A novel Mn⁴⁺-activated layered oxide-fluoride perovskite-type KNaMoO₂F₄ red phosphor for wide gamut warm white...

Mn⁴⁺-activated oxide-fluoride phosphors are attractive for application in a wide range of solid-state lighting devices because of their distinct red emission at about 630 nm and the abundant storage of Mn ions. However, the zero-phonon line (ZPL) of Mn⁴⁺ ions is too weak to be detected in most host materials due to the magnetic dipole nature. In this article, we introduce a co-precipitation method for synthesizing a Mn⁴⁺-doped oxyfluoride perovskite KNaMoO₂F₄ phosphor containing [MoO₂F₄]^2- building units. The electron paramagnetic resonance (EPR) spectra are consistent with the presence of a MnF₆^2- species at g = 1.991. The KNaMoO₂F₄:0.01Mn⁴⁺ phosphor exhibits strong absorption under blue light and an internal quantum yield (IQE) of 65.8%. Attributed to the distorted octahedral environment of the Mn⁴⁺ ions, visible ZPL emission was detected at 625 nm. Based on theoretical calculations, the Mn⁴⁺ ions in the KNaMoO₂F₄ host exist in a strong crystal field with a high D_q/B value of ∼3.86. A series of photoluminescence-dependent low-temperature spectra indicates that the Mn⁴⁺ emissive state experiences weak electron-phonon interactions upon calculating the Huang-Rhys factor. Benefiting from the ZPL, a warm white-light-emitting diode is achieved using YAG:Ce³⁺ and KNaMoO₂F₄:Mn⁴⁺ as color converters, in which the color rendering index was R_a = 83.5 and CCT = 4490 K.