Michiyoshi Tanaka

Highly ordered structure of icosahedral quasicrystals in Zn-Mg-RE (RE ≡ rare earth metals) systems

Abstract A new group of stable icosahedral phases (i-phases) in the Zn-Mg-RE system were found to have an ideal composition close to Zn50 Mg42RE8 (RE ≡ Y, Gd, Tb, Dy, Ho or Er). The new i-phases exhibit a highly ordered and nearly perfect face-centred icosahedral lattice as revealed by electron and X-ray diffraction and high-resolution electron microscopy. Powder X-ray diffraction indicates that the i-phase has long-range structural order with a correlation length over 1000 A, which is the largest found in the Frank-Kasper group. These new i-phases have the common valence concentration (about 2.08) and could be regarded as a new class of Hume-Rothery alloys.

Electron Optical Studies of Barium Titanate Single Crystal Films

Thin single crystal films of barium titanate prepared by chemical thinning of single crystal plates are studied by transmission electron microscopy and electron diffraction. The films as thin as 1000 A are found to have the same lattice parameters as the bulk crystals, giving no evidence for the existence of the anomalous surface layer proposed by Kanzig et al. The surface of barium titanate is found to change substantially when heated above 500°C in air and vacuo, invalidating their evidence for an unusually high Curie temperature of their anomalous surface layer. Ferroelectric 90° and 180° domains are observed by electron microscopy. The contrast of 90° domains is simple diffraction contrast. The contrast of 180° domains is due to the failure of Friedels law, which is known to be possible to take place in electron diffraction by dynamical diffraction effect. Dislocations, tracks of fission fragments from uranium and their interaction with domains are observed.

Production of zigzag-type BN nanotubes and BN cones by thermal annealing

Abstract Boron nitride nanotubes (BNT) and boron nitride cones (BN cones) were produced by thermal annealing of a mixed powder of β-rhombohedral boron and hexagonal boron nitride (h-BN) at 1200°C under lithium vapor. The BNTs were found to grow from the edge of h-BN particles along one of 〈10 1 0〉 directions of the particles. The BNTs consist of zigzag-type layers whose 〈10 1 0〉 direction is parallel to the tube axis. The BN cones with an apex angle of about 20° were discovered. It was revealed that the BN cones were not formed by a helically wound conical BN layer but by a pile of monolayer BN cones.

Refinement of crystal structural parameters using two-dimensional energy-filtered CBED patterns.

A new method to refine crystal structural parameters using convergent-beam electron diffraction (CBED), which is applicable to nanometre-size crystal structure analysis, is proposed. This method is based on the fitting between theoretical calculations and experimental intensities of energy-filtered two-dimensional CBED patterns containing higher-order Laue-zone (HOLZ) reflections. The use of HOLZ reflections is essential for the method because small displacements of atoms can be sensitively detected using HOLZ reflections with large reciprocal vectors. For this purpose, a new Omega-filter transmission microscope (JEM-2010FEF), which can take energy-filtered CBED patterns up to a high angle with a small distortion, and a new analysis program to refine structural parameters, which is based on many-beam Bloch-wave calculations and nonlinear least-squares fitting, have been developed. As a test example, a positional parameter and isotropic and anisotropic Debye-Waller factors of CdS have been refined. Two-dimensional CBED patterns calculated with the refined parameters show very good agreement with the experimental ones, and the refined values of the parameters also agree well with the result of a single-crystal X-ray diffraction experiment. Important problems of the analysis procedure are discussed item by item.

Electron Energy-Loss Spectra of Carbon Nanotubes

Carbon nanotubes were investigated by means of electron energy-loss spectroscopy. Two peaks due to the π plasmon and the π+σ plasmon were observed. The energy of the π+σ plasmon peaks varied from 22.0 eV to 24.5 eV, which roughly agrees with the average plasmon energy of graphite. A shoulder due to single electron excitations was observed at 13 eV, which was not observed in graphite. There were two kinds of nanotubes which exhibited their respective π plasmon peaks at 5.2 eV and 6.4 eV. The peaks in the dielectric function obtained by Kramers-Kronig analysis of the spectra were broader than those of graphite probably due to the curving of the graphitic sheets.

Point-group determination by convergent-beam electron diffraction

The method of point-group determination from convergent-beam electron diffraction patterns has been established by Buxton, Eades, Steeds & Rackham [Philos. Trans. R. Soc. London (1976), 281, 171-194]. However, Table 2 given by them is inconvenient for practical purposes, since many symmetries of the dark-field and ±G dark-field patterns are not given and are left for the readers consideration. The table is improved and completed with the help of some new symmetry symbols and illustration of symmetries. The new table makes the point-group determination easy and quick. The symmetries of the symmetrical many- beam convergent-beam electron diffraction patterns have been studied by Tinnappel [PhD Thesis (1975), Tech. Univ. Berlin] using group theory. It is shown that the graphical method used by Buxton et al. can reveal the symmetries of these patterns. A method of point-group determination which uses three types of symmetrical many-beam patterns, the hexagonal six- beam, square four-beam and rectangular four-beam patterns, is described. This method requires only one photograph in determining most diffraction groups. This fact means that the method is more convenient and reliable than that of Buxton et al., since their method requires two or three photographs for most cases. Experimental results which verify the theoretical ones are given. The characteristic features of the symmetrical many-beam method are discussed.

Structural Study of an Al 72Ni 20Co 8 Decagonal Quasicrystal Using the High-Angle Annular Dark-Field Method

High-angle annular dark-field (HAADF) images of an Al72Ni20Co8 decagonal quasicrystal have been taken for the first time. It has been found that the transition metals form pentagonal and star-shaped cluster columns with radii of about 0.4 nm. The atom-cluster columns with a 2 nm diameter of the alloy, which have been considered as the basic structural unit with decagonal symmetry so far, have been revealed to have no decagonal symmetry. These atom-cluster columns have been found to be decomposed into the pentagonal and star-shaped cluster columns, corresponding clusters existing in the Al13Fe4 monoclinic approximant. The tiling of the pentagonal and star-shaped clusters has been found to agree with the rhombic Penrose lattice for an area with a diameter greater than 50 nm.

Electron Microscopic Studies on Domain Structure of PbZrO3

The domain structure of a typical antiferroelectric substance, lead zirconate, is examined in detail fori the first time by electron diffraction and electron microscopy. The 90? and 60? domain configurations can be consistently understood on the basis of the pseudo-cubic unit cell. 180? domains, which are characteristic of the antiferroelectric phase, are observed. The displacement vectors between the domains are determined as ?[21n], ?[2n] and ?[02n] (n=0 or 2) by the ?-fringe theory for off-Bragg settings. Crystal structure images of the domains reveal how the unit cells connect across the domain boundary. 90? and 180? domains are found in the intermediate phase between the antiferroelectric and the paraelectric phases. The fact that 180? domains are observed is important evidence indicating that the phase is ferroelectric.

Electron energy-loss spectra of single-shell carbon nanotubes

Transmission electron energy-loss spectra of single-shell carbon nanotubes were measured and compared with those of multishell carbon nanotubes and graphite. Two peaks due to the π plasmon and the π+σ plasmon were observed at 5.8 eV and 20.6 eV, respectively. The energy of the π plasmon takes a value between those of two kinds of multishell tubes. The energy of the π+σ plasmon is lower than those of multishell tubes and graphite by 2 eV and 6 eV, respectively. The 1s→π* and 1s→σ* transition peaks of the single-shell tubes are much broader than those of the multishell tubes and graphite. The reason for the broadening may be due to the strong curving of the graphitic sheets.

Crystal Chirality and Helicity of the Helical Spin Density Wave in MnSi. II. Polarized Neutron Diffraction

Recent theoretical and experimental studies have shown the single handed helicity of the helical spin density wave in MnSi. In order to determine unambiguously the sense of the helicity we have carried out detailed experiments using pulsed polarized neutrons. Combining the result with that of the experiment on the crystal chirality, we have determined the left-handed or anticlockwise helicity for the left-handed chirality of the crystal symmetry. This leads to the positive sign of the antisymmetric exchange interaction in the left-handed chirality.