High Seebeck Coefficient and Unusually Low Thermal Conductivity Near Ambient Temperatures in Layered Compound Yb_2–xEu_xCdSb₂

Abstract

Zintl phases are promising thermoelectric materials because they are composed of both ionic and covalent bonding, which can be independently tuned. An efficient thermoelectric material would have regions of the structure composed of a high-mobility compound semiconductor that provides the "electron−crystal" electronic structure, interwoven (on the atomic scale) with a phonon transport inhibiting structure to act as the "phonon−glass". The phonon−glass region would benefit from disorder and therefore would be ideal to house dopants without disrupting the electron−crystal region. The solid solution of the Zintl phase, Yb 2−x Eu x CdSb 2 , presents such an optimal structure, and here we characterize its thermoelectric properties above room temperature. Thermoelectric property measurements from 348 to 523 K show high Seebeck values (maximum of ∼269 μV/K at 523 K) with exceptionally low thermal conductivity (minimum ∼0.26 W/m K at 473 K) measured via laser flash analysis. Speed of sound data provide additional support for the low thermal conductivity. Density functional theory (DFT) was employed to determine the electronic structure and transport properties of Yb 2 CdSb 2 and YbEuCdSb 2 . Lanthanide compounds display an f-band well below (∼2 eV) the gap. This energy separation implies that f-orbitals are a silent player in thermoelectric properties; however, we find that some hybridization extends to the bottom of the gap and somewhat renormalizes hole carrier properties. Changes in the carrier concentration related to the introduction of Eu lead to higher resistivity. A zT of ∼0.67 at 523 K is demonstrated for Yb 1.6 Eu 0.4 CdSb 2 due to its high Seebeck, moderate electrical resistivity, and very low thermal conductivity.

Exploratory Work in the Quaternary System of Ca–Eu–Cd–Sb: Synthesis, Crystal, and Electronic Structures of New Zintl Solid Solutions

“…The AE 1 site, offering a larger available volume, accumulates most of the Eu in the structure, whereas the smaller AE 2 site shows a minute Eu occupancy of about 2%, in line with a larger ionic radius of Eu 2+ versus Ca 2+ . A similar trend was previously observed for the Yb 2 CdSb 2 -type solid solutions, Sr 2− x A x CdSb 2 ( A = Ca, Yb), Ba 2− x A x CdSb 2 ( A = Ca, Sr, Eu, Yb) [39], and Yb 2− x Eu x CdSb 2 [24,39].…”

“…In accordance with the simple electron counting in the frame of the Zintl concept, CaEuCdSb 2 displays an electronic bandgap of about 0.5 eV. This value is larger than the 0.1 eV bandgap calculated for the isostructural YbEuCdSb 2 [24]. This is likely is an electronic effect due to electronegativity differences between Yb and Ca, suggesting lower iconicity of the bonding in the latter compound.…”

“…Strong correlations of the Eu 4f orbitals considered by applying the GGA + U approach with U eff = 7 eV (rotationally invariant formulation [22]). For consistency with the previous theoretical work on d 10 metal antimonides [23,24], we also introduced U eff = 2 eV for the Cd 4d states. Spin polarization was treated in a simple ferromagnetic model.…”

Inorganic thermoelectric materials: A review

“…Best performing oxide‐based materials highlighting (A) highest σ, (B) low κ t and (C) ZT value via miscellaneous optimization techniques …”

Experimental and Theoretical Investigations for the Quaternary Mixed‐Cation Zintl Phase Ca1.82(1)Eu0.18CdSb2

“…Some of those include the A 14 MSb 11 (A = Ca, Yb; M = Al, Mn), A 11 Pn 10 (A = Ca, Yb, Eu; Pn = Sb, Bi, Sn), A 5 M 2 Pn 6 (A = Ca, Yb; M = Al, In; Pn = Sb, Ge) systems, and AM 2 Pn 2 (A = Ca, Yb, Eu; M = Mg, Zn, Cd; Pn = Sb, Bi) systems . Recently, the Yb 2‐ x Eu x CdSb 2 series having the Eu substitution for Yb in the A 2 CdSb 2 (A = Ca, Yb, Eu, Sr) system was reported for the high Z T value due to its low lattice conductivity originated from the 2‐dimensional (2‐D) crystal structure and the Yb 2+ /Eu 2+ mixed‐cations . However, despite the similarities of Yb and Ca in terms of the cationic sizes as well as electronegativities, the Ca 2‐ x Eu x CdSb 2 series having the Ca 2+ /Eu 2+ mixed‐site has not been thoroughly investigated in the A 2 CdSb 2 system.…”