Yuli Yan

Electronic structure and thermoelectric properties of the Zintl compounds Sr5Al2Sb6 and Ca5Al2Sb6: first-principles study

Previous experimental work showed that Zn-doping only slightly increased the carrier concentration of Sr5Al2Sb6 and the electrical conductivity improved barely, which is very different from the results of Zn-doping in Ca5Al2Sb6. To understand their different thermoelectric behaviors, we investigated their stability, electronic structure, and thermoelectric properties using first-principles calculations and the semiclassical Boltzmann theory. We found that the low carrier concentration of Zn-doped Sr5Al2Sb6 mainly comes from its high positive formation energy. Moreover, we predict that a high hole concentration can possibly be realized in Sr5Al2Sb6 by Na or Mn doping, due to the negative and low formation energies of Na- and Mn-doped Sr5Al2Sb6, especially for Mn doping (−6.58 eV). For p-type Sr5Al2Sb6, the large effective mass along Γ–Y induces a large Seebeck coefficient along the y direction, which leads to the good thermoelectric properties along the y direction. For p-type Ca5Al2Sb6, the effective mass along Γ–Z is always smaller than those along the other two directions with increasing doping degree, which induces its good thermoelectric properties along the z direction. The analysis of the weight mobility of the two compounds confirms this idea. The calculated band structure shows that Sr5Al2Sb6 has a larger band gap than Ca5Al2Sb6. The relatively small band gap of Ca5Al2Sb6 mainly results from the appearance of a high density-of-states peak around the conduction band bottom, which originates from the Sb–Sb antibonding states in it.

Electronic structure and thermoelectric properties of Zintl compounds A3AlSb3 (A = Ca and Sr): first-principles study

Experimentally synthesized Zn-doped Sr3AlSb3 exhibited a smaller carrier concentration than Zn-doped Ca3AlSb3, which induces a lower thermoelectric figure of merit (ZT) than Zn-doped Ca3AlSb3. We used first-principles methods and the semiclassical Boltzmann theory to study the reason for this differing thermoelectric behavior and explored the optimal carrier concentration for high ZT values via p-type and n-type doping. The covalent AlSb4 tetrahedral arrangement exhibited an important effect on the electronic structure and thermoelectric properties. p-type Ca3AlSb3 may exhibit good thermoelectric properties along its covalent AlSb4 chain due to its double band degeneracy at the valence band edge and small effective mass along its one-dimensional chain direction. Zn doping the Al site exhibited higher formation energy for Sr3AlSb3 than Ca3AlSb3, which explains the lower carrier concentration for Zn-doped Sr3AlSb3 than Zn-doped Ca3AlSb3. The double band degeneracy at the valence band edge for Ca3AlSb3 may also help to increase the carrier concentration. Sr3AlSb3 containing isolated Al2Sb6 dimers can exhibit a high thermoelectric performance via heavy p-type doping with a carrier concentration above 1 × 1020 holes per cm3. Moreover, the ZT maxima for the n-type Sr3AlSb3 can reach 0.76 with a carrier concentration of 4.5 × 1020 electrons per cm3.

Ag-Mg antisite defect induced high thermoelectric performance of α-MgAgSb

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.