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.