Solution combustion synthesis of high-entropy rare earth oxide Ce <sub>0.2</sub>La <sub>0.2</sub>Gd <sub>0.2</sub>Y <sub>0.2</sub>Lu <sub>0.2</sub>O <sub>1.6</sub>:Eu <sup>3+</sup>phosphor with intense blue-light excitable red emission for solid-state lighting
Ziqing Yin, Yufeng Mao, Shikao Shi, Jiye Wang, Ruilong Zong
Abstract
Red-light-emitting phosphors capable of being well excited with blue light are highly desirable in solid-state lighting. In this work, a novel Eu<sup>3+</sup>-activated high-entropy rare earth oxide Ce<sub>0.2</sub>La<sub>0.2</sub>Gd<sub>0.2</sub>Y<sub>0.2</sub>Lu<sub>0.2</sub>O<sub>1.6</sub>:<i>x</i>Eu<sup>3+</sup> (<i>x</i> = 4–16 mol%) phosphor was successfully prepared by solution combustion reaction for the first time. The multi-composition rare earth oxide has a specific cubic fluorite structure, which is almost the same as that of the pure CeO<sub>2</sub> despite the tiny ceria composition in the sample, demonstrating the formation of a high-entropy composite solid solution. To our surprise, the high-entropy phosphor exhibits extremely intense red emission at 613 nm, corresponding to the <sup>5</sup>D<sub>0</sub>→<sup>7</sup>F<sub>2</sub> characteristic transition of Eu<sup>3+</sup> under the excitation of blue light at 466 nm. The luminescence internal quantum yield (QY) for the optimal high-entropy phosphor (<i>x</i> = 12 mol%) reaches nearly 50% and can further increase to 67.8% through a subsequent heat-treatment process at 1400 °C. The QY result is much superior to that of previously reported Eu<sup>3+</sup>-activated CeO<sub>2</sub> as well as Y<sub>2</sub>Ce<sub>2</sub>O<sub>7</sub> and La<sub>2</sub>Ce<sub>2</sub>O<sub>7</sub> low-entropy composite oxides (QYs are approximately 10%–20%). Moreover, the high-entropy oxide phosphor also shows better luminescence thermal stability than low-entropy oxides, as confirmed from the temperature-dependent photoluminescence emission spectra. The tremendous improvement in optical properties depends closely upon the high-entropy and other related effects. The novel high-entropy rare earth oxide phosphor is beneficial to be used in the field of solid-state lighting owing to the coincidence of excitation of blue light with the emission of InGaN light-emitting diode (LED) chips.