Fluoride Inside: Syntheses, Structure, and Optical Properties of the REFWO4 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.