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

Area of Science:

• Inorganic Chemistry
• Solid-State Chemistry
• Materials Science

Background:

• The 2+ oxidation state of gold (Au²⁺) is rare due to relativistic effects destabilizing its 5d orbitals.
• Au²⁺ is typically stabilized by Au-Au bonding or redox non-innocent ligands.
• Perovskite structures offer a versatile platform for stabilizing unusual oxidation states.

Purpose of the Study:

• To synthesize and characterize a novel perovskite containing stable, mononuclear Au²⁺ sites.
• To investigate the electronic and optical properties of the Au^2+/3+ mixed-valence state.
• To explore the characteristics of Au²⁺ coordinated to simple ligands.

Main Methods:

• Single-crystal X-ray diffraction for structural characterization.
• ¹⁹⁷Au Mössbauer spectroscopy, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements for oxidation state assignment.
• Density functional theory (DFT) calculations for theoretical validation.
• Optical absorption and electronic conductivity measurements.

Main Results:

• Successful synthesis and ambient stability of the perovskite Cs₄Au^IIAu^III₂Cl₁₂ with mononuclear Au²⁺ sites.
• Confirmation of the Au²⁺ oxidation state through spectroscopic and magnetic measurements.
• Cs₄Au^IIAu^III₂Cl₁₂ exhibits a 0.7 eV lower optical absorption onset and a 10³-fold higher electronic conductivity compared to Cs₂Au^IAu^IIICl₆.

Conclusions:

• The perovskite Cs₄Au^IIAu^III₂Cl₁₂ provides a stable platform for studying the rare Au²⁺ ion and its mixed-valence properties.
• This material demonstrates significantly altered optical and electronic properties compared to known gold perovskites.
• The findings open new avenues for exploring exotic gold oxidation states in extended solids.