A Na2MoO2F4:Mn4+ phosphor with red luminescence peaking at 625 nm and a ZPL/v6 intensity ratio of 243%

Abstract

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 Mn^{4+ 2}E_g → ⁴A_2g 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.