Fluoride Inside: Syntheses, Structure, and Optical Properties of the RE FWO4 Representatives with RE = Y, Sm–Lu

The reactions of WO3, RE2O3, and REF3 (RE = Y, Sm–Lu) with an excess of CsBr as flux lead to the formations of coarse single crystals with the composition REFWO4. In the crystal structure (space group P1̅; a = 471–467, b = 617–589, c = 694–676 pm; α ≈ 89°, β = 83–81°, γ ≈ 75°; Z = 2), 8-fold coordinated RE3+ cations are present in the shape of distorted trigonal dodecahedra consisting of five O2– and three F– anions. One of the latter contacts is gradually detached throughout the series from RE = Sm to Lu, so that the coordination number of Sm3+ in SmFWO4 can be considered as eight, while for LuFWO4 the coordination number of Lu3+ is more accurately described as seven plus one. The W6+ cations are surrounded by six O2– anions in distorted octahedral shapes, and these polyhedra are fused together via common edges chains running along [100]. Both YFWO4 and GdFWO4 were activated with Eu3+ and Tb3+ cations to investigate the usability of these compounds as host materials for luminescence. Besides steady-state excitation and emission spectra, time-resolved measurements at 80 K and at room temperature were performed.