Critical mode and band-gap-controlled bipolar thermoelectric properties of SnSe

TL;DR

This study combines neutron diffraction and density functional theory to analyze SnSe's crystal structure and electronic properties, revealing how structural distortions and band gap changes influence its thermoelectric performance.

Contribution

It provides a detailed structural and electronic analysis of SnSe, linking phase transitions and band gap variations to its thermoelectric properties, which was not previously understood.

Findings

Structural distortion modes are active over a wide temperature range.

Band gap reduction causes bipolar conductivity onset.

Controlled doping can optimize Seebeck effects along different directions.

Abstract

The reliable calculation of electronic structures and understanding of electrical properties depends on an accurate model of the crystal structure. Here, we have reinvestigated the crystal structure of the high-zT thermoelectric material tin selenide, SnSe, between 4 and 1000 K using high-resolution neutron powder diffraction. Symmetry analysis reveals the presence of four active structural distortion modes, one of which is found to be active over a relatively wide range of more than +/-200 K around the symmetry-breaking Pnma-Cmcm transition at 800 K. Density functional theory calculations on the basis of the experimental structure parameters show that the unusual, step-like temperature dependencies of the electrical transport properties of SnSe are caused by the onset of intrinsic bipolar conductivity, amplified and shifted to lower temperatures by a rapid reduction of the band gap between 700 and 800 K. The calculated band gap is highly sensitive to small out-of-plane Sn displacements observed in the diffraction experiments. SnSe with a sufficiently controlled acceptor concentration is predicted to produce simultaneously a large positive and a large negative Seebeck effect along different crystal directions.