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Register a new userThermoelectric properties of (Ba,K)Cd2As2 crystallized in the CaAl2Si2-type structure
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As-Based Zintl compounds Ba1-xKxCd2As2 crystallized in the CaAl2Si2-type structure (space group P3-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.
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We present results of electronic band structure, Fermi surface and electron transport properties calculations in orthorhombic $n$- and $p$-type SnSe, applying Korringa-Kohn-Rostoker method and Boltzmann transport approach. The analysis accounted for temperature effect on crystallographic parameters in $Pnma$ structure as well as the phase transition to $CmCm$ structure at $T_csim 807 $K. Remarkable modifications of conduction and valence bands were notified upon varying crystallographic parameters within the structure before $T_c$, while the phase transition mostly leads to jump in the band gap value. The diagonal components of kinetic parameter tensors (velocity, effective mass) and resulting transport quantity tensors (electrical conductivity $sigma$, thermopower $S$ and power factor PF) were computed in wide range of temperature ($15-900 $K) and, hole ($p-$type) and electron ($n-$type) concentration ($10^{17}-10^{21}$ cm$^{-3}$). SnSe is shown to have strong anisotropy of the electron transport properties for both types of charge conductivity, as expected for the layered structure. In general, $p$-type effective masses are larger than $n$-type ones. Interestingly, $p$-type SnSe has strongly non-parabolic dispersion relations, with the pudding-mold-like shape of the highest valence band. The analysis of $sigma$, $S$ and PF tensors indicates, that the inter-layer electron transport is beneficial for thermoelectric performance in $n$-type SnSe, while this direction is blocked in $p$-type SnSe, where in-plane transport is preferred. Our results predict, that $n$-type SnSe is potentially even better thermoelectric material than $p$-type one. Theoretical results are compared with single crystal $p$-SnSe measurements, and good agreement is found.
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