To Luminesce or to Change Valence? Insight into the Wavelength Dependency of the Reversible Valence Switching of Europium in Sr₃SiO₅

A good control over the valence state of dopants in luminescent materials or phosphors is important for the development of highly efficient phosphors for white light-emitting diodes (LEDs). Detailed spectroscopic studies allow us to reveal optically induced charge transfer processes and elucidate the underlying mechanisms in phosphors with additional functionalities such as photochromism or persistent luminescence. However, the spectroscopic study of the valence switching of europium has scarcely been reported. Here, we report on the Sr3SiO5:Eu phosphor, in which photo-reduction (Eu3+ → Eu2+) and photo- or thermal-oxidation (Eu2+ → Eu3+) reactions are demonstrated. The variation of the illumination wavelength influences the efficiency of both the photo-reduction/oxidation and the accompanying dynamic process, especially when the two opposite reactions occur simultaneously. Temperature-dependent annealing indicates a large trap depth for the electron trapped by Eu3+. The good stability of Eu2+ obtained by photo-reduction and the repeatability of the Eu2+/Eu3+ valence switching are confirmed as well. Furthermore, the application of optical information storage is demonstrated based on this phosphor. The results of this work may not only improve the understanding of Eu2+/Eu3+ valence change during illumination but also allow the development of new functional luminescent materials

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%

Short-Chain Alcohols Strip X‑Type Ligands and Quench the Luminescence of PbSe and CdSe Quantum Dots, Acetonitrile Does Not

The effect of short-chain alcohols and acetonitrile on the ligand shell composition and the photoluminescence quantum yield of purified PbSe and CdSe quantum dots is analyzed by solution NMR and photoluminescence spectroscopy. We find that short-chain alcohols induce the release of X-type carboxylate ligands with a concurrent reduction of the photoluminescence quantum yield, while acetonitrile does not. We interpret this difference in terms of the protic or aprotic character of both nonsolvents, where only the protic alcohols can provide the protons needed to desorb carboxylate ligands. We find similar differences between short-chain alcohols and acetonitrile when used as nonsolvents during the purification of crude synthesis products, a result stressing the importance of using aprotic nonsolvents for nanocrystal purification or processing

First-Principles Study of Antisite Defect Configurations in ZnGa₂O₄:Cr Persistent Phosphors

Zinc gallate doped with chromium is a recently developed near-infrared emitting persistent phosphor, which is now extensively studied for in vivo bioimaging and security applications. The precise mechanism of this persistent luminescence relies on defects, in particular, on antisite defects and antisite pairs. A theoretical model combining the solid host, the dopant, and/or antisite defects is constructed to elucidate the mutual interactions in these complex materials. Energies of formation as well as dopant, and defect energies are calculated through density-functional theory simulations of large periodic supercells. The calculations support the chromium substitution on the slightly distorted octahedrally coordinated gallium site, and additional energy levels are introduced in the band gap of the host. Antisite pairs are found to be energetically favored over isolated antisites due to significant charge compensation as shown by calculated Hirshfeld-I charges. Significant structural distortions are found around all antisite defects. The local Cr surrounding is mainly distorted due to a Zn_Ga antisite. The stability analysis reveals that the distance between both antisites dominates the overall stability picture of the material containing the Cr dopant and an antisite pair. The findings are further rationalized using calculated densities of states and Hirshfeld-I charges

Exploring Lanthanide Doping in UiO-66: A Combined Experimental and Computational Study of the Electronic Structure

Lanthanide-based metal–organic frameworks show very limited stabilities, which impedes their use in applications exploiting their extraordinary electronic properties, such as luminescence and photocatalysis. This study demonstrates a fast and easy microwave procedure to dope UiO-66, an exceptionally stable and tunable Zr-based metal–organic framework. The generally applicable synthesis methodology is used to incorporate different transition metal and lanthanide ions. Selected experiments on these newly synthesized materials allow us to construct an energy scheme of lanthanide energy levels with respect to the UiO-66 host. The model is confirmed via absolute intensity measurements and provides an intuitive way to understand charge transfer mechanisms in these doped UiO-66 materials. Density functional theory calculations on a subset of materials moreover improve our understanding of the electronic changes in doped UiO-66 and corroborate our empirical model

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

Incommensurate Modulation and Luminescence in the CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) Red Phosphors

Scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> with <i>B</i>′, <i>B</i>″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)­D incommensurately modulated structures with superspace group <i>I</i>4₁/<i>a</i>(α,β,0)­00­(−β,α,0)­00, while the structures of all tungstates are (3 + 1)­D incommensurately modulated with superspace group <i>I</i>2/<i>b</i>(<i>αβ</i>0)­00. In both cases the modulation arises because of cation-vacancy ordering at the <i>A</i> site. The prominent structural motif is formed by columns of <i>A</i>-site vacancies running along the <i>c</i>-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd³⁺ by the larger Eu³⁺ at the <i>A</i>-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo⁶⁺ by W⁶⁺ switches the modulation from (3 + 2)­D to (3 + 1)­D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the <i>A</i> sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> phosphors emit intense red light dominated by the ⁵D₀–⁷F₂ transition at 612 nm, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. The intensity of the ⁵D₀–⁷F₂ transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1

Incommensurate Modulation and Luminescence in the CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) Red Phosphors

Scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> with <i>B</i>′, <i>B</i>″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)­D incommensurately modulated structures with superspace group <i>I</i>4₁/<i>a</i>(α,β,0)­00­(−β,α,0)­00, while the structures of all tungstates are (3 + 1)­D incommensurately modulated with superspace group <i>I</i>2/<i>b</i>(<i>αβ</i>0)­00. In both cases the modulation arises because of cation-vacancy ordering at the <i>A</i> site. The prominent structural motif is formed by columns of <i>A</i>-site vacancies running along the <i>c</i>-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd³⁺ by the larger Eu³⁺ at the <i>A</i>-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo⁶⁺ by W⁶⁺ switches the modulation from (3 + 2)­D to (3 + 1)­D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the <i>A</i> sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> phosphors emit intense red light dominated by the ⁵D₀–⁷F₂ transition at 612 nm, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. The intensity of the ⁵D₀–⁷F₂ transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1