Novel high-efficiency Eu 3+ -activated Na 2 Gd 2 B 2 O 7 red-emitting phosphors with high color purity.

In this study, a series of Na 2 (Gd 1- x Eu x ) 2 B 2 O 7 (abbreviated as: Na 2 Gd 2 B 2 O 7 : x Eu 3+ ; 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 Eu 3+ 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 Eu 3+ ions through dipole-dipole interactions. The calculated internal quantum efficiency of the Na 2 Gd 2 B 2 O 7 :0.35Eu 3+ 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 Eu 3+ , the lifetimes decreased due to the increase in non-radiative transition. Importantly, the present Na 2 Gd 2 B 2 O 7 :0.35Eu 3+ 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 BaMgAl 10 O 17 :Eu 2+ blue phosphors, (Ba,Sr) 2 (SiO 4 ):Eu 2+ green phosphors, and the as-prepared Na 2 Gd 2 B 2 O 7 :0.35Eu 3+ 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 Na 2 Gd 2 B 2 O 7 :Eu 3+ phosphors may be used as the red emitting component in pc-WLEDs.