How the Unconventional Oxidation State Au ²⁺ Is Stabilized in the Halide Perovskite Cs ₄ Au ₃ Cl ₁₂ : A First-Principles Study of Its Polaron Crystal Nature

Gold in crystalline compounds is typically only stable in oxidation states Au 1+ and Au 3+ . Even compounds with nominal Au 2+ usually disproportionate into Au 1+ and Au 3+ . Recently, Cs 4 Au 3 Cl 12 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 Au 2+ 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 [Au 2+ Cl 4 ] 2– and [Au 3+ Cl 4 ] 1– square planar motifs, and the crystal lacks a smooth pathway for Au 2+ to disproportionate into Au 1+ and Au 3+ without creating dangling bonds. The electronic states of Au are contained within each AuCl 4 motif, which allows for the Au 2+ state to be localized and isolated electronically. The Au 2+ sites form an ordered structure, which is driven by a strong repulsive interaction between [Au 2+ Cl 4 ] 2– motifs due to their lattice distortion. By considering a redox reaction, we show that Cs 4 Au 3 Cl 12 has the maximal density of Au 2+, and further reduction will induce a delocalized state. Cs 4 Au 3 Cl 12 has a distinctive electronic structure, with a narrow gap, isolated HOMO and LUMO bands strongly localized at the Au sites, and magnetization at the Au 2+ sites, making Cs 4 Au 3 Cl 12 unique among quantum materials. Cs 4 Au 3 Cl 12 can be a testbed to explore novel gold chemistry, opening opportunities to control the oxidation state through engineering of lattice distortions.