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.
@article{arxiv.2604.09193,
title = {The hidden ferroelectric chiral ground state of silver niobate},
author = {Safari Amisi and Fernando Gómez-Ortiz and Eric Bousquet and Philippe Ghosez},
journal= {arXiv preprint arXiv:2604.09193},
year = {2026}
}