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 and Au. Even compounds with nominal Au usually disproportionate into Au and Au. Recently, CsAuCl 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 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 [AuCl] and [AuCl] square planar motifs, and the crystal lacks a smooth pathway for Au to disproportionate into Au and Au without creating dangling bonds. The electronic states of Au are contained within each AuCl motif, which allows for the Au state to be localized and isolated electronically. The Au sites form an ordered structure, which is driven by a strong repulsive interaction between [AuCl] motifs due to their lattice distortion. By considering a redox reaction, we show that CsAuCl has the maximal density of Au, and further reduction will induce a delocalized state. CsAuCl 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 sites, making CsAuCl unique among quantum materials. CsAuCl can be a testbed to explore novel gold chemistry, opening opportunities to control the oxidation state through engineering of lattice distortions.