Study Of The Structure Of Icosahedral AlPdMn Quasicrystal By Means Of Quantitative Convergent Beam Electron Diffraction

In this thesis, the shape and volume of the atomic surfaces of icosahedral AlPdMn quasicrystal are refined by using the quantitative convergent beam electron diffraction technique in combination with describing the shape of atomic surfaces with symmetry-adapted functions. A modified non-spherical model of icosahedral AlPdMn quasicrystal is obtained. Comparing with the spherical atomic surface model of icosahedral AlPdMn quasicrystal, the percentage of unphysically short interatomic distances is reduced. Based on the modified non-spherical model of icosahedral AlPdMn quasicrystal, the cluster structures and the atomic structures of surfaces of icosahedral AlPdMn quasicrystal are investigated. It is shown that, the building block of the icosahedral AlPdMn phase is the incomplete pseudo-Mackay cluster. The layer with the largest spacing to one of its neighbours also possesses the maximum surface atomic density. Such layers are thought most likely to be the terminal surfaces. The surface perpendicular to the fivefold axis is the most stable surface, and the surface perpendicular to the threefold axis is the least stable surface among the three high-symmetry surfaces. No flat surface perpendicular to the pseudo-twofold axis could be formed. It is easier to cleave the icosahedral AlPdMn quasicrystal along the plane perpendicular to the fivefold axis than in other directions.Using the quantitative convergent beam electron diffraction technique, the structure factors of nine symmetry inequivalent reflections, which are strongest according to the intensities of X-ray diffraction, are refined. The charge density map and the bonding charge density map are constructed using the refined structure factors of nine symmetry inequivalent reflections in combination with the modified non-spherical model of icosahedral AlPdMn quasicrystal. The numbers of electrons, which different atoms in icosahedral AlPdMn quasicrystal gain or lose, are calculated. The numbers of valence electrons per atom in individual pseudo-Mackay clusters are also calculated. The distribution of the bonding charge density in icosahedral AlPdMn quasicrystal indicates that the bonding charge is localized and distributed along certain directions. Such bonds with characteristic localization and direction are covalent bands. The results about the number of electrons, which an atom gains or loses, show that identical atoms can have different valences because of chemically and structurally different local environments. The number of valence electrons per atom in a pseudo-Mackay cluster is 1.69.