Hidden Chiral Ferroelectricity in AgNbO$_3$ Perovskite

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.

Figures (4)

• Figure 1: (a) Group--subgroup tree highlighting the relationship of $Pbcm$ and $R3$ with respect to $Pm\bar{3}m$. (b) Free-energy contour map of the coupling between the $\Gamma_{4}^{-}$ and $M_{3}^{+}$ modes, see Table S2 for the fitting parameters. (c,d) Crystal structures of $Im\bar{3}$ and $R3$, respectively. (e,f) Atomic displacement patterns of the $\Gamma_{4}^-$ mode for positive and negative polarizations, respectively. Cyan, green, and purple arrows denote the in-plane ($ab$-plane) projections of the displacements of O atoms in different layers. Curved arrows indicate the sense of rotation of the O atoms. The insets show the vibrational components of the O atoms along the $c$ axis. Clockwise and counterclockwise rotations of the O atoms about the $c$ axis correspond to left- and right-handedness, respectively.
• Figure 2: Evolution of (a) the energy difference $\Delta E$ between the $R3$ and $Im\bar{3}$ phases, (b) the spontaneous polarization $P$, (c) the band gap $E_{\mathrm{g}}$, (d) the largest gyration-tensor component $g_{21}$ associated with natural optical activity (NOA), and (e) the largest second-order nonlinear-optical coefficient $d_{16}$, as functions of the amplitude of the $\Gamma_{4}^{-}$ mode, $Q_{\Gamma_{4}^{-}}$, which connects the $R3$ and $Im\bar{3}$ phases.
• Figure 3: (a,c) Spin texture of the lowest conduction band near the $\Gamma$ point and the momentum-resolved Berry curvature for left-handed ($+$ polarization) $R3$ AgNbO$_3$, respectively. (b,d) Spin texture of the lowest conduction band near the $\Gamma$ point and the momentum-resolved Berry curvature for right-handed ($-$ polarization) $R3$ AgNbO$_3$, respectively.
• Figure 4: (a) Circular dichroism (CD) spectra of the $R3$ phase for different distortion amplitudes of the polar $\Gamma_{4}^{-}$ mode. (b) Circular photogalvanic effect (CPGE) tensor component ($\beta_{\mathrm{zz}}$) of the $R3$ phase as a function of the distortion amplitude of the polar $\Gamma_{4}^{-}$ mode.