Crystal structure, optical properties and thermal properties of M₂[W₂O₃(SO₄)₆] (M = Y, Eu, Tb, Lu, Bi)

Sulfatotungstates constitute a rather new host structure materials that may exhibit interesting optical properties. The compounds M₂[W₂O₃(SO₄)₆], with M = Y, Eu, Tb, Lu, and Bi, were synthesised as phase pure materials under solvothermal conditions in oleum. They crystallise isotypic to each other in the monoclinic space group C2/c (no. 15) with Z = 4, determined by single-crystal X-ray diffraction. Their structures feature corner-connected sulfate and tungstate units, the latter showing a disorder of the tungstate cations with a thread-like arrangement along the b-axis. The metal cations M are eight-fold-coordinated and show typical red (M = Eu) or green (M = Tb) luminescence in fluorescence spectra due to their f-f transitions. The f-d transition of Tb³⁺ reveals rather weak coordination behaviour of the host structure. Thermal analysis showed relatively high temperature stabilities of the compounds, increasing as the ionic radii of the metal cations decrease. Additionally, the compounds were characterised by FT-IR and UV-vis measurements while using Tauc plots to determine their band gaps. The lone-pair activity of the bismuth compound was elucidated by geometrical calculations based on the structural data.