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The article presents a study examining Soft enharmonic phonons and ultralow thermal conductivity in Mg3(Sb, Bi)2 thermoelectrics. It found a large phonon softening and flattening of low-energy transverse acoustic phonons in Mg3X2 compared to the ternary analogs and traced to a specific Mg-X bond, which markedly enlarges the scattering phase-space.

The candidate thermoelectric compounds Mg 3 Sb 2 and Mg 3 Bi 2 show excellent performance near ambient temperature, enabled by an anomalously low lattice thermal conductivity (κ l ) comparable to those of much heavier PbTe or Bi 2 Te 3 Contrary to common mass-trend expectations, replacing Mg with heavier Ca or Yb yields a threefold increase in κ l in CaMg 2 Sb 2 and YbMg 2 Bi 2 Here, we report a comprehensive analysis of phonons in the series A Mg 2 X 2 ( A = Mg, Ca, and Yb; X = Bi and Sb) based on inelastic neutron/x-ray scattering and first-principles simulations and show that the anomalously low κ l of Mg 3 X 2 has inherent phononic origins. We uncover a large phonon softening and flattening of low-energy transverse acoustic phonons in Mg 3 X 2 compared to the ternary analogs and traced to a specific Mg- X bond, which markedly enlarges the scattering phase-space, enabling the threefold tuning in κ l These results provide key insights for manipulating phonon scattering without the traditional reliance on heavy elements., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

In electrically conductive solids, the Wiedemann-Franz law requires the electronic contribution to thermal conductivity to be proportional to electrical conductivity. Violations of the Wiedemann-Franz law are typically an indication of unconventional quasiparticle dynamics, such as inelastic scattering, or hydrodynamic collective motion of charge carriers, typically pronounced only at cryogenic temperatures. We report an order-of-magnitude breakdown of the Wiedemann-Franz law at high temperatures ranging from 240 to 340 kelvin in metallic vanadium dioxide in the vicinity of its metal-insulator transition. Different from previously established mechanisms, the unusually low electronic thermal conductivity is a signature of the absence of quasiparticles in a strongly correlated electron fluid where heat and charge diffuse independently., (Copyright © 2017, American Association for the Advancement of Science.)