EVIDENCE OF INTERMEDIATE STATES BETWEEN APPROXIMANT CRYSTAL AND QUASI-CRYSTAL

We report transmission electron microscopy investigations on the Al6Li3Cu (quasicrystal)-Al5Li3Cu (crystal) interface. This interfacial area is not sharp but displays intermediate states. A detailed analysis of the diffraction patterns reveals that the transformation involves a planar defect organization. The defect network, periodically organized at low defect density, becomes aperiodic when the defect density increases. This complex organization suggests that diffusion rates are very low in quasicrystals. New conditions for annealing Al6Li3Cu quasicrystals are then determined that subsequently improve the quasicrystalline order. Fine lines of diffuse scattering in annealed samples still remain as imperfections. We interpret these lines as a precursor state of a new rhombohedral approximant crystal. These observations provide a sequence of intermediate states to discuss in relation to the deviation from icosahedral symmetry in diffraction patterns and diffusive/displacive transformations.