The hidden ferroelectric chiral ground state of silver niobate

TL;DR

First-principles calculations uncover a hidden, chiral ferroelectric ground state in silver niobate, explaining its complex low-temperature structure and optical activity.

Contribution

The study reveals a previously overlooked rhombohedral ferroelectric phase with structural chirality as the thermodynamic ground state of silver niobate.

Findings

Identifies a stable rhombohedral ferroelectric phase with R3 symmetry.

Shows the phase is structurally chiral and exhibits natural optical activity.

Suggests kinetic limitations may hinder experimental observation of this phase.

Abstract

Silver niobate is a conventional perovskite oxide compound, known to exhibit a rich polymorphism. Although often classified as antiferroelectric, its low-temperature structure remains unclear. Here, first-principles calculations reveal a previously overlooked and unusual rhombohedral ferroelectric phase with $R3$ symmetry that emerges as the thermodynamic ground state despite its close energetic competition among previously proposed structures. Remarkably, this phase is structurally chiral, with chirality emerging improperly from the coupling between polarization and in-phase rotations of the oxygen octahedra along [111], producing a ferri-chiral state with incomplete cancellation of local chiral motifs. As a consequence, the phase exhibits significant natural optical activity comparable to that of quartz. Although energetically favored, its experimental observation may be hindered by kinetic limitations, potentially contributing to the ongoing controversy surrounding the low-temperature structure of silver niobate.