Direct evidence of strong local ferroelectric ordering in a thermoelectric semiconductor

TL;DR

This study provides direct evidence of local ferroelectric ordering in the thermoelectric semiconductor AgSbSe2, revealing nanometer-scale ferroelectric domains despite its cubic centrosymmetric structure, which could enable new multifunctional applications.

Contribution

It demonstrates the existence of local ferroelectric domains in AgSbSe2 through direct polarization switching measurements, highlighting a novel ferroelectric behavior in a centrosymmetric thermoelectric material.

Findings

AgSbSe2 exhibits nanometer-scale ferroelectric domains.

Local ferroelectricity arises from structural distortions and lone pair activity.

Ferroelectric domains can be switched by external electric fields.

Abstract

It is thought that the proposed new family of multi-functional materials namely the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching we show that the recently discovered thermoelectric semiconductor $AgSbSe_{2}$ has local ferroelectric ordering. Using piezo-response force microscopy, we demonstrate the existence of nanometer scale ferroelectric domains that can be switched by external electric field. These observations are intriguing as $AgSbSe_{2}$ crystalizes in cubic rock salt structure with centro-symmetric space group (Fm-3m) and therefore no ferroelectricity is expected. However, from high resolution transmission electron microscopy (HRTEM) measurement we found the evidence of local superstructure formation which, we believe, leads to local distortion of the centro-symmetric arrangement in $AgSbSe_{2}$ and gives rise to the observed ferroelectricity. Stereochemically active $5s^{2}$ lone pair of Sb can also give rise to local structural distortion, which creates ferroelectricity in $AgSbSe_{2}$.