Strong Lattice Softening Induced by Atomic Mismatch in Meta‐Phase Thermoelectrics

Abstract Conventional strategies for suppressing lattice thermal conductivity κ _L typically focus on maximizing phonon scattering to reduce phonon mean free path. Such reductions, however, are limited to the interatomic spacing or phonon wavelength. Alternatively, herein, an effective approach is proposed to lower phonon velocity by introducing atoms with significant atomic mismatch into the crystal lattice of three meta‐phases. Specifically, substituting Te for S in Ag ₈ SnS ₆ and Cu ₂ S, or Sn for Si in Mg ₂ Si considerably increases the atomic mass and weakens the chemical bonding, causing notable reductions in the sound velocity. This reduction further leads to an amorphous‐like, extremely low lattice thermal conductivity κ _L across the whole temperature range. Consequently, we achieve outstanding thermoelectric performance in these atomic mismatched meta‐phases, with a maximum zT of 1.0 for Ag ₈ SnS _4.99 Te, 1.1 for Mg ₂ Si _0.5 Sn _0.5 , and 2.0 for Cu ₂ S _0.5 Te _0.5 . The work demonstrates a new approach to manipulating thermal conductions through lattice softening, providing a promising pathway for designing high‐performance thermoelectric materials.