Unusual Ferroelectricity Induced by the Jahn-Teller Effect: A Case Study on Lacunar Spinel Compounds

TL;DR

This paper explains how the Jahn-Teller effect can induce ferroelectricity in lacunar spinel compounds like GaV4S8, revealing a new mechanism for multiferroic materials with potential for electric control of magnetism.

Contribution

The study provides a group theory analysis showing how Jahn-Teller distortions can lead to ferroelectricity in non-centrosymmetric materials, exemplified by GaV4S8, and suggests a new pathway for designing multiferroics.

Findings

GaV4S8 exhibits ferroelectric Jahn-Teller distortion due to non-centrosymmetric parent phase.

The material is both ferroelectric and ferromagnetic, enabling electric control of magnetization.

Strong electron-phonon interactions involving low-energy phonon modes drive the ferroelectricity.

Abstract

The Jahn-Teller effect refers to the symmetry-lowering geometrical distortion in a crystal (or non-linear molecule) due to the presence of a degenerate electronic state. Usually, the Jahn-Teller distortion is not polar. Recently, GaV4S8 with the lacunar spinel structure was found to undergoes a Jahn-Teller distortion from cubic to ferroelectric rhombohedral structure at TJT = 38K. In this work, we carry out a general group theory analysis to show how and when the Jahn-Teller effect gives rise to ferroelectricity. On the basis of this theory, we find that the ferroelectric Jahn-Teller distort in GaV4S8 is due to the non-centrosymmetric nature of the parent phase and strong electron-phonon interaction related to two low-energy T2 phonon modes. Interestingly, GaV4S8 is not only ferroelectric, but also ferromagnetic with the magnetic easy axis along the ferroelectric direction. This suggests that GaV4S8 is a multiferroic in which an external electric field may control its magnetization direction. Our study not only explains the Jahn-Teller physics in GaV4S8, but also paves a new way for searching and designing new ferroelectrics and multiferroics.