Barium dierbium(III) tetrasulfide

Barium dierbium(III) tetrasulfide, BaEr2S4, crystallizes with four formula units in the orthorhombic space group Pnma in the CaFe2O4 structure type. The asymmetric unit contains two Er, one Ba, and four S atoms, each with .m. site symmetry. The structure consists of channels formed by corner- and edge-sharing ErS6 octahedra in which Ba atoms reside. The resultant coordination of Ba is that of a bicapped trigonal prism.


Related literature
The unit-cell parameters of BaEr 2 S 4 , which crystallizes in the CaFe 2 O 4 structure type (Decker & Kasper, 1957), were previously determined from X-ray powder diffraction data (Patrie et al., 1964). For related structures, see:  ;Narducci et al. (2000); Carpenter & Hwu (1992); Flahaut et al. (1965); Schurz & Schleid (2011). For synthetic details, see: Bugaris & Ibers (2008); Haneveld & Jellinek (1969). For standardization of structural data, see: Gelato & Parthé (1987 Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008b); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008b); molecular graphics: Crys-talMaker (Palmer, 2012); software used to prepare material for publication: SHELXL97. Orange needles of BaEr 2 S 4 were obtained in a solid-state reaction. The compound was synthesized previously (Patrie et al., 1964), and its unit cell parameters were determined from X-powder diffraction data. In the BaLn 2 S 4 family (Ln = rare earth element) no structures have been determined from single-crystal data but that of the closely related compound BaLu 2 S 4 has (Schurz & Schleid, 2011). Here, from X-ray diffraction single-crystal data we find that BaEr 2 S 4 crystallizes in the CaFe 2 O 4 structure type (Decker & Kasper, 1957) with four formula units in space group Pnma of the orthorhombic system. In the asymmetric unit there are two Er, one Ba, and four S atoms, each with site symmetry .m.. A projection of the structure down [010] is shown in Figure 1. The structure consists of ErS 6 octahedra that form dimers by edge-sharing.
Four such dimers form an infinite channel by corner-sharing in the (010) plane. Each channel is filled by one Ba atom.
Each Er1 atom is octahedrally coordinated to one S2, three S3, and two S4 atoms; each Er2 atom is coordinated to three S1, two S2, and one S4 atom. The interatomic Er -S distances at 2.6706 (10) to 2.7376 (7) Å compare favorably to those of 2.672 (4) to 2.720 (4) Å in the structure of BaLu 2 S 4 (Schurz & Schleid, 2011). As there are no S -S bonds in the structure, formal oxidation states may be assigned as Ba 2+ , Er 3+ , and S 2-.

Experimental
In an exploration of the quaternary solid-state Ba/Er/U/S system, orange needles of BaEr 2 S 4 were obtained instead in a two-step reaction. Uranium powder was obtained by hydridization and decomposition of 238 U turnings (Oak Ridge National Laboratory) (Bugaris & Ibers, 2008;Haneveld & Jellinek, 1969). The other reactants were used as obtained. In the first step, a mixture consisting of powdered 238 U (20.9 mg, 0.088 mmol), Er (14.0 mg, 0.084 mmol), BaS (42.7 mg, 0.252 mmol), and S (8.0 mg, 0.25 mmol) was loaded into a carbon-coated fused-silica tube under an Ar atmosphere in a glove box. The tube was evacuated to 10 -4 Torr, and flame sealed. It was placed in computer-contolled furnace, heated to 1273 K in 48 h, held there for 8 d, then cooled to 293 K at 3 K/h. In the second step, the resultant black powder was ground and mixed thoroughly with 50 mg of Sb 2 S 3 . This mixture was re-loaded into a carbon-coated fused-silica tube, evacuated, sealed, and then placed in a computer-controlled furnace The tube was heated to 1273 K in 24 h, held there for 4 d, then cooled to 293 K at 2 K/h. Orange needles were obtained in about 50 wt% yield. Analysis of these orange crystals on an EDX-equipped Hitachi S-3400 SEM showed the presence of Ba, Er, and S in the approximate ratio 1:2:4 but no U. The other products were black crystals of Sb 2 S 3 and US 2 .

Refinement
The structure was standardized by means of the program STRUCTURE TIDY ( (Gelato & Parthé, 1987