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A novel Tl3PbI5 crystal has been studied both experimentally and theoretically. Complex measurements of the X-ray photoelectron core-level and valence-band spectra for the pristine and Ar(+)-ion irradiated surfaces of a Tl3PbI5 single crystal grown by the Bridgman-Stockbarger method were performed in order to clarify their principal properties (charge carriers mobility, effective inter-band distances, effective absorption etc.) relevant for optoelectronic applications. The principal role of two heavy cations - Tl and Pb - is explored. The X-ray photoelectron spectroscopy results reveal a high chemical stability of the Tl3PbI5 single crystal surface which makes it very promising for technological applications. Theoretical band-structure calculations for the Tl3PbI5 compound reveal that the I 5p states dominate in the top of the valence band and play a crucial role in the formation of the optical features and charge carrier mobility. The bottom of the Tl3PbI5 valence band is formed mainly by the admixture of Tl 6s and Pb 6s states, while the unoccupied Pb 6p and Tl 6p states dominate at the bottom of the conduction band. The band energy dispersion related to effective masses and the charge carrier mobility is studied in detail. Crucially, the theoretical calculations reveal an indirect band gap for Tl3PbI5, which indicates a strong influence of the electron-phonon interaction on the observed optoelectronic features. The temperature measurements of the fundamental absorption have shown that the band energy gap of Tl3PbI5 increases from 2.29 to 2.39 eV when the temperature changes from 300 to 100 K.

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