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

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 κ_total ∼ 0.42 W m⁻¹ K⁻¹ because of a high degree of anharmonicity as estimated from the large Gr$u¨$neisen parameter (γ ∼ 4.07) and low Debye temperature (θ_d ∼ 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 n_e ∼ 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.