Ag-Mg antisite defect induced high thermoelectric performance of α-MgAgSb
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Ag-Mg antisite defect induced high thermoelectric performance of ?-MgAgSb
Zhenzhen Feng 0
Jihua Zhang 0 1
Yuli Yan 0
Guangbiao Zhang 0
Chao Wang 0
Chengxiao Peng 0
Fengzhu Ren 0
Yuanxu Wang 0
Zhenxiang Cheng 0 2
0 Institute for Computational Materials Science, School of Physics and Electronics, Henan University , Kaifeng, 475004 , China
1 Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Guizhou Education University , 115 Gaoxin Road, Guiyang, 550018 , China
2 Institute for Superconducting and Electronic Materials , Innovation
OPEN Published: xx xx xxxx 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 ?-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 ?-MgAgSb by increasing the number of band valleys near the Fermi level. For Li-doped ?-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 ?-MgAgSb with 14 conducting carrier pockets.
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Thermoelectric materials can perform direct conversion between electrical and thermal energy. Thermoelectric
performance is quantified by the figure of merit, ZT = S2?T /?, where S is the Seebeck coefficient, ? is the
electrical conductivity, T is the absolute temperature, and ? is the total thermal conductivity, which consists of both
electronic (?e) and lattice (?l) components1?3. A high ZT value indicates good thermoelectric properties, and one
therefore should try to increase the power factor (S2?) and decrease the thermal conductivity (? = ?e+?l). A large
power factor can be achieved by (a) increasing the density of states near the Fermi level (by forming localized
resonant states4, 5 or increasing band degeneracy6?16), and (b) by increasing the energy dependence of the carrier
mobility using energy filtering17, 18. Meanwhile, forming a solid solution19?21 and creating strong lattice
anharmonicity22?28 can achieve low lattice thermal conductivity. A recent study has proposed that engineering
atomic-scale native point defects can simultaneously optimize the thermal and electrical performances of
thermoelectric materials29, 30, which is becoming an attractive strategy to improve ZT values. Native point defects play
important roles in conduction in semiconductors, and they can change the band structure29.
The ? phase of MgAgSb31 shows superior thermoelectric properties in the low temperature range31?44. Great
efforts have been devoted to understanding and enhancing the unique thermoelectric properties of ?-MgAgSb.
The carrier concentration of ?-MgAgSb-based materials is relatively low at room temperature, which leads to its
high electrical resistivity. To overcome this limitation, extrinsic doping, including Na doping35, Cu doping36. In
doping38, and changing the Sb content41 have been used to increase the carrier concentration of ?-MgAg0.97Sb0.99
or ?-MgAgSb, although the electrical resistivity (1?4.5 ? 10?5 ??m) is still larger than those of good
thermoelectric materials, such as CoSb3 (0.3?1 ? 10?5 ??m)45 and Bi2Te3 (1?1.5 ? 10?5 ??m)46. Liu et al. used Li doping
to increase the carrier concentration of MgAg0.97Sb0.99, and a high average ZT39 of 1.1 from 300 K to 548 K was
achieved.
Intrinsic defects represent another effective way to tune the carrier concentration to enhance the
thermoelectric performance. Moreover, extrinsic point defects strongly influence the native point defects. Recently, Liu et al.
reported that Ag vacancy could increase the ZT for ?-MgAgSb30. Moreover, the Ag vacancy concentration can
be tuned by the hot pressing temperature, which they denoted as the recovery effect. Therefore, it is necessary to
explore the conditions for forming intrinsic defects and their influence on the electronic structure.
In this work, the chemical potentials and defect formation energies of native point defects and Li doping in
?-MgAgSb at all possible charge states are studied by using density functional theory. We found that the defect
formation energies strongly depend on the chemical potentials. Ag vacancies and Ag-Mg antisites (Ag on Mg
sites) are the dominant defects that act as shallow acceptors, which determine the p-type conduction. Moreover,
the AgMg point defect in ?-MgAgSb may have higher ZT than the Ag vacancy. For Li-doped ?-MgAgSb, the
doping formation energies strongly depend on the chemical potentials. Under more Mg-rich conditions, Li will
substitute on Ag sites (LiAg) r (...truncated)