Electron-density distribution of approximants of the icosahedral Al-based alloys by the maximum-entropy method and the Rietveld refinement

The electron-density distributions of two approximants, metallic Al12Re constructed of 13 atoms icosahedra with the center Re atoms and non-metallic α-Al–Mn–Si constructed of 12 atoms icosahedra without the center atom, are obtained using the maximum entropy method with synchrotron radiation powder data. Since the minimum electron-density of Al12Re is about one hundred times larger than that of α-Al–Mn–Si, Al12Re has free electron background and apparently metallic bonding. α-Al–Mn–Si is found to have strong covalent bonds in the Mackay icosahedron and those between the glue atom sites and the Mackay icosahedron. The covalency of Al (or Si) icosahedron and that between Al (or Si) and Mn atoms are considered to be the origin of the pseudogap in the α-Al–Mn–Si rather than the Hume–Rothery mechanism.