A stable ideally quasiperiodic phase exists in a small range of concentration, close to the composition Al62Cu25.5Fe12.5. Reducing the iron content, or replacing small amounts of copper by aluminium, lead to icosahedral alloys which exhibit around 650-degrees-C structural transformations of unclear nature: in the X-ray powder diffraction pattern, the peak profiles become purely Lorentzian (Al62.3Cu25.3Fe12.4) or diffuse << side-bands >> appear in the tails of the Bragg peaks (Al63Cu24.5Fe12.5). In the last case long annealing treatments eventually transform the Bragg peaks into diffuse peaks located at positions clearly off the ideal icosahedral symmetry. Small deviations from this composition range lead to Bragg peaks with shoulders whatever the heat-treatment may be; perfect icosahedral order is never obtained for these compositions (Al63.25Cu24.5Fe12.25, Al64Cu24Fe12, Al63Cu25Fe12).
