Abstract
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 λ = 393nm, 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.
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