Moisture-Resistant Mn4+-Doped Core-Shell Structured Fluoride Red Phosphor Exhibiting High Luminous Efficacy for Warm White LEDs.

K2TiF6:Mn4+ is a highly efficient narrow-band emission red phosphor with promising applications in white LEDs and wide-gamut displays. Nevertheless, the poor moisture-resistant property of this material hinders its further commercialization. Herein, we introduce a convenient reverse cation exchange strategy to construct a core-shell structured K2TiF6:Mn4+@K2TiF6 phosphor. The outer pure K2TiF6 shell can not only act as a shield for preventing moisture in the air to hydrolyze inner MnF62- group, but also effectively cut off the path of energy migration to surface defects, thus increasing the emission efficiency, especially for the phosphors doped with high Mn4+ concentration. The packaged white LED by employing this material as red phosphor exhibits an extraordinarily high luminous efficacy of 162 lm W-1, a CCT of 3510 K and a color rendering index of 93 (Ra). Particularly, the aging tests performed on this device at 85 ºC and 85% humidity for 480 h show that the luminous efficacy retains as high as 89% with respect to its initial value. These findings demonstrate a substantial advance towards the commercial application of K2TiF6:Mn4+@K2TiF6 phosphor.