Relationship between the density of states effective mass and carrier concentration of thermoelectric phosphide Ag6Ge10P12 with strong mechanical robustness
Hiromasa Namiki, Masahiro Kobayashi, Kengo Nagata, Yoshiki Saito, Naoki Tachibana, Yuichi Ota
Abstract
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.