Boosting phonon-induced luminescence in red fluoride phosphors via composition-driven structural transformations
In this study, a series of (KxNa1−x)2SiF6:Mn4+ red phosphors with systematic composition variations of alkali metals was synthesized via a low-temperature full-solution approach. Driven by composition variations, a sequence of continuous structural phase transformations, i.e., from trigonal to mixed, and then to orthorhombic, and eventually to a cubic phase, is evidently observed in this series of red phosphors. More excitingly, phonon-induced luminescence is promoted as the most efficient and dominant light emission mechanism in a cubic phosphor of K2SiF6:Mn4+ at room temperature. As a result, the overall emission intensity of cubic K2SiF6:Mn4+ is increased by more than fivefold with respect to that of trigonal Na2SiF6:Mn4+. High-resolution X-ray diffraction, electron paramagnetic resonance and micro-Raman scattering experiments consistently reveal a decisive relationship between fluorescence properties and crystalline structures.