New route to Bi+-doped crystals: Preparation and NIR luminescence of K, Rb and Cs ternary chlorides, containing univalent bismuth

In Lewis acidic melts it is possible to obtain the metal cations in unusually low oxidation (subvalent) states with unusual electronic configurations. Specifically, univalent bismuth cation possesses the open shell 6p² configuration with several low-lying electronic exited states, causing the absorption bands in visible/NIR and intense long-lived (hundreds of μs) and broad luminescence in NIR [1-4]. Quantum chemical estimation of Bi⁺ ionic radius in different coordination states gives the values, which place the radius of Bi⁺ very close to Rb⁺ cation. This fact open the possibility to prepare the crystal hosts with alkali cations of similar size (K⁺, Rb⁺, Cs⁺), which can be doped with Bi⁺ by isomorphous substitution. The experimental investigation of NIR luminescence in bismuth-doped RbPb₂Cl₅ [1], KAlCl₄ [2,3], KMgCl₃ [3] and CsI [4] crystals support this conclusion.