Crystalline anharmonicity and ultralow thermal conductivity in layered Bi₂GeTe₄ for thermoelectric applications

Anharmonicity related to crystal structures strongly affects the phonon propagation and reduces lattice thermal conductivity (κ_l). Associated with a higher order of anharmonicity, thermoelectric (TE) materials with a large Grüneisen parameter (γ) possess ultralow thermal conductivity (κ_total), which plays a decisive role in engineering of their TE performances. We report on the structural and TE transport properties of layered Bi₂GeTe₄, which belongs to the Bi₂Te₃ family of TE materials albeit with a larger unit cell. Bi₂GeTe₄ shows the electron dominated metallic nature throughout the temperature range with a carrier density of n = 8.23 × 10¹⁹ cm⁻³ at 300 K. Bi₂GeTe₄ exhibits an ultralow κ_total ∼ 0.52 W m⁻¹ K⁻¹ at 350 K with a high degree of anharmonicity as estimated by a large value of γ ∼ 4.24. Here, the calculated average phonon speed (ν_avg = 1.54 km s⁻¹) and larger bond lengths support the observations of ultralow κ_total. The sufficiently lower value of κ_total makes Bi₂GeTe₄ a good candidate for high performance TE materials through appropriate electronic transport modulation.