High Thermoelectric Performance and Low Lattice Thermal Conductivity in Lattice-Distorted High-Entropy Semiconductors AgMnSn1–xPbxSbTe4

We investigate the structure and thermoelectric properties of a new high-entropy solid solution system AgMnSn1–xPbxSbTe4 (x = 0, 0.25, 0.5, 0.75, and 1), which crystallizes in the rock-salt NaCl structure with cations Ag, Mn, Sn, Pb, and Sb randomly disordered over the Na site. Our density functional theory calculations indicate that AgMnSn1–xPbxSbTe4 exhibits complex multi-peak valence band structures, whose energy difference is lower than 0.11 eV, leading to effective band convergence and thus high density of states effective mass m* and Seebeck coefficients. As a consequence, AgMnSn0.25Pb0.75SbTe4 has a peak ZT of 1.3 at 773 K and a desirable average ZT value of 0.8 in the temperature range of 400–773 K. In addition, we propose the lattice distortion degree (i.e., δ) as an important indicator of thermoelectric performance for high-entropy materials. Specifically, with the gradual increase in δ, the lattice thermal conductivity decreases monotonically from 0.90 W m–1 K–1 for AgMnSnSbTe4 (i.e., δ = 0.205) to 0.54 W m–1 K–1 for AgMnPbSbTe4 (i.e., δ = 0.230) at 300 K. Meanwhile, the generalized material parameter Bx*/B0* and ZT increase monotonically from 1 and 0.11 for δ = 0.205 to 3.15 and 0.29 for δ = 0.230 at 300 K.