Nanoscale Sn off-centering behind low thermal conductivity in SnSe thermoelectric

TL;DR

This study reveals that persistent Sn off-centering distortions in SnSe across its phase transition contribute to its ultra-low thermal conductivity, driven by a complex crossover involving ferroelectric-like instabilities.

Contribution

It uncovers the complex local atomic structure and off-centering distortions in SnSe, linking them to its low thermal conductivity and phase transition behavior.

Findings

Sn off-centering persists in high-temperature phase

Transition is a complex crossover, not purely order-disorder or displacive

Sn off-centering is key to ultra-low thermal conductivity

Abstract

The local atomic structure of SnSe was characterized across its orthorhmbic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn off-centering distortions persist in the high symmetry high temperature phase, with symmetry different from that of ordered distortions below the transition. The analysis implies that the transition is neither order-disorder nor displacive, but rather a complex crossover where the character of coupling changes from 3D-like at low temperature to 2D-like at high temperature. Robust ferro-coupled SnSe intra-layer distortions suggest a ferroelectric-like instability as the driving force. Complex local Sn off-centering is integral to the ultra-low lattice thermal conductivity mechanism in SnSe.