Photoluminescence of Mn2+ in the Borosulfate Zn[B2 (SO4 )4 ] : Mn2+ -A Tool to Detect Weak Coordination Behavior of Ligands

The impact of the surrounding ligand field is successfully exploited in the case of Eu2+ to tune the emission characteristics of inorganic photoactive materials with potential application in, e.g., phosphor-converted white light-emitting diodes (pc-wLEDs). However, the photoluminescence of Mn2+ related to intraconfigurational 3d5 -3d5 transitions is also strongly dependent on local ligand field effects and has been underestimated in this regard so far. In this work, we want to revive the idea how to electronically tune the emission color of a transition metal ion in inorganic hosts by unusual electronic effects in the metal-ligand bond. The concept is explicitly demonstrated for the weakly coordinating layer-like borosulfate ligand in the Mn2+ -containing solid solutions Zn1-x Mnx [B2 (SO4 )4 ] (x = 0, 0.03, 0.04, 0.05, 0.10). Zn[B2 (SO4 )4 ]:Mn2+ shows orange narrow-band luminescence at 590 nm, which is an unusually short wavelength for octahedrally coordinated Mn2+ and indicates an uncommonly weak ligand field. On the other hand, the analysis of the interelectronic Racah repulsion parameters reveals ionic Mn-O bonds with values close to the Racah parameters of the free Mn2+ ion. Overall, this strategy demonstrates that electronic control of the metal-ligand bond can be a tool to make Mn2+ a potent alternative emitter to Eu2+ for inorganic phosphors.

Narrow-Band Deep-Blue Emission and Superior Thermal Stability of Fluoroaluminate Phosphor Based on Tungsten Bronze-Type Mineral Structure

Screening novel narrow-band phosphors inspired by natural mineral structures is urgently demanded for improving the performance of phosphor-converted light-emitting diodes. In this work, a novel narrow-band deep-blue-emitting tungsten bronze-type KCaAl₂F₉:Eu²⁺ phosphor with superior thermal stability is successfully synthesized. Structural analysis shows that the representative KCaAl₂F₉:0.013Eu²⁺ phosphor crystallizes in an orthorhombic space group C222₁ with a rigid network. The rigid [AlF₆]^3- octahedrons are linked together by sharing corners to build endless [AlF₆]^3-_∞ chains, further stacking with each other in a highly cross-linked way to establish the rigid network of the KCaAl₂F₉ host. Benefiting from the rigid microenvironment, the developed phosphor not only shows a narrow-band deep-blue emission with a full width at half maximum of 45 nm and a high color purity of 92%, but it also exhibits the superior thermal stability with an emission loss of only 10% at 423 K, demonstrating its application potential in bridging the deep-blue spectral cavity toward sunlight-like full-spectrum lighting. In addition, the concentration/temperature quenching behaviors of KCaAl₂F₉:Eu²⁺ phosphor are systematically investigated. By revealing the specific structure-property relationship of tungsten bronze-type KCaAl₂F₉:Eu²⁺ phosphor, the present study provides a significant guide for identifying the novel narrow-band deep-blue-emitting component applicable to full-spectrum warm white light-emitting diode devices.

Polymorphism and polymorph-dependent luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2

Building on studies of monoclinic Li₃SiNO₂, a polymorph, β-Li₃SiNO₂, with a previously unknown structure type was synthesized. The β-phase crystallizes in the orthorhombic space group Pbca (no. 61) with lattice parameters of a = 18.736(2), b = 11.1267(5), c = 5.0897(3) Å, and a cell volume of V = 1057.2(1) ų. Using high-temperature solid-state reactions in sealed tantalum tubes, it was possible to obtain high purity samples (<5 wt% of side phase LiSi₂N₃ according to Rietveld analysis) containing exclusively one or the other polymorph, depending solely on the cooling rate. In contrast to the monoclinic phase, orthorhombic β-Li₃SiNO₂ additionally contains a third layer and shows a layer-sequence of the type ABCB. Doped with the activator ion Eu²⁺, the new polymorph exhibits an intense yellow emission (λ_max = 586 nm, fwhm = 89 nm, 0.33 eV, 2650 cm^-1) under irradiation with UV to blue light. Hence, the structural difference between the two polymorphs goes along with a significant blue-shift of 16 nm. The results from single-crystal diffraction and single-grain luminescence measurements were confirmed by Rietveld analysis of bulk samples and powder luminescence data.

Li2 Ba4 Al2 Ta2 N8 O, the First Barium Nitridoalumotantalate with BCT-Zeolite Type Structure

Single-crystals of Li₂ Ba₄ Al₂ Ta₂ N₈ O:Eu²⁺ were synthesized from Ba₃ N₂ , Al₂ O₃ , Li₃ N, Eu₂ O₃ , and lithium metal by a high-temperature solid-state reaction in a weld shut tantalum ampule. The crystal structure of Li₂ Ba₄ Al₂ Ta₂ N₈ O was determined by single-crystal X-ray diffraction and it crystallizes in the orthorhombic space group Pnnm (no. 58) with the lattice parameters a=1006.71(3), b=1026.58(3), c=607.10(2) pm, and a volume of V=0.62742(3) nm³ . The compound is built up from AlN₄ and TaN₄ tetrahedra, which form a three-dimensional network corresponding to the BCT-zeolite type structure. Li₂ Ba₄ Al₂ Ta₂ N₈ O is homeotypic to Li₂ Sr₄ Si₄ N₈ O and Li₂ Sr₄ Al₂ Ta₂ N₈ O but, additionally, it could be successfully doped with the activator ion Eu²⁺ and hence features an experimental observed overall emission at λ_max =565 nm (fwhm=89 nm) consisting of a superposition of two adjusted emission bands at λ_max =557 nm (fwhm=69 nm) and at λ_max =604 nm (fwhm=102 nm).

Ba4Al7Li28.08O26.92N1.08, the Barium Oxonitridolithoaluminate with a Highly Condensed LiO4 Tetrahedra Framework

The new compound Ba₄Al₇Li_28.08O_26.92N_1.08 consists of AlO₄/AlO₃N tetrahedra, 10-fold coordinated Ba²⁺ cations, and a highly condensed edge- and corner-sharing LiO₄ tetrahedra framework, which leads to a degree of condensation greater than 1. The first barium oxonitridolithoaluminate was synthesized by a high-temperature solid-state reaction in a weld-shut tantalum ampoule and the crystal structure has been determined by single-crystal X-ray diffraction. Ba₄Al₇Li_28.08O_26.92N_1.08 crystallizes in the monoclinic space group P2₁/m (no. 11) with the lattice parameters a = 1052.41(3), b = 615.93(2), c = 1088.45(4) pm, β = 98.86(1)°, and a volume of V = 0.69712(4) nm³. In addition, Ba₄Al₇Li_28.08O_26.92N_1.08 doped with the activator ion Eu²⁺, exhibits a broad band emission with a maximum at λ_max = 524 nm (2.34 eV) with a fwhm of 112 nm (4373 cm^-1/0.54 eV), which can be described by a superposition of two adjusted emission bands at λ_max = 515 nm (2.41 eV) with a fwhm of 70 nm (2704 cm^-1/0.34 eV), and at λ_max = 574 nm (2.18 eV) with a fwhm of 127 nm (4127 cm^-1/0.51 eV).

The crystal structure and luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2+

Obtained by a high temperature solid-state synthesis, the compound Li3SiNO2:Eu2+ was characterized via SCXRD, PXRD, and luminescence spectroscopy. This revealed a new structure type and interesting luminescence properties.

Rb[Li5 Si2 O7 ] - A Latecomer in the Family of Alkali Lithosilicates Hiding a Green-Emitting Lithosilicate

In order to expand the field of alkali lithosilicates, a new representative of the substance class with a previously unknown structure type was found based on solid-state synthesis. The novel compound with the sum formula Rb[Li5 Si2 O7 ] crystallizes in the orthorhombic space group Pbcm (no. 57) with a=7.6269(3), b=9.5415(4), and c=9.4095(3) Å by means of single-crystal X-ray diffraction. The structure consists of a highly condensed lithosilicate framework, built up of corner- and edge-linked [LiO4 ]-tetrahedra and [Si2 O7 ]-units, and the rubidium ions aligned in channels. Suitable crystals of the material were obtained using sealed tantalum ampoules as reaction tube at a temperature of 750 °C. The new compound was further characterized via powder diffraction, Rietveld analysis, and EDX measurements. At first glance, Eu2+ -doped Rb[Li5 Si2 O7 ] reveals an intense green luminescence. In-depth crystal analysis shows that a core-shell formation is present even for apparently high quality single-crystals. As a minority phase, the known green phosphor RbLi[Li3 SiO4 ]2 :Eu2+ is the origin of the luminescence, representing a tiny core inside of the particles surrounded by a large matrix of transparent Rb[Li5 Si2 O7 ] dominating the single-crystal diffraction pattern.

Broadband NaK2Li[Li3SiO4]4:Ce Alkali Lithosilicate Blue Phosphors

Phosphors with a rigid and symmetrical structure are urgently needed. The alkali lithosilicate family (A[Li₃SiO₄]) has been extensively studied with a narrow emission band due to its unique cuboid-coordinated environment and rigid structure. However, here we demonstrate for the first time Ce-doped NaK₂Li[Li₃SiO₄]₄ phosphors with a broad emission band, a high internal quantum efficiency (85.6%), and excellent thermal stability. Photoluminescence indicates the Ce's preference to occupy the Na⁺ site, leading to a strong blue color emission with peak maxima at 417 and 450 nm. Temperature- and pressure-dependent photoluminescence reveals thermal stability and a phase transition. Moreover, the X-ray absorption near-edge structure reveals the mixing of Ce³⁺ and Ce⁴⁺ in the materials; this result differs from that of Eu²⁺-doped A[Li₃SiO₄] phosphors. The charge compensation process is then proposed to explain this difference. This study not only provides insights into Ce-doped UCr₄C₄-type phosphors but also explains the charge compensation mechanism of the aliovalent doping process.

A Double-Band Emitter with Ultranarrow-Band Blue and Narrow-Band Green Luminescence

Understanding the origin and mechanisms of luminescence is a crucial point when it comes to the development of new phosphors with targeted luminescence properties. Herein, a new phosphor belonging to the substance class of alkali metal lithosilicates with the generalized sum formula Cs4-x-y-z Rbx Nay Liz [Li3 SiO4 ]4 :Eu2+ is reported. Single crystals of the cyan-emitting UCr4 C4 -type phosphor show a peculiar double-band luminescence with one ultranarrow emission band at 473 nm and a narrow emission band at 531 nm under excitation with UV light (λexc =408 nm). Regarding occupation of the channels by the light metal ions, investigations of single-crystal XRD data led to the assumption that domain formation with distinct lithium- and sodium-filled channels occurs. Depending on which of these channels hosts the activator ion Eu2+ , a green or blue emission results. The herein-presented results shed new light on the luminescence process in the well-studied UCr4 C4 -type alkali metal lithosilicate phosphors.