Hidden Chiral Ferroelectricity in AgNbO$_3$ Perovskite

TL;DR

This paper discovers a new chiral ferroelectric phase in AgNbO$_3$ with electrically switchable chirality and optical properties, advancing the understanding and potential applications of lead-free ferroelectric materials.

Contribution

It reports a previously unknown chiral ferroelectric phase in AgNbO$_3$, demonstrating electric control of chirality and associated optical effects, which was not known before.

Findings

Discovery of a chiral ferroelectric phase with space group R3.

Electrical switching of chiral optical responses like circular dichroism.

Large spontaneous polarization with low switching barrier.

Abstract

AgNbO$_3$ is a lead-free perovskite with considerable potential for energy storage and optoelectronic applications, yet its low-temperature crystal structure has remained controversial. In this Letter, we revisit its low-energy structural landscape using a systematic first-principles structural search based on symmetry-adapted phonon-mode theory. We uncover a previously unreported chiral ferroelectric phase with space group $R3$, which exhibits a large spontaneous polarization and a low polarization switching barrier, enabling polarization reversal under electric fields. Crucially, the structural chirality of this phase is intrinsically locked to the ferroelectric polarization, allowing electrical control of the chiral handedness. Consequently, chiral optical responses--including circular dichroism, circular photogalvanic effect, optical activity, and second-order nonlinear optics--can be reversibly switched by an external electric field. These results not only clarify the complex low-temperature structural behavior of AgNbO$_3$ but also establish a rare purely inorganic platform for electric-field-tunable chirality, opening a pathway toward ultrafast, electrically controlled chiral optoelectronics.