A comprehensive structure model for the decagonal quasicrystalline aluminum-cobalt-copper phase and its crystalline approximants

A comprehensive structure model for and a microstructural characterization of the decagonal Al-Co-Cu phase and its crystalline approximants have been undertaken. Unlike a crystalline structure, long-range translational order is absent in the structures of quasicrystals. This aperiodic nature limits the ability of conventional X-ray methods in the determination of quasicrystalline structures. To overcome this difficulty, X-ray diffraction, electron microscopy, and scanning tunneling microscopy were used collectively in this investigation to approach the structures of the Al-Co-Cu decagonal phase and its crystalline approximants.; The investigation started with the structures of the approximant phases to the decagonal phase because the approximants are understood to bear a close structural resemblance to the decagonal phase. Conventional concepts of crystalline structures were used to understand the structures of the approximants. Attention was then directed to the transition from the crystalline approximants to the quasicrystalline state. On the basis of the structural information about the local atomic configurations obtained from the study of the approximants, the structure of the decagonal phase was derived and tested with the experimental results.; According to the proposed model, there are 990 atoms in a C-centered orthorhombic unit cell of one of the approximants (sample approximant) with edge lengths, 98.98 x 32.16 x 4.13 A. The structure of this approximant is built up of two primary atom clusters (pentagonal polyhedra). A "periodic" (to a certain extent) arrangement of these atom clusters leads to an approximant structure and a random configuration of the clusters yields a quasicrystalline structure. Orientational order persists in both approximant and quasicrystalline structures. The periodic arrangement is inherently accompanied by a special type of structural defect due to the inconsistency of the local 5-fold symmetry with the crystallographic symmetry for a long-range periodic order. Removal of the defects occurs during the transition from the approximant to quasicrystalline state, which reduces the internal energy of the structure.