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

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.