Raman and density functional theory studies of lutecium fluoride and oxyfluoride structures in molten FLiNaK

Combined with Raman spectroscopy and density functional theory (DFT) calculations, the micro-structures in molten FLiNaK-LuF₃ and FLiNaK-LuF₃-Li₂O systems were studied. Both LuF₅^2- (D_3h) and LuF₆^3- (O_h) anions were identified in molten FLiNaK, and their relative content varied with the concentration of LuF₃. For regarding the affection of oxygen anion, Li₂O was added into the molten FLiNaK-LuF₃ (20 mol%) sample. The lutecium oxyfluoride anion Lu₂OF₈^4- was firstly formed which possesses a linear LuOLu geometry with two LuF₄ moieties bridged by one single oxygen atom. Further increasing the Li₂O content to 10 mol% resulted in the formation of two species, which belonged to the Lu₂O₂F₄^2- and Lu₂O₂F₆^4- anions. When increased the concentration of Li₂O to 20 mol%, a new species appeared which was approximate to the oxide structure.