High‐Pressure Syntheses of Lanthanide Polysulfides and Polyselenides LnX 1.9 ( Ln = Gd–Tm, X = S, Se)

The polysulfides Ln S 1.9 and polyselenides Ln Se 1.9 of the lanthanide metals from gadolinium to thulium were prepared by high‐pressure high‐temperature synthesis. The compounds adopt the tetragonal CeSe 1.9 structure type in space group P 4 2 / n (No. 86) with lattice parameters of 8.531 Å ≤ a ≤ 8.654 Å and 15.563 Å ≤ c ≤ 15.763 Å for the sulfides and 8.869 Å ≤ a ≤ 9.076 Å and 16.367 Å ≤ c ≤ 16.611 Å for the selenides. The atomic pattern consists of puckered double slabs [ LnX ] + and planar chalcogenide layers with ten possible chalcogen positions, of which eight are occupied by chalcogen atoms forming dinuclear X 2 2– dianions, one by a single X 2– ion and one remaining vacant. This resembles a √5×√5 × 2 superstructure of the ZrSSi aristotype. Structural relationships to the aristotype and the related lanthanide polychalcogenides LnX 2– δ ( Ln = La–Tm, X = S–Te, δ = 0 – 0.3) are discussed. The samples synthesized under high‐pressure conditions ( p > 1 GPa) decompose slowly under release of sulfur or selenium at ambient conditions. The crystal structure of a partially degraded thulium‐polysulfide sample can be described as an incommensurate variant of the original TmS 1.9 motif. Additionally, the isostructural ternary compound Gd(S 1–x Se x ) 1.9 was synthesized and characterized by powder X‐ray diffraction.