Al-Cu-Ru: An icosahedral alloy without phason disorder

We present high-resolution x-ray diffraction mesaurements of the recently discovered face-centered icosahedral phase of ${\mathrm{Al}}_{65}$ ${\mathrm{Cu}}_{20}$ ${\mathrm{Ru}}_{15}$. Analysis of the diffraction-peak profiles shows that this alloy is better ordered than the simple-icosahedral alloys previously studied. In particular, the peak broadening is well accounted for by simple lattice strain, with no measurable dependence of the peak widths on phason momentum. This implies a significantly diminished role for phason disorder in this system relative to the simple-icosahedral alloys.