Rb₄CuSb₂Cl₁₁ and Rb₂In_0.91(0.2)Sb_0.09Cl₅·H₂O: Wide Band Gap 0D Metal Halide Semiconductors

Herein, we report the discovery, structural and photophysical characterization of a new zero-dimensional (0D) lead-free all-inorganic halide, Rb₄CuSb₂Cl₁₁, which adopts a new structure type. Single-crystal X-ray diffraction (SCXRD) shows that the structure consists of isolated, distorted seesaw [SbCl₄]^- and trigonal planar [CuCl₃]^2- units, separated by Rb⁺ cations that provide charge balance. Optoelectronic measurements and density functional theory (DFT) calculations indicate an indirect band gap of 2.89 eV, making it a candidate for wide-bandgap optoelectronic applications. Electrical resistivity was measured at 1.29 × 10¹⁰ Ω·cm, and the trap-state density (n_trap) was found to be 7.44 × 10¹⁰ cm^-3. Attempts to synthesize substitution analogs of Rb₄CuSb₂Cl₁₁ led to the synthesis of Rb₂In_0.91(0.2)Sb_0.09Cl₅·H₂O, which was erroneously reported as Rb₂SbCl₅O in a previous study. Rb₂In_0.91(0.2)Sb_0.09Cl₅·H₂O adopts a vacancy-ordered perovskite structure and exhibits broad-band yellow emission under UV excitation. The measured photoluminescence quantum yield (PLQY) for Rb₂In_0.91(0.2)Sb_0.09Cl₅·H₂O is 18.2%. These findings add to the growing class of quaternary metal halides with multiple cation and anion compositions, expanding the chemical phase space for the discovery of new materials with functional properties.