Preparation, Characterization, and Unexpected Cathodoluminescence of Polycrystalline TlPbI₃

A polycrystalline powder of orange-red TlPbI₃ was prepared from equimolar amounts of TlI and PbI₂ by solid-state synthesis in an evacuated silica ampule at 400 °C. This reaction was analyzed for the first time by differential scanning calorimetry: During the first temperature cycle, it was revealed that small amounts of precursors sublimate before the reaction mixture melts congruently and recrystallizes, forming the title compound. During the second temperature cycle, the melting point for the title compound was determined to be 345 °C. Rietveld refinement of room-temperature powder X-ray diffraction data shows that orthorhombic TlPbI₃ crystallizes in the space group Cmcm with the lattice parameters a = 462.16(4), b = 1487.12(15), and c = 1181.43(11) pm for Z = 4. Optical studies by UV-Vis diffuse reflectance spectroscopy result in an absorption edge at λ_abs = 568 nm and an experimentally determined band gap at E_g = 2.23 eV for polycrystalline TlPbI₃. Temperature-dependent cathodoluminescence studies on the polycrystalline sample show a broad signal at lower temperatures, which is blue-shifted by roughly 180 nm compared with previous reports.