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

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 Ag₈SnS₆ and Cu₂S, or Sn for Si in Mg₂Si 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 Ag₈SnS_4.99Te, 1.1 for Mg₂Si_0.5Sn_0.5, and 2.0 for Cu₂S_0.5Te_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.