Crystal chemical investigations on TiU8S17 and U36Lu21S93I3 - ordered and disordered multinary uranium sulfides

Abstract

The crystal chemistry of uranium sulfides is characterized by a large diversity in composition, crystal structures, oxidation states and physical properties. This holds also for TiU₈S₁₇ and U₃₆Lu₂₁S₉₃I₃, of which large single crystals up to several mm in length were grown by chemical vapor transport using iodine as a transport agent. TiU₈S₁₇ crystallizes with the monoclinic CrU₈S₁₇ structure type (C2/m, a = 13.3477(2) Å, b = 8.4927(2) Å, c = 10.4836(2) Å, β = 101.207(2)°) featuring distorted octahedra [TiS₆] and bicapped trigonal prisms [US₈]. Interatomic distances and X-ray photoelectron spectra indicate tetravalent uranium. Disordered U₃₆Lu₂₁S₉₃I₃ represents a new structure type ($\text{R}\overline{3}\text{0}$, 13.3841(2) Å, 20.6447(2) Å) with an average crystal structure exhibiting square antiprisms [LuS₈], bi- and tricapped trigonal prisms [US₈] and [US₉] and distorted octahedra [IU₆]. Lutetium and sulfur atoms are disordered similar to Ce₅₃Fe₁₂S₉₀X₃, La₅₃Fe₁₂S₉₀X₃ (X = Cl, Br, I) and La₅₂Fe₁₂S₉₀. This might be caused by positional shifts of iodine atoms away from the center of the polyhedron, thus avoiding unusually long U–I distances and severely disrupting the order of neighboring lutetium and sulfur atoms. The results of chemical analyses (ICP-MS, EDX), electron microscopy (TEM) and X-ray photoelectron spectroscopy support the composition and crystal structure of U₃₆Lu₂₁S₉₃I₃ and indicate mixed valency of uranium and charge balance according to the crystal chemical formula (U^+III)₁₈(U^+IV)₁₈(Lu^+III)₂₁(S^-II)₉₃(I^–I)₃.