Surface lattice enhancement of red-emitting fluorides enabled by embedding small cations

Abstract

The poor moisture resistance of Mn⁴⁺-activated fluoride red phosphors restricts their practical applications. Herein, this work proposes the embedment of small radius cations (Si⁴⁺ or Ge⁴⁺) into the inert shell (K₂TiF₆) constructed on the surface of K₂TiF₆:Mn⁴⁺ particles, which increases the covalence of the surface lattice and thus leads to a significant improvement of the stability of the fluoride. By using multidimensional microstructural characterization techniques, we confirmed the construction of a heterogeneous shell (K₂Ti_1−xSi_xF₆ and K₂Ti_1−yGe_yF₆) and systematically investigated the construction process. Compared to the untreated K₂TiF₆:Mn⁴⁺, the external quantum efficiencies of the K₂TiF₆:Mn⁴⁺@K₂Ti_1−xSi_xF₆ and K₂TiF₆:Mn⁴⁺@K₂Ti_1−yGe_yF₆ heterogeneous core–shell particles improved by 2–3%, and 99% and 88% of the initial luminescence intensity were maintained after boiling in water for 50 min, respectively, which are significantly better than that (25%) of the homogeneous K₂TiF₆:Mn⁴⁺@K₂TiF₆. The aging of red and white light-emitting diode devices at a high temperature (85 °C) and a high humidity (85%) shows that the heterogeneous core–shell structures have higher stability than their homogeneous counterparts. The surface lattice enhancement strategy proposed in this work is useful as a reference for improving the properties of other under-stable materials.