Low and Anisotropic Thermal Conductivity in Mixed-Valent Sn$_2$S$_3$

TL;DR

This study reveals that mixed-valent Sn$_2$S$_3$ exhibits low, anisotropic thermal conductivity due to anharmonic rattling vibrations of Sn(II) atoms, with potential implications for thermoelectric materials.

Contribution

It introduces a combined Boltzmann and Wigner model analysis of Sn$_2$S$_3$, highlighting the role of lone pair interactions and rattling atoms in low thermal conductivity.

Findings

Sn$_2$S$_3$ has low thermal conductivity along the c-axis.

Sn(II) atoms act as rattlers causing anharmonic vibrations.

The low thermal conductivity is weakly temperature-dependent.

Abstract

Compounds of Sn, such as SnSe and SnS, exhibit novel phonon characteristics and low thermal conductivity, making them emerging star materials in the thermoelectric family. In this work, through the Boltzmann transport equation scheme and the Wigner thermal transport model, quasi-1D mixed-valent Sn$_2$S$_3$ were found to exhibit a low thermal conductivity along c-axis with a weak temperature dependence. The low thermal conductivity is attributed to the anharmonic rattling vibrations of weakly bonded Sn(II) atoms, which are influenced by the coulomb interaction of lone pairs at adjacent Sn(II) atoms. The rattling of Sn(II) induces low-frequency flat optical phonons and avoids crossing behavior. The atomic displacements and mean square displacement (MSD) analysis reveal that Sn(II) atoms exhibit significantly greater and anisotropic displacements compared to Sn(IV) and S, confirming that Sn(II) behaves as a rattler. The results obtained from this work suggest an opportunity to discover low thermal conductivity in mixed-valent compounds.