Novel high-efficiency Eu³⁺-activated Na₂Gd₂B₂O₇ red-emitting phosphors with high color purity

In this study, a series of Na₂(Gd_1-xEu_x)₂B₂O₇ (abbreviated as: Na₂Gd₂B₂O₇:xEu³⁺; x = 0, 0.05, 0.15, 0.35, 0.45, and 0.50) phosphors were synthesized by conventional solid state reaction approach. The structure and morphology of the as-prepared phosphors were studied by X-ray diffraction, scanning electron microscopy and elemental mapping. The results indicated that the phosphors had micro-size particles with monoclinic phases. The photoluminescence excitation and emission study indicated that the as-prepared phosphors could give rise to efficient red emissions under near-ultraviolet excitation. The optimal doping concentration of Eu³⁺ ions was x = 0.35, and the corresponding deduced critical distance (R_c) was about 9.4 Å. The concentration quenching mechanism was dominated by energy-transfer among the Eu³⁺ ions through dipole-dipole interactions. The calculated internal quantum efficiency of the Na₂Gd₂B₂O₇:0.35Eu³⁺ red phosphor was 70.8% and the color purity was as high as 99%. Furthermore, decay dynamics revealed that with the increase in the content of Eu³⁺, the lifetimes decreased due to the increase in non-radiative transition. Importantly, the present Na₂Gd₂B₂O₇:0.35Eu³⁺ phosphors also exhibited good thermal stability, and the emission intensity at 150 °C was about 56% of that at 30 °C. Furthermore, a warm white light-emitting diode (WLED) device was fabricated with commercial BaMgAl₁₀O₁₇:Eu²⁺ blue phosphors, (Ba,Sr)₂(SiO₄):Eu²⁺ green phosphors, and the as-prepared Na₂Gd₂B₂O₇:0.35Eu³⁺ red phosphors as well as a 395 nm LED chip. The device exhibited CIE coordinates of (0.4367, 0.3987), a high color rendering index (CRI = 92.2), a low correlated color temperature (CCT = 2960 K), and a luminous efficacy of 40.65 lm W^-1. The observed results strongly indicate that the as-prepared Na₂Gd₂B₂O₇:Eu³⁺ phosphors may be used as the red emitting component in pc-WLEDs.