Kaichi Saito

Al13M4-type structures and atomic models of their twins

Abstract From the structural analysis of Al 13 Fe 4 and Al 13 Co 4 phases as well as a new Al 13 (Pd, Fe) 4 phase with single-crystal X-ray diffraction, their structures were examined in terms of tilings of two types of pentagonal columns, which have similar atomic arrangements in the three phases. It has been found that the tilings of the pentagonal columns in these phases are characterized as arrangements of squashed hexagonal tiles formed by an edge-sharing linkage of the pentagonal columns, and that there is a rule for arrangements of the pentagonal columns forming the hexagonal tiles. It has been found from high-resolution electron microscopy observations that the structures of twins in the three phases can be interpreted by this topological rule for arrangements of the pentagonal columns.

Crystal structure of a new 1/1-rational approximant for the Al–Cu–Ru icosahedral phase

Abstract The structure of a cubic Al 57.3 Cu 31.4 Ru 11.3 phase, which is referred to as an 1/1-rational approximant of an Al–Cu–Ru icosahedral phase, was determined by means of a single crystal X-ray structural analysis: space group Pm 3 (No. 200), a =12.3773(8) A, V =1896.2(2) A 3 , MoKα ( λ =0.71073 A); atoms/cell=122.6, F (000)=2645, D calc =5.04 Mg/m 3 , R =0.0495 for the observed 1140 reflections with F obs. >4.0 σ ( F obs. ). Cu- and Ru-rich icosidodecahedral atom clusters are recognized at the 1 a and 1 b sites of the unit cell, respectively. The atom clusters possess Ru centers and the surrounding first shells of about 2.5 A in diameter show heavily disordered features with partial breakdown of the local icosahedral symmetry. These chemically and topologically decorated Mackay-like atom clusters should be classified as the fundamental structural units for the F-type Al–Cu–Ru icosahedral phase.

The crystal stucture of a gamma-(Al-Cu-Ru-Si) cubic phase: A new approximant phase for the Al-Cu-Ru icosahedral quasicrystal

The crystal structure of a gamma-(Al-Cu-Ru-Si) cubic phase (Al Cu Ru Si ), considered to be closely related to that of the Al-Cu55.1 14.6 20.2 10.1 Ru icosahedral quasicrystal, has been determined by single-crystal X-ray diffraction. The atomic model, refined to a final R value of 0.0346, has about 262 atoms with an atomic ratio of Al Cu Ru in a cubic unit cell with a 66.3 13.5 19.8 lattice constant a = 1.538 0(2) nm and the space group Fm 3. The fundamental structure is characterized by a sc arrangement of dodecahedral atom clusters with an interval of 0.77 nm, which corresponds to a lattice constant of a 1/0 cubic approximant phase for the icosahedral quasicrystals. The chemical ordering of the constituents in the dodecahedral cluster produces a superstructure with a lattice constant twice that of a 1/0 cubic approximant phase.

Real space observation of skyrmion polycrystallization and its domain boundary behavior in FeGe1-xSix

In magnetic skyrmion crystals (SkXs), the quality of skyrmion ordering affects the motion under electric current and temperature gradient. However, the crystalline nature itself remains an open issue. We report the observation of skyrmion polycrystallization in FeGe1−xSix as a function of x by Lorentz transmission electron microscopy. With increasing x, the SkX changes from single crystalline to polycrystalline, concomitantly with the formation of the interfaces referred to as domain boundaries. In situ observations have revealed that the domain boundary formation competes the tendency to accommodate the differences between orientations of the SkX.

Determination of Quasi-Crystallographic Orientations of Al–Pd–Mn Icosahedral Phase by Means of Light Figure Method

The light figure method has been utilized for the determination of quasi-crystallographic orientations of a bulk single quasi-crystal of Al–Pd–Mn icosahedral phase. Prior to the light figure experiment, anodic etching was carried out in a solution of CH3OH+HNO3 (volume ratio 3:1), which revealed the occurrence of well-faceted micropits consisting of etch pits and microvoids on specific surface areas, showing approximately twofold, threefold or fivefold symmetry. The facet planes of the etch pits as well as those of the microvoids, when irradiated by a narrow light beam passing through a hole in the center of a screen, reflected the beam effectively, thus giving rise to a well-defined light figure with a specific symmetry on the screen. It is demonstrated that the light figure method functions effectively for the identification of principal symmetry directions of the quasi-crystal.

Direct Observations of Aperiodic Arrangements of Transition-Metal Atoms in Al–Co–Ni Decagonal Quasicrystals by Cs-Corrected HAADF-STEM

HAADF (high-angle annular detector dark-field) images with Cs-corrected scanning transmission electron microscopy (STEM) have been observed for a W-(AlCoNi) crystalline phase and two-types of decagonal quasicrystals in Al71.5Co25.5Ni3 and Al72.5Co17.5Ni10 alloys. We have secured positive evidence that three-dimensional arrangements of transition-metal (TM) atoms of decagonal quasicrystals can be directly derived from the arrangements of bright dots in HAADF-STEM images, which correspond to individual TM atoms, by reference to results on HAADF-STEM observation and the structure of W-(AlNiCo) determined by X-ray diffraction analysis. We could conclude that pure TM atomic sites and mixed TM sites (with Al atoms) on A and B planes stacking along the periodic axis are located at the lattice points of a Penrose lattice with a bond length of 0.25 nm. In both planes atomic sites form pentagonal tilings with bond lengths of 0.47 nm and 0.76 (=0.47⋅τ) nm, respectively, in both the Al71.5Co25.5Ni3 and Al72.5Co17.5Ni10 decagonal quasicrystals, whose structures were formally characterized as rhombic and pentagonal tilings of atom columnar clusters with a bond length of 2 nm.

Morphological study of micropits formed by anodic etching of an Al-Pd-Mn icosahedral quasicrystal

Faceted micropits obtained by electrochemical etching of a single quasicrystal of an Al-Pd-Mn icosahedral phase have been examined by optical and electron microscopy. Anodic etching of the flat surface of a fivefold plane in a solution composed of CH3OH and HNO3(3:1 in volume ratio) reveals a variety of icosahedrally faceted micropits which originated from pre-existing microvoids and etch pits of pentagonal pyramid shape due to the other origins, probably structural defects inside the specimen. As the dissolution progresses, the micropits make successive changes in their shapes, ending in pentagonal pyramids before they vanish.

Local structures appearing in an intermediate state of transformation from Al–Pd decagonal to Al3Pd crystalline phases

Abstract The structures of an Al–Pd decagonal quasicrystal and an Al 3 Pd crystal are characterized as two-dimensionally quasiperiodic and periodic arrangements of the same columnar clusters of atoms, respectively. In order to study the transformation process from quasiperiodic to periodic arrangements of the atom clusters, melt-quenched Al 3 Pd alloys, annealed at various temperatures, have been examined by electron diffraction and high-resolution electron microscopy. In the intermediate state of the transformation, we have found locally periodic arrangements with special intervals, some of which are not in the Al 3 Pd crystal. Based on the periodic arrangements in the intermediate state of the transformation, metastable structures between the quasiperiodic to periodic arrangements of the atomic clusters have been discussed.

Dissolution patterns caused by etching of low-indexed surfaces of Al-Pd-Mn icosahedral and Al-Cu-Co decagonal quasicrystals

Abstract Dissolution patterns caused by anodic etching of an Al–Pd–Mn icosahedral quasicrystal and chemical etching of an Al–Cu–Co decagonal quasicrystal were examined by optical and electron microscopy as well as by atomic force microscopy. Appropriate etching treatments of these quasicrystals, i.e. the anodic etching of the Al–Pd–Mn quasicrystal in a solution of CH 3 OH–HNO 3 , and the chemical etching of the Al–Cu–Co quasicrystal in a solution of HF–HNO 3 –H 2 O resulted in occurrence of dissolution patterns composed of a number of well-facetted micropits (and hillocks for the Al–Cu–Co alloy as well) on low-indexed surfaces of the specimens. The morphological features of these etch-figures are discussed in relation to the internal structural symmetries.

Transmission Electron Microscopy and Atomic Force Microscopy Studies of an Al–Pd–Mn Thin Film Prepared by a Combined Technique of Vacuum Deposition and Thermal Annealing

A thin-film sample of Al–Pd–Mn ternary alloy has been produced by a vacuum deposition technique using dual evaporation sources, followed by thermal annealing. The microstructure of the film has been examined by transmission electron microscopy (TEM) and by atomic force microscopy (AFM). The TEM observations reveal that the film has a granular texture of highly ordered icosahedral phase islands embedded in the matrix consisting mainly of multiple-twinned crystals exhibiting a pseudo-tenfold symmetry. It is shown by AFM that the icosahedral islands form protuberant structures with the average height of approximately 50 nm.