Non‐Rigid Band Structure in Mg₂Ge for Improved Thermoelectric Performance

Abstract

Magnesium silicide and its solid solutions are among the most attractive materials for thermoelectric generators in the temperature range of 500-800 K. However, while n-type Mg 2 (Si,Ge,Sn) materials show excellent thermoelectric performance, the corresponding p-type solid solutions are still inferior, mainly due to less favorable properties of the valence bands compared to the conduction bands. Here, Li doped Mg 2 Ge with a thermoelectric figure of merit zT of 0.5 at 700 K is reported, which is four times higher than that of p-type Mg 2 Si and double than that of p-type Mg 2 Sn. The reason for the excellent properties is an unusual temperature dependence of Seebeck coefficient and electrical conductivity compared to a standard highly doped semiconductor. The properties cannot be captured assuming a rigid band structure but well reproduced assuming two parabolic valence bands with a strong temperature dependent interband separation. According to the analysis, the difference in energy between the two bands decrease with temperature, leading to a band convergence at around 650 K and finally to an inversion of the band positions. The finding of a combination of a light and a heavy band that are non-rigid with temperature can pave the way for further optimization of p-type Mg 2 (Si,Ge,Sn).

Synergistic modulation of the thermoelectric performance of melt-spun p-type Mg2Sn via Na2S and Si alloying

“…(e) Diagram of S 2 s and 1/k L of various Mg 2 (Sn,Si) materials. (f)Comparison between the peak zT of our (Mg 2 Sn 0.9 Si 0.1 ) 0.93 (Na 2 S) 0.07 and previously reported data 18,26,29,34,39,53,57,58,63,64.…”

“…Comparison between the peak zT of our (Mg 2 Sn 0.9 Si 0.1 ) 0.93 (Na 2 S) 0.07 and previously reported data. 18,26,29,34,39,53,57,58,63,64…”

Machine-learned model Hamiltonian and strength of spin-orbit interaction in strained Mg2X (X = Si, Ge, Sn, Pb)

“…These Mg 2 X solid solutes can desirably change the electronic, mechanical, and macroscopic properties of these metallic matrices. [1][2][3][4][5]12] Furthermore, Mg 2 X(X = Si, Ge, Sn) are chemically and thermally stable, nontoxic, resistive against oxidation, economically inexpensive, environmentally friendly, relatively light weight, and therefore have a great potential for large-scale production. [2,7,8] More importantly, they possess a high figure of merit and can serve as good solidstate thermoelectric materials for converting waste heat to electricity.…”

“…For instance, one main approach followed to manipulate the electrical transport of these compounds was by intercalation with differing elements. [4,5,[7][8][9][10][11][12][13][14][15][16][17][18][19] It was found that the intercalation can result in nonlinearity in the band gap, enhanced spin-orbit interaction (SOI), and band splitting. The controllable and efficient thermoelectric materials are promising in designing micro-scale self-powered sensors, solar thermal elements, and waste heat recovery devices.…”

“…Additionally, there are multiple works discussing various aspects of these compounds by using an effective singleband parabolic approximation. [8,12,14] The common assumption of these works is the homogeneity of the alloys and generalization of the rigid-band structure approach to obtain the band structure of the intermediate alloy systems. Motivated by the above, we have constructed multiorbital tight-binding (MOTB) models, accounting for the {s, p, d, s * } orbitals.…”