Disorder in Al-Li-Cu and Al-Mn-Si icosahedral alloys

Faceted dendrites of icosahedral Al6Li3Cu have been studied by high-resolution x-ray scattering. The samples display long-range icosahedral symmetry both in their diffraction patterns and in their macroscopic morphology. Despite the appearance of welldefined facets, the samples have a high degree of atomic disorder. The Bragg peaks have symmetry-dependent shapes and widths that scale linearly with G⊥ (phason momentum). The peak widths are surprisingly similar to those found in icosahedral Al-Mn-Si alloys in both their absolute magnitude and their dependence on G ⊥. The origin of these features in models for the icosahedral phase is discussed.