Study of Al-Cu Hume-Rothery alloys and their relationship to the electronic properties of quasicrystals

The stability of quasicrystals is related to the Hume-Rothery mechanism known to take place in many intermetallics, especially Al-Cu alloys. In order to assess to what extent the electronic density of states in aperiodic Al-Cu-Fe compounds is sensitive to Hume-Rothery stabilization, we present experimental measurements by soft x-ray emission and absorption spectroscopies of the aluminium electronic states in a series of Al-Cu crystalline compounds. Some of them, like icosahedral Al-Cu-Fe, are characterized by nearly spherical Brillouin zones. We observe the opening of a narrow but definite pseudo-gap in such Al-Cu compounds. We point out a strong interaction between Cu d states and the Al states in the middle of the valence band which is well described by a Fano-like effect. Some other characteristic features of our spectra are interpreted on the basis of band structure calculations. A comparison to previous data published for quasicrystalline and approximant phases stresses the importance for the stability of these compounds of sp-d hybridization at the Fermi level. It also leads to the conclusion that the effect of Hume-Rothery interaction is dramatically enhanced by the hierarchical structure of quasicrystals rather than by the almost spherical shape of the Brillouin zone.