Oxidfluoridsulfide der Lanthanoide vom Formeltyp M6O2F8S3 (M=La–Nd, Sm, Gd) / Oxide Fluoride Sulfides of the Lanthanoids with the Formula M6O2F8S3 (M = La–Nd, Sm, Gd)

Abstract

The lanthanoid(III) oxide fluoride sulfides M6O2F8S3 (M = La-Nd, Sm, Gd) can be obtained by reacting the rare-earth metals, their trifluorides and sesquioxides with elemental sulfur in appropriate molar ratios at 850 °C flux-assisted by NaCl in gas-tightly sealed niobium or tantalum ampoules. All compounds are colorless, except for those containing M = Pr with green, M = Nd with lilac and M = Sm with yellow color. They form transparent single crystals as needles or rods. The M6O2F8S3 representatives crystallize in a hexagonal structure (space group: P63/m; a≈1382 - 1326, c≈398 - 376 pm, c/a≈0:288 - 0.284; Z =2) with two different crystallographic M3+ positions. The (M2)3+ cations reside in ninefold coordination of anions arranged as tricapped trigonal prisms formed by three F-, four mixed-occupied O2-/F- and two S2- anions, resembling the unique M3+ coordination sphere of the M3OF5S-type oxide fluoride sulfides. The (M1)3+ cations are surrounded by square antiprisms built of four O2-/F- and S2- anions each, which are capped by one F- anion each, again resulting in a ninefold coordination similar to that of the A-MFS-type fluoride sulfides. While the crystallographically unique S2- anions have six cationic neighbors arranged in trigonal prisms, there are four different light-anion positions. Two of them, representing only fluoride anions, are situated in a triangular environment of cations ((F1)-: planar, (F2)-: non-planar). The mixed-occupied light-anion positions (F3)-=(O3)2- and (F4)-=(O4)2- exhibit tetrahedral coordination spheres with a ratio F- :O2- =2 : 1. The M6O2F8S3 arrangement is characterized by an empty hexagonal channel structure created by (F1)- anions with a potential of accommodating alkali-metal cations like Na+. All single-crystal X-ray structure investigations did not indicate any ordering of the O2- and F- anions, but MAPLE calculations revealed a slightly favored oxygen occupation for the (F4)-=(O4)2- rather than for the (F3)-=(O3)2- position. Electron-beam microanalyses gave no evidence for the incorporation of either Na+ or Cl- ions stemming from NaCl used as the flux.