Journal of Industrial and Engineering Chemistry, Vol.92, 218-225, December, 2020
Synthesis and luminescence properties of β-NaRE0.95Eu0.05F4 (RE¼Y, Lu)
On the base of β-NaYF4 and β-NaLuF4 compounds, optimal synthesis methods were selected to obtain highly crystalline Eu3+-doped compounds in the hexagonal (β) structure. The compounds β-NaY0.95Eu0.05F4 and β-NaLu0.95Eu0.05F4 were synthesized by hydrothermal synthesis. The concentration of Na2EDTA introduced into the reaction system as a modifier affects the shape and size of the product from elongated hexagonal prisms to quartz-like crystals. The thermal behavior of the compounds has been studied; the temperature of the polymorphic transition from hexagonal to cubic structure is 680 °C for β-NaY0.95Eu0.05F4 and 624 °C for β-NaY0.95Eu0.05F4, the melting point of α-NaRE0.95Eu0.05F4 is about 900 °C. Under excitation with UV-light of l = 393 nm, β-NaY0.95Eu0.05F4 and β-NaLu0.95Eu0.05F4 show the emission lines of direct Eu3+ f-f transitions, with the presence of the emission from higher Eu3+ excited states (up to 5H3) in both samples, which is responsible for the more orange color of emission instead of the usual red for Eu3+. Emission decay times are slightly longer for β-NaY0.95Eu0.05F4 than for β-NaLu0.95Eu0.05F4, which is the result of a bigger unit cell for the former, leading to longer interatomic distances between Eu3+ ions in the lattice. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed.