Ultralow Thermal Conductivity and Thermoelectric Properties of Bi4GeTe7 with an Intrinsic van der Waal Heterostructure

TL;DR

This study reports that layered Bi4GeTe7 exhibits ultralow thermal conductivity and promising thermoelectric properties due to its van der Waal heterostructure, high anharmonicity, and topological features, making it suitable for thermoelectric applications.

Contribution

It demonstrates that Bi4GeTe7's unique layered structure results in ultralow thermal conductivity and notable thermoelectric performance, highlighting its potential as a topological thermoelectric material.

Findings

Thermal conductivity of 0.42 W/mK at room temperature.

Seebeck coefficient of 82 μV/K at 380 K.

ZT value of approximately 0.24 at 380 K.

Abstract

Ternary chalcogenides, having large crystalline unit cell and van der Waal 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, \k{appa}total 0.42 Wm-1K-1 because of high degree of anharmonicity as estimated from large Gruneisen parameter ({\gamma} 4.07) and low Debye temperature ({\theta}d 135 K). The electron dominated charge transport has been realized from the Seebeck coefficient, S - 82 uV/K, at 380 K, and Hall carrier concentration of ne ~ 9.8 x 1019 cm-3 at 300 K. Observation of weak antilocalization (WAL), due to spin-orbit coupling (SOC) of heavy Bi and Te, advocate Bi4GeTe7 to be a topological quantum material also. 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, the Bi4GeTe7 is a good material for TE applications.