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 Al62Cu25.5Fe12.5 (perfect sample) and Al63Cu24.5Fe12.5 (imperfect sample). The former is a perfect quasi-crystal with sharp Bragg peaks over the whole temperature range from room temperature up to 800-degrees-C. The latter exhibits morphological evolution to a low-temperature imperfect quasi-periodic state starting around 650-degrees-C. Spectra were recorded at 790-degrees-C, 750-degrees-C, 675-degrees-C, 625-degrees-C and room temperature with an incident wavelength of 7 angstrom (i.e. 77-mu-eV elastic resolution). In both samples at high temperature a quasi-elastic component (a Lorentzian of width GAMMA = 95-mu-eV) is observed which disappears around 650-degrees-C and is attributed tentatively to phason hopping. At 790-degrees-C the crystalline beta-phase Al50Cu25Fe25 does not show such a quasi-elastic component. We clarify different concepts of phason existing in the literature.