Hydrothermal approach and luminescent properties for the synthesis of orthoniobates GdNbO4:Ln3+ (Ln = Dy, Eu) single crystals under high-temperature high-pressure conditions

Abstract

Single crystal GdNbO₄:Ln³⁺ (Ln = Dy, Eu) phosphors were prepared via a high-temperature high-pressure hydrothermal procedure at 650 °C under autogenous pressure. X-ray diffraction, field emission scanning electron microscopy, photoluminescence, Raman and XPS were utilized to characterize the as-synthesized phosphors. XRD reveals that the samples begin to crystallize at 550 °C and a pure GdNbO₄ phase can be obtained at 650 °C. FE-SEM images indicate that GdNbO₄:Ln³⁺ (Ln = Dy, Eu) samples consist of fine sheets with a size of 50–100 μm. Under UV excitation, the GdNbO₄:Eu³⁺ and GdNbO₄:Dy³⁺ phosphors showed the characteristic emissions of Eu³⁺ are ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4), and Dy³⁺ (⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions), respectively. The preparation method presented here dramatically lowered the traditional temperature of 2300 °C or above in the Czochralski method. The GdNbO₄:Dy³⁺ single crystals showed a bright white emission under different excitation wavelengths with a relatively high quantum yield of 21.7%. The GdNbO₄:0.05Eu³⁺ exhibited excellent bright red luminescence at 612 nm under near-UV excitation, narrowed emission spectra, room temperature luminescence lifetimes of milliseconds and maximum quantum efficiencies of 43.2%.