A Na2MoO2F4:Mn4+ phosphor with red luminescence peaking at 625 nm and a ZPL/v6 intensity ratio of 243%
A K2SiF6:Mn4+ phosphor with narrow line-type red emission plays an important role in white light-emitting diode (WLED) backlights for liquid crystal displays, but this phosphor exhibits weak zero phonon line (ZPL) emission. The ZPL intensity of the Mn4+ 2Eg → 4A2g transition can be enhanced after relaxing the orbital selection rule. We dope Mn4+ into a host lattice with mixed-anions to introduce a distortion of local coordination octahedra around Mn4+ and relax the orbital selection rule. Remarkably, the Na2MoO2F4:Mn4+ phosphor with [MoO2F4] mixed-anion coordination exhibits a dominant luminescence peak at 625 nm (ascribed to the ZPL) and the ZPL to Stokes v6 intensity ratio reaches an unprecedented high value of 243% under 480 nm excitation. Density functional theory calculations indicate that the most energy-efficient pathway for maintaining charge balance during Mn4+-doping involves the substitution of [MoO2F4]2− by [MnF6]2−, which generates a strong crystal field with a high Dq/B value of 4.05 for Mn4+ in Na2MoO2F4. An optimal composition Na2MoO2F4:1.5%Mn4+ (nominal) exhibits a high color purity of 99.3% and a short luminescence lifetime of 0.62 ms, suggesting its potential in fast-response backlight display applications. When incorporated into WLEDs, the red-emitting Na2MoO2F4:Mn4+ phosphor contributes to a broad color gamut (∼108.3% NTSC). Comparative analysis of a series of Mo6+-based oxyfluoride phosphors demonstrates that Mn4+ more readily exhibits intense ZPL and fast luminescence decay occurs when Mo6+ is coordinated in an O/F ordered octahedron.
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