Relationship between the density of states effective mass and carrier concentration of thermoelectric phosphide Ag6Ge10P12 with strong mechanical robustness

• Published in: Materials Today Sustainability Vol. 18; p. 100116
• Main Authors: Namiki, H., Kobayashi, M., Nagata, K., Saito, Y., Tachibana, N., Ota, Y.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.06.2022
• Subjects: Carrier tuning; Eco-friendly thermoelectric material; Electronic structure; Mechanical properties; Pudding-mold band; Thermoelectric properties; Pudding-mold band; Mechanical properties; Electronic structure; Carrier tuning; Thermoelectric properties; Eco-friendly thermoelectric material
• ISSN: 2589-2347; 2589-2347
• DOI: 10.1016/j.mtsust.2022.100116

A ternary phosphide Ag6Ge10P12 containing no toxic elements has attracted much attention as an eco-friendly thermoelectric material. This study reveals the relationship between the density of states effective mass mDOS∗ and carrier concentration n for achieving higher thermoelectric performance in Ag6Ge10P12. The Seebeck coefficient S of Ag6Ge10−xGaxP12 (0.0 ≤ x ≤ 0.25) with various carrier concentrations is unexpectedly improved by increasing n. Scrutinizing electrical transport properties, including the S and electrical conductivity σ, and electronic structure indicated that the improved S is owing to the enhanced mDOS∗, which originated from tuning the Fermi level in a valence band with multi-valley and pudding-mold bands. The power factor S2σ is enhanced by both improved S and σ. The total thermal conductivity κtot monotonically decreases with increasing x because of the decrease in the lattice thermal conductivity κL. Combining the improved S2σ and reduced κtot, the maximum ZT value of Ag6Ge9.875Ga0.125P12 at 390 K reaches ∼0.33 with the optimal carrier concentration n ∼7.0 × 1020 cm−3. The present results demonstrate a guideline for enhancing the thermoelectric performance of Ag6Ge10P12 by breaking the trade-off relationship between the S and σ through n. Moreover, Young's modulus E and nanoindentation hardness H of Ag6Ge10-xGaxP12 are greater than 125 GPa and 9 GPa, respectively, comparable to those of thermoelectric Si–Ge alloy. These findings and insights in the present study will serve as a basis for enhancing the thermoelectric performance and fabricating the thermoelectric module for eco-friendly phosphide Ag6Ge10P12. [Display omitted] •The large density of states effective mass originated from the valence band with the coexistence of multi-valley and pudding-mold bands leads to high thermoelectric performance for Ag6Ge10P12.•The density of states effective mass of Ag6Ge10P12 is dramatically changed by shifting the Fermi level in the valence band, optimized with carrier concentration n ∼5–7 × 1020 cm−3.•Mechanical robustness of Ag6Ge10P12 is superior to state-of-the-art thermoelectric materials with similar lattice thermal conductivity.•Strong mechanical robustness originates from the covalent framework that shares vertices of tetrahedron GeP4 with strong Ge–P bonding.