Isolated and Condensed Ru(In/Sn) ₆ Octahedra Clustering Together: A Mixed Valence Phase of Ru ₄ Sn ₂₀ In ₂ O ₂₁ with Ru ⁰ , Ru ⁺ , and Ru ²⁺ Species

A new quaternary phase compound, Ru₄Sn₂₀In₂O₂₁, was synthesized by using a reactive tin-flux method. Single-crystal X-ray diffraction analysis reveals that this compound crystallizes in a monoclinic structure with the noncentrosymmetric space group <i>Cm</i>. The structure consists of Ru-centered (Sn/In)₆ octahedra, featuring both condensed and isolated layers, which are surrounded by tetrahedrally disordered and ordered SnO₃ units. Formal oxidation state analysis, supported by both experimental data and theoretical calculations, indicates a rare mixed valence of Ru atoms: Ru⁰, Ru⁺, and Ru²⁺. Tin atoms in corner-sharing sites primarily exhibit Sn⁺, while the remaining tin and indium atoms are present as Sn²⁺ and In⁺, respectively. Electronic band structure calculations suggest the presence of a direct band gap, consistent with the observed photophysical properties. Photoluminescence measurements show a peak at 620 nm, corresponding to a red emission. Given its direct band gap and red emission, Ru₄Sn₂₀In₂O₂₁ exhibits significant potential for optoelectronic applications, such as light-emitting diodes (LEDs) and photovoltaic devices.