Soft anharmonic phonons and ultralow thermal conductivity in Mg ₃ (Sb, Bi) ₂ thermoelectrics

Abstract

The candidate thermoelectric compounds Mg3Sb2 and Mg3Bi2 show excellent performance near ambient temperature, enabled by an anomalously low lattice thermal conductivity (κl) comparable to those of much heavier PbTe or Bi2Te3. Contrary to common mass-trend expectations, replacing Mg with heavier Ca or Yb yields a threefold increase in κl in CaMg2Sb2 and YbMg2Bi2. Here, we report a comprehensive analysis of phonons in the series AMg2X2 (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 κl of Mg3X2 has inherent phononic origins. We uncover 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, enabling the threefold tuning in κl. These results provide key insights for manipulating phonon scattering without the traditional reliance on heavy elements.

Synergistic Effect of Band and Nanostructure Engineering on the Boosted Thermoelectric Performance of n‐Type Mg3+δ(Sb, Bi)2 Zintls

“…[16] Such a unique crystal structure leads to an intrinsic low κ l in Mg 3 Sb 2 . [16,21,22] Besides, Mg 3 Sb 2 possesses a narrow bandgap of 0.17 eV, confirmed by our first-principles density functional theory (DFT) calculations, as shown in Figure S1, Supporting Information, which indicates that Mg 3 Sb 2 is a suitable thermoelectric material with potentially high σ and good S. Besides, Mg 3 Sb 2 was reported to have relatively high mechanical performance, which can be further improved by Nd-doping. [23] Generally, pristine Mg 3 Sb 2 usually presents a p-type semiconducting behavior due to the native Mg vacancies (V Mg ) in the matrix, which are mainly derived from the low defect formation energy of V Mg .…”

Origin of the unique thermoelectric transport in Mg3(Sb,Bi)2: absence of d-orbital bonding in crystal cohesion

“…30 Logically, the softened acoustic phonon branch in Mg 3 Sb 2 can be attributed to the weak ‘Mg1–Sb’ bonding. 25 Fig. 5d presents the phonon DOS of Mg 3 Sb 2 and CaMg 2 Sb 2 .…”

“…20,24 This phonon soening was believed to be responsible for the unusually low lattice thermal conductivity of Mg 3 (Bi,Sb) 2 . 20,24,25 The electronic structure and phonon spectrum calculations indicate that all the anomalous properties of Mg 3 (Sb,Bi) 2 may have the same intrinsic origin.…”

Strongly Anharmonic Phonons and Their Role in Superionic Diffusion and Ultralow Thermal Conductivity of Cu7PSe6

“…At 5 K, the peak at 7 meV is well defined, but it considerably broadens on warming and merges with the elastic line above 250 K. Materials with a significantly anharmonic potential energy surface (including nonsuperionic systems) exhibit strongly temperature-dependent phonon spectra. [60][61][62][63] Superionic materials are generally highly anharmonic, with some very shallow potential energy basins allowing easy migration of cations, and therefore exhibit prominent anharmonic behaviors. In particular, we observe pronounced spectral shifts in the lowenergy regions (few meV) close the elastic line (E ≈ 0 meV), with an evolution toward a quasielastic response (Figure 2a,b).…”