High‐Pressure Syntheses of Lanthanide Polysulfides and Polyselenides LnX_1.9 (Ln = Gd–Tm, X = S, Se)

Abstract

The polysulfides LnS1.9 and polyselenides LnSe1.9 of the lanthanide metals from gadolinium to thulium were prepared by high‐pressure high‐temperature synthesis. The compounds adopt the tetragonal CeSe1.9 structure type in space group P42/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 X22– dianions, one by a single X2– 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 LnX2–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 TmS1.9 motif. Additionally, the isostructural ternary compound Gd(S1–xSex)1.9 was synthesized and characterized by powder X‐ray diffraction.

LaTe1.82(1): modulated crystal structure and chemical bonding of a chalcogen-deficient rare earth metal polytelluride

“…Considering only these distances, three different motifs can be distinguished: a Te 8 unit, probably consisting of smaller anions, like Te 2À , Te 2 2À , bent Te 3 2À and linear Te 3 4À anions, as well as isolated Te 2À anions surrounded by different anionic Te entities. Eightmembered rings of chalcogen atoms and isolated X 2À anions are, as already pointed out, common motifs in the crystal structures of the rare earth metal sulfides and selenides with compositions of RES 1.9 (RE = La-Nd, Sm, Gd-Tm) (Doert, Graf, Lauxmann et al, 2007;Tamazyan et al, 2000;Mü ller et al, 2012), RESe 1.9 (RE = La-Nd, Sm, Gd-Tm) (Grupe & Urland, 1991;Plambeck-Fischer et al, 1989;Urland et al, 1989;Mü ller et al, 2012), RE 8 S 15-(RE = Y, Tb-Ho) (Doert et al, 2012), RE 8 Se 15-(RE = Y, Gd-Er; = 0 0.3) (Doert, Dashjav et al, 2007) and RESe 1.85 (RE = La-Nd or Sm) (Doert, Graf, Schmidt et al, 2007;Graf & Doert, 2009).…”

“…This is achieved by four Se 2 2À anions and one isolated Se 2À anion. This kind of charge-ordered superstructure has only been reported once for a rare earth metal telluride, namely for CeTe 1.9 (Ijjaali & Ibers, 2006), whereas many examples are known for the rare earth metal polysulfides and polyselenides (Doert, Graf, Lauxmann et al, 2007;Grupe & Urland, 1991;Plambeck-Fischer et al, 1989;Urland et al, 1989;Dashjav et al, 2000;Mü ller et al, 2012).…”

Rare Earth Metal PolytelluridesRETe1.8(RE= Gd, Tb, Dy) – Directed Synthesis, Crystal and Electronic Structures, and Bonding Features

“…This example stands out for its complex layer, whereas CeTe 1.9 , another example of a deficient rare earth telluride, shows a motive similar to its sulfide and selenide analogues . CeTe 1.9 [ P 4 2 / n, a = 1002.61(5) pm, c = 1813.4(1) pm] adopts the CeSe 1.9 structure type, which has been found for the sulfides and selenides of RE = La – Nd, Sm, Gd – Tm . This CeSe 1.9 type is characterized by a pinwheel‐like arrangement of four X 2 2– anions surrounding a vacancy.…”

“…Thermoanalytical and tensimetric studies reveal that polysulfides and polyselenides LnX 2– δ generally decompose at elevated temperatures by a stepwise release of molecular chalcogen X 2 until the sesquichalcogenide Ln 2 X 3 is finally reached , , . Polychalcogenides of the smaller rare‐earth metals have a higher decomposition pressure which is the reason that these dichalcogenides are not accessible by syntheses under ambient pressure conditions.…”

“…Polychalcogenides of the smaller rare‐earth metals have a higher decomposition pressure which is the reason that these dichalcogenides are not accessible by syntheses under ambient pressure conditions. The increasing decomposition tendency with decreasing lattice parameters of the polychalcogenides is attributed to growing anti‐bonding interactions between the chalcogenide anions in the planar layers , …”