Synthesis under high pressure: crystal structure and properties of cubic Dy₃₆O₁₁F₅₀[AsO₃]₁₂ ∙ H₂O

The multi-anionic compound with the composition Dy₃₆O₁₁F₅₀[AsO₃]₁₂ ∙ H₂O, which can be described in the non-centrosymmetric cubic space group F $$ c, already shows an unusually large unit cell with an axis of a = 2587.59(14) pm. Its crystal structure exhibits isolated ψ¹-tetrahedral [AsO₃]^3- anions, but both the coordination numbers and the linking schemes of the Dy³⁺-centered polyhedra differ significantly from the mostly layered structures described so far in literature. (Dy1)³⁺ is sevenfold coordinated by oxygen atoms and F^- anions, forming a capped trigonal prism [(Dy1)O_4.333F_2.667]^8.333-, and the remaining two cations (Dy2)³⁺ and (Dy3)³⁺ both reside in an eightfold coordination of anions. In both cases they form slightly distorted square antiprisms, which have the compositions of [(Dy2)O_3.667F_4.333]^8.667- and [(Dy3)O_4.667F_3.333]^9.667-, respectively. Some of the oxygen atoms are not part of ψ¹-[AsO₃]^3- tetrahedra, but occur as O^2- anions and one of these even shares a common crystallographic position with fluoride (F^-). It is also worth mentioning that the single crystals were obtained as comparatively large cubes with an edge length of several 100 µm providing very good data with regard to single-crystal X-ray diffraction. To verify the simultaneous presence of oxygen and fluorine, electron-beam microprobe analysis was carried out, and a single-crystal Raman spectrum ruled out the presence of hydroxide anions or protonated [AsO₃]^3- groups, but proved the interstitial crystal-water molecules, which could not be determined precisely by the crystal-structure refinement.