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We investigate the transport properties of bulk Ca2YZ (Y = Au, Hg; Z = As, Sb, Bi, Sn and Pb) by a combination method of first-principles and Boltzmann transport theory. The focus of this article is the systematic study of the thermoelectric properties under the effect of a spin–orbit coupling. The highest dimensionless figure of merit (ZT) of Ca2AuAs at optimum carrier concentration are 1.23 at 700 K. Interestingly enough, for n-type Ca2HgPb, the maximum ZT are close to each other from 500 K to 900 K and these values are close to 1, which suggests that semimetallic material can also be used as an excellent candidate for thermoelectric materials. From another viewpoint, at room temperature, the maximum PF for Ca2YZ are greater than 3 mW m−1 K−2, which is very close to that of ∼3 mW m−1 K−2 for Bi2Te3 and ∼4 mW m−1 K−2 for Fe2VAl. However, the room temperature theoretical κl of Ca2YZ is only about 0.85–1.6 W m−1 K−1, which is comparing to 1.4 W m−1 K−1 for Bi2Te3 and remarkably lower than 28 W m−1 K−1 for Fe2VAl at same temperature. So Ca2YZ should be a new type of promising thermoelectric material at room temperature.

The previous experimental work showed that n-type low-temperature (LT) SnSe doped with BiCl3 simultaneously exhibited larger Seebeck coefficient (S), higher electrical conductivity (σ), and lower thermal conductivity (κ) compared with polycrystalline LT SnSe. Here, the first-principles method was used to explore how the independent adjustment to the tangled thermoelectric (TE) transport coefficients of n-type LT SnSe was achieved and whether there exist the similar adjustment mechanisms in n-type high-temperature (HT) SnSe. Cl doping increases the electron-pockets and the total density of states (TDOS) near the Fermi level (EF). This results in a high σ and an obvious decrease in S. Whereas, Bi doping introduces two half-filled bands into the band gap. The two bands originate from the 6s “lone-pairs” of Bi atoms and the sigma bonds formed by Bi atoms and their nearest Se atoms. This contributes to maintaining a relative large S and a low κ, but this cannot effectively enhance σ. Fortunately, the coexistence of Cl ions and Bi ions can make Bi/Cl codoped LT (or HT) SnSe set a high σ and a relatively large S in one through independent controlling the electronic structures and charge distribution near the EF. Further, we predict that Bi/Cl co-doping is also an effective strategy to improve the TE performance of HT SnSe.

The magneto-Seebeck effect has recently attracted considerable attention because of its novel fundamental physics and future potential application in spintronics. Herein, employing first-principles calculations and the spin-resolved Boltzmann transport theory, we have systematically investigated the electronic structures and spin-related transport properties of Co2FeAl/MgO/Co2FeAl multilayers with parallel (P) and anti-parallel (AP) magnetic alignment. Our results indicate that the sign of tunneling magneto-Seebeck (TMS) value with Co2/O termination is consistent with that of the measured experimental result although its value (-221%) at room temperature is smaller than the experimental one (-95%). The calculated spin-Seebeck coefficients of the Co2/O termination with P and AP states and the FeAl/O termination with the AP state are all larger than other typical Co2MnSi/MgO/Co2MnSi heterostructures. By analyzing the geometries, electronic structures, and magnetic behaviors of two different terminations (Co2/O and FeAl/O terminations), we find that the two terminations in the interface region form anti-bonding and bonding states, reconstructing the energy gap, changing the magnetic moment of O atoms, and improving the spin-polarization (-82%). This phenomenon can be ascribed to the charge transfer and hybridization between Co/Fe 3d and O 2p states, which also results in a bowknot orbital shape of Co atoms with Co2/O termination and an ankle shape of Co atoms with FeAl/O termination far away from the interface. Moreover, there are spin-splitting transmission gaps with the Co2/O-termination around the Fermi level, while the transmission gaps with the FeAl/O-termination are closed and thus show a typical metallic character. Our findings will guide the experimental design of magneto-Seebeck devices for future spintronic applications.

Ba 3 Al 2 As 4 exhibits an unusual anisotropic electrical conductivity, that is, the electrical conductivity along the chain is smaller than those along other two directions. The results is conflict with previous conclusion for Ca 5 M 2 Pn 6 . Earlier studies on Ca 5 M 2 Pn 6 showed that a higher electrical conductivity could be obtained along the chain. The band decomposed charge density is used to explain such unusual behavior. Our calculations indicate the existence of a conductive pathway near the Fermi level is responsible for the electrons transport. Further, the Ba–As bonding of Ba 3 Al 2 As 4 has some degree covalency which is novel for the Zintl compounds.

The highest thermoelectric figure of merit (ZT) value of 2.6 has been experimentally determined in p-type single crystalline SnSe. In this work, we propose possible method to further enhance the thermoelectric performance of SnSe by comparing different thermoelectric performances of p- and n-type SnSe using the first-principles method and the semiclassical Boltzmann theory. There are relatively stronger intra-layer interactions between Sn and Se atoms for the layered single crystalline SnSe. The Sn p and s electrons near the Fermi level are pushed away along the a-direction because of the anti-bonding interaction between Sn and Se atoms. The pushed away electrons would hinder holes transport in the a-direction. This leads to the lowest electrical conductivity and ZT value along the a-direction for the p-type single crystalline SnSe. For n-type SnSe, the Sn p and Se p electrons are primarily distributed along the a-direction near the Fermi level. This would lead to high electrical conductivity and large Seebeck coefficient simultaneously in the a-direction. Combined with its ultralow thermal conductivity in the a-direction, the ZT value (∼3.1 at 770 K by a rough estimation) of n-type SnSe along the a-direction is probably much higher than that of p-type one. The peak values of S 2 σ τ of the p-type SnSe correspond to different carrier concentrations along the a- and b-directions. This is one reason for the low experimental peak ZT value of 0.6 for the p-type polycrystalline SnSe at 750 K. For the n-type SnSe, the S 2 σ τ can attain their peak values almost at the same carrier concentration along the a-, b-, and c-directions. This suggests that the power factor value of n-type polycrystalline SnSe would be much higher than that of p-type one at 770 K.

Electronic band structure is vital in determination the performance of thermoelectric materials. What is the optimum electronic structure for the largest figure of merit? To answer the question, we studied the relationship between the thermoelectric properties and the electronic band structure under the assumption of isotropic elastic scattering, within the context of Chasmar-Stratton theory. The results show that whether the anisotropic band structure and the effective mass of the carrier are beneficial to improving the thermoelectric properties. The scattering mechanism and the shape of the Fermi surface play a decisive role. Regardless of scattering mechanism type, a larger valley degeneracy is always beneficial to thermoelectric materials.

The electronic structure and transport properties of A 5 M 2 Pn 6 (A = Ca, Sr, Ba; M = Ga, Al, In; Pn = As, Sb) are investigated by using the first-principles calculations and Boltzmann transport theory, respectively. The results show that the order of the width of these band gaps is Ca 5 Ga 2 As 6 (0.37 eV) > Ca 5 Al 2 Sb 6 (0.35 eV) > Ca 5 In 2 Sb 6 (0.32 eV) > Sr 5 In 2 Sb 6 (0.29 eV) > Ba 5 In 2 Sb 6 (0.27 eV) > Ca 5 Ga 2 Sb 6 (0.088 eV). For intrinsic A 5 M 2 Pn 6 , increasing the band gap is better for improving their thermoelectric properties. For doped A 5 M 2 Pn 6 , increasing the band gap is helpful to improve the high-temperature thermoelectric properties of A 5 M 2 Pn 6 . However, at 300 K, when the band gap is small enough, regardless of n -type or p -type doping, A 5 M 2 Pn 6 exhibits good thermoelectric properties. Our analysis shows that the band gaps can be roughly adjusted by changing the electronegativity difference between Pn and A, and subtly adjusted by changing the electronegativity difference between Pn and M atoms. So the electronegativity difference among the constituent element is a very important factor affecting the thermoelectric properties.

Based on ab initio electronic structure calculations and Boltzmann transport theory, the size dependence of thermoelectric properties of ZnO single-walled nanotubes (SWNTs) and nanowires was investigated. There is an optimal carrier concentration yielding the maximum value of ZT at room temperature. The optimal carrier concentration and the maximum value of ZT depend on the diameters and structure of ZnO. The maximum value of ZT for ZnO SWNTs are remarkably higher than that of ZnO nanowires. The 9.60 A ZnO SWNT possess the highest ZT value, 0.322, which is nearly 3-fold higher than that of the best experimental samples at room temperature.

Spin-dependent thermoelectric effects in Aharonov–Bohm (AB) interferometer with four Rashba quantum dots (QDs) coupled to external two one-dimensional semi-infinite metallic leads are investigated theoretically. The basic thermoelectric transport characteristics, like charge (spin) Seebeck coefficient, and the charge (spin) figure of merits, have been calculated in terms of Green's function formalism. It is found that a pure spin-up (spin-down) Seebeck coefficient can be generated by the coaction of the magnetic flux and the Rashba spin-orbit interaction (RSOI) at room temperature.

Engineering atomic-scale native point defects has become an attractive strategy to improve the performance of thermoelectric materials. Here, we theoretically predict that Ag-Mg antisite defects as shallow acceptors can be more stable than other intrinsic defects under Mg-poor‒Ag/Sb-rich conditions. Under more Mg-rich conditions, Ag vacancy dominates the intrinsic defects. The p-type conduction behavior of experimentally synthesized α-MgAgSb mainly comes from Ag vacancies and Ag antisites (Ag on Mg sites), which act as shallow acceptors. Ag-Mg antisite defects significantly increase the thermoelectric performance of α-MgAgSb by increasing the number of band valleys near the Fermi level. For Li-doped α-MgAgSb, under more Mg-rich conditions, Li will substitute on Ag sites rather than on Mg sites and may achieve high thermoelectric performance. A secondary valence band is revealed in α-MgAgSb with 14 conducting carrier pockets.

The previous experimental work showed that Hf- or Zr-doping has remarkably improved the thermoelectric performance of FeNbSb. Here, the first-principles method was used to explore the possible reason for such phenomenon. The substitution of X (Zr/Hf) atoms at Nb sites increases effective hole-pockets, total density of states near the Fermi level (EF), and hole mobility to largely enhance electrical conductivity. It is mainly due to the shifting the EF to lower energy and the nearest Fe atoms around X atoms supplying more d-states to hybrid with X d-states at the vicinity of the EF. Moreover, we find that the X atoms indirectly affect the charge distribution around Nb atoms via their nearest Fe atoms, resulting in the reduced energy difference in the valence band edge, contributing to enhanced Seebeck coefficients. In addition, the further Bader charge analysis shows that the reason of more holes by Hf-doping than Zr in the experiment is most likely derived from Hf atoms losing less electrons and the stronger hybridization between Hf atoms and their nearest Fe atoms. Furthermore, we predict that Hf/Sn co-doping may be an effective strategy to further optimize the thermoelectric performance of half-Heusler (HH) compounds.

The structural, electronic, and thermoelectric properties of InSe nanotubes (InSeNTs) are investigated using first-principle calculations based on density functional theory (DFT). The thermoelectric transport coefficients of InSeNTs at room temperature are calculated within the semiclassical Boltzmann theory. Calculated total energies show that (2,2) InSeNT is more stable than other ones. As a consequence of high Seebeck coefficient and reasonable electrical conductivity, its thermoelectric powerfactor with respect to relaxation time is much larger than those of other studied InSeNTs and is nearly 10 times larger than that of BiSb nanotube. The light and heavily bands appear together around the Fermi level in (2,2) InSeNT, inducing its high Seebeck coefficient and reasonable electrical conductivity. (2,2), (4,0), and (6,0) InSeNTs are metallic. (3,3), (4,4), (6,6), and (8,0) InSeNTs are semiconducting. It is found that each Se atom in the semiconducting InSeNTs is coordinated by two In atoms, and that in ...

Band engineering is one of the effective approaches for designing ideal thermoelectric materials. Introducing an intermediate band in the band gap of semiconducting thermoelectric compounds may largely increase the carrier concentration and improve the electrical conductivity of these compounds. We test this hypothesis by Pb doping in Zintl Ca5In2Sb6. In the current work, we have systematically investigated the electronic structure and thermoelectric performances of substitutional doping with Pb on In sites at a doping level of 5% (0.2 e per cell) for Ca5In2Sb6 by using density functional theory combined with semi-classical Boltzmann theory. It is found that in contrast to Zn doping, Pb doping introduces a partially filled intermediate band in the band gap of Ca5In2Sb6, which originates from the Pb s states by weakly hybridizing with the Sb p states. Such an intermediate band dramatically increases the electrical conductivity of Ca5In2Sb6 and has little detrimental effect on its Seebeck coefficient, which may increase its thermoelectric figure of merit, ZT. Interestingly, a maximum ZT value of 2.46 may be achieved at 900 K for crystalline Pb-doped Ca5In2Sb6 when the carrier concentration is optimized. Therefore, Pb-doped Ca5In2Sb6 may be a promising thermoelectric material.

The effects of doping ZnO nanowires with Al, Ga and Sb on their electronic structure and thermoelectric properties are investigated by first-principles calculations. We find that the band gap of ZnO nanowires is narrowed after doping with Al and Ga, while band gap broadening is observed in Sb doped ZnO nanowires. The lattice thermal conductivity of ZnO nanowires is obtained based on the Debye-Callaway model. The thermoelectric properties of ZnO nanowires were calculated using the BoltzTraP code. The results show that there exists an optimal carrier concentration yielding the maximum value of ZT for Al, Ga and Sb doped ZnO nanowires at room temperature. The maximum value of ZT, 0.147, is obtained for Ga doped ZnO nanowires, when the carrier concentration is 3.62 × 10(19) cm(-3). The figure of merit ZT of Sb doped ZnO nanowires is higher than that of Ga doped ZnO nanowires when the temperature is between 400 K and 1200 K. We also find that Al doped ZnO nanowires always have poor thermoelectric properties, which means that the Al dopant may not be the optimal choice for ZnO nanowires in thermoelectric applications.

BiCuSeO is an attractive material for its high temperature stability, high Seebeck coefficient, and low lattice thermal conductivity. However, its electrical conductivity is low. To enhance the thermoelectric properties of BiCuSeO further, we investigated the material's electronic structure and transport property using first-principles calculations. We determine that the low electrical conductivity may originate from strong ionic bonding and without a Cu–Se conductive pathway forming near the Fermi level. Moreover, although p-type BiCuSeO has a high thermopower due to the high density of states near the Fermi level, its electrical conductivity is relatively low because no conductive pathway for carrier transport exists along the c axis. In contrast, a conductive pathway is formed between Bi and Cu atoms at the conduction-band minimum along the c axis. This feature would lead to high electrical conductivity for n-type BiCuSeO. With relatively high electrical conductivity and slightly decreased thermopower, n-type BiCuSeO exhibits a higher power factor than that of p-type BiCuSeO. The maximum ZT value could be improved by 32% for n-type BiCuSeO compared with the prevailing p-type doping approach at 920 K.

Ca5Ga2As6 gives us a chance to achieve a high thermoelectric performance with a normal bulk system. This is mainly due to the fact that its complex chemical structure can not only supply an ultra-low lattice thermal conductivity but also show a high Seebeck coefficient. In this paper, we have discussed the lattice, electronic, and transport properties of Ca5Ga2As6 for elucidating its high thermoelectric performance. In our calculations, density-functional theory and Boltzmann transport theory were used. For intrinsic Ca5Ga2As6, the valence band effective mass is larger than the conduction band effective mass, which leads to its positive sign of S over the whole temperature range. By studying the anisotropy of its transport properties, we find that the transport properties of p-type Ca5Ga2As6 are better than that of the n-type one, which mainly comes from the large anisotropy of its band structure. Moreover its transport properties along the z direction are much better than those along the x and y directions, which can be attributed to its anisotropic one-dimensional lattice structure. At different temperatures, the peak value of ZeT appears at different carrier concentrations, which makes it possible to obtain a highest efficiency under large temperature gradients. The carrier concentrations corresponding to large ZeT are in the range of 0.033–0.5 e per uc, which is equal to 5.19 × 1019 cm−3 to 7.87 × 1020 cm−3. The largest value of ZeT is equal to 0.95.