Evidence for Phason Hopping in Icosahedral AlFeCu Quasi-Crystals

By means of time-of-flight inelastic neutron scattering we have studied two samples with nominal compositions Al 62 Cu 25.5 Fe 12.5 (perfect sample) and Al 63 Cu 24.5 Fe 12.5 (imperfect sample). The former is a perfect quasi-crystal with sharp Bragg peaks over the whole temperature range from room temperature up to 800 °C. The latter exhibits morphological evolution to a low-temperature imperfect quasi-periodic state starting around 650 °C. Spectra were recorded at 790 °C, 750 °C, 675 °C, 625 °C and room temperature with an incident wavelength of 7 Å ( i.e. 77 μeV elastic resolution). In both samples at high temperature a quasi-elastic component (a Lorentzian of width Γ = 95 μeV) is observed which disappears around 650 °C and is attributed tentatively to phason hopping. At 790 °C the crystalline β-phase Al 50 Cu 25 Fe 25 does not show such a quasi-elastic component. We clarify different concepts of phason existing in the literature.