Soft anharmonic phonons and ultralow thermal conductivity in Mg<sub>3</sub>(Sb, Bi)<sub>2</sub> thermoelectrics.

TitleSoft anharmonic phonons and ultralow thermal conductivity in Mg3(Sb, Bi)2 thermoelectrics.
Publication TypeJournal Article
Year of Publication2021
AuthorsJ Ding, T Lanigan-Atkins, M Calderón-Cueva, A Banerjee, DL Abernathy, A Said, A Zevalkink, and O Delaire
JournalScience Advances
Start Pageeabg1449
Date Published05/2021

The candidate thermoelectric compounds Mg<sub>3</sub>Sb<sub>2</sub> and Mg<sub>3</sub>Bi<sub>2</sub> show excellent performance near ambient temperature, enabled by an anomalously low lattice thermal conductivity (κ<sub>l</sub>) comparable to those of much heavier PbTe or Bi<sub>2</sub>Te<sub>3</sub> Contrary to common mass-trend expectations, replacing Mg with heavier Ca or Yb yields a threefold increase in κ<sub>l</sub> in CaMg<sub>2</sub>Sb<sub>2</sub> and YbMg<sub>2</sub>Bi<sub>2</sub> Here, we report a comprehensive analysis of phonons in the series <i>A</i>Mg<sub>2</sub> <i>X</i> <sub>2</sub> (<i>A</i> = Mg, Ca, and Yb; <i>X</i> = Bi and Sb) based on inelastic neutron/x-ray scattering and first-principles simulations and show that the anomalously low κ<sub>l</sub> of Mg<sub>3</sub> <i>X</i> <sub>2</sub> has inherent phononic origins. We uncover a large phonon softening and flattening of low-energy transverse acoustic phonons in Mg<sub>3</sub> <i>X</i> <sub>2</sub> compared to the ternary analogs and traced to a specific Mg-<i>X</i> bond, which markedly enlarges the scattering phase-space, enabling the threefold tuning in κ<sub>l</sub> These results provide key insights for manipulating phonon scattering without the traditional reliance on heavy elements.

Short TitleScience Advances