POSITRON-ANNIHILATION STUDY OF EQUILIBRIUM DEFECTS IN AL-CU-FE FACE-CENTERED-ICOSAHEDRAL QUASI-CRYSTALS

In situ Doppler-broadening temperature scans and room temperature posi tron-lifetime measurements are reported for Al63CU25Fe12, Al62.5CU25Fe 12.5, Al62CU25.5Fe12.5, and Al62Cu22.8Fe15.2 face-centered-icosahedral (FCI) quasicrystalline samples. Quenched-in disorder has been observe d and found to anneal out of the samples above 200 degrees C. A quasic rystal-to-microcrystal phase transition has been observed in two of th e samples (Al63Cu25Fe12 and Al62Cu22.8Fe15.2), indicating an instabili ty of the FCI phase at low temperatures. No such transition is observe d in the other two samples (Al62.5CU25Fe12.5 and Al62Cu25.5Fe12.5). It is shown that intrinsic structural vacancies, possessing only Al-atom nearest neighbors, exist in both the microcrystal and FCI phases and act as saturation traps for the positrons. At similar to 200 degrees C , thermal activation of dynamic Al-atom phasons in the FCI phase resul ts in distortions of the intrinsic structural vacancies as the nearest neighbor Al atoms begin to hop within double-well potentials. At high er temperatures (525-750 degrees C depending upon the sample stoichiom etry) dynamic Cu-atom phasons are activated and observed to coincide w ith the onset of plasticity in the FCI phase.