Crystal structure, spectroscopic properties, and magnetic behavior of the fluoride-derivatized lanthanoid(III) ortho-oxomolybdates(VI) LnF[MoO4] (Ln=Sm–Tm)

Blue Excitement: The Lanthanide(III) Chloride Oxidomolybdates(VI) Ln3Cl3[MoO6] (Ln = La, Pr, and Nd) and Their Spectroscopic Properties

The lanthanide(III) chloride oxidomolybdates(VI) with the empirical formula Ln₃Cl₃[MoO₆] (Ln = La, Pr, and Nd) were synthesized by solid-state reactions utilizing the respective lanthanide trichloride, lanthanide sesquioxide (where available), and molybdenum trioxide together with lithium chloride as a fluxing agent. The title compounds crystallize in hexagonal space group P6₃/ m ( a = 942-926 pm, c = 542-533 pm, Z = 2). Besides tetracapped trigonal prismatically coordinated Ln³⁺ cations, noncondensed trigonal prismatic [MoO₆]^6- entities are found in the crystal structure. In addition to X-ray diffraction, the title compounds were also characterized by single-crystal Raman and infrared spectroscopy as well as measurements to determine their magnetic susceptibility and behavior at low temperatures. The most outstanding properties of the Ln₃Cl₃[MoO₆] representatives (Ln = La, Pr, and Nd), however, are of an optical nature, because their band gaps, determined by diffuse reflectance spectroscopy, show a significant shift toward lower energies compared to those of other rare-earth metal chloride molybdates with a different polyhedral arrangement. This culminates in La₃Cl₃[MoO₆]:Eu³⁺ exhibiting luminescence, which can be excited in the visible range of the electromagnetic spectrum by a blue light-emitting diode.

On halide derivatives of rare-earth metal(III) oxidomolybdates(VI) and -tungstates(VI)

Halide derivatives of rare-earth metal(III) oxidomolybdates(VI) have been investigated comprehensively over the last decade comprising the halogens fluorine, chlorine, and bromine. Iodide-containing compounds are so far unknown. The simple composition REXMoO4 (RE=rare-earth element, X=halogen) is realized for X=F almost throughout the complete lanthanide series as well as for yttrium. While ytterbium and lutetium do not form any fluoride derivative, for lanthanum, only a fluoride-deprived compound with the formula La3FMo4O16 is realized. Moreover, molybdenum-rich compounds with the formula REXMo2O7 are also known for yttrium and the smaller lanthanoids. For X=Cl the composition REClMoO4 is known for yttrium and the whole lanthanide series, although, four different structure types were identified. Almost the same holds for X=Br, however, only two different structure types are realized in this class of compounds. In the case of halide derivatives of rare-earth metal(III) oxidotungstates(VI) the composition REXWO4 is found for chlorides and bromides only, so far. Due to the similar size of Mo6+ and W6+ cations, the structures found for the tungstates are basically the same as for the molybdates. With the larger lanthanides, the representatives for both chloride and bromide derivates exhibit similar structural motifs as seen in the molybdates, however, the crystal structure cannot be determined reliably. In case of the smaller lanthanoids, the chloride derivatives are isostructural with the respective molybdates, although the existence ranges differ slightly. The same is true for rare-earth metal(III) bromide oxidotungstates(VI).