Evidence of intermediate states between approximant crystal and quasicrystal

Abstract

We report transmission electron microscopy investigations on the Al₆Li₃Cu (auasicrystal)-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 Iow defect density, becomes aperiodic when the defect density increases. This complex organization suggests that diffusion rates are very Iow in auasicrystals. New conditions for annealing Al₆Li₃Cu auasicrystals are then determined that subsequently improve the auasicrystalline 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 seauence of intermediate states to discuss in relation to the deviation from icosahedral symmetry in diffraction patterns and diffusive/displac-ive transformations.