How unconventional oxidation state Au$^{2+}$ is stabilized in halide perovskite Cs$_4$Au$_3$Cl$_{12}$: a first-principles study of its polaron crystal nature

Abstract

Gold in crystalline compounds is typically only stable in oxidation states Au1+ and Au3+. Even compounds with nominal Au2+ usually disproportionate into Au1+ and Au3+. Recently, Cs4Au3Cl12 was synthesized, where gold took the 2+ state in the bulk. Here, we investigate this compound using first-principles calculations and show that stabilization of the Au2+ ion is through the formation of a polaron crystal. The electronic and phononic structure suggest that the bonding network can be interpreted as a collection of [Au2+Cl4]2- and [Au3+Cl4]1- square planar motifs, and the crystal lacks a smooth pathway for Au2+ to disproportionate into Au1+ and Au3+. The electronic states of Au are contained within each [AuCl4] motif, which allows for the Au2+ state to be localized and isolated electronically. The Au2+-sites form an ordered structure, which is driven by a strong repulsive interaction between [Au2+Cl4]2- motifs due to their lattice distortion. The electron-phonon coupling between Au2+ and Cl explains the stability of Au2+, which suggests this material to be interpreted as a polaron crystal. By considering redox reaction, we show that Cs4Au3Cl12 has the maximal density of Au2+, and further oxidation will induce a delocalized state. Cs4Au3Cl12 has distinctive electronic structure, with a narrow gap, isolated HOMO and LUMO bands strongly localized at the Au-sites, and magnetization at the Au2+-sites making Cs4Au3Cl12 unique among quantum materials. Magnetism in gold is rare, and Cs4Au3Cl12 can be a testbed to explore novel gold chemistry as well as polaron crystal transport. The strategy to stabilize an unconventional oxidation state through engineering of lattice distortions is quite general; therefore, we propose that a similar approach will be applicable to a wide variety of transition metal compounds.