Hetero‐Composition and Homo‐Structure Decoupling Charge and Phonon Properties in p‐Type Thermoelectric SnAgBiTeSe ₂

ABSTRACT The inherent compromise between charge mobility and phonon transport necessitates hierarchical structural modularity to achieve a high figure‐of‐merit ( ZT ) in homogeneous materials. Herein, shifting our focus from conventional grain boundaries to grain interiors, we leverage configurational entropy and metavalent bonding to design a novel SnAgBiTeSe 2 solid solution alloy with a single cubic crystal structure, representing a true hetero‐composition/homo‐structure (heC/hoS) system. Due to its highly polarizable metavalent bonds, the material maintains a uniform macroscopic symmetry, with strain fluctuations at the nanoscale preventing macroscopic phase separation. This dual mechanism strongly suppresses phonon propagation without significantly compromising charge transport. Its electronic structure exhibits a pronounced Rashba effect, which simultaneously modulates both valley and spin degeneracies. This leads to the formation of highly converged, multi‐spin‐split bands that facilitate efficient charge carrier transport. Moreover, owing to the suppression of Bi Ag antisite defect enabled by the heC/hoS architecture, slight Ag doping successfully induces rare p‐type conductivity. These features collectively yield a record‐low lattice thermal conductivity ( κ l = 0.35 Wm −1 K −1 at 300 K) and high ZT values of 0.42 at 300 K and 0.6 at 402 K for SnAg 1.03 Bi 0.97 TeSe 2 , outperforming all reported AgBiSe 2 ‐based counterparts across the 300–500 K temperature window.