Yb3F4S2: A mixed-valent ytterbium fluoride sulfide according to YbF2 center dot 2YbFS

Attempts to synthesize ytterbium(III) fluoride sulfide (YbFS) from 2:3:1-mo lar mixtures of ytterbium metal (Yb), elemental sulfur (S) and ytterbium tr ifluoride (YbF3) after seven days at 850 degreesC in silica-jacketed gastig ht-sealed are-welded tantalum capsules result in the formation of the mixed -valent ytterbium(II,III) fluoride sulfide Yb3F4S2 (tetragonal, I4/mmm; a = 384,61(3), c = 1884,2(4) pm; Z = 2) instead. The almost single-phase produ ct becomes even single-crystalline and emerges as black shiny platelets wit h square cross-section when equimolar amounts of NaCl are present as fluxin g agent. Its crystal structure can be described as a sheethed intergrowth a rrangement of one layer of CaF2-type YbF2 followed by two layers of PbFCl-t ype YbFS parallel (001). Accordingly there are two chemically and crystallo graphically different ytterbium cations present. One of them (Yb2+) is surr ounded by eight fluoride anions in a cubic fashion, the other one (Yb3+) ex hibits a capped square-antiprismatic coordination sphere consisting of four F- and five S2- anions. In spite of being structurally very plausible, the obvious ordering of the differently charged ytterbium in terms of a locali zed mixed valency can only be fictive because of the black colour of Yb3F4S 2 which rather suggests charge delocalization coupled with polaron activity .