Alkali Lithosilicates: Renaissance of a Reputable Substance Class with Surprising Luminescence Properties

A Robust Anti-Thermal-Quenching Phosphor Based on Zero-Dimensional Metal Halide Rb3InCl6:xSb3

High-power phosphor-converted white light-emitting diodes (hp-WLEDs) have been widely involved in modern society as outdoor lighting sources. In these devices, due to the Joule effect, the high applied currents cause high operation temperatures (>500 K). Under these conditions, most phosphors lose their emission, an effect known as thermal quenching (TQ). Here, we introduce a zero-dimensional (0D) metal halide, Rb3InCl6:xSb3+, as a suitable anti-TQ phosphor offering robust anti-TQ behavior up to 500 K. We ascribe this behavior of the metal halide to two factors: (1) a compensation process via thermally activated energy transfer from structural defects to emissive centers and (2) an intrinsic structural rigidity of the isolated octahedra in the 0D structure. The anti-TQ phosphor-based WLEDs can stably work at a current of 2000 mA. The low synthesis cost and nontoxic composition reported here can herald a new generation of anti-TQ phosphors for hp-WLED.

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.

Spectral properties and self-reduction of Eu3+ to Eu2+ in aluminosilicate oxyfluoride glass

Eu-doped aluminosilicate oxyfluoride glass prepared via a melt-quenching method was investigated using X-ray diffraction, absorption spectroscopy, X-ray fluorescence spectrometry, photoluminescence spectroscopy and fluorescence decay curves. We found that the reduction of Eu3+ to Eu2+ ions occurred in the glass prepared in air. The emission spectra showed that the intensity of 4f65d → 4f7 transition of Eu2+ ions varied with increasing incident beam wavelength. Meanwhile, the fluorescence lifetimes of Eu3+: 5D0 → 7F2 monitored at 617 nm in the glass change with the variation of excitation wavelength. The energy transfer between Eu2+ and Eu3+ and the emission mechanisms of Eu2+ ions in the glass were also discussed.

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.

Narrow-band Rb1-yKyNa3(Li3SiO4)4:Eu2+(0 ≤ y ≤ 1) cyan-blue phosphors for full-spectrum white LEDs

A series of Rb_1-yK_yNa₃(Li₃SiO₄)₄:Eu²⁺(0 ≤ y ≤ 1) phosphors were successfully synthesized through a high-temperature solid-state reaction. The introduction of K⁺ into the RbNa₃(Li₃SiO₄)₄:Eu²⁺ phosphor to partially or completely replace Rb⁺ allows the emission spectrum to be modulated from blue (λ = 473 nm, FWHM = 22.5 nm) to a narrow cyan band (λ = 485 nm, FWHM = 21.1 nm). As the K⁺ ion content increases, the space group of the phosphor evolves from I4/M to I41/A. The complete replacement of Rb⁺ by K⁺ results in the KNa₃(Li₃SiO₄)₄:Eu²⁺ cyan phosphor, which shows excellent thermal stability (the comprehensive emission loss is only 8% at 150 °C) and can be used to fill the cyan light gap in white LED devices. By adding the KNa₃(Li₃SiO₄)₄:Eu²⁺ cyan phosphor in packaging with yellow and red phosphors, the color rendering index is increased from 90.2 to 97.1 and the correlated color temperature improved to 3658 K. These results indicate that the cyan phosphor has important application value in full-spectrum white LEDs.

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.

Tailoring of White Luminescence in a NaLi3 SiO4 :Eu2+ Phosphor Containing Broad-Band Defect-Induced Charge-Transfer Emission

Single-component materials with white-light emission are ideal for lighting applications. However, it is very challenging to achieve white luminescence in single-dopant activated solid phosphors. Herein, white NaLi₃ Si_{1- x} O₄ :Eu²⁺ materials are designed via defect engineering and synthesized by reducing the Si content (0.15 ≤ x ≤ 0.25). Stochiometric NaLi₃ SiO₄ :Eu²⁺ exhibits a narrow-band blue emission at 469 nm, ascribed to the 5d → 4f transition of Eu²⁺ at highly symmetric cuboid Na sites, while samples with Si content reduced by 15-25% display white emission with two peaks at 472 nm and 585 nm. The newly appeared broadband yellow peak arises from charge-transfer transitions involving Eu²⁺ and nearby defects, as verified by an unusual bandwidth, a large Stokes shift, and a long decay time. A single-component white light-emitting diode device is fabricated by employing a white phosphor to demonstrate a color-rendering index of 82.9. This result provides a new design strategy for single-component white-light materials with broad-band defect-induced charge-transfer emission.

Highly efficient blue-emitting phosphor of Sr[B8O11(OH)4]:Eu2+ prepared by a self-reduction method

A highly efficient blue-emitting phosphor of Sr[B₈O₁₁(OH)₄]:xEu²⁺ was synthesized though a medium-high temperature boric acid melting method by means of a self-reduction mechanism. The quantum yield and color purity of Sr[B₈O₁₁(OH)₄]:6%Eu²⁺ are both as high as 99%. The PL intensity of Sr[B₈O₁₁(OH)₄]:6%Eu²⁺ at 150 °C remains 84% of that at 25 °C.

Achievement of narrow-band blue-emitting phosphors KScSr1-y Ca y Si2O7:Bi3+ by the migration of luminescence centers

In recent years, efforts have been made to develop narrow-band emission phosphors with excellent performance. Herein, a series of KScSr_1−yCa_ySi₂O₇:0.07Bi³⁺ narrow-band phosphors were synthesized by a co-substitution method, and the crystal structure, the occupancy of activated ions and luminescence properties were studied in detail. The substitution of Ca²⁺ for Sr²⁺ ions resulted in the migration of the activated Bi³⁺ from the K site to Sr site, accompanied by the regulation of the emission peak from 410 nm to 455 nm, the peak emission half width from 52 nm to 40 nm, and the color purity from the original 78% to 88%. In addition, a warm white LED with low CCT = 3401 K, CRI = 95.5, and CIE color coordinates of (0.3447, 0.3682) has been obtained through the combination of KSS_0.6C_0.4S:0.07Bi³⁺ with a commercial green and red phosphor on a UV (370 nm) chip. The results not only provided a strategy based on the manipulation of chemical composition and crystal structure to tune spectral distribution, but also broadens the choice of activators of narrow-band blue-emitting phosphors.

Narrow-band blue emitting phosphor KScSr_1−yCa_ySi₂O₇:0.07Bi³⁺ with FWHM 40 nm.

Borate Hydrides as a New Material Class: Structure, Computational Studies, and Spectroscopic Investigations on Sr5 (BO3 )3 H and Sr5 (11 BO3 )3 D

The unprecedented borate hydride Sr5 (BO3 )3 H and deuteride Sr5 (11 BO3 )3 D crystallizing in an apatite-related structure are reported. Despite the presence of hydride anions, the compound decomposes only slowly in air. Doped with Eu2+ , it shows broad-band orange-red emission under violet excitation owing to the 4f6 5d-4f7 transition of Eu2+ . The observed 1 H NMR chemical shift is in good agreement with previously reported 1 H chemical shifts of ionic metal hydrides as well as with quantum chemical calculations and very different from 1 H chemical shifts usually found for hydroxide ions in similar materials. FTIR and Raman spectroscopy of different samples containing 1 H, 2 H, nat B, and 11 B combined with calculations unambiguously prove the absence of hydroxide ions and the sole incorporation of hydride ions into the borate. The orange-red emission obtained by doping with Eu2+ shows that the new compound class might be a promising host material for optical applications.

Dual-Shelled RbLi(Li3 SiO4 )2 :Eu2+ @Al2 O3 @ODTMS Phosphor as a Stable Green Emitter for High-Power LED Backlights

The stability of luminescent materials is a key factor for the practical application in white light-emitting diodes (LEDs). Poor chemical stability of narrow-band green-emitting RbLi(Li₃ SiO₄ )₂ :Eu²⁺ (RLSO:Eu²⁺ ) phosphor hinders their further commercialization even if they have excellent stability against thermal quenching. Herein, we propose an efficient protection scheme by combining the surface coating of amorphous Al₂ O₃ and hydrophobic modification by octadecyltrimethoxysilane (ODTMS) to construct the moisture-resistant dual-shelled RLSO:Eu²⁺ @Al₂ O₃ @ODTMS composite. The growth mechanisms of both the Al₂ O₃ inorganic layer and the silane organic layer on the phosphor surface are investigated. The results remarkably improve the water-stability of this narrow-band green emitter. The evaluation of the white LED by employing this composite as the green component demonstrates that RLSO:Eu²⁺ @Al₂ O₃ @ODTMS is a promising candidate for the high-performance display backlights, and this dual-shelled strategy provides an alternative method to improve the moisture-resistant property of humidity-sensitive phosphors.

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.

Novel narrow-band blue-emitting Cs3Zn6B9O21:Bi3+ phosphor with superior thermal stability

A novel Bi³⁺-doped CsZn₆B₉O₂₁ blue phosphor shows an extraordinary narrow-band emission with a FWHM of only 50 nm and excellent thermal stability.

SrAl2-xLi2+xO2+2xN2-2x:Eu2+ (0.12 ≤ x ≤ 0.66)-Tunable Luminescence in an Oxonitride Phosphor

Starting from the recently published narrow band red phosphor SALON, a tunable oxonitride-phosphor can be derived by introducing disorder into the structure. To achieve this, the oxygen content of the reaction mixture is increased, thereby prohibiting the oxygen/nitrogen ordering observed in SALON. The resulting compound is isotypic to UCr₄C₄ and exhibits mixed oxygen/nitrogen and lithium/aluminum sites. Further variation of the oxygen/nitrogen ratio revealed that the structure remains stable over a wide range of compositions. The compound can therefore be described by the general sum formula SrAl_2-xLi_2+xO_2+2xN_2-2x with x ranging between 0.12 and 0.66. When doped with Eu²⁺, the title compound exhibits an intense luminescence upon excitation with blue light. The maximum of this emission varies depending on the oxygen content and can be tuned to values between 581 nm (x = 0.66) and 672 nm (x = 0.12).

RbKLi2[Li3SiO4]4:Eu2+ an ultra narrow-band phosphor

The new phosphor RbKLi2[Li3SiO4]4:Eu2+ was synthesized by different high-temperature solid-state reactions. It is accessible from the alkali metal carbonates, SiO2, and Eu2+ as a luminous active cation either in closed tantalum ampoules or by conventional solid-state reaction in nickel crucibles, under a constant flow of forming gas. The structure of the thereby received rod-shaped crystals was solved and refined on the basis of single crystal X-ray diffraction data. The compound crystallizes isostructurally to CsKNa2[Li3SiO4]4 and forms a highly condensed network of LiO4 and SiO4 tetrahedra [I4/m (no. 87), Z = 2, a = 10.9508(6) and c = 6.3334(3) Å]. It is a new member of the recently discovered family of alkali lithosilicate phosphors. Under excitation with UV to blue light, the compound exhibits interesting luminescence properties. Depending on the mode of synthesis, either green or blue luminescence of the samples is observed. Both emission profiles can be described as ultra-narrow-banded, since the full width at half maximum (fwhm) is below 0.2 eV. The green phosphor shows an emission maximum at 532 nm with a fwhm of 43.5 nm (0.193 eV) and the blue one at 474 nm with a fwhm of 24.8 nm (0.137 eV). Furthermore, the material presented here allows a more detailed localization of the luminescence center inside the structure, which may allow a better understanding of the luminescence properties of many other alkali lithosilicate phosphors.

Simultaneous bifunctional application of solid-state lighting and ratiometric optical thermometer based on double perovskite LiLaMgWO6:Er3+ thermochromic phosphors

Both efficient light emitting diodes and highly sensitive (2.24% K⁻¹) ratiometric thermometer were obtained based on the LiLaMgWO₆:Er³⁺ thermochromic phosphor.

Narrow-band green emission of Eu2+ in a rigid tunnel structure: site occupations, barycenter energy calculations and luminescence properties

As a new member of an oxide-based family with a UCr₄C₄-related type structure, CsKNaLi(Li₃SiO₄)₄:Eu²⁺ was synthesized successfully for the first time and its rigid tunnel structure leads to narrow-band green emission, which can be used for white light generation.

Emerging ultra-narrow-band cyan-emitting phosphor for white LEDs with enhanced color rendition

Phosphor-converted white LEDs rely on combining a blue-emitting InGaN chip with yellow and red-emitting luminescent materials. The discovery of cyan-emitting (470-500 nm) phosphors is a challenge to compensate for the spectral gap and produce full-spectrum white light. Na_0.5K_0.5Li₃SiO₄:Eu²⁺ (NKLSO:Eu²⁺) phosphor was developed with impressive properties, providing cyan emission at 486 nm with a narrow full width at half maximum (FWHM) of only 20.7 nm, and good thermal stability with an integrated emission loss of only 7% at 150 °C. The ultra-narrow-band cyan emission results from the high-symmetry cation sites, leading to almost ideal cubic coordination for UCr₄C₄-type compounds. NKLSO:Eu²⁺ phosphor allows the valley between the blue and yellow emission peaks in the white LED device to be filled, and the color-rendering index can be enhanced from 86 to 95.2, suggesting great applications in full-spectrum white LEDs.

Alkali Lithosilicates: Renaissance of a Reputable Substance Class with Surprising Luminescence Properties

A hitherto unknown synthetic access to alkali lithosilicates, a substance class first described by Hoppe in the 1980s, is reported. With the synthesis and characterization of NaK₇ [Li₃ SiO₄ ]₈ , a new representative has been discovered, expanding the family of known alkali lithosilicates. Astonishingly, NaK₇ [Li₃ SiO₄ ]₈ and the already established alkali lithosilicates Na[Li₃ SiO₄ ] as well as K[Li₃ SiO₄ ] display unforeseen luminescence properties, when doped with Eu²⁺ . Na[Li₃ SiO₄ ]:Eu²⁺ exhibits an ultra-narrow blue, K[Li₃ SiO₄ ]:Eu²⁺ a broadband, and NaK₇ [Li₃ SiO₄ ]₈ :Eu²⁺ a yellow-green double emission upon excitation with near-UV to blue light. Consequently, all of the investigated substances of this class of compounds are highly interesting phosphors for application in phosphor converted LEDs.