistivity have been measured at low temperatures for single-grained icosahedral quasicrystals of Al 6 Li 4 Cu, Al 65 Cu 25 Fe 10 and Al 72 Pd 20 Mn 8 , and a decagonal quasicrystal of Al 70 Ni 15 Co 15 . Except for the periodic direction in the decagonal quasicrystal where the resistivity is ≈50 μΩ cm, the resistivity ranges from 500 to 10 000 μΩ cm and increases with decreasing temperature with a maximum between 20 and 70 K, with the exception of the AlLiCu alloy. Magnetoresistance at helium temperaturesis positive and large (Δρ/ρ ≑ 5% at 8 T, 2 K) in AlPdMn, very small in AlNiCo and negative in AlLiCu. These results are discussed in terms of the theory of weak localisation that takes into account the spin-orbit scattering of electrons.

New Stable Icosahedral Quasicrystal in Mg-Pd-Al System

A thermodynamically stable icosahedral phase (I-phase) has been found to form in Mg-Pd-Al alloys in the compositional range 13 to 15 at%Pd and 42 to 44 at%Al. The X-ray diffraction measurements have shown that the I-phase is of the Frank-Kasper type (FK-type), as in Al-Li-Cu and Mg-Ga-Zn alloys. The stability of the I-phase has been confirmed by the result that it shows no transformation by annealing for 40 h at temperatures just below the melting point. The electrons per atom ratio (e/a) for the Mg-Pd-Al I-phase is ~2.15, which is nearly equal to those for other stable FK-type I-phases. This result suggests that the energy-band structure plays a dominant role in the formation of I-phases.

Computer experiment on the electron-irradiation-induced crystalline-amorphous transition

Abstract By introducing vacancy-interstitial pairs, both f.c.c. and b.c.c. model crystals constructed by computer have been transformed to an amorphous structure in accordance with electron-irradiation experiments. The crystalline-amorphous (C-A) transition occurs only gradually, without being affected by the presence of an amorphous nucleus and is completed at 0·6 to 1·0 displacement per atom, much larger than the experimental estimates. These results suggest that the apparently sharp C-A transition which is experimentally observed is brought about by an accelerated damage rate accompanying disordering, rather than by an ordinary nucleation and growth process of a new phase. The tendency towards amorphization is discussed in terms of disordering energy.

Atomic packing geometries and lattice properties of a model decagonal phase

Abstract A model binary decagonal phase and the monoclinic Al13Fe4 phase, a crystal approximant to the decagonal phase, have been produced by using interatomic potentials. Voronoi polyhedra analysis of the structures has shown that the atomic packing geometries of the two phases are similar: (1) the most abundant edge number of the faces of the Voronoi polyhedra is 5, (2) the distributions of Voronoi volumes are separated into large and small atom sites and (3) large and small atoms have large and small coordination numbers respectively. Tetrahedra-octahedra packing analysis has shown that the atomic packing characteristics of the decagonal phase are between the two types of icosahedral phases previously studied. Vibrational density of states spectra computed for the decagonal phase have revealed no distinct anisotropy between the tenfold direction and the direction perpendicular to it.

Approximant crystals of icosahedral Mg–(Ga, Al)–Zn alloys

Abstract It has been found in Mg39.5(Ga1−xAlx)20.5Zn40.0 alloys that an icosahedral phase produced by melt-spinning transforms to rational approximant phases of the icosahedral quasicrystal on heating. In the alloy with the composition x = 0.2, the icosahedral phase transforms to a 3/2–2/1–2/1 orthorhombic approximant phase with a = 3.693 nm, b = 2.296 nm and c = 2.268 nm. In the alloy with x = 0.7, the icosahedral phase transforms to a 1/1 cubic approximant phase with a = 1.421 nm at 653 K and successively to a 2/1 cubic approximant with a = 2.300 nm at around 668 K on heating. Structural relations of the approximant phases to the icosahedral phase are discussed, in the framework of the projection scheme, on the basis of X-ray diffraction data.

Effects of plastic deformation on the electrical conductivity of SmS single crystals

Abstract Single crystals of semiconducting SmS, grown by a Bridgman method, have been compressed at room temperature by a few per cent, and the effects of deformation on the electrical conductivity have been investigated between room and liquid-helium temperatures. For deformed crystals, the conductivity measured below 20 K in a direction parallel to the edge dislocations is orders of magnitude larger than that of the undeformed crystals. This increase is interpreted in terms of the existence of a metallic region along the core of the edge dislocation. The activation energy of the conductivity at around liquid-nitrogen temperature exhibits an anisotropy after deformation, which is explained by segregation of donor Sm interstitials in the strain field of edge dislocations.