Biomedical subjects

Akiji Yamamoto

Publications and source records attributed to Akiji Yamamoto.

6 recordsLinked to original sources

PubMed2006

Atomic structure of the binary icosahedral Yb-Cd quasicrystal.

Icosahedral quasicrystals (i-QCs) are long-range ordered solids that show non-crystallographic symmetries such as five-fold rotations. Their detailed atomic structures are still far from completely understood, because most stable i-QCs form as ternary alloys suffering from chemical disorder. Here, we present the first detailed structure solution of i-YbCd(5.7), one of the very few stable binary i-QCs, by means of X-ray structure determination. Three building units with unique atomic decorations arrange quasiperiodically and fill the space. These also serve as building units in the periodic approximant crystals. The structure is not only chemically feasible, but also provides a seamless structural understanding of the i-YbCd(5.7) phase and its series of related i-QCs and approximant crystals, revealing hierarchic features that are of considerable physical interest.

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PubMed2006

Superspace description of the homologous series Ga4Tim-4O2m-2 with the crystallographic shear structure based on that of rutile.

A generalized approach is proposed for the structure description of the homologous series Ga(4)Ti(m-4)O(2m-2) with crystallographic shear (CS) structures based on that of rutile. A (3 + 1)-dimensional model for an ideal CS structure is built up in connection with the CS operation in three-dimensional space. Its structural parameters are determined by m, the shear plane and the shear vector. The Rietveld fitting of the X-ray diffraction profile of Ga(4)Ti(13)O(32) (m = 17) was successfully carried out using the ideal CS structure as an initial model, where modulation functions of atomic positions are inherently discontinuous and sawtooth-like. The deviation of a real structure from the ideal one was described by a few additional Fourier terms. This method was confirmed to be efficient for reducing the number of structural parameters required in the refinement in comparison to the conventional three-dimensional description.

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PubMed2005

Incommensurate crystallographic shear structure of BaxBi(2-2x)Ti(4-x)O(11-4x) (x = 0.275).

The title compound generates diffraction patterns which are indexable within the framework of the higher-dimensional description of incommensurate structures. However, it is difficult to discriminate the main reflections from the satellite ones. This paper has clarified that the structure can be treated as a strongly modulated structure with sawtooth-like modulation functions and is classified as an incommensurate crystallographic shear (CS) structure. The structure consists of domains isostructural to beta-Bi(2)Ti(4)O(11) and domain boundaries composed of TiO(6) octahedra. Ba and Bi ions are accommodated in the cavities between TiO(6) octahedra in the domain. Domain boundaries are aperiodically inserted, in contrast to the usual CS structures, forming an incommensurate structure.

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PubMed2005

Generalized quasiperiodic patterns and superstructures in quasicrystals.

Patterns with octagonal and icosahedral symmetries obtained from periodic two-dimensional (2D) 8-grids and three-dimensional (3D) 12-grids by the dual method are shown to be superstructures of the Beenker pattern and the 3D Penrose pattern with the same edge length. The superstructures are described in the same spaces as those of the latter (4D for the Beenker and 6D for the 3D Penrose patterns) in the section method but their lattice constants are doubled. The occupation domains and the diffraction patterns for these cases are given.

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PubMed2004

Section method for projected structures of icosahedral quasicrystals and its application to electron-microscopy-image and surface analyses.

A section method for projected structures of icosahedral quasicrystals (IQCs) is given. A structure projected along a specified direction can be calculated directly from a six-dimensional periodic structure by this method. The method concludes that all peaks in high-resolution transmission electron-microscopy images of an IQC have different projected atom densities in general and leads to different chemical compositions and densities for all atom layers, suggesting that all surfaces of an IQC are different. Its application to icosahedral Al-Pd-Mn quasicrystals is shown.

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PubMed2004

A five-dimensional model of dodecagonal Ta-Te quasicrystals with fractal occupation domains.

The consideration of two approximants of the first stable dodecagonal Ta-Te quasicrystal shows that the five-dimensional (5D) space group of the dodecagonal phase is P12m2(12(5)mm) and the structure consists of five layers within a 10 A period. A 5D quasicrystal model leading to the two approximate structures by the introduction of appropriate linear phason strains is derived by the section method. The model shows 20 A dodecagonal clusters (DDC) arranged at each vertex of the square-triangle dodecagonal pattern and has fractal occupation domains, suggesting that the quasicrystal has either a deterministic or a random square-triangle packing of the DDC.

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