Stabilizing Au²⁺ in a mixed-valence 3D halide perovskite

Although Cu²⁺ is ubiquitous, the relativistic destabilization of the 5d orbitals makes the isoelectronic Au²⁺ exceedingly rare, typically stabilized only through Au-Au bonding or by using redox non-innocent ligands. Here we report the perovskite Cs₄Au^IIAu^III₂Cl₁₂, an extended solid with mononuclear Au²⁺ sites, which is stable to ambient conditions and characterized by single-crystal X-ray diffraction. The 2+ oxidation state of Au was assigned using ¹⁹⁷Au Mössbauer spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements, with comparison to paramagnetic and diamagnetic analogues with Cu²⁺ and Pd²⁺, 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 Au²⁺ ion coordinated to simple ligands. Compared with the perovskite Cs₂Au^IAu^IIICl₆, which has been studied since the 1920s, Cs₄Au^IIAu^III₂Cl₁₂ exhibits a 0.7 eV reduction in optical absorption onset and a 10³-fold increase in electronic conductivity.