Covalent Bonds in AlMnSi Icosahedral Quasicrystalline Approximant

Electron density distributions were obtained using the maximum entropy method with synchrotron radiation powder data. In the metallic ${\mathrm{Al}}_{12}\mathrm{Re}$, metallic bonding was observed for the icosahedral ${\mathrm{Al}}_{12}$ cluster with central Re atom. In the nonmetallic $\alpha -\mathrm{AlMnSi}$ $1/1$ approximant, covalent bonds were found in the electron density distribution of the Mackay icosahedral cluster without central atom. Rather than the Hume-Rothery mechanism, the covalency of Al (Si) icosahedron and that between Al (Si) and Mn atoms is considered to be the origin of the pseudogap and nonmetallic behavior of $\alpha -\mathrm{AlMnSi}$.