Low Thermal Conductivity in Single Crystalline Mg₃Bi₂ and Its Thermopower Enhanced by Electron-Phonon Interaction

Promising thermoelectric materials are usually those of "phonon-glass electron-crystal" (PGEC) compounds, with low thermal conductivity and high carrier mobility. In metallic materials, strong electron-phonon interaction usually causes an increase of Seebeck coefficient (S) at low temperature, due to an extra electrical current driven by heat-carrying phonons, named phonon drag effect. Here, this study reports that single crystalline metallic Mg₃Bi₂ has low lattice thermal conductivity of ≈0.49 W m^-1 K^-1 at 285 K, and corresponding mean free path of phonons (L_ph) is ≈0.48 nm, with carrier mobility of ≈54.2 cm² V^-1 s^-1 around room temperature. It is found that S exhibits an increase as a "hump" ≈20 K, and phonon drag effect (S_ph) contributes to ≈80%, significantly higher than diffusive electrons. Meanwhile, S_ph is positively proportional to L_ph, where coefficient of S_ph/L_ph is ≈4.6 × 10² µV K^-1 µm^-1, twice that of CrSb₂ and FeSb₂, and relative strength of electron-phonon interaction is ≈0.12. The L_ph-intercept of S_ph/L_ph approaches to ≈4.68 nm, where phonons can be strongly scattered before interacting with electrons, leading to a negligible phonon drag effect. The findings shed light on fundamental understanding of thermoelectric transport and exploring novel thermoelectric materials.