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

The thermoelectric properties of Tsai-type Au-Al-RE (RE: Yb, Tm, Gd) icosahedral quasicrystals (QCs) and their approximant crystals (ACs) have been investigated from 373 K to 873 K. Comparison of their thermoelectric properties above room temperature is of great interest since resonant states are likely to be present in the electronic density of states near the Fermi level for the Au-Al-Yb QC, which should influence the thermoelectric properties and, hence, the dimensionless figure of merit. The electrical conductivity of Au-Al-Yb ACs varies significantly, i.e. from 3000 Ω^-1 cm^-1 to 5200 Ω^-1 cm^-1, with changing the Au/Al composition whereas the other compounds have conductivities of ∼3000 Ω^-1 cm^-1. Such an anomalous behavior of the Au-Al-Yb ACs is understood to be due to the existence of a resonant peak originating from hybridization of Yb f-states and Au/Al sp-states. A maximum power factor of 230 μW m^-1 K^-2 at 873 K is obtained in 1/1-Au₄₈Al₃₇Yb₁₅ AC, which is significantly higher than those of Al-based icosahedral phases at the same temperature. On the other hand, the room-temperature thermal conductivities for Au-Al-RE compounds range from 3 to 4 W m^-1 K^-1, reflecting their complex crystal structures with a large unit cell and mixed and/or partially occupied sites. Consequently, both 1/1-Au₅₀Al₃₅Yb₁₅ and Au₄₈Al₃₇Yb₁₅ ACs have similar maximum figures of merit of 0.014 at 873 K.