Site-Selective Occupancy of Eu2+ Toward Blue-Light-Excited Red Emission in a Rb3 YSi2 O7 :Eu Phosphor

Broadband spectral cyan emission phosphor for full-spectrum LED caused by interstitial site occupation

The phosphor with a highly condensed, rigid framework structure and a single crystallographic site often exhibit symmetrical narrow-band emission. It is challenging to achieve broadband emission by doping Eu2+ ions in similar structures. Here, we propose to control the occupation and quenching concentration of Eu2+ ions in a single-site matrix Sc2Si2O7 to increase efficiency and precise regulation of luminescence spectra substantially. The analysis of photoluminescence spectroscopy through steady-state, transient-state, and Gaussian fitting techniques has discovered two emission centers despite the presence of a single rare-earth substitution site. The theoretical calculations and bond valence sum subsequently prove that Eu2+ ions prefer substituting the Sc3+ and interval sites to emit intense cyan light. Under 340 nm excitation, broad cyan-emission (FWHM = 115 nm) is exhibited with a high quantum yield of 60.67 %. The present phosphor exhibits pronounced thermal stability, and the emission intensity can still keep 68.3 % at 170 °C compared to that at atmospheric temperature. The Sc2Si2O7: Eu2+ phosphor boasts exceptional potential as a highly efficient cyan component in full-spectrum WLEDs. By replacing the blue light component commonly found in WLEDs, the intelligent and healthy alternative Sc2Si2O7: Eu2+ phosphor can effectively decrease the harmful blue light. This work also highlights the critical need to analyze local phosphor distortions upon rare-earth substitution, especially in single crystallographic site structures.

Quantifying the interfacial triboelectricity in inorganic-organic composite mechanoluminescent materials

Mechanoluminescence (ML) sensing technologies open up new opportunities for intelligent sensors, self-powered displays and wearable devices. However, the emission efficiency of ML materials reported so far still fails to meet the growing application requirements due to the insufficiently understood mechano-to-photon conversion mechanism. Herein, we propose to quantify the ability of different phases to gain or lose electrons under friction (defined as triboelectric series), and reveal that the inorganic-organic interfacial triboelectricity is a key factor in determining the ML in inorganic-organic composites. A positive correlation between the difference in triboelectric series and the ML intensity is established in a series of composites, and a 20-fold increase in ML intensity is finally obtained by selecting an appropriate inorganic-organic combination. The interfacial triboelectricity-regulated ML is further demonstrated in multi-interface systems that include an inorganic phosphor-organic matrix and organic matrix-force applicator interfaces, and again confirmed by self-oxidization and reduction of emission centers under continuous mechanical stimulus. This work not only gives direct experimental evidences for the underlying mechanism of ML, but also provides guidelines for rationally designing high-efficiency ML materials.

Millimeter-Sized K2MnF6:Si4+, NH4+ Red-Luminescent Crystals with High Absorption Efficiency (AEmax of 93.5%) and External Quantum Efficiency (EQEmax of 68.9%) Grown by Cooling-Induced Crystallization

Luminescent bulk crystals exhibit fewer grain boundaries and defects compared with conventional microsized powdery ones. Herein, targeting Mn4+-activated fluoride crystals with a sharp line-type red luminescence spectrum, we propose a new cooling-induced crystallization method to grow the fluoride crystals. By this new method, we successfully grew millimeter-sized K2MnF6:Si4+, NH4+ crystals, featuring an AEmax (absorption efficiency) of 93.5% and an EQEmax (external quantum efficiency) of 68.9%, which are among the best values for Mn4+-activated fluoride red phosphors. The influence of doping Si4+ and/or NH4+ in K2MnF6 on the local coordination structure and luminescence properties was studied. The anomalous thermal quenching behaviors were discussed, the luminescence decay from the excited state was compared, and the origin for the high quantum efficiencies was analyzed.

Ce3+-Doped Li2Ca5Gd(BO3)5 Phosphors with Multiple Luminescent Centers and High Pressure Sensitivity under Near UV Excitation

The performance of Ce3+-based phosphors under mechanical high pressures becomes attractive due to the potential application as a visual pressure sensor. Li2Ca5Gd(BO3)5 was selected as the host for the Ce3+ doping. Rietveld refinements reveal that rare earth cations occupy M1, M2, and M3 sites, and indeed, the photoluminescent spectra of Li2Ca5Gd1-xCex(BO3)5 (0.005 ≤ x ≤ 0.15) exhibit the characteristic of multiple activators, defined as CeI, CeII, and CeIII, with the maximal emission wavelength at ∼444, 419, and 378 nm, respectively. The optimal internal and external quantum efficiencies are 86.29% for x = 0.005 and 20.26% for x = 0.10, respectively, under the NUV excitation at 363 nm. In-situ high pressure emission spectra under 375 nm excitation exhibit an overall red-shift, and the linear pressure susceptibilities up to 6.7 GPa for CeI and CeII centers are -390 and -279 cm-1/GPa, respectively, which is probably the largest among Ce3+-doped oxides and oxysalts. Due to the above superiorities, Ce3+-doped LCGB possesses a high potential as a visual pressure sensor, and this is a successful study on the structure-property relationship of inorganic materials.

Limited energy migration and circumscribed multiphonon relaxation produced non-concentration quenching in a novel dazzling red-emitting phosphor

White LED applications are still constrained by extremely efficient narrow band red emitting phosphors. Meanwhile, the concentration quenching induced by energy migration is the main reason that limits the emission intensity of a red emitting phosphor. Therefore, developing a novel red emitting material with energy migration limitations seems necessary. Here, we proposed and realized the non-concentration quenching doping of Eu3+ ions in a Sr9Y2-2xW4O24:xEu3+ (0 ≤ x ≤ 1.0) phosphor for the first time by means of host preferential selection. By clearly investigating the crystal structure and luminescence kinetics, the long-distance between the nearby Eu3+ ions and the low phonon energy are the main reasons that suppress the energy migration and the cross-relaxation among Eu3+ ions. These advantages result in a high internal (90.47%) and external quantum efficiency (42.1%) of Sr9Eu2W4O24. With the help of the Judd-Ofelt theory and the large value of oscillator strength Ω2, Eu3+ ions are verified to occupy the non-symmetric lattice site with high color purity (94.4%). In addition, only 5.2% emission intensity loss at 140 °C can guarantee its application in LED devices. Moreover, the SYWO:Eu3+ phosphor has high thermal tolerance, high color stability, excellent moisture resistance and superior physical/chemical stability, and thus has broad practical spectral application prospects. The prepared WLED shows superior performance, and the calculated NTSC values are as high as 101.8% and 104.7%, respectively. For comparison, the optical performances of the Sr9Y2W4O24:Eu3+ phosphor outperform those of the standard commercial red phosphors, Y2O3:Eu3+ and Y2O2S:Eu3+, and almost match that of K2MnF6. These results may pave the way for fresh approaches to the study of high-performance Eu3+-activated phosphors.

Synthesis, Photoluminescent Characteristics and Eu3+-Induced Phase Transitions in Sr3Zr2O7:Eu3+ Red Phosphors

Designing phosphors that are excited by blue light is extraordinarily important for white light-emitting diodes (w-LEDs). In the present study, a new Ruddlesden-Popper type of SZO:xEu³⁺ (x = 0.01~0.10) phosphors was developed using solid-state reactions. Interestingly, a Eu³⁺ doping-induced phase transformation from the Sr₃Zr₂O₇ (cubic) to the SrZrO₃ (orthorhombic) phase was observed, and the impact of the occupied sites of Eu³⁺ ions on the lifetime of Sr₃Zr₂O₇:xEu³⁺ phosphors is discussed in detail. Diffuse reflectance spectroscopy results showed that the band gap of SZO:xEu³⁺ phosphors gradually increased from 3.48 eV for undoped Sr₃Zr₂O₇ hosts to 3.67 eV for SZO:0.10Eu³⁺ samples. The fluorescence excitation spectrum showed that ultraviolet (300 nm), near-ultraviolet (396 nm) and blue light (464 nm) were all effective excitation pump sources of Sr₃Zr₂O₇:xEu³⁺ phosphors, and the strongest emission at 615 nm originated from an electric dipole transition (⁵D₀→⁷F₂). CIE coordinates moved from orange (0.5969, 0.4267) to the reddish-orange region (0.6155, 0.3827), and the color purity also increased. The fabricated w-LED was placed on a 460 nm chip with a mixture of YAG:Ce³⁺ and SZO:0.1Eu³⁺ samples and showed "warm" white light with a color rendering index (CRI) of 81.8 and a correlation color temperature (CCT) of 5386 K, indicating great potential for application in blue chip white LEDs.

Equivalent chemical substitution in double-double perovskite-type ALaLiTeO6:Mn4+ (A = Ba2+, Sr2+, Ca2+) phosphors enabling wide range crystal field strength regulation and efficient far-red emission

Mn⁴⁺-activated phosphors have shown wide prospective applications in phosphor-converted white light-emitting diodes (pc-WLEDs) and pc-LEDs used in illumination and indoor plant cultivation, respectively. Recently, double perovskites A₂B'B''O₆ with a tunable crystal structure and versatile octahedral sites have been extensively studied as good host matrixes for Mn⁴⁺-emitters to realize tunable far-red emissions. Herein, a series of double-double perovskite-type ALaLiTeO₆:Mn⁴⁺ (A = Ba, Ba_0.5Sr_0.5, Sr, Sr_0.5Ca_0.5, Ca) phosphors were synthesized and structurally characterized, and the correlations between their structure and luminescence were also studied systematically. With a decrease of the A-cation size, an increased distortion in the average structure and a structure symmetry lowering (I2/m → P2₁/_n) were observed for ALaLiTeO₆:Mn⁴⁺. In contrast, on the local scale, the degree of (Li/Te)O₆-octahedral distortion is positively correlated with the ΔIR value, which is the ionic radius difference between A²⁺ and La³⁺. The local structural changes were found to be irrelevant to the significant improvements in photoluminescence properties. In combination with careful spectroscopic analysis, we deciphered that a decreased A-cation is in fact helpful for the enhancements in crystal field strength (D_q/B = 2.12-2.82) and Mn-O covalent bonding, thereby resulting in an improved quantum efficiency, a suppressed nonradiative transition, and a redshift in photoluminescence spectra. Amongst the ALaLiTeO₆:Mn⁴⁺ phosphor series, CaLaLiTeO₆:Mn⁴⁺ exhibits the highest external quantum efficiency of 70.1% and internal quantum efficiency of 96.4% and superior thermal stability (93.3%@423 K), making CaLaLiTeO₆:Mn⁴⁺ very promising as far-red phosphors for pc-LEDs. The findings of this work will serve as a new guide for rational design of high-performance Mn⁴⁺-activated double-double perovskite-type far-red phosphors.

Eu3+ -activated red phosphor Ca3 YAl3 B4 O15 with low thermal quenching behaviour

This paper reports a sequence of a Ca₃ YAl₃ B₄ O₁₅ :xEu³⁺ red phosphor prepared using a high-temperature solid-state reaction. At the excitation of 396 nm, the samples emitted intense red emission centred at ~623 nm, which could be attributed to the ⁵ D₀ →⁷ F₂ transition of the Eu³⁺ ion. The results showed that the optimum Eu³⁺ doping concentration of Ca₃ YAl₃ B₄ O₁₅ :Eu³⁺ phosphor was x = 80 mol%, and the concentration quenching mechanism of Ca₃ YAl₃ B₄ O₁₅ :Eu³⁺ red phosphor belonged to the exchange coupling between Eu³⁺ ions. The Commission Internationale de l'éclairage (CIE) coordinates and colour purity of Ca₃ Y_0.2 Al₃ B₄ O₁₅ :0.8Eu³⁺ were calculated as (0.6375, 0.3476) and 95.5%, respectively. Moreover, the red emission of the obtained phosphor Ca₃ YAl₃ B₄ O₁₅ :0.8Eu³⁺ exhibited a low thermal quenching behaviour with an intensity retention rate of 92.85% at 150°C. The above results manifest that the Eu³⁺ -activated Ca₃ YAl₃ B₄ O₁₅ phosphor is predicted to be a promising red luminescent component for white light-emitting diodes.

Stabilization of Eu2+ in Li2B4O7 with the BO3 network through U6+ co-doping and defect engineering

Owing to the unique 4f-5d transitions and the involvement of 5d electrons, the divalent europium (Eu²⁺) ion is extensively used as a dopant ion in luminescent materials for phosphor-converted light emitting diodes (pc-LEDs) and other technological applications. Earlier reports in most of the cases have shown that the reduction of Eu³⁺ to Eu²⁺ requires very high temperatures and large hydrogen flux. In this study, a co-doping strategy with higher valent U⁶⁺ ions was utilized to successfully stabilize Eu²⁺ ions in the Li₂B₄O₇ (LTB) host with both the BO₃ and BO₄ network in low H₂ flux of only 8%. It is postulated that charge transfer occurs from U to Eu, resulting in the reduction of the charged state of Eu and the reaction probably proceeds via the formation of paramagnetic transient [U⁵⁺-Eu³⁺] species in the co-doped LTB. The same is also believed to be facilitated by the enhanced formation of Li-O type vacancy clusters in co-doped samples and enhanced oxygen vacancies in a reducing atmosphere. We believe this work will pave a new pathway for stabilizing the unusual oxidation state of lanthanides and transition metal ions through co-doping with hexavalent uranium ions.

Sr/Ba substitution induced higher thermal stability far red-emitting Ba1-ySryLaLiWO6:Mn4+ phosphors for plant growth applications

A series of red-emitting BaLaLiWO₆:Mn⁴⁺ (BLLW:Mn⁴⁺) phosphors were successfully synthesized by a high-temperature solid-state reaction method. The crystal structure and luminescence properties of the obtained samples were systematically investigated. The emission spectra exhibited a deep red emission band peaking at 716 nm with a full width at half-maximum (FWHM) of 44 nm under 340 nm excitation. The optimal Mn⁴⁺ molar concentration was about 1.2%. In addition, the luminescence mechanism was analyzed using a Tanabe Sugano energy level diagram. With the substitution of Sr for Ba, there was a red shift in the emission spectrum and a blue shift in the excitation spectrum. The emission intensity of BLLW:1.2%Mn⁴⁺ at 150 °C was about 22% of the initial value at room temperature. In contrast, the emission intensity of SrLaLiWO₆:1.2%Mn⁴⁺ still maintained 79% of the initial emission intensity at room temperature at 150 °C. This was due to the fact that with the substitution of Sr for Ba, the W-O bond length gradually decreases, which gradually enhanced the crystal field strength of Mn⁴⁺.

Recent Developments of Microscopic Study for Lanthanide and Manganese Doped Luminescent Materials

Luminescent materials are indispensable for applications in lighting, displays and photovoltaics, which can transfer, absorb, store and utilize light energy. Their performance is closely related with their size and morphologies, exact atomic arrangement, and local configuration about photofunctional centers. Advanced electron microscopy-based techniques have enabled the possibility to study nanostructures with atomic resolution. Especially, with the advanced micro-electro-mechanical systems, it is able to characterize the luminescent materials at the atomic scale under various environments, providing a deep understanding of the luminescent mechanism. Accordingly, this review summarizes the recent achievements of microscopic study to directly image the microstructure and local environment of activators in lanthanide and manganese (Ln/Mn²⁺ )-doped luminescent materials, including: 1) bulk materials, the typical systems are nitride/oxynitride phosphors; and 2) nanomaterials, such as nanocrystals (hexagonal-phase NaLnF₄ and perovskite) and 2D nanosheets (Ca₂ Ta₃ O₁₀ and MoS₂ ). Finally, the challenges and limitations are highlighted, and some possible solutions to facilitate the developments of advanced luminescent materials are provided.

Multichannel Control of PersL/Upconversion/Down-Shifting Luminescence in a Single Core-Shell Nanoparticle for Information Encryption

Persistent luminescence (PersL) has been attracting substantial attention in diverse frontier applications such as optical information security and in vivo bioimaging. However, most of the reported PersL emissions are based on the dopants instead of the host matrix, which also plays an important role. In addition, there are few works on the PersL-based multifunctional nanoplatform in nanosized materials. Here, we report a class of novel nanostructure designs with PersL, upconversion, and down-shifting luminescence to realize the fine-tuning of emission colors under different excitation modes including steady-state irradiation, time-gating, and PersL generation. Blue, orange, and green emissions were easily achieved in such a single nanoparticle under suitable excitation modes. Moreover, the physical origin of the PersL of the CaF₂ matrix was discussed by simulating the energy band structure with Ca_xF_y defects. Our results provide new opportunities for the design of a new class of multifunctional materials, showing great promise in the field of information encryption security and multilevel anticounterfeiting.

Self-reduction-induced BaMgP2O7:Eu2+/3+: a multi-stimuli-responsive phosphor for X-ray detection, anti-counterfeiting and optical thermometry

Mixed-valence Eu^2+/3+-activated phosphors have attracted wide attention due to their excellent luminescence tunability. Steady control of the Eu²⁺/Eu³⁺ ratio is the key to achieving reproducible Eu^2+/3+ co-doped materials. In this work, BaMgP₂O₇:xEu^2+/3+ (BMPO:Eu, x = 0.001-0.20) was successfully prepared by the traditional solid-state method in air. Eu³⁺ undergoes selective self-reduction at Ba²⁺ sites surrounded by a [P₂O₇] framework, leading to quantitive Eu²⁺/Eu³⁺. The phosphors exhibit a blue-violet emission band at ∼410 nm due to 5d-4f transitions of Eu²⁺ and a group of red emission peaks from ⁵D₀-⁷F_J of Eu³⁺. Controllable multicolor emissions are realized by regulating the Eu content and excitations. A linear response of overall luminescence intensity to irradiation dose makes the phosphor appropriate for X-ray detection. The combination of UV-blue excitation-dependent color evolution and X-ray luminescence qualifies the phosphors with great potential for multi-level anti-counterfeiting. In addition, Eu³⁺ presents abnormal anti-thermal quenching, so that the fluorescence intensity ratio (FIR) of Eu²⁺/Eu³⁺ changes in the temperature range of 300-520 K, suggesting a promising application in optical thermometry. Therefore, selectively partial self-reduction in a multi-cationic host is an effective strategy to design mixed-valence co-doped materials, providing a multiplicity of applications.

Enhanced luminescence performances of BaLaMgTaO6:Mn4+ red phosphor by Bi3+, Ca2+ doping for indoor plant lighting supplementary LED

A new perovskite BaLaMgTaO₆:Mn⁴⁺ (BLMTO:Mn⁴⁺) red phosphor was synthesized for the first time via the high-temperature solid-state method. The emission band of the phosphor ranges from 650 to 750 nm, which matches well with the absorption band of P_FR and P_R. By doping of Bi³⁺ and Ca²⁺ ions in the BLMTO:Mn⁴⁺ phosphor, a 4.76-fold enhancement in the luminescence emission intensity was achieved. The optimized BLMTO:0.5%Mn⁴⁺, 1.5%Bi³⁺, 2%Ca²⁺ phosphor exhibited a high quantum efficiency of 65% and a high color purity of 98.1% with the chromaticity coordinate (CIE) at (0.733, 0.267). Finally, a LED device was fabricated with the BLMTO:0.5%Mn⁴⁺, 1.5%Bi³⁺, 2%Ca²⁺ phosphor for further agricultural lighting, which emits warm white light with a low color temperature of 3549 K. The result indicates that the BLMTO:Mn⁴⁺, Bi³⁺, Ca²⁺ phosphors have a potential for applications in agricultural cultivations.

Eu2+ Doping Concentration-Induced Site-Selective Occupation and Photoluminescence Tuning in KSrScSi2O7:Eu2+ Phosphor

Regulation of Eu²⁺ dopants in different cation sites of solid-state materials is of great significance for designing multicolor phosphors for light-emitting diodes (LEDs). Herein, we report the selective occupation of Eu²⁺ for multiple cationic sites in KSrScSi₂O₇, and the tunable photoluminescence from blue to cyan is realized through Eu²⁺ doping concentration-dependent crystal-site engineering. Eu²⁺ preferably occupies the K and Sr sites in KSrScSi₂O₇ at a low doping concentration, resulting in a 440 nm blue emission. As the Eu²⁺ concentration increases, a new Eu²⁺ substitution pathway is triggered, that is, Eu²⁺ enters the Sc site, leading to the red-shifted emission spectra from 440 to 485 nm. The doping mechanism and photoluminescence properties are corroborated by structural analysis, optical spectroscopy study, and density functional theory calculations. The optical properties of the as-fabricated white LEDs are studied, which demonstrates that these phosphors can be applied to full-spectrum phosphor-converted LEDs. This study provides a new design strategy to guide the development of multicolor Eu²⁺-doped oxide phosphors for lighting applications.

Achieving highly thermostable red emission in singly Mn2+-doped BaXP2O7 (X = Mg/Zn) via self-reduction

The non-rare earth doped red phosphors are attracting wide attention for warm-white lighting and indoor plant cultivation applications. The Mn2+-doped phosphors have well spectral tunability and great potential to generate...

Extension of Spectral Shift Controls from Equivalent Substitution to an Energy Migration Model Based on Eu2+/Tb3+-Activated Ba4-xSrxGd3-xLuxNa3(PO4)6F2 Phosphors

Single-phase phosphors with tunable emission colors are crucial to develop high-performance white light-emitting diodes since they are valuable to improve the energy efficiency, color rendering index, and correlated color temperature. Most of the studies have been conducted to control the spectral shifts via a polyhedral distortion or chemical unit cosubstitution strategy. The combination of host optimization and dopant activator design in a single-phase phosphor system is very rare. Herein, a partial substitution strategy of [Ba²⁺-Gd³⁺] by [Sr²⁺-Lu³⁺] has been employed in Ba_4-xSr_xGd_3-x-yLu_xNa₃(PO₄)₆F₂/5% Eu²⁺ (x = 0-0.40) phosphors. Also, the energy migration from Eu²⁺ to Tb³⁺ ions has been investigated in as-prepared samples. Consequently, the emitted signal is observed to shift from 470 to 575 nm derived from equivalent substitutions, which is attributed to specific performance by the emission profile of Eu²⁺, and such results are closely related to splitting of the crystal field and energy transfer among various luminescent centers. Moreover, the tunable yellowish-green emitting material has been assembled by incorporating ion pairs (Eu²⁺ → Tb³⁺) into the Ba_3.85Sr_0.15Gd_2.85Lu_0.15Na₃(PO₄)₆F₂, and their relative ratios are varied. The corresponding Eu²⁺ → Tb³⁺ energy migration process is assigned to be the dipole-quadrupole interaction by the Inokuti-Hirayama model. This work provides rational guidance for the design and discovery of new products with tunable emission colors, originating from the cosubstitution strategy and energy conversion model.

Design of highly efficient deep-red emission in the Mn4+ doped new-type structure CaMgAl10O17 for plant growth LED light

The auxiliary light equipment for plant growth requires phosphor-converted light-emitting-diodes (pc-LEDs) with high luminous efficiency and a stable structure, and the properties of phosphors highly determine the performance of the pc-LEDs. This work reports a deep-red emitting phosphor with an ultra-wide response range which is regarded as CaMgAl₁₀O₁₇:Mn⁴⁺. The absorption range spans the ultraviolet, near-ultraviolet, blue, and green light regions from 250 to 550 nm. Under the excitation of the best excitation position at 343 nm, deep-red light at 654 nm is emitted, and the quantum efficiency is as high as 86.7%. The luminous efficiency of the two pc-LED devices prepared based on CaMgAl₁₀O₁₇:Mn⁴⁺ with 395 and 460 nm chips reached 54.3 and 59.6 lm W^-1, respectively. The spectra of the two pc-LEDs exhibit high resemblance to the absorption spectra of chlorophyll A and B in plant growth photosynthesis. These indicate that the CaMgAl₁₀O₁₇:Mn⁴⁺ phosphor can be an excellent candidate for plant growth LED light.

Research on Molecular Structure and Electronic Properties of Ln3+ (Ce3+, Tb3+, Pr3+)/Li+ and Eu2+ Co-Doped Sr2Si5N8 via DFT Calculation

We use density functional theory (DFT) to study the molecular structure and electronic band structure of Sr₂Si₅N₈:Eu²⁺ doped with trivalent lanthanides (Ln³⁺ = Ce³⁺, Tb³⁺, Pr³⁺). Li⁺ was used as a charge compensator for the charge imbalance caused by the partial replacement of Sr²⁺ by Ln³⁺. The doping of Ln lanthanide atom causes the structure of Sr₂Si₅N₈ lattice to shrink due to the smaller atomic radius of Ln³⁺ and Li⁺ compared to Sr²⁺. The doped structure's formation energy indicates that the formation energy of Li⁺, which is used to compensate for the charge imbalance, is the lowest when the Sr2 site is doped. Thus, a suitable Li⁺ doping site for double-doped lanthanide ions can be provided. In Sr₂Si₅N₈:Eu²⁺, the doped Ce³⁺ can occupy partly the site of Sr₁²⁺ ([SrN₈]), while Eu²⁺ accounts for Sr₁²⁺ and Sr₂²⁺ ([SrN₁₀]). When the Pr³⁺ ion is selected as the dopant in Sr₂Si₅N₈:Eu²⁺, Pr³⁺ and Eu²⁺ would replace Sr₂²⁺ simultaneously. In this theoretical model, the replacement of Sr²⁺ by Tb³⁺ cannot exist reasonably. For the electronic structure, the energy level of Sr₂Si₅N₈:Eu²⁺/Li⁺ doped with Ce³⁺ and Pr³⁺ appears at the bottom of the conduction band or in the forbidden band, which reduces the energy bandgap of Sr₂Si₅N₈. We use DFT+U to adjust the lanthanide ion 4f energy level. The adjusted 4f-CBM of Ce_Sr1Li_Sr1-Sr₂Si₅N₈ is from 2.42 to 2.85 eV. The energy range of 4f-CBM in Pr_Sr1Li_Sr1-Sr₂Si₅N₈ is 2.75-2.99 eV and its peak is 2.90 eV; the addition of Ce³⁺ in Eu_Sr1Ce_Sr1Li_Sr1 made the 4f energy level of Eu²⁺ blue shift. The addition of Pr³⁺ in Eu_Sr2Pr_Sr2Li_Sr1 makes part of the Eu²⁺ 4f energy level blue shift. Eu²⁺ 4f energy level in Eu_Sr2Ce_Sr1Li_Sr1 is not in the forbidden band, so Eu²⁺ is not used as the emission center.

Glass crystallization making red phosphor for high-power warm white lighting

Rapid development of solid-state lighting technology requires new materials with highly efficient and stable luminescence, and especially relies on blue light pumped red phosphors for improved light quality. Herein, we discovered an unprecedented red-emitting Mg₂Al₄Si₅O₁₈:Eu²⁺ composite phosphor (λ_ex = 450 nm, λ_em = 620 nm) via the crystallization of MgO-Al₂O₃-SiO₂ aluminosilicate glass. Combined experimental measurement and first-principles calculations verify that Eu²⁺ dopants insert at the vacant channel of Mg₂Al₄Si₅O₁₈ crystal with six-fold coordination responsible for the peculiar red emission. Importantly, the resulting phosphor exhibits high internal/external quantum efficiency of 94.5/70.6%, and stable emission against thermal quenching, which reaches industry production. The maximum luminous flux and luminous efficiency of the constructed laser driven red emitting device reaches as high as 274 lm and 54 lm W^-1, respectively. The combinations of extraordinary optical properties coupled with economically favorable and innovative preparation method indicate, that the Mg₂Al₄Si₅O₁₈:Eu²⁺ composite phosphor will provide a significant step towards the development of high-power solid-state lighting.

Thermally stable and highly efficient red-emitting Eu3+-doped Cs3GdGe3O9 phosphors for WLEDs: non-concentration quenching and negative thermal expansion

Red phosphor materials play a key role in improving the lighting and backlit display quality of phosphor-converted white light-emitting diodes (pc-WLEDs). However, the development of a red phosphor with simultaneous high efficiency, excellent thermal stability and high colour purity is still a challenge. In this work, unique non-concentration quenching in solid-solution Cs₃Gd_1 - xGe₃O₉:xEu³⁺ (CGGO:xEu³⁺) (x = 0.1-1.0) phosphors is successfully developed to achieve a highly efficient red-emitting Cs₃EuGe₃O₉ (CEGO) phosphor. Under the optimal 464 nm blue light excitation, CEGO shows a strong red emission at 611 nm with a high colour purity of 95.07% and a high internal quantum efficiency of 94%. Impressively, this red-emitting CEGO phosphor exhibits a better thermal stability at higher temperatures (175-250 °C, >90%) than typical red K₂SiF₆:Mn⁴⁺ and Y₂O₃:Eu³⁺ phosphors, and has a remarkable volumetric negative thermal expansion (coefficient of thermal expansion, α = -5.06 × 10^-5/°C, 25-250 °C). By employing this red CEGO phosphor, a fabricated pc-WLED emits warm white light with colour coordinates (0.364, 0.383), a high colour rendering index (CRI = 89.7), and a low colour coordinate temperature (CCT = 4508 K). These results indicate that this highly efficient red-emitting phosphor has great potential as a red component for pc-WLEDs, opening a new perspective for developing new phosphor materials.

Complex crystal structure and photoluminescence of Bi3+-doped and Bi3+/Eu3+ co-doped Ca7Mg2Ga6O18

The multiple and site-selective occupancies of Bi³⁺ activators in Ca₇Mg₂Ga₆O₁₈:xBi³⁺ lead to a continuous red-shift of the broad emission band upon increasing the Bi³⁺-content.

Structure, luminescence of Eu2+ and Eu3+ in CaMgSi2O6 and their co-existence for the excitation-wavelength/temperature driven colour evolution

Luminescent materials with controllable colour evolution features are demanded for the development of multi-level anti-counterfeiting technologies. Here we report the structural and luminescence properties of CaMgSi2O6:Ln (Ln = Eu2+, Eu3+, Eu2+/3+) samples in detail and reveal their excitation-wavelength/temperature driven colour evolution characteristics. By tuning either the excitation-wavelength (276, 304, 343, 394 nm) or temperature (in the 330-505 K range), the designed samples with co-existing Eu2+/Eu3+ ions can achieve diverse and controllable colour evolution from red, to pink, purple and blue. This shows their potential application in anti-counterfeiting with the help of sophisticated pattern design. In addition, the underlying mechanism of the Stokes shift of the Eu2+ emission and valence stability of both Eu2+/Eu3+ ions in CaMgSi2O6 are also studied in depth. These results are valuable for designing colour-controllable luminescent materials based on the co-existence of the Eu2+/Eu3+ ions for anti-counterfeiting applications.

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.

Data-Driven Photoluminescence Tuning in Eu2+-Doped Phosphors

Discovery of rare earth phosphors has generally relied on the chemical intuition and time-intensive trial-and-error synthesis; therefore, finding new materials assisted by data-driven computations is urgent. Herein, we utilize a regression model to predict the emission wavelengths of Eu²⁺-doped phosphors by revealing the relationships between the crystal structure and luminescence property. The emission wavelengths of [Rb_(1-x)K_(x)]₃LuSi₂O₇:Eu²⁺ (0 ≤ x ≤ 1) phosphors, as examples for the data-driven photoluminescence tuning, are successfully predicted on the basis of the existing data of only eight systems, also consistent with the experimental results. These phosphors can be excited by blue light and exhibit broad-band red and near-infrared emission ranging from 619 to 737 nm. These findings in Eu²⁺-doped silicate phosphors indicate that data-driven computations through the regression mode would have bright application in discovering novel phosphors with a target emission wavelengths.

Massive red shift of Ce3+ in Y3Al5O12 incorporating super-high content of Ce

In light emitting diodes, Y₃Al₅O₁₂:Ce (YAG:Ce) is used as a yellow phosphor in combination with blue LEDs but lacks a red component in emission. Therefore, considerable efforts have been directed toward shifting the emission of YAG:Ce to longer wavelengths. In this study, a Y₃Al₅O₁₂ (YAG) crystal incorporating a high content of Ce, (Y_1-x Ce _x )₃Al₅O₁₂ (0.006 ≦ x ≦ 0.21), was successfully prepared by a polymerized complex method in which low-temperature annealing (650-750 °C) was employed prior to sintering at 1080 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis indicated that the obtained sample was a single phase YAG crystal with x ≤ 0.21. Interestingly, orange-red emission was observed with x ≥ 0.07 with UV-blue light irradiation. With excitation at 450 nm, the emission peak increases from 538 nm (x = 0.006) to 606 nm (x = 0.21). This massive red shift in the high-x region was not observed without the 1^st step of low-temperature annealing, which implied that low-temperature annealing was essential for incorporating a high concentration of Ce. The precursor formed by low-temperature annealing was amorphous at x = 0.04, whereas CeO₂ nanocrystals were formed in the amorphous material with x ≥ 0.11, based on the XRD and TEM results. CeL_III X-ray absorption edge structure revealed that Ce existed as Ce⁴⁺ in the precursor and Ce³⁺ in the obtained crystal. It was speculated that CeO₂ was formed at low temperature, releasing oxygen, with sintering at 1080 °C, leading to the incorporation of Y³⁺ in the Ce-O framework. The lattice constant increased significantly from 12.024 Å to 12.105 Å with increasing x, but the crystal field splitting did not increase and was constant from x = 0.06 to x = 0.21. Hence, the massive red shift in emission was not explained by the large crystal field splitting, but instead by the Stokes shift.

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.

A rapid in situ synthesis of wide-spectrum CD@BaCl2 phosphors via anti-solvent recrystallization for white LEDs

A facile and rapid in situ recrystallization strategy that can anchor carbon dots in an inorganic barium chloride solid medium is applied to produce wide-spectrum hybrid CD@barium chloride phosphors that show good photoluminescence for WLED aplications.

Divalent europium-doped near-infrared-emitting phosphor for light-emitting diodes

Near-infrared luminescent materials exhibit unique photophysical properties that make them crucial components in photonic, optoelectronic and biological applications. As broadband near infrared phosphors activated by transition metal elements are already widely reported, there is a challenge for next-generation materials discovery by introducing rare earth activators with 4f-5d transition. Here, we report an unprecedented phosphor K₃LuSi₂O₇:Eu²⁺ that gives an emission band centered at 740 nm with a full-width at half maximum of 160 nm upon 460 nm blue light excitation. Combined structural and spectral characterizations reveal a selective site occupation of divalent europium in LuO₆ and K2O₆ polyhedrons with small coordination numbers, leading to the unexpected near infrared emission. The fabricated phosphor-converted light-emitting diodes have great potential as a non-visible light source. Our work provides the design principle of near infrared emission in divalent europium-doped inorganic solid-state materials and could inspire future studies to further explore near-infrared light-emitting diodes.

Mn4+ activated Al2O3 red-emitting ceramic phosphor with excellent thermal conductivity

An Al₂O₃:Mn⁴⁺, Mg²⁺ red emitting ceramic phosphor, which can be effectively excited by ultraviolet and blue light, was successfully synthesized via solid-state reaction in an oxygen and air atmosphere. The ceramic sintered in oxygen atmosphere has higher optical transmittance and stronger luminescence intensity than the ceramic sintered in the air, which is more suitable for LED application. Since the structure of α-Al₂O₃ is very simple, it is convenient to study the factors affecting the Mn⁴⁺ luminescence. The crystal-strength parameter Dq, Racah parameters B and C, and the nephelauxetic ratio β₁ were calculated to investigate the influence of crystal field strength and nephelauxetic effect on the emission of Mn⁴⁺ in the Al₂O₃ host. The ratio of Dq to B was 1.74, which was lower than 2.2. This indicated that the Mn⁴⁺ ions in the α-Al₂O₃ host were in a weak crystal field environment. Under the 395 nm and 460 nm excitations, quantum yields (QY) of the sample were measured to be 46% and 28.7%, respectively. The density measured by the Archimedes method was 3.61 g/cm³. The ceramic also showed an excellent thermal conductivity value, which was as high as 26.27 W·m^-1·K^-1@30 °C.