Hiroyuki Takakura

A stable binary quasicrystal.

All stable quasicrystals known so far are composed of at least three metallic elements. Sixteen years after the discovery of the quasicrystal, we describe a stable binary quasicrystalline alloy in a cadmium–ytterbium (Cd–Yb) system. The structure of this alloy represents a new class of packing of 66-atom icosahedral clusters whose internal structure breaks the icosahedral symmetry. The binary quasicrystal offers a new opportunity to investigate the relation between thermodynamic stability and quasiperiodic structure, as well as providing a basis for the construction of crystallographic models.

Quasi-Unit-Cell Model for an Al-Ni-Co Ideal Quasicrystal based on Clusters with Broken Tenfold Symmetry

We present new evidence supporting the quasi-unit-cell description of the Al72Ni20Co8 decagonal quasicrystal which shows that the solid is composed of repeating, overlapping decagonal cluster columns with broken tenfold symmetry. We propose an atomic model which gives a significantly improved fit to electron microscopy experiments compared to a previous proposal by us and to alternative proposals with tenfold symmetric clusters.

The structure of a decagonal Al72Ni20Co8 quasicrystal.

The structure of a decagonal Al72Ni20Co8 quasicrystal with space group P10(5)/mmc has been determined on the basis of a single-crystal X-ray data set using the five-dimensional description. The best-fit model structure based on a cluster model having lower symmetry than the decagonal symmetry with 103 parameters gives wR = 0.045 and R = 0.063 for 449 reflections. The structure is well described by the hexagon, boat and star tiling with an edge length of 6.36 A and is very consistent with recent high-resolution electron-microscopy images. The refined structure is compared with previously discussed model structures including cluster-based models having 20 A tenfold symmetric clusters.

Quantum critical state in a magnetic quasicrystal

Quasicrystals are metallic alloys that possess long-range, aperiodic structures with diffraction symmetries forbidden to conventional crystals. Since the discovery of quasicrystals by Schechtman et al. in 1984, there has been considerable progress in resolving their geometric structure. For example, it is well known that the golden ratio of mathematics and art occurs over and over again in their crystal structure. However, the characteristic properties of the electronic states--whether they are extended as in periodic crystals or localized as in amorphous materials--are still unresolved. Here we report the first observation of quantum (T = 0) critical phenomena of the Au-Al-Yb quasicrystal--the magnetic susceptibility and the electronic specific heat coefficient arising from strongly correlated 4f electrons of the Yb atoms diverge as T→0. Furthermore, we observe that this quantum critical phenomenon is robust against hydrostatic pressure. By contrast, there is no such divergence in a crystalline approximant, a phase whose composition is close to that of the quasicrystal and whose unit cell has atomic decorations (that is, icosahedral clusters of atoms) that look like the quasicrystal. These results clearly indicate that the quantum criticality is associated with the unique electronic state of the quasicrystal, that is, a spatially confined critical state. Finally we discuss the possibility that there is a general law underlying the conventional crystals and the quasicrystals.

Crystal and quasicrystal structures in Cd?Yb and Cd?Ca binary alloys

We show that the structures of cubic Cd6Yb and Cd6Ca crystals are closely related to those of recently found stable icosahedral Cd-Yb and Cd-Ca alloys. These structures are described as a periodic packing of rhombohedra having different atomic decoration in terms of three-dimensional Penrose tiling. A new type of icosahedral cluster composed of 66 atoms is found. The innermost shell of the cluster consisting of four Cd atoms breaks its entire icosahedral symmetry.

Crystal structure of a hexagonal phase and its relation to a quasicrystalline phase in Zn-Mg-Y alloy

The crystal structure of a Zn-Mg-Y hexagonal phase, considered to be related to that of the quasicrystalline phase, has been determined by single-crystal X-ray diffraction. The atomic model, refine...

Hexagonal superstructures in the Zn–Mg–rare-earth alloys

We show that three types of hexagonal Zn–Mg–rare-earth (RE) ternary compound are formed in the vicinity of Zn6Mg3RE1. An electron diffraction study revealed that these phases are related with a lattice parameters by a ratio 3:5:7 but with approximately the same c parameters. A preliminary structural model for the phase with medium a parameter has been proposed by high-resolution electron microscopy, based on the structures of the other two phases recently determined. It is shown that the two-dimensional hexagonal superstructures can be described by two kinds of Zn–Mg clusters A and B, linked by RE atoms.

Production of single quasicrystals and their electrical resistivity in the Al-Pd-Re system

Single Al-Pd-Re icosahedral quasicrystals with a maximum diameter of 5mm have been grown by a slow cooling method on the basis of a partial phase diagram determined in the present study. Laue X-ray and electron diffraction verified the highly ordered structure of the single icosahedral quasicrystals. The electrical resistivity rho of the single quasicrystals was measured to be 2000- 4000muOmegacm at 300K and 3000-6000muOmegacm at 2K, revealing a negative temperature dependence with a rho4.2K/rho300K value smaller than 2.

Single Crystal Growth of the Icosahedral Zn-Mg-Ho Quasicrystal

Single crystal growth of the icosahedral Zn-Mg-Ho quasicrystal has been performed using the Bridgeman method. Composition of the starting alloy was selected as Zn46Mg51Ho3, so that the icosahedral phase directly precipitates from the liquidus phase. A large single quasicrystal about 0.5 cm3 in size was obtained, and its composition was determined as Zn60Mg31Ho9. The obtained quasicrystal shows an extremely high structural-perfection evidenced by the X-ray Laue reflection and neutron diffraction methods.

Phason fluctuations in i-AlPdRe and i-CdYb phases

Abstract We present a high resolution X-ray diffraction study on two single grain phases: the i-AlPdRe phase which is supposed to be isostructural to the i-AlPdMn one and the new binary CdYb phase. Both the Bragg peak width (sensitive to uniform phason strain distribution) and the diffuse scattering (sensitive to long wavelength phason fluctuations) have been studied. In the i-AlPdRe phase, we find an anisotropic distribution of diffuse scattering intensity around the Bragg peaks. Its intensity is related to Qper, demonstrating its relation to phason fluctuations. The amount of diffuse scattering is similar to what was previously found in the AlPdMn phase, but the shape anisotropy is markedly different. The i-CdYb phase presents a good structural quality. Its Bragg reflection shows an isotropic broadening which scales with Qper, as a consequence of uniform phason strain distribution with a magnitude similar to what was found in the i-ZnMgRe phases. There is also a clear anisotropic distribution of diffuse scattering intensity located around the Bragg peaks. It is related to long wavelength phason fluctuations with an anisotropy pointing toward a threefold type instability.