Systematics of Disorder in Quasiperiodic Material

Abstract

X-ray diffraction and transmission electron microscope (TEM) measurements were performed on icosahedral A1Mn and A1MnSi samples. Quasicrystalline order was obtained via different sample treatments: quenching, laser annealing, and furnace annealing of amorphous samples. In quenched bulk AIMn samples, the icosahedral grains are embedded in a crystalline Al matrix. The Al matrix is strongly reduced or rendered noncrystalline by the addition of Si. Samples prepared by laser annealing and quenching show a high degree of disorder. This shows up, in x-ray diffraction, as a large average peak line width. The furnace annealed samples show apparent homogeneous grains by TEM darkfield methods, and the smallest peak widths in x-ray diffraction. However, the grain sizes are much larger than the x-ray correlation lengths. These samples have peak widths which correlate strongly with G_⊥ (phason momentum). We show that the data are consistent with either frozen-in phason strain or random icosahedral packing. These two models predict surprisingly similar but potentially distinguishable scattering widths, differing in their detailed dependence on G_⊥ .