Ultralow thermal conductivity and thermoelectric properties of Bi4GeTe7 with an intrinsic van der Waals heterostructure

Abstract

Ternary chalcogenides, having large crystalline unit cells and van der Waals stacking of layers, are expected to be poor thermal conductors and good thermoelectric (TE) materials. We are reporting that layered Bi4GeTe7 with alternating quintuplet-septuplet layers of Bi2Te3 and Bi2GeTe4 has an ultralow thermal conductivity of κtotal ∼ 0.42 W m−1 K−1 because of a high degree of anharmonicity as estimated from the large Gru¨neisen parameter (γ ∼ 4.07) and low Debye temperature (θd ∼ 135 K). The electron dominated charge transport has been realized from the Seebeck coefficient, S ∼ −82 μV/K, at 380 K and a Hall carrier concentration of ne ∼ 9.8 × 1019 cm−3 at 300 K. Observation of weak antilocalization due to the spin–orbit coupling of heavy Bi and Te also advocates Bi4GeTe7 to be a topological quantum material. The cross-sectional transmission electron microscopy images show the inherent stacking of hetero-layers, which are leading to a large anharmonicity for poor phonon propagation. Thus, being a poor thermal conductor with a TE figure of merit, ZT ∼ 0.24, at 380 K, Bi4GeTe7 is a good material for TE applications.