Dielectric softening in the halide double perovskites $A_2$Au$_2X_6$ ($A$: Cs, Rb; $X$: Cl, Br, I) via a strain-mediated pseudotriggered mechanism

TL;DR

This work tackles the existence and origin of polar ferroelectric behavior in Pb-free halide double perovskites $A_2$Au$_2X_6$ by combining density functional theory with group-theoretical symmetry analysis. It reveals a metastable polar phase $I4mm$ in $Rb_2Au_2I_6$ near the nonpolar ground state $C2/m$ and shows that Jahn-Teller–driven strains indirectly soften the polar mode via a strain-mediated pseudotriggered mechanism, outweighing direct JT–polar coupling. The results quantify a polarization of about $5.0~\mu{\rm C/cm^2}$ and a switching barrier of ~14 meV per 10-atom formula unit, with HSE06 predicting a gap near $0.9$ eV, suggesting feasible electric-field switchability and favorable dielectric screening. The authors propose stabilization via epitaxial strain or mixed A-site cations and discuss broader applicability to JT-active oxide double perovskites, highlighting implications for dielectric tunability and potential photoferroelectric functionality in Pb-free materials.

Abstract

Halide perovskites have emerged as promising candidates for next generation photovoltaic applications, attracting significant attention in recent years. Through first-principles calculations combined with group-theoretical analyses, we investigate the structural phase diagram of Pb-free Jahn-Teller-active $A_2$Au$_2X_6$ ($A$: Cs, Rb; $X$: Cl, Br, I) double perovskites. Our study identifies a previously unreported ferroelectric phase, where the softening of the polar mode$, $key to ferroelectricity, is driven by an unconventional and indirect coupling with improper strains originating from Jahn-Teller distortions. The proposed strain mediated \textit{pseudo}triggered mechanism offers an alternative pathway to enhance the static dielectric constant or even promote (photo-)ferroelectricity, addressing challenges such as defects, excitons, and charge scattering that hinder photovoltaic efficiency. More broadly, this unique mechanism could be extended to oxide double perovskites and opens up a new type of ferroelectric phase transition worthy of future investigation.

Figures (10)

• Figure 1: (a) Phonon spectrum of the high-symmetry cubic perovskie phase of RbAuI$_3$ as a representative of the gold halide perovskites $A$Au$X_3$ ($A$: Cs, Rb; $X$: Cl, Br, I). (b) Condensation of the most unstable phonon mode with $R^-_3$ symmetry leads to a tetragonal $I4/mmm$ phase with alternating elongated and compressed octahedra. The magenta, gold, and purple spheres represent the Rb, Au, and I atoms, respectively.
• Figure 2: (a) Crystal structure of the ground state $C2/m$ phase of Rb$_2$Au$_2$I$_6$. The red arrows indicate the directions of octahedral tilts. (b) The metastable $I4mm$ polar structure of Rb$_2$Au$_2$I$_6$. Polar displacements of the cations and the direction of net out-of-plane polarization ($\mathbf{P}$) are denoted by blue and green arrows, respectively.
• Figure 3: Polarization switching path in the metastable $I4mm$ phase of Rb$_2$Au$_2$I$_6$.
• Figure 4: Energy landscape as a function of the polar mode amplitude at different percentage values of the strain modes (both hydrostatic and tetragonal) produced by the JT distortion in the $I4/mmm$ phase.
• Figure 5: Schematic diagram illustrating (a) the conventional triggered mechanism and (b) the novel pseudotriggered mechanism. In the conventional triggered mechanism, a cooperative biquadratic coupling exists directly between the two primary order parameters (JT and polar distortions) in the free energy expansion. In contrast, the pseudotriggered mechanism involves an indirect coupling between the two primary modes, mediated by a secondary order parameter (strain) through individual cooperative linear-quadratic couplings, leading to an effective biquadratic coupling between the primary modes—hence the term pseudotriggered.