Hetero-Composition and Homo-Structure Decoupling Charge and Phonon Properties in p-Type Thermoelectric SnAgBiTeSe₂

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 SnAgBiTeSe₂ 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^-1K^-1 at 300 K) and high ZT values of 0.42 at 300 K and 0.6 at 402 K for SnAg_1.03Bi_0.97TeSe₂, outperforming all reported AgBiSe₂-based counterparts across the 300-500 K temperature window.