Anomalous Thermopower and High ZT in GeMnTe ₂ Driven by Spin’s Thermodynamic Entropy

Abstract

NaxCoO2 was known 20 years ago as a unique example in which spin entropy dominates the thermoelectric behavior. Hitherto, however, little has been learned about how to manipulate the spin degree of freedom in thermoelectrics. Here, we report the enhanced thermoelectric performance of GeMnTe2 by controlling the spin’s thermodynamic entropy. The anomalously large thermopower of GeMnTe2 is demonstrated to originate from the disordering of spin orientation under finite temperature. Based on the careful analysis of Heisenberg model, it is indicated that the spin-system entropy can be tuned by modifying the hybridization between Te-p and Mn-d orbitals. As a consequent strategy, Se doping enlarges the thermopower effectively, while neither carrier concentration nor band gap is affected. The measurement of magnetic susceptibility provides a solid evidence for the inherent relationship between the spin’s thermodynamic entropy and thermopower. By further introducing Bi doing, the maximum ZT in Ge0.94Bi0.06MnTe1.94Se0.06 reaches 1.4 at 840 K, which is 45% higher than the previous report of Bi-doped GeMnTe2. This work reveals the high thermoelectric performance of GeMnTe2 and also provides an insightful understanding of the spin degree of freedom in thermoelectrics.

Synergistically improving the thermoelectric and mechanical performance for p-type MnGe1−xSbxTe2 alloys

“…28,35 The decrease in carrier concentration may lead to a decrease in the exchange integral, which leads to an increase in entropy and an increase in the Seebeck coefficient. 36,37 Therefore, in our work, the Ge 1-x MnSb x Te 2 sample exhibited lower carrier concentration and higher mobility. As shown in Fig.…”

Discordant Distortion in Cubic GeMnTe2 and High Thermoelectric Properties of GeMnTe2-x%SbTe

“…(a) ZT values for GeMnTe 2 - x %SbTe. (b) ZT ave with the comparison of literature values. ,,, (c) Quality factor analysis showing the increase of ZT values.…”

“…Meanwhile, a high ZT ave of 0.96 in the temperature range of 400−823 K is achieved for the sample with x = 15.1, which outperforms the reported ZT ave values for GeMnTe 2 -related materials (Figure 7b). 42,43,52,64 The enhancement of ZT values is attributed to the synergistic optimization of κ L and Fermi level (η) (Figure 7c). The pristine GeMnTe 2 exhibits a quality factor (B) of 0.55 at 823 K, which is enhanced to 0.7 due to the reduction of κ L in GeMnTe 2 -15.1%SbTe.…”

“…Apart from the carrier concentrations, the density-of-state (DOS) effective mass ( m D *) is another important parameter influencing the Seebeck coefficient. Thus, the Pisarenko plot (single parabolic model with an assumption of acoustic phonon scattering) is typically used to estimate the DOS effective mass at 300 K (Figure a). ,, It can be seen that the DOS effective mass of 9–14 m e is needed to fit the Seebeck coefficient data and carrier concentration. Particularly, with increasing x , the m D * gradually increases from 9.5 m e for the pristine sample to 13 m e for the sample with x = 17.4 (Figure b), which can account for the increase of the Seebeck coefficient.…”

“…(a) Pisarenko plot including data from the literature at 300 K. ,, (b) DOS effective mass as a function of Sb contents. (c) Jonker plot at 300 and 823 K. (d) Weighted mobility for samples with different contents of SbTe.…”