Non-Rare-Earth K2XF7:Mn4+(X = Ta, Nb): A Highly-Efficient Narrow-Band Red Phosphor Enabling the Application in Wide-Color-Gamut LCD

The advent of wide color-gamut LED backlight technology greatly improves the user experience in visions. The bottleneck restricting progress of this technology is to develop red phosphor with satisfactory spectral features and low cost. Herein, a novel non-rare-earth K2XF7:Mn4+ (X = Ta, Nb) was successfully designed and synthesized, exhibiting an admirable quantum efficiency of ∼93.5%, an extremely narrow FWHM of ∼2.3 nm, and a high color purity of ∼99.6%. Unlike the previously reported Mn4+ activated fluorides, Mn4+ in K2XF7 suffers highly distorted octahedral environment in C1 (or C2v) group symmetry, enabling the appearance of intense zero phonon line and distinctive vibronic transitions. Systematical high-resolution spectroscopic analyses down to 10 K disclose the influences of strong crystal field, weak nephelauxetic effect, and intermediate electron-phonon coupling are responsible for the peculiar spectral features of Mn4+ in K2XF7. Impressively, a wide color-gamut reaching up to 86.7% NTSC is realized, demonstrating great validity of K2XF7:Mn4+ for LCD backlights. The present study not only brings a brand-new kind of Mn4+ activated host to the sight of phosphor community (the host can be extended to surges of tantalum/niobium fluorides), but also may enlighten researchers to design highly distorted environment to achieve unique spectral properties of Mn4+.