Isolated and Condensed Ru(In/Sn)6 Octahedra Clustering Together: A Mixed Valence Phase of Ru4Sn20In2O21 with Ru0, Ru+, and Ru2+ 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 Cm. 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.