A low-temperature self-flux route to prepare hexagonal rare earth fluorides and manipulation of upconversion luminescence in rodlike NaYbF4:Tm3+/Fe3+ crystals

Abstract

A low-temperature self-flux strategy has been applied to fabricate a large collection of pure hexagonal phase rare earth fluorides NaREF₄ and REF₃ (RE = rare earth) using NaNO₃, RE(NO₃)₃·xH₂O and NH₄F with low melting points as the initial raw and semifinished materials. It is found that the stable structure and morphology evolution of hexagonal rare earth fluorides are highly dependent upon the type of rare earth ion, the reaction temperature and the reaction time. After incorporating Fe³⁺ ions at less than 40 mol% into NaYbF₄:Tm³⁺ crystal, the resultant samples still maintain a hexagonal crystalline structure and rod-like morphology. The NaYb_0.89Tm_0.01Fe_0.10F₄ (89Yb1Tm10Fe) material exhibits the strongest upconversion (UC) emission and its maximal absolute UC quantum yield (UCQY) is 0.45% under the illumination of a laser diode with pumping energy density of 45 W cm⁻². Strikingly, applying a magnetic field of 15 kOe compels its UCQY to advance up to 0.56%. Moreover, the intrinsic mechanism of Fe³⁺-induced UC luminescence enhancement has also been explored at length. These advances in enhancing UC emissions and UCQYs under an applied magnetic field offer exciting opportunities for important photonic applications.