T. Ishimasa

Photoemission study of an Al-Cu-Fe icosahedral phase

Photoemission spectra of the Al-Cu-Fe quasicrystal with the icosahedral phase were studied at room temperature. The line shape of the overall observed spectrum is like a large peak on a plateau cut-off at EF. The large peak observed at about 4 eV below EF seems to originate mainly from the 3d state of Cu atoms. The bump was observed at about 1 eV below EF. The high resolution experiment showed an anomalous valley at EF in the electronic density of states. This means that the dip at EF in the DOS has a half-width 0.3-0.4 eV and that its depth is 70% of the value for a no-dip case.

Magnetic properties and the electronic structure of a stable Al-Cu-Fe icosahedral phase

The stable Al-Cu-Fe icosahedral phase showed almost no local magnetic moment at low temperatures, and a dependence of the magnetic susceptibility linear to the square of the temperature above about 150 K. The former is interpreted by the condition of the local magnetic moment formation according to the theory of Friedel and Anderson, and the latter affords strong experimental evidence for the band gap corresponding to the Brillouin zone inferred for strong diffraction spots. An experimental value of the mass density is obtained, in order to estimate the electron number density.

Phase transition and diffuse scattering studies in the Al-Cu-Fe ternary system

The reversible phase transformation between the crystalline multidomain structure of the rhombohedral phase to the perfect icosahedral quasicrystalline state at high temperature has been studied by in situ X-ray diffraction on a four-circle diffractometer using synchrotron radiation. The transient state known as the 5-fold modulated state has been observed and its evolution with temperature is characterised. Diffuse scattering located close to the Bragg reflections is observed in the studied temperature range. Its evolution with temperature shows a reversible transformation with an important change of shape in agreement with the proximity of a 5-fold phason instability.

High-temperature magnetic properties of a stable Al65Cu20Fe15 icosahedral phase

The field and temperature dependence of the magnetisation is studied in a stable Al-Cu-Fe icosahedral phase, in the temperature region between 77 and 1050 K and in magnetic fields up to 18 kOe. The sample shows a good quality of structural perfection in X-ray diffraction and electron microscope observations as compared with known icosahedral phases. The magnetic susceptibility shows an increase from -1.5*10-7 to 4.5*10-7 EMU g-1 with a rise in temperature from 150 to 1050 K, which has not been observed icosahedral phases studied to date.

Stability of the F2-(Al-Pd-Mn) phase

The stability of the F2-(Al-Pd-Mn) phase has been studied by in-situ neutron diffraction on a single quasicrystal with composition Al69.8Pd21.4Mn8.8. We find that the F2 phase is not stable and corresponds to a transient state in the process of the transformation of the icosahedral phase to the F2M phase. The icosahedral-to-F2Mphase transition occurs at around 715oC. In the F2 phase a large amount of diffuse scattering is located close to the icosahedral Bragg reflection in place of the S 1 superstructure reflections characteristic of the F2m phase.

Magnetic and electrical properties of the single-grained Al-Cu-Fe icosahedral phase

Magnetization of a single-grained quasi-crystal of nominal composition 66.32 at.% Al-20.41 at.% Cu-13.27 at.% Fe was investigated in the temperature range from 2 to 250 K and for magnetic fields below 5 T along fivefold, threefold and twofold axes. No anisotropy was found, which is consistent with the symmetry consideration based on the symmetry of the diffraction spots. Electrical conductivity along the twofold axis showed a minimum at around 20 K and a positive longitudinal magnetoresistance at low temperatures. The conductivity is discussed on the basis of the weak-localization theory.

Resonant photoemission study of the Al-Cu-Fe icosahedral phase

Photoemission spectra of the valence band in an Al-Cu-Fe icosahedral phase are studied at excitation photon energies near the Fe threshold. The constant-initial-state spectra in the binding energy region between 0-2 eV show a resonant characteristic. The spectrum has an asymmetric shape, in which the weak peak is on the lower photon energy side and the strong dip is on the higher side. Its resonant energy seems to be slightly smaller than the 3p3d core excitation energy of an Fe natural form. The Fe 3d bands are located in the region ranging from EF to 2-2.5 eV below it, with the maximum contribution at around 0.7 eV binding energy. The smaller binding energy of Fe 3d bands suggests that Fe atoms have small numbers of extra electrons transferred from the other elements. The smaller binding energy (corresponding exactly to Fe 3d states sitting just below EF) is consistent with Friedels suggestion that the stability of the icosahedral phase is enhanced by a crossing effect of the 3d states with a nearly free electron-like state below EF which enhances the energy gap of the Brillouin zone.

Phason diffuse scattering in the icosahedral quasicrystalline phases Zn–X–Sc, X = Co, Ag, Mg

The strong sharp Bragg reflections that occur in diffraction patterns of all real crystals are used by conventional X-ray crystallography to deduce the average repetitive arrangements of atoms or molecules. Diffuse scattering, on the other hand, contains information about the deviations from the average (i.e., different types of disorder) and gives structural information on a scale that goes beyond that of the average unit cell and extends over a range of 0.1nm-100nm. In many important materials, it is this extended range of structural information that is crucial in determining the unique or novel properties of the material, rather than the average unit cell structure. We review the development of our diffuse scattering methodology using a variety of examples taken from different fields to illustrate the kind of information that can be obtained by current day methods information that is simply not available from the Bragg experiment. Amongst these will be the zeolite mordenite, the organic pen tach loroni t robenzene , which i s one of a se r ies o f chloronitrobenzene compounds whose anomalous dielectric properties are thought to be related to their disorder, the relaxor ferroelectric PZN and finally examples of organic molecular crystals that exhibit polymorphism a topic of crucial interest to the pharmaceutical industry. We gratefully acknowledge the support of the Australian Research Council, the Australian Synchrotron Research Program, the Access to Major Research Facilities Program and the Australian Partnership for Advanced Computing.