Thermal-stability synergy improvement of Sm3+ and Eu3+ in Ca3.6In3.6(PO4)6: the effect of local symmetry

Abstract

The thermal stability of phosphors, particularly for the red component, is a major factor restricting the quality improvement of PC-WLEDs. Numerous researchers have invested considerable time and effort into developing excellent thermally stable red phosphors. Herein, a synergistic strategy of enhancing symmetry based on homogeneity co-doping was proposed to enhance the luminous performance of phosphors. As a typical application case, using Ca_3.6In_3.6(PO₄)₆ as a host, a series of single-doped and co-doped Eu³⁺, Sm³⁺ red phosphors were designed and synthesized. In contrast to single-doped phosphors, Eu³⁺, Sm³⁺ co-doped phosphors show reinforced UV absorption and improved thermal stability. At room temperature, the fluorescence lifetime of Sm³⁺ decreases to a certain extent as the Eu³⁺ concentration increases, indicating the energy transfer from Sm³⁺ to Eu³⁺. The co-admixture of Eu³⁺ and Sm³⁺ increased crystal local symmetry, which succeeded in triggering the synergy enhancement of thermal stability for Eu³⁺ or Sm³⁺ doped phosphors. Moreover, the internal quantum efficiency of Ca_3.6In_{(1−x−y)3.6}(PO₄)₆:0.05Eu³⁺,0.04Sm³⁺ phosphor highly reaches 82.8%. A warm white light-emitting diode based on Ca_3.6In_{(1−x−y)3.6}(PO₄)₆:0.05Eu³⁺,0.04Sm³⁺ with low CCT (4694 K), high Ra (86.9), and CIE chromaticity coordinate (0.3445, 0.3253) was fabricated. The present results expect that Eu³⁺, Sm³⁺ co-activated Ca_3.6In_3.6(PO₄)₆ phosphors are suitable for application in high-power LEDs. Most importantly, this study provides new insight into the relationship between local symmetry and thermal stability and offers a creative strategy for developing high-quality phosphors in the future.