Hydrothermal synthesis of BaLu2F8:Ln3+ crystals: phase/morphology evolution, energy transfer and tunable multicolor luminescence

Abstract

BaLu_2−xF₈:xLn³⁺ crystals have been synthesized via a one-step hydrothermal route without any surfactants. X-ray diffraction (XRD), scanning electron microscopy (SEM), and down-shift/up-conversion photoluminescence spectroscopy were used to characterize the samples. Pure monoclinic BaLu₂F₈ crystals were obtained at pH = 7 with NaBF₄ as a fluoride source. Doping 0.06 mmol Ln³⁺ ions from Ho³⁺ to La³⁺ and doping Yb³⁺ ions from 0.1 to 0.4 mmol both have caused dramatic changes to the product's morphology from prismatic microrods of about 10 μm to particles of about 3 μm aggregated by flakes. The differences in the phase, size and shape of the final products were greatly related to the fluoride source and pH values, which have been studied in detail. In addition, surfactants also affected the phase and morphology of the BaLu₂F₈ crystals. Multicolor down-shift emission was achieved in the BaLu_1.94F₈:0.06Ln³⁺ (Ln = Sm, Eu, Tb, Dy, Er and Tm) crystals under UV lamp excitation at 365 nm, and multicolor UC luminescence has been achieved in BaLu₂F₈:Yb³⁺,Ln³⁺ (Ln = Er, Tm, Ho) systems. When the Yb³⁺–Er³⁺ ions are co-doped into the host, the mechanism for the red emission (⁴F_9/2 → ⁴I_15/2) retains the tri-photon process like the single-doped Er³⁺ ion, but the mechanism for the green emission (²H_11/2/⁴S_3/2 → ⁴I_15/2) is a two-photon process. Furthermore, the UC luminescence mechanism for the co-doped Yb³⁺–Ho³⁺ pairs is also a tri-photon process in the BaLu₂F₈ matrix. The as-prepared BaLu₂F₈:Ln³⁺ phosphors have potential applications in the fields of light emitting devices, three-dimensional displays and backlights.