Chiral lone-pair helices with handedness coupling to electric-strain fields

TL;DR

This study demonstrates ferrochirality in specific tungsten bronzes, revealing an achiral-to-chiral phase transition driven by helical Bi3+ displacements, with potential for electric-field-controlled chirality switching and piezoelectric effects.

Contribution

It reports the first observation of ferrochirality in tetragonal tungsten bronzes and links chirality to a helical atomic displacement, proposing electric-strain coupling as a control mechanism.

Findings

Achiral-to-chiral phase transition observed in K4Bi2Nb10O30 and Rb4Bi2Nb10O30

Chirality originates from helical Bi3+ displacements

Coupling between chirality and piezoelectric response identified

Abstract

Ferrochiral materials with an achiral-to-chiral phase transition and switchable chirality have unique application opportunities, enabling control of the angular momentum of circularly polarized lattice vibrations (chiral phonons) and chirality-related electronic phenomena. Materials that fall into this class are, however, extremely rare, and often accompanied by other types of ferroic order that interfere with the ferrochiral responses. In this work, we demonstrate ferrochirality in two tetragonal tungsten bronzes, K4Bi2Nb10O30 and Rb4Bi2Nb10O30. Using high-resolution X-ray powder diffraction combined with transmission electron microscopy, we solve the incommensurately modulated and chiral structures. Temperature dependent X-ray powder diffraction reveals that both materials undergo an achiral-to-chiral phase transition from P4/mbm to P4212(00{\gamma})q00. The chirality originates from a cooperative helical displacement of Bi3+ atoms perpendicular to the c direction and represents the primary order parameter. As a secondary effect of the ferrochiral order, a spatially varying piezoelectric response is observed, consistent with the polycrystalline nature of the investigated materials. Through invariant analysis, an external electric-strain-field coupling with the piezoelectricity is proposed as a conjugate field for switching chirality, establishing tetragonal tungsten bronzes as a versatile playground for the emergent field of ferrochirality.