title: Metallic-covalent bonding conversion and thermoelectric properties of Al-based icosahedral quasicrystals and approximants.
abstract: In this article, we review the characteristic features of icosahedral cluster solids, metallic-covalent bonding conversion (MCBC), and the thermoelectric properties of Al-based icosahedral quasicrystals and approximants. MCBC is clearly distinguishable from and closely related to the well-known metal-insulator transition. This unique bonding conversion has been experimentally verified in 1/1-AlReSi and 1/0-Al12Re approximants by the maximum entropy method and Rietveld refinement for powder x-ray diffraction data, and is caused by a central atom inside the icosahedral clusters. This helps to understand pseudogap formation in the vicinity of the Fermi energy and establish a guiding principle for tuning the thermoelectric properties. From the electron density distribution analysis, rigid heavy clusters weakly bonded with glue atoms are observed in the 1/1-AlReSi approximant crystal, whose physical properties are close to icosahedral Al-Pd-TM (TM: Re, Mn) quasicrystals. They are considered to be an intermediate state among the three typical solids: metals, covalently bonded networks (semiconductor), and molecular solids. Using the above picture and detailed effective mass analysis, we propose a guiding principle of weakly bonded rigid heavy clusters to increase the thermoelectric figure of merit (ZT) by optimizing the bond strengths of intra- and inter-icosahedral clusters. Through element substitutions that mainly weaken the inter-cluster bonds, a dramatic increase ofZTfrom less than 0.01 to 0.26 was achieved. To further increaseZT, materials should form a real gap to obtain a higher Seebeck coefficient.
authors.affiliation: Department of Advanced Materials Science, The University of Tokyo, Kiban-toh 502, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan.
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Takagiwa
authors.affiliation: Department of Advanced Materials Science, The University of Tokyo, Kiban-toh 502, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan.
authors.forename: Kaoru
authors.initials: K
authors.lastname: Kimura
keywords: MEM/Rietveld; chemical bond; electron density; icosahedral cluster; metal-insulator transition; quasicrystal; thermoelectric material