Stabilizing Au 2+ in a mixed-valence 3D halide perovskite

Although Cu 2+ is ubiquitous, the relativistic destabilization of the 5d orbitals makes the isoelectronic Au 2+ exceedingly rare, typically stabilized only through Au–Au bonding or by using redox non-innocent ligands. Here we report the perovskite Cs 4 Au II Au III 2 Cl 12 , an extended solid with mononuclear Au 2+ sites, which is stable to ambient conditions and characterized by single-crystal X-ray diffraction. The 2+ oxidation state of Au was assigned using 197 Au Mössbauer spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements, with comparison to paramagnetic and diamagnetic analogues with Cu 2+ and Pd 2+ , respectively, as well as to density functional theory calculations. This gold perovskite offers an opportunity to study the optical and electronic transport of the uncommon Au 2+/3+ mixed-valence state and the characteristics of the elusive Au 2+ ion coordinated to simple ligands. Compared with the perovskite Cs 2 Au I Au III Cl 6 , which has been studied since the 1920s, Cs 4 Au II Au III 2 Cl 12 exhibits a 0.7 eV reduction in optical absorption onset and a 10 3 -fold increase in electronic conductivity.