Efficient and stable Sr3Eu2B4O12 red phosphor benefiting from low symmetry and distorted local environment

Abstract

The development of suitable red phosphors to obtain improved white color stands a good chance to serve in the new generation of white light-emitting diodes. Owing to multi-elements via doping and oxidation of reduced valence state of lanthanide or transition metal ions, most of the reported phosphors usually suffer from complex synthetic processes and unstable color of the lighting industry cycle. In this work, we present a new red emitting and stable Sr₃Eu₂B₄O₁₂ phosphor with regard to its special structure. It crystallizes as an orthorhombic cell, with Sr and Eu atoms co-occupying three different lattice sites in the space group of Pnma (no. 62). It is proposed that the long bond distance between activators minimizes the content quenching, while the high disorder of location restricts the thermal quenching. This phosphor emits bright red light with good color purity under UV excitation, with the luminescence intensity and quantum yield tunable via the fabrication temperature. Through a preliminary optimization of the synthesis process, the Sr₃Eu₂B₄O₁₂ phosphor prepared at 1250 °C has high quantum yields of about 94.7% and excellent thermal stability of 85.6% intensity retention at 150 °C relative to the initial value at room temperature. The calculated Judd–Ofelt intensity parameters (Ω₂, Ω₄) further clarified that the Eu³⁺ site in Sr₃Eu₂B₄O₁₂ had lower symmetry without an inversion center, and more distorted local environment and structural rigidity of the host, predicting excellent thermal stability. Finally, a warm pc-WLED device has been produced by mixing as-prepared Sr₃Eu₂B₄O₁₂ powders and commercial BaMgAl₁₀O₁₇:Eu²⁺ and (Sr, Ba)₂SiO₄:Eu²⁺ phosphors, which exhibits a high color rendering index (R_a = 83.4) along with a color temperature at around 4102 K. The present work indicates that the Sr₃Eu₂B₄O₁₂ phosphor is an efficient red component with excellent thermal stability for white-light production of near-UV-excited w-LEDs.