Crystalline anharmonicity and ultralow thermal conductivity in layered Bi2GeTe4for thermoelectric applications

Abstract

Anharmonicity related to crystal structures strongly affects the phonon propagation and reduces lattice thermal conductivity (κl). Associated with a higher order of anharmonicity, thermoelectric (TE) materials with a large Grüneisen parameter (γ) possess ultralow thermal conductivity (κtotal), 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 κtotal ∼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 (νavg = 1.54 km s-1) and larger bond lengths support the observations of ultralow κtotal. The sufficiently lower value of κtotal makes Bi2GeTe4 a good candidate for high performance TE materials through appropriate electronic transport modulation.