We report the crystal structure and magnetic properties of the Zn1−xMnxO compounds synthesized by a combustion method. The Zn1−xMnxO compounds with x=0–0.1 crystallize in the wurtzite ZnO structure. The lattice parameters a and c of Zn1−xMnxO increase linearly with the Mn content, indicating that Mn2+ ions substitute for Zn2+ ions. Scanning electron microscopy shows that the average particle radius of Zn0.95Mn0.05O is about 40 nm. From the Curie–Weiss behavior of susceptibility at high temperature, it was found that the Mn–Mn interaction is dominated by antiferromagnetic coupling with effective nearest-neighbor exchange constant J about −32 K and the large negative value of the Curie–Weiss temperature.
Motivated by the recent experimental synthesis of atomic-thick SnTe [Liu et al., Science 353(6296), 274 2016] exhibiting a layered orthorhombic phase similar to SnSe, we carried out systematic investigations on its electronic, thermoelectric, and phonon transport properties based on a combination of density functional theory and Boltzmann transport theory. Our results indicate that the monolayer is dynamically stable with a band gap of 1.05 eV. A considerable figure of merit (ZT) is predicted to be 2.9 for n-type doping and 2.2 for p-type doping along the armchair direction at a moderate carrier concentration of 1020 cm−3. The electronic band structure and the Fermi surface with multi-valleys lead to band convergence and anisotropic transport behavior. The synergistic optimization of Seebeck coefficient and electrical conductivity is achieved in anisotropic monolayer SnTe, due to the independence of carrier relaxation time and directional effective mass. A maximum power factor of 37 mW/(mK2) can be achieved for the n-type SnTe monolayer along the armchair direction, almost two times as high as that in the zigzag direction. However, the anisotropy of intrinsic lattice thermal conductivity is relatively low and strong phonon anharmonicity is found due to the coexistence of weak bonding and resonant bonding.
Serials of Mn doping by substituting Cd sites on Cu2CdSnSe4 are prepared by the melting method and the spark plasma sintering (SPS) technique to form Cu2Cd1−xMnxSnSe4. Our experimental and theoretical studies show that the moderate Mn doping by substituting Cd sites is an effective method to improve the thermoelectric performance of Cu2CdSnSe4. The electrical resistivity is decreased by about a factor of 4 at 723 K after replacing Cd with Mn, but the seebeck coefficient decreases only slightly from 356 to 289 μV/K, resulting in the significant increase of the power factor. Although the thermal conductivity increases with the doping content of Mn, the figure of merit (ZT) is still increased from 0.06 (x = 0) to 0.16 (x = 0.10) at 723 K, by a factor of 2.6. To explore the mechanisms behind the experimental results, we have performed an ab initio study on the Mn doping effect and find that the Fermi level of Cu2CdSnSe4 is shifted downward to the valence band, thus improving the hole concentration and enhancing the electrical conductivity at the low level doping content. Optimizing the synthesis process and scaling Cu2Cd1−xMnxSnSe4 to nanoparticles may further improve the ZT value significantly by improving the electrical conductivity and enhancing the phonon scattering to decrease the thermal conductivity.
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