Red Mn⁴⁺-Doped Fluoride Phosphors: Why Purity Matters

Traditional light sources, e.g., incandescent and fluorescent lamps, are currently being replaced by white light-emitting diodes (wLEDs) because of their improved efficiency, prolonged lifetime, and environmental friendliness. Much effort has recently been spent to the development of Mn⁴⁺-doped fluoride phosphors that can enhance the color gamut in displays and improve the color rendering index, luminous efficacy of the radiation, and correlated color temperature of wLEDs used for lighting. Purity, stability, and degradation of fluoride phosphors are, however, rarely discussed. Nevertheless, the typical wet chemical synthesis routes (involving hydrogen fluoride (HF)) and the large variety of possible Mn valence states often lead to impurities that drastically influence the performance and stability of these phosphors. In this article, the origins and consequences of impurities formed during synthesis and aging of K₂SiF₆:Mn⁴⁺ are revealed. Both crystalline impurities such as KHF₂ and ionic impurities such as Mn³⁺ are found to affect the phosphor performance. While Mn³⁺ mainly influences the optical absorption behavior, KHF₂ can affect both the optical performance and chemical stability of the phosphor. Moisture leads to decomposition of KHF₂, forming HF and amorphous hydrated potassium fluoride. As a consequence of hydrate formation, significant amounts of water can be absorbed in impure phosphor powders containing KHF₂, facilitating the hydrolysis of [MnF₆]^2– complexes and affecting the optical absorption of the phosphors. Strategies are discussed to identify impurities and to achieve pure and stable phosphors with internal quantum efficiencies of more than 90%

Crystal Structure and Luminescent Properties of R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) Red Phosphors

The R₂(MoO₄)₃ (R = rare earth elements) molybdates doped with Eu³⁺ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (α) and orthorhombic (β′) R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) modifications were prepared by solid-state reaction, and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic β′-phases could be synthesized only by quenching from high temperature to room temperature for Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ in the Eu³⁺-rich part (<i>x</i> > 1) and for all Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ solid solutions. The transformation from the α-phase to the β′-phase results in a notable increase (∼24%) of the unit cell volume for all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Sm, Gd) solid solutions. The luminescent properties of all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) solid solutions were measured, and their optical properties were related to their structural properties. All R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) phosphors emit intense red light dominated by the ⁵D₀→​⁷F₂ transition at ∼616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO₈ to RO₇ for the α- and β′-modification, respectively. The Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ solid solutions are the most efficient emitters in the range of 0 < <i>x</i> < 1.5, but their emission intensity is comparable to or even significantly lower than that of Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ for higher Eu³⁺ concentrations (1.5 ≤ <i>x</i> ≤ 1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the ultraviolet–visible–infrared regions of the EELS spectrum

Crystal Structure and Luminescent Properties of R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) Red Phosphors

The R₂(MoO₄)₃ (R = rare earth elements) molybdates doped with Eu³⁺ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (α) and orthorhombic (β′) R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) modifications were prepared by solid-state reaction, and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic β′-phases could be synthesized only by quenching from high temperature to room temperature for Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ in the Eu³⁺-rich part (<i>x</i> > 1) and for all Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ solid solutions. The transformation from the α-phase to the β′-phase results in a notable increase (∼24%) of the unit cell volume for all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Sm, Gd) solid solutions. The luminescent properties of all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) solid solutions were measured, and their optical properties were related to their structural properties. All R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) phosphors emit intense red light dominated by the ⁵D₀→​⁷F₂ transition at ∼616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO₈ to RO₇ for the α- and β′-modification, respectively. The Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ solid solutions are the most efficient emitters in the range of 0 < <i>x</i> < 1.5, but their emission intensity is comparable to or even significantly lower than that of Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ for higher Eu³⁺ concentrations (1.5 ≤ <i>x</i> ≤ 1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the ultraviolet–visible–infrared regions of the EELS spectrum