CuPrOS: No impressive acronym, rather a real quaternary chalcogenide with stuffed PbFCl-type crystal structure

During attempts to synthesize copper(I) praseodymium(III) disulfide (CuPrS2 ) from 1:1:2-molar mixtures of the elements (Cu, Pr and S) after seven days at 850 degreesC in sealed evacuated silica tubes the almost quantitative f ormation of the quaternary chalcogenide CuPrOS indicated that some massive oxygen-contaminant (most probably surface-oxidized praseodymium metal) must have been present. The unexpected product becomes even single-crystalline and emerges as yellowish green transparent platelets with square cross-sect ion when equimolar amounts of CsCl; work as fluxing agent. The crystal stru cture of CuPrOS (tetragonal, P4/nmm; a = 394.19(4), c = 843.98(9) pm; c/a = 2.141, Z = 2) can be described as an anionic PbFCl-type arrangement accord ing to (2)(infinity){[(Pr3+)(O2-)(4/4)(S2-)(4/4)](-)} with Pr3+ in square-a ntiprismatic coordination of four F- and four S2- anions which is "stuffed" by Cu+ cations within tetrahedral interstices between the S2- double layer s for charge balance. Another description emphasizes two kinds of layers (a nionic (2)(infinity){[(Cu+)(S2-)(4/4)](-)} and cationic (2)(infinity){[O2-) (Pr3+)(4/4)](+)} ones) consisting of condensed [(Cu+)(S2-)(4)](7-) and [(O2 -)(Pr3+)(4)](10+) tetrahedra, respectively, each of which share four of the ir six edges. Both types of square layers exhibit the same topology and are alternatingly sheethed parallel (001). The most important internuclear dis tances range from 233 (d(O-Pr), 4x) via 243 (d(Cu-S), 4x) up to 320 pm (d(P r-S), 4x).