Orbital Alignment for High Performance Thermoelectric YbCd₂Sb₂ Alloys

Abstract

As a typical class of Zintl thermoelectrics, AB2C2 (A = Eu, Yb, Ba, Ca, Mg; B = Zn, Cd, Mg, and C = Sb, Bi) compounds have shown a superior thermoelectric performance, largely stemming from the existence of multiple transporting bands in both conduction types. Being similar to many III–V and elemental semiconductors, the transport of holes in AB2C2 Zintls usually involves multiple valence bands with extrema at the Brillouin zone center Γ. However, these valence bands, originating from different orbitals, are unnecessarily aligned in energy due to the crystal field splitting. Formation of solid solutions between constituent compounds having opposite arrangements in energy of band orbitals is believed to be particularly helpful for thermoelectric enhancements, because orbital alignment increases band degeneracy while alloy defects scatter phonons. These effects are simultaneously realized in this work, where the p orbitals of anions in YbCd2–x Zn x Sb2 alloys are well-aligned for maximizing the electronic performance, and meanwhile high-concentration Cd/Zn substitutions are introduced for minimizing the lattice thermal conductivity. As a result, a significantly enhanced thermoelectric figure of merit, zT ∼ 1.3, is achieved, being a record among AB2C2 Zintls in p-type. This work demonstrates not only YbCd2–x Zn x Sb2 alloys as efficient thermoelectrics but also orbital alignment as an effective strategy for advancing thermoelectrics.

Insights into the design of thermoelectric Mg3Sb2 and its analogs by combining theory and experiment

“…This powerful strategy is confirmed in several alloys including YbCd alloys, which combines with reducing thermal conductivities by the alloy scattering to result in a peak zT of 0.87 at 850 K. 36 As shown by Wang et al, optimizing electrical transport performance induced by minimizing the p orbital splitting energy as well as minimizing the lattice thermal conductivity by the point defect scattering leads to an optimal zT of~1.3 at 700 K in the YbCd 2−x Zn x Sb 2 alloys. 35 In addition to the approach of forming solid solutions, biaxial strain engineering can be applied to continuously tune Δ so as to optimize electrical transport performance in thin-film TE materials (see Fig. 4b).…”

“…1). Experimentally, strategies such as doping, [45][46][47]49,50 alloying, 15,16,18,19,21,23,25,29,35,36 and controlling vacancy concentrations 27 have been used to optimize electrical transport properties and to reduce thermal conductivity through point defect scattering. As nearly all these compounds are persistently p-type, it is a remarkable breakthrough that the low-cost Te-doped Mg 3 Sb 2 -based compounds were recently discovered to show exceptional n-type TE properties at low and intermediate temperatures ( Fig.…”

Point defect approach to enhance the thermoelectric performance of Zintl-phase BaAgSb

“…In addition, the guest atoms or vacancies can suppress the lattice thermal conductivity by strengthening phonon scattering [22][23][24]. Indeed, high thermoelectric efficiency has been realized in some Zintl phases, such as YbZn 2 Sb 2 [25,26], Ca 9 Zn 4.5 Sb 9 [27,28], Ca 5 Al 2 Sb 6 [29], and Yb 14 MnSb 11 [30]. Our previous studies reported the thermoelectric performance of the ZrBeSi-type Zintl-phase Eu 2 ZnSb 2 , and a ZT value of 1.0 was achieved at 823 K [31].…”

Achieving High Thermoelectric Performance in Rare-Earth Element-Free CaMg 2 Bi 2 with High Carrier Mobility and Ultralow Lattice Thermal Conductivity

“…As one of the two major categories of 1-2-2 type materials, Sb-based system has been paid lots of attention and is in-depth studied. However, it is a pity that the toxic element Cd is always inevitable among Sb-based materials with maximal ZT in excess of 1 [35][36][37][38][39], which is not in keeping with the concept of environmental friendliness. Out of the environmental consideration, peoples gradually turn their concerns to the analogous Bi-based Zintl materials, AMg 2 Bi 2 (A = Ca, Yb, Eu) [40,41].…”