Abstract
As-Based Zintl compounds Ba1-xKxCd2As2 crystallized in the CaAl2Si2-type structure (space group P3[combining macron]m1) were prepared using solid-state reactions followed by hot-pressing. We have successfully substituted K for Ba up to x = 0.08, producing hole-carrier doping with concentrations up to 1.60 × 1020 cm-3. We have determined the band-gap value of non-doped BaCd2As2 to be 0.40 eV from the temperature dependence of the electrical resistivity. Both the electrical resistivity and the Seebeck coefficient decrease with hole doping, leading to a power factor value of 1.28 mW m-1 K-2 at 762 K for x = 0.04. A first-principles band calculation shows that the relatively large power factor mainly originates from the two-fold degeneracy of the bands comprising As px,y orbitals and from the anisotropic band structure at the valence-band maximum. The lattice thermal conductivity is suppressed by the K doping to 0.46 W m-1 K-1 at 773 K for x = 0.08, presumably due to randomness. The effect of randomness is compensated by an increase in the electronic thermal conductivity, which keeps the total thermal conductivity approximately constant. In consequence, the dimensionless figure-of-merit ZT reaches a maximum value of 0.81 at 762 K for x = 0.04.
Highlights
High-performance thermoelectric materials are desirable for energy conservation, as they can generate electrical power from waste heat
Thermoelectric performance is characterized by the dimensionless figure-of-merit ZT = S2T/ρκ, where S is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the total thermal conductivity
We have succeeded in synthesizing single-phase Ba1−xKxCd2As2 samples over the concentration range x ≤ 0.08 (Fig. 2)
Summary
High-performance thermoelectric materials are desirable for energy conservation, as they can generate electrical power from waste heat. The 122 Zintl compounds are a promising system.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23] In particular, Mg3Sb2 exhibits a ZT value of 1.65 at 725 K, with n-type electronic conductivity, where the Mg atoms are located at two atomic sites in the ratio of 1 : 2 so that it is categorized as 122 system.[17,18] As for the p-type materials, Yb(Zn,Cd)2Sb2 and (Ca,Yb,Eu)Mg2Bi2 exhibit ZT values of 1.2 at 700 K and 1.3 at 873 K, respectively.[19,20,21,22,23] Those compounds crystallize in the CaAl2Si2-type structure[24,25] with the space group P3m1 (Fig. 1) They consist of alternately stacked anionic layers and cation sheets. We have explored the thermoelectric properties of (Ba,K)Cd2As2 crystallized in the CaAl2Si2-type structure
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