Dielectric Softening in Halide Double Perovskites via an unusual Pseudo-Triggered Mechanism

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 novel mechanism could be extended to oxide double perovskites and opens up a new type of ferroelectric phase transition worthy of future investigation.