Icosahedral quasicrystal structure determination - Al-Cu-Li /
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| Other Authors: | , , |
| Format: | Thesis Book |
| Language: | English |
| Published: |
1992.
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| Subjects: | |
| Online Access: | Link to OAKTrust copy |
| Abstract: | The discovery of quasicrystals by Shechtman el al. in 1984 challenged a century old belief that periodicity and structural order are essentially equivalent and, consequently, that ordered icosahedral structures are impossible. Although extensive studies have been made since then on this new kind of material, one of the most important as well as the most fundamental questions, "Where are the atoms?", is still unanswered. This dissertation reports the theoretical study of this problem - the determination of quasicrystal atomic structures. The procedures of the study are the following: (1) A survey is made to discuss all the existing approaches to the atomic structure determination for quasicrystals, and my approach to the study of this problem is framed. Necessary theories and concepts which are important to our study are introduced. (2) A new method is developed for phasing the quasicrystal diffraction data (neutron, X-ray diffraction). The method is based on the fact that many quasicrystals have related periodic crystals whose structures are known. For such quasicrystals, a mathematical model is built which explicitly relates quasicrystals to their related periodic crystals. Such a relationship can lead to finding the unknown phases of quasicrystal diffraction peaks. (3) A test of the method is made on a simple theoretical system, in which both the structure of the quasicrystal and its related periodic crystal are known. One such system is the icosahedral quasiperiodic Ammann tiling decorated with point scatterers (Dirac δ scattering potential) on vertices. A remarkable success is achieved in reconstructing the phases of the quasicrystal. Also a detailed evaluation of the method is accomplished through the study of this simple known system. (4) The method is applied to a real quasicrystal system. The best candidate is the icosahedral quasicrystal i(Al[.570]Cu[.108]Li[.322]), where single-grain X-ray as well as neutron diffraction data are available, and the structure of the related large-unit-cell crystal of R(Al[.564]Cu[.116]Li[.320]), is well known. The reconstructed phases give the density of scatterers, which can then be used in the analysis of the atomic structure, and provide a guide to the final structure modeling... |
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| Item Description: | Typescript (photocopy). Vita. "Major subject: Physics." |
| Physical Description: | xi, 193 leaves : illustrations ; 29 cm |
| Bibliography: | Includes bibliographical references. |