Positron annihilation studies of icosahedral quasicrystals and their approximants in the Al-Cu-Ru-(Si) alloy systems

The positron lifetimes for the icosahedral quasicrystal Al62.4CU25.4RU12.2 and its cubic approximants (1/1-Al58Cu31.5Ru10.5, 1/1-Al68Cu7Ru17Si8, and 1/0-Al55Cu15Ru20Si10), two-detector coincident Doppler broadening for the icosahedral quasicrystal Al62.4aCu25.4Ru12.2 and its 1/1-Al68Cu7Ru17Si8 cubic approximant, and the Doppler broadening obtained by making use of a slow positron beam for the 1/1-Al58Cu31.5Ru10.5 cubic approximant have been measured. Structurally intrinsic trapping sites giving rise to saturation trapping were detected by lifetime measurements. The chemical environments of the trapping sites in the icosahedral quasicrystal Al62.4Cu25.4Ru12.2 and the 1/1-Al68Cu7Ru17Si8 cubic approximant were determined by coincident Doppler broadening techniques to be dominantly surrounded by A1 atoms. The positron diffusion length in the 1/1-Al58Cu31.5Ru10.5 cubic approximant derived from the measured S parameter measured by means of a slow positron beam was similar to180 Angstrom, which is clearly too short, probably due to the high concentration of trapping sites as described above. The atomic structures of the icosahedral quasicrystal Al62.4Cu25.4Ru12.2 and its variety of approximants are discussed and compared to the present proposed model.