Keiichi Edagawa

New type of decagonal quasicrystal with superlattice order in Al–Ni–Co alloy

Abstract A new type of decagonal phase having superlattice order has been found in an AI70Ni15Co15 alloy by means of the electron diffraction technique. Most of the grains in the Al70Ni15Co15 sample heat treated at 600°C for 216h exhibit superlattice order. In contrast, superlattice order has not been seen for the Al65Cu20Co15 decagonal phase. The relation between the structures with and without superlattice order is discussed using a four-dimensional description of the decagonal quasicrystalline structure.

Order-disorder transformation in an Al[sbnd]Ni[sbnd]Co decagonal qnasicrystal

Abstract The temperature dependence of the degree of order has been studied by X-ray diffractometry for the superlattice-type decagonal quasicrystal recently found in the Al-Ni-Co alloy system. On raising the temperature, all the superlattice peaks start to lose intensity at about 750[ddot]C, above which two different behaviours have been observed, the superlattice peaks of one group keep about 40% of the original intensity at 850[ddot]C, while those of the other vanish almost completely at the same temperature. The appearance of the two different behaviours of the superlattice peaks is attributable to the existence of two independent order parameters in this system. The superlattice peaks regain the lost intensity by subsequent low-temperature annealing, indicating that the intensity changes are reversible. A possibility of realization of a novel type of ordering is suggested for the present superlattice decagonal quasicrystal.

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.

Al?Pd?Re Icosahedral Quasicrystals and Their Low Electrical Conductivities

In the Al–Pd–Re system, two kinds of icosahedral phases (I-phases) with different quasilattice constants have been found to form at 1173 K. The I-phase with larger quasilattice constant is formed in a wide composition range at 1173 K: 13–25 at.%Pd and 10–15 at.%Re. The conductivity of single-I-phase samples varies greatly with the alloy composition and also from sample to sample for the same nominal composition, and is generally quite low; in some samples of Al70Pd20Re10 it is 0.5 Ω-1cm-1 at 4.2 K, the lowest value ever reported for quasicrystals. The magnetoconductivity at helium temperatures changes systematically with the conductivity value of the sample; it is negative at low temperature with its magnitude decreasing with decreasing conductivity and becomes positive for the lowest conductivity sample. The conductivity behavior cannot be interpreted either in the framework of the weak localization or in that of conventional variable range hopping.

Production of High-Quality Thin-Film Samples of Al-Cu-Fe Icosahedral Quasicrystal

Thin-film samples of Al-Cu-Fe icosahedral quasicrystal have been successfully produced by vacuum deposition with an electron beam evaporation apparatus followed by annealing. Coating the film surfaces with alumina has been found to be essential in avoiding compositional change during annealing. X-ray diffractometry and electrical resistivity measurements have shown that the annealed sample having the icosahedral phase is of high quality.

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...

Phonon–phason coupling in decagonal quasicrystals

Phonon–phason coupling in decagonal quasicrystals has been discussed in terms of phonon strains induced in crystal approximants. Elastically, decagonal quasicrystals can be classified into two groups having different symmetries in the coupling constant tensor; one has only one independent coupling constant, K 4, and the other has two such constants, K 4 and K 5. It has been shown that the orthorhombic and monoclinic approximants formed in various alloy systems can be interpreted as resulting from the two different types of decagonal quasicrystals and that a slight distortion in the unit cell of the monoclinic phases is attributable to the effect of one of the coupling constants, K 5. Phonon strains induced in monoclinic approximants of the Al–Co system have been calculated from their lattice constants and the coupling constants K 4 and K 5 have been evaluated.

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.