Uranium's valency in U3S5

U3S5 has been prepared by chemical transport reaction and investigated usin g X-ray powder diffraction, FTIR spectroscopy, electrical resistivity measu rements, and X-ray photoelectron spectroscopy. U3S5 is a semiconductor with a thermal band gap E-g = 78.1(4) meV (298 K < T < 50 K), which closes grad ually to 3.4(4) meV for T < 25 K. Photoelectron spectroscopy on single crys tals of U3S5 and beta-US2 suggest a mixed valency of uranium in U3S5. Physi cal and structural data are consistent with a mixed-valent model (U3+)(2)U4 + (S2-)(5.) A brief survey of literature data on crystal structure and phys ical properties of uranium sulfides and selenides is given. (C) 2000 Academ ic Press.