Concentration and thermal quenching of SrGdLiTeO<sub>6</sub>: Eu<sup>3+</sup> red-emitting phosphor for white light-emitting diode

Abstract

A series of SrGd<sub>1-<i>x</i></sub>LiTeO<sub>6</sub>:<i>x</i>Eu<sup>3+</sup> (<i>x</i>=0.1-1) red-emitting phosphors, prepared by high-temperature solid-state reaction at 1100℃, is thoroughly investigated by means of X-ray diffraction, diffuse reflectance spectra, photoluminescence spectra, and electroluminescence spectra. These double-perovskite-type phosphors crystallize into monoclinic systems with space group <i>P</i>2<sub>1</sub>/<i>n</i>(14), accommodate Eu<sup>3+</sup> in a highly distorted <i>C</i><sub>1</sub> site symmetry without inversion center, and facilitate the enhancing of the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> hypersensitive transition. The excitation spectra, emission spectra and decay curves indicate that the optimum doping concentration of Eu<sup>3+</sup> is <i>x</i>=0.6. The SrGd<sub>0.4</sub>LiTeO<sub>6</sub>:0.6Eu<sup>3+</sup> presents the strongest excitation peak at 395 nm, which is adequate for near-UV light-emitting diode (LED) pumping; meanwhile, it exhibits an intense red emission with chromaticity coordinates of (0.6671, 0.3284), an asymmetry ratio of 7.56, a color purity of 98.6%, and a luminous efficacy of radiation of 249 lm/W. The fluorescence lifetime is 721 μs, from which the internal quantum efficiency is determined to be 89.7% via the Judd-Ofelt analysis. The formula proposed by van Uiter (van Uitert L G 1967 <i>J. Electrochem. Soc</i>. <b>114</b> 1048), is used to elucidate the energy transfer mechanism. However, the plot of log(I/<i>x</i>)-log(<i>x</i>) produces a confusing index <i>s</i>=4.26, which makes it difficult to distinguish the dipole-dipole interaction from the exchange interaction. After analyzing the reason of error, we present a new plot of log(I<sub>0</sub>'/I-1)-log(<i>x</i>), in which I<sub>0</sub>'=I<sub>0</sub>/<i>x</i><sub>0</sub> and I'=I/<i>x</i>, with <i>x</i><sub>0</sub> corresponding to the low doping content without nonradiative energy transfer. This plot gives rise to <i>s</i>=5.25, a more reasonable value for the dipole-dipole interaction. The integrated emission intensity at 423 K is as high as 85.2% of that at ambient temperature. The thermal activation energy is determined to be 0.2941 eV according to the model based on a temperature-dependent pathway through a charge transfer state. The prototypical LED based on it can emit a bright red light beam. In conclusion, the phosphor exhibits favorable luminous efficiency, color purity and thermal stability of luminescence, which promises solid-state lighting and display applications.