Yb 3 F 4 S 2 : Ein gemischtvalentes Ytterbiumfluoridsulfid gemäß YbF 2 · 2 YbFS

Yb 3 F 4 S 2 : A mixed‐valent Ytterbium Fluoride Sulfide according to YbF 2  · 2 YbFS Attempts to synthesize ytterbium(III) fluoride sulfide (YbFS) from 2 : 3 : 1‐molar mixtures of ytterbium metal (Yb), elemental sulfur (S) and ytterbium trifluoride (YbF 3 ) after seven days at 850 °C in silica‐jacketed gastight‐sealed arc‐welded tantalum capsules result in the formation of the mixed‐valent ytterbium(II,III) fluoride sulfide Yb 3 F 4 S 2 (tetragonal, I4/mmm; a = 384,61(3), c = 1884,2(4) pm; Z = 2) instead. The almost single‐phase product becomes even single‐crystalline and emerges as black shiny platelets with square cross‐section when equimolar amounts of NaCl are present as fluxing agent. Its crystal structure can be described as a sheethed intergrowth arrangement of one layer of CaF 2 ‐type YbF 2 followed by two layers of PbFCl‐type YbFS parallel (001). Accordingly there are two chemically and crystallographically different ytterbium cations present. One of them (Yb 2+ ) is surrounded by eight fluoride anions in a cubic fashion, the other one (Yb 3+ ) exhibits a capped square‐antiprismatic coordination sphere consisting of four F – and five S 2– anions. In spite of being structurally very plausible, the obvious ordering of the differently charged ytterbium in terms of a localized mixed valency can only be fictive because of the black colour of Yb 3 F 4 S 2 which rather suggests charge delocalization coupled with polaron activity.