How the Unconventional Oxidation State Au²⁺ Is Stabilized in the Halide Perovskite Cs₄Au₃Cl₁₂: A First-Principles...

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 structures 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⁺ without creating dangling bonds. 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. By considering a redox reaction, we show that Cs₄Au₃Cl₁₂ has the maximal density of Au²⁺, and further reduction will induce a delocalized state. Cs₄Au₃Cl₁₂ has a 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. Cs₄Au₃Cl₁₂ can be a testbed to explore novel gold chemistry, opening opportunities to control the oxidation state through engineering of lattice distortions.