Diffraction Experiments on Quasicrystals and Related Phases

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Structural studies are essential for understanding formation, growth stability and physical properties of quasicrystals and related phases. Diffraction is a powerful tool for studying order and disorder in these materials. Electron diffraction has played a key role in the discovery of icosahedral [1, 2], decagonal [3] and dodecagonal [4] quasicrystals and their characterization. In many quasicrystalline alloys, the size of quasicrystalline grains being small (typically 1 um), only electron diffraction and X-ray or neutron powder diffraction methods were feasible. Inconvenient with electron diffraction is the importance of multiple Bragg scattering making intensity calculations generally hard to handle. The disadvantage of powder methods is that the three-dimensional diffraction patterns of all individual small single grains are angularly averaged and reduced to one-dimensional informations. It is one of the reasons to go further in the structure determination of these materials, since the beginning, that efforts were undertaken to prepare large single grains Of course, this rests on the assumption that the quasicrystal is stable at least in some composition and temperature range. With the elaboration of large single grains with triacontahedral shapes, Al6Li3Cu was the first reported stable icosahedral quasicrystal [5]. This opened the way for structure determination by X-ray or neutron diffraction. The use of X-ray synchrotron beam combining high-flux and high-resolution is particularly well-suited for studying very weak diffraction peaks that allow to distinguish between quasiperiodic order and complex domain arrangements of approximant phases.