Origin of Low Thermal Conductivity in In₄Se₃

In4Se3 is an attractive n-type thermoelectric material for midrange waste heat recovery, owing to its low thermal conductivity (∼0.9 W·m- 1·K-1 at 300 K). Here, we explore the relationship between the elastic properties, thermal conductivity, and structure of In4Se3. The experimentally determined average sound velocity (2010 m·s-1), Young's modulus (47 GPa), and Debye temperature (198 K) of In4Se3 are rather low, indicating considerable lattice softening. This behavior, which is consistent with low thermal conductivity, can be related to the complex bonding found in this material, in which strong covalent In-In and In-Se bonds coexist with weaker electrostatic interactions. Phonon dispersion calculations show that Einstein-like modes occur at ≈30 cm-1. These Einstein-like modes can be ascribed to weakly bonded In+ cations located between strongly bonded [(In3)5+(Se2-)3]- layers. The Grüneisen parameter for the soft-bonded In+ at the frequencies of the Einstein-like modes is large, indicating a high degree of bond anharmonicity and hence increased phonon scattering. The calculated thermal conductivity and elastic properties are in good agreement with experimental results.