Electronic Instability and Anharmonicity in SnSe

TL;DR

This paper investigates the electronic and lattice origins of strong anharmonicity in SnSe, revealing how electronic instabilities lead to ultralow thermal conductivity, which is key for thermoelectric performance.

Contribution

It identifies the Jahn-Teller electronic instability as the driver of anharmonic lattice behavior in SnSe, linking electronic structure to thermal properties.

Findings

Jahn-Teller instability causes high-temperature lattice distortion.

Coupling of phonon modes explains phase transition mechanism.

Electronic-lattice coupling underpins ultralow thermal conductivity.

Abstract

The binary compound SnSe exhibits record high thermoelectric performance, largely because of its very low thermal conductivity. The origin of the strong phonon anharmonicity leading to the low thermal conductivity of SnSe is investigated through first-principles calculations of the electronic structure and phonons. It is shown that a Jahn-Teller instability of the electronic structure is responsible for the high-temperature lattice distortion between the Cmcm and Pnma phases. The coupling of phonon modes and the phase transition mechanism are elucidated, emphasizing the connection with hybrid improper ferroelectrics. This coupled instability of electronic orbitals and lattice dynamics is the origin of the strong anharmonicity causing the ultralow thermal conductivity in SnSe. Exploiting such bonding instabilities to generate strong anharmonicity may provide a new rational to design efficient thermoelectric materials.