Engineering Thermoelectric Performance of α‐GeTe by Ferroelectric Distortion

Abstract

The rhombohedral α‐GeTe can be approximated as a slightly distorted rock‐salt structure along its [1 1 1] direction and possesses superb thermoelectric performance. However, the role of such a ferroelectric‐like structural distortion on its transport properties remains unclear. Herein, we performed a systematic study on the crystal structure and electronic band structure evolutions of Ge1‐xSnxTe alloys where the degree of ferroelectric distortion is continuously tuned. It is revealed that the band gap is maximized while multiple valence bands are converged at x = 0.6, where the ferroelectric distortion is the least but still works. Once undistorted, the band gap is considerably reduced, and the valence bands are largely separated again. Moreover, near the ferro‐to‐paraelectric phase transition Curie temperature, the lattice thermal conductivity reaches its minima because of significant lattice softening enabled by ferroelectric instability. We predict a peak ZT value of 2.6 at 673 K in α‐GeTe by use of proper dopants which are powerful in suppressing the excess hole concentrations but meanwhile exert little influence on the ferroelectric distortion.

Contrasting the Roles of Cu Interstitials and Sb Substitutions in Regulating Ferroelectric Distortions and Thermoelectric Properties of α-GeTe

“…Moreover, the bond angle (β) increases from 82.2°to 90°. 31,51 As x increases from 0 to 0.08 in Ge 1−x Sb x Te, d S gradually increases and d L decreases. Meanwhile, β keeps rising and amounts to 83.8°.…”

“…Our previous study has shown that, in α-GeTe, the energies of ∑ and L valence bands first decrease and then increase as the ferroelectric distortion gradually decreases to zero, reaching the extreme value in the middle stage. 31 The modulation of ferroelectric distortion can achieve multiple valence band convergence, giving rise to an overall high band degeneracy (N V ). Therefore, we further performed the temperature-variant Hall test on two chosen samples, Ge 0.95 Sb 0.05 Te and Cu 0.03 GeTe, to investigate the cause of variation in electrical properties.…”

“…High-purity raw materials of Ge (5N), Sb (5N), Cu (5N), and Te (6N) were utilized for the synthesis of samples with nominal compositions of Ge 1−x Sb x Te (x = 0−0.12) and Cu y GeTe (y = 0− 0.05). The details of synthetic procedures can be found in ref 31.…”

“…As we know, the pristine α-GeTe possesses an intrinsically high carrier concentration of ∼10 21 cm –3 at ambient temperature. − Neutralizing the excess charge carriers in α-GeTe is a prerequisite of optimizing ZT . − On the other hand, GeTe experiences a phase transition at ∼700 K, from a paraelectric structure (cubic) to the ferroelectric structure (rhombohedral). , The phase transition is accompanied by the displacement of Ge atoms . Prior studies have shown that both the electronic band structures and the lattice dynamics are significantly affected by the strong ferroelectric distortion, which are advantageous for ZT improvement . The frequently used dopants in GeTe, such as Mn, Pb, − Sm, Sb, and Bi, , seem to influence the ferroelectric distortion, but the underlying mechanism remains unclear.…”

“…30 Prior studies have shown that both the electronic band structures and the lattice dynamics are significantly affected by the strong ferroelectric distortion, which are advantageous for ZT improvement. 31 The frequently used dopants in GeTe, such as Mn, 32 Pb, 33−36 Sm, 37 Sb, 38 and Bi, 39,40 seem to influence the ferroelectric distortion, but the underlying mechanism remains unclear.…”