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 of ZT from less than 0.01 to 0.26 was achieved. To further increase ZT, materials should form a real gap to obtain a higher Seebeck coefficient.

Improvement of the Thermoelectric Performance of Pseudogap and Narrow-Gap Compounds via Theoretical Calculations

“…Kimura et al proposed a guiding principle to enhance the thermoelectric properties of complex cluster solids, such as quasicrystals: weakly bonded rigid heavy clusters. 11,12) We succeeded in a dramatic enhancement of the zT parameter from less than 0.01 to 0.26 through the optimization of the sample's composition and via elemental substitution to weaken inter-cluster bonds; a zT value of 0.26 was achieved at 473 K for a Ga-substituted AlPdMn quasicrystal®this is the highest value reported thus far for stable quasicrystals and related approximant crystals [ Fig. 3(a)(b)].…”

Quo Vadis Quasicrystals?

“…A growing number of experimental and numerical simulation evidences now suggest that a main factor determining the remarkable properties of both decagonal and icosahedral QCs is related to physicochemical aspects involving the nearest and next-nearest atomic neighbours, thereby highlighting the important role of chemical bonding in the emergence of the unusual physical properties of QCs [47][48][49][50]. In fact, whenever there are wave functions overlapping among contiguous atoms in a solid lattice (a concept closely related to that of chemical bonding), the possibility of charge delocalization (i.e., extended character of the electrons) naturally emerges.…”

Thermoelectric properties of Tsai-type Au–Al–RE (RE: Yb, Tm, Gd) quasicrystals and approximants

“…The discovery of icosahedral (i-) Al-based [1] and Cd-based [2] quasicrystals (QCs) opened up a new field of research in physics, chemistry, and materials science looking at various aspects such as the origin of phase stability [3,4], structure determination [5], diffusionless order-disorder transition [6], and characteristic surface properties [7], in addition to the peculiar transport properties such as electrical [8], magnetic [9], and thermoelectric properties [10]. A recent review article on the potential applications of QCs and related approximant crystals (ACs) discussed properties related to mechanical reinforcement, hydrogen storage, and catalytic action [11].…”

“…Thermoelectric QC and AC materials have promising applications because of two representative features; one is the pseudogap formation at the Fermi level, E F , and the other is their complex crystal structures. Other reviews have focused on theoretical and experimental studies on thermoelectrics [10,12,13]. Recently, Takeuchi et al proposed a novel practical application of Al-based QCs as thermal rectifiers by controlling their unusual electron thermal conductivity [14].…”