A highly-distorted octahedron with a C2v group symmetry inducing an ultra-intense zero phonon line in Mn4+-activated oxyfluoride Na2WO2F4

Owing to the magnetic dipole nature, the zero phonon line (ZPL) of Mn4+:E-2 -> (4)A(2) transition is weak unless Mn4+ is situated at a site deviating from the centrosymmetric nature. Herein, we report a brand-new oxyfluoride, Na2WO2F4:Mn4+(NWOF:Mn4+), which shows unprecedented intense red ZPL at similar to 620 nm along with relatively weak vibronic transitions under blue light excitation. This peculiar spectral feature is demonstrated to be originated from the highly distorted octahedral coordination environment in the C-2v group symmetry surrounding Mn4+. High-resolution spectroscopic studies at 10 K disclose the fine structured electronic/vibronic transitions of Mn4+:E-2, 2T(1) -> (4)A(2) and the weak electron-phonon interaction (Huang-Rhys factor S < 1) on the Mn4+ emissive state. Benefiting from the intense ZPL, an ultra-high color rendering index with Ra = 92.7 and R9 = 90.0 is achieved in the w-LED using YAG:Ce3+ and NWOF:Mn4+ as color converters, and a wide color gamut of 107.1% NTSC in the w-LED using CsPbBr3 quantum dots and NWOF:Mn4+ is obtained. Herein, we first demonstrate that Mn4+-activated oxyfluorides have great potential in w-LED lighting and display applications. Our study can also enlighten researchers to design highly distorted octahedral sites for Mn4+ doping to achieve an ultra-intense ZPL.