Cs₃Cu₄In₂Cl₁₃ Nanocrystals: A Perovskite-Related Structure with Inorganic Clusters at A Sites

An effort to synthesize the Cu(I) variant of a lead-free double perovskite isostructural with Cs₂AgInCl₆ resulted in the formation of Cs₃Cu₄In₂Cl₁₃ nanocrystals with an unusual structure, as revealed by single-nanocrystal three-dimensional electron diffraction. These nanocrystals adopt a A₂BX₆ structure (K₂PtCl₆ type, termed vacancy ordered perovskite) with tetrahedrally coordinated Cu(I) ions. In the structure, 25% of the A sites are occupied by [Cu₄Cl]³⁺ clusters (75% by Cs⁺), and the B sites are occupied by In³⁺. Such a Cs₃Cu₄In₂Cl₁₃ compound prepared at the nanoscale is not known in the bulk and is an example of a multinary metal halide with inorganic cluster cations residing in A sites. The stability of the compound was supported by density functional theory calculations that also revealed that its bandgap is direct but parity forbidden. The existence of the Cs₃Cu₄In₂Cl₁₃ structure demonstrates that small inorganic cluster cations can occupy A sites in multinary metal halides.