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

Abstract

Owing to the magnetic dipole nature, the zero phonon line (ZPL) of Mn⁴⁺:²E → ⁴A₂ 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 ∼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⁺:²E, ²T₁ → ⁴A₂ 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.