Solvochemical Synthesis and Crystal Structure of the Fluoride-Derivatized Early Lanthanoid(III) ortho- Oxidomolybdates(VI) LnF[MoO_4] (Ln = Ce-Nd)

摘要

The development of a new solvochemical synthesis route resulted in single crystals of lanthanoid(III) fluoride oxidomolybdates(VI) with the formula LnF[MoO_4] for the early lanthanoids (Ln = Ce-Nd); to date, these compounds could not be obtained by solid-state reactions. The preparation strategy comprises the dropwise combination of an alkaline aqueous solution containing molybdenum trioxide, sodium hydroxide and sodium fluoride with an acidic aqueous solution of the lanthanoid(III) nitrate. After the purification of the obtained gel, the noncrystalline product has to be dried at 120 °C and converted into a crystalline phase by a final thermal treatment for 24 h at 850 °C in evacuated silica ampoules. The crystal structure of the lanthanoid(III) fluoride oxidomolybdates(VI) with formula LnF[MoO_4] (Ln = Ce-Nd) is not isotypic to those for the smaller rare-earth metals. The title compounds crystallize monoclinically in the space group P2_1 (a = 703-694, b = 660-653, c = 939-933 pm and β ≈ 106°) with four formula units per unit cell. The structure contains crystallographically unique Ln~(3+) cations, which are surrounded by two F- and six O~(2-) anions to form distorted bicapped trigonal prisms (CN = 8). These polyhedra are fused together by two common edges (e) consisting of two oxide anions to form ∞1 {[LnFO~e_(4/2)O~t_(2/1)]~(6-)} strands, which are finally condensed through shared fluoride vertices (v) to build up a three-dimensional ∞3 {[LnF~v_(2/2)O~e_(4/2)O~t_(2/1)]~(6-)} network that still contains terminal (t) oxygen atoms. The Mo~(6+) cations reside in the tetrahedral voids of the aforementioned arrangement. This is the fundamental difference compared to the structures of the representatives with the smaller lanthanoids, in which the Mo~(6+) cations interconnect anionic ∞2 {[LnF~e_(2/2)-O~e_(4/2)O~t_(2/1)]~(6-)} layers to achieve their tetrahedral oxygen coordination spheres. Vibrational spectroscopic measurements such as bulk solid-state and single-crystal Raman spectroscopy as well as diffuse reflectance spectroscopy (DRS) were performed for the crystalline lanthanoid(III) fluoride oxidomolybdates(VI) and the noncrystalline samples of the dried raw products.