High zT and Its Origin in Sb‐doped GeTe Single Crystals

Abstract

A record high zT of 2.2 at 740 K is reported in Ge0.92Sb0.08Te single crystals, with an optimal hole carrier concentration ≈4 × 1020 cm−3 that simultaneously maximizes the power factor (PF) ≈56 µW cm−1 K−2 and minimizes the thermal conductivity ≈1.9 Wm−1 K−1. In addition to the presence of herringbone domains and stacking faults, the Ge0.92Sb0.08Te exhibits significant modification to phonon dispersion with an extra phonon excitation around ≈5–6 meV at Γ point of the Brillouin zone as confirmed through inelastic neutron scattering (INS) measurements. Density functional theory (DFT) confirmed this phonon excitation, and predicted another higher energy phonon excitation ≈12–13 meV at W point. These phonon excitations collectively increase the number of phonon decay channels leading to softening of phonon frequencies such that a three‐phonon process is dominant in Ge0.92Sb0.08Te, in contrast to a dominant four‐phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improving thermoelectric (TE) performance.

General strategies to improve thermoelectric performance with an emphasis on tin and germanium chalcogenides as thermoelectric materials

“…All these effects led to a ZT ∼ 1.85 at 725 K when x = 0.10. 386 A peak ZT ∼ 2.2 at 740 K in Sb doped GeTe has been reported by Vankayala et al 387 for Ge 0.92 Sb 0.08 Te single crystals. The extreme reduction in κ lattice was explained theoretically as well as experimentally in this work.…”

“…, to reduce the phase transition temperature of GeTe. 373,384,385,387,388,399,407 These techniques can be applied to other materials that face similar phase transition problems in their operating temperature range.…”

Realizing high thermoelectric performance in GeTe by defect engineering on cation sites

“…Therefore, co-doping methods of group V elements and elements Mn, 20,23 Cd, 21 Ga, 24 In, 25 Ti, 16 Cr, 18 or V, 17 on cation sites are commonly used and the group V elements generally refer to Bi 11,26 and Sb. 27–29 They can not only decrease the phase transition temperature, but also significantly reduce n H to 1–3 × 10 20 cm −1 , leading to excellent TE properties with the maximum ZT ( ZT max ) over 1.5. Therefore, defect engineering by co-doping to reduce the n H to the ideal level and prompt the adjustment of the electronic band structure can greatly improve the thermoelectric properties of GeTe materials.…”

Влияние термической предыстории на свойства эффективных термоэлектрических сплавов Ge-=SUB=-0.86-=/SUB=-Pb-=SUB=-0.1-=/SUB=-Bi-=SUB=-0.04-=/SUB=-Te-=SUP=-*-=/SUP=-

“…Эти усилия привели, как следует из публикаций, к значительным успехам. Параметр термоэлектрической эффективности ZT = S 2 σ κ T (здесь S -коэффициент термоэдс, σ -электропроводность, κ -теплопроводность, T -абсолютная температура) [2][3][4], согласно литературным данным, достигает значений, значительно превышающих 2 [5][6][7][8][9][10][11][12][13]. Однако, хотя публикации с высокими значениями параметра ZT в соединениях на основе PbTe, GeTe появились около 10 лет назад [14,15], сведений об их практическом применении до сих пор нет.…”