Oxide Fluoride Sulfides of the Lanthanoids with the Formula M _6O_2F_8S_3 (M = La-Nd, Sm, Gd)

摘要

The lanthanoid(III) oxide fluoride sulfides M_6O _2F_8S_3 (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 M_6O_2F_8S_3 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 crystallographicM~(3+) positions. The (M2)~(3+) cations reside in ninefold coordination of anions arranged as tricapped trigonal prisms formed by three F-, four mixed-occupied O~(2-)=F- and two S~(2-) anions, resembling the unique M~(3+) coordination sphere of the M3OF5S-type oxide fluoride sulfides. The (M1)~(3+) cations are surrounded by square antiprisms built of four O~(2-)=F- and S~(2-) 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 S~(2-) 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~-:O ~(2-) =2: 1. The M_6O_2F_8S_3 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 O~(2-) 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.