Vacancy-Ordered Quadruple Halide Perovskites: All-Inorganic Noble-Metal Semiconductors with Infrared Absorption

A unique family of inorganic halide perovskites based on noble metals was successfully synthesized, using the “stick & square” approach, where the octahedral units of the perovskite framework can be substituted with 4-coordinate square and 2-coordinate dumbbell units, orthogonally assembled to generate a perovskite-like network. The compounds have a general formula of A4Au2Pd□X12 (A = Cs, Rb; X = Cl, Br, I; □ = ordered-vacancy site), crystallizing in a pseudocubic unit cell with 4 × 4 × 4 enlargement with respect to the perovskite aristotype, AMX3. The compounds consist of square planar complexes of [AuIIIX4]2– and [PdIIX4]2– ions, randomly distributed across the crystallographic sites of the unit cell. The high symmetry of the compounds is revealed by the isolation of euhedral crystals with a rare deltoidal icositetrahedral morphology. X-ray photoelectron spectroscopy (XPS) was employed to validate the oxidation states of the elements and demonstrate the changes in the spectral response as a function of the electronegativity changes in the halide ion. All compounds are black in color, with sharp absorption edges in the near-infrared range, with an experimentally determined bandgap in the 0.8–1.55 eV range. Density functional theory (DFT) calculations based on a metal-site-ordered model reveal an indirect bandgap between the halide-p orbital-dominated valence band maximum (VBM) and the gold-dz2-dominated conduction band minimum (CBM) in the iodide analogue, changing to a palladium-d orbital-dominated VBM for the bromide and chloride analogues. The present work demonstrates the rich complexity of the late transition metal halide chemistry and illustrates that the “stick & square” approach is a viable route to access complex perovskite derivatives. A4Au2Pd□X12 compounds hold special merits in the fact that they possess a suitable bandgap for photovoltaic applications and potentially infrared electronics, while at the same time being stable against air and water, the major enemies of typical p-block halide perovskites.