Iron dynamics in Al–Cu–Fe quasicrystals and approximants: Mössbauer and neutronexperiments

Abstract

We present new results on the iron dynamics in the icosahedral quasicrystal i-AlCuFeand two cubic approximants as well as the non-approximant Al–Cu–Fe cubic B2phase. Conventional Mössbauer spectroscopy is used as well as, for the i-AlCuFephase, high Doppler velocity Mössbauer spectroscopy and quasielastic neutronscattering for samples with different Fe isotope contents. We show that in thei-phase the Fe Lamb–Mössbauer recoilless fraction decreases below that predicted forlattice vibrations alone for temperatures above about 550 K. This decrease iscorrelated with the onset of a quasielastic signal seen in both Mössbauer and neutronbackscattering spectroscopy, which indicates the presence above 550 K of Fe jump processesconfined in a local cage. The timescale of the Fe jumps (660 ps at 1000 K) and theirtemperature dependence differ widely from those of Cu jumps in the same i-AlCuFequasicrystal. From the temperature dependence of the quadrupole splitting of the57Fe Mössbauer spectrum, one can distinguish two kinds of Fe jumps, one starting at550 K and the second above 800 K. In the two cubic approximants, a loss in the Ferecoilless fraction also occurs above 550 K, revealing the same kind of Fe dynamics asin the i-phase but the effect is smaller. On the other hand, no anomalous Fedynamics (other than lattice vibrations) is detected in the B2-AlCuFe phase. Sincethe cubic approximants possess similar local configurations as the quasicrystal,we conclude that locally a Penrose tile description is appropriate. This showsthat the detected Fe jumps can be interpreted in terms of phason-like local tilingflips.