Disorder in Al-Li-Cu and Al-Mn-Si Icosahedral Alloys

Faceted dendrites of icosahedral Al$_{6}$Li$_{3}$Cu 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 well-defined facets, the samples have a high degree of atomic disorder. The Bragg peaks have symmetry-dependent shapes and widths that scale linearly with G$_{\perp}$ (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$_{\perp}$. The origin of these features in models for the icosahedral phase is discussed.