Phosphor-converted light-emitting diodes (PC-LEDs) are widely used in various fields due to their long lifetime and high energy efficiency. In particular, blue-green-emitting phosphors have the potential to fill the cyan gap in white LEDs and to be used in display indicators for autonomous driving. A new blue-green-emitting Ba2LiAlSi2O8:Eu2+ phosphor was discovered through exploratory experiments in the BaO-Li2O-Al2O3-SiO2 quasi-quaternary system using a single-particle-diagnosis approach. Single-crystal X-ray diffraction analysis revealed that Ba1.96Eu0.04LiAlSi2O8 crystallizes in a space group of Pna21 (No. 33) with a = 8.04521(11) Å, b = 19.0484(2) Å, c = 5.02228(6) Å, and Z = 4. The crystal structure comprises LiO4, AlO4, and SiO4 tetrahedra, which orderly align and form a framework by sharing apical oxygen atoms. Ba atoms are surrounded by eight and seven oxygen atoms in the framework. Density functional theory calculations corroborated the Al and Si arrangement in the Ba2LiAlSi2O8 crystal structure. A single-phase powder of the Ba2LiAlSi2O8:Eu2+ phosphor was successfully obtained via a solid-state reaction. This phosphor exhibited a blue-green luminescence peak at 497 nm with a full width at half-maximum of 85 nm under 372 nm excitation. The internal and external quantum efficiencies were 51.0% and 42.7%, respectively. The peak intensity at 150 °C was 67% of that at room temperature. We fabricated pc-LEDs based on 405 nm LED chips combined with Ba2LiAlSi2O8:Eu2+ phosphor, and the CIE chromaticity coordinates were in the blue-green region. These results indicate that the new Ba2LiAlSi2O8:Eu2+ phosphor is a promising candidate for future LED technologies.

This study demonstrates an interpretable, data-driven framework for analyzing compositional trends in Eu2+-activated phosphors. A simple linear model (ridge regression) was constructed using only atomic fractions as features, enabling quantification of the relative influence of each constituent element on the peak emission wavelength through an elemental contribution coefficient (ECC). The ECC is defined as a dataset-conditioned index derived from model coefficients, enabling comparisons of elemental tendencies within the assumptions of the model. The trends observed in the ECC are qualitatively consistent with established empirical rules in phosphor chemistry, thereby supporting the interpretability of the approach. At the same time, systematic discrepancies reveal inherent limitations of composition-based linear models, particularly their inability to separate competing physical effects, such as centroid shift and crystal field splitting. These findings highlight both the potential and the limitations of interpretable models as analytical tools for extracting trends from data and complementing chemical intuition. This study presents a pathway for using machine learning not only as a predictor but also as a tool for exploring data-driven insights in materials science.

Phosphors exhibiting narrow-band emission are key materials for LED backlights in next-generation liquid crystal displays (LCDs) that require high color reproducibility. We successfully realized a hypothetical composition, SrLi3AlO4 (SLAO):Eu2+, which is expected to exhibit narrow-band emission, by inspiring from reported SrLiAl3N4 (SLAN):Eu2+. Using a single-particle-diagnosis approach, SLAO was selectively extracted from calcined powder mixture involving multiple phases, followed by crystal structure analysis and optical property characterization. SLAO:Eu has a similar cubic structure as SLAN, while it shows narrow-band yellow-green emission at 521 nm and 567 nm with full widths at half maximum (FWHM) of 39 nm and 40 nm, respectively. The emission characteristic is comparable with that of quantum dots, demonstrating (i) the feasibility of material design based on the coordination environment similarities and (ii) the resultant SLAO:Eu as a promising phosphor for the next-generation display applications.

Cr3+-activated near-infrared emitting phosphor has a potential application in plant growth and sensing technologies. Herein, we synthesized near-infrared emitting La1_x/3Al1_x_yTixO3:yCr3+ phosphor by a solid-state reaction. La1_x/3Al1_x_yTixO3:yCr3+ (x = 0.2, y = 0.003) showed the near-infrared emission at 751 nm with a full width at half-maximum of 41 nm under 351 nm excitation. The internal and external quantum efficiencies were 54.4 and 25.5 %, respectively. The luminescence intensities at 150 degrees C of the La1_x/3Al1_x_yTixO3:yCr3+ (x = 0.1, 0.2, and 0.3, y = 0.003) were 32, 17, and 3 % of those at room temperature, respectively. The thermal quenching effect strongly occurred with increasing the amount of Ti and it was related to the change in the absorbance edge of the host material La1_x/3Al1_x_yTixO3.

A polycrystalline ceramic disk containing a mixture of new silicates of Eu2+, such as NaEuSc(Si2O7), Na4Eu2Sc2(Si2O7)(SiO4)2, and Na7Eu2Sc3(Si2O7)2(SiO4)2, was synthesized by heating a mixture of Na2O, Eu2O3, Si, and SiO2 in a stainless-steel container at 1190 degrees C. The crystal structures of the grains obtained by crushing the disk were analyzed using single-crystal X-ray diffraction (XRD). Streaks were observed between the fundamental reflections in the a* direction in the XRD pattern of NaEuSc(Si2O7). The statistical average structure of NaEuSc (Si2O7) (monoclinic, a = 8.6951(3) & Aring;, b = 5.4662(2) & Aring;, c = 13.4778(4) & Aring;, /f = 108.875(4)degrees, and space group P21/ c) was analyzed using a split-site model based on the structure of KSrSc(Si2O7). Na4Eu2Sc2(Si2O7)(SiO4)2 is orthorhombic (a = 5.48570(10) & Aring;, b = 14.3556(3) & Aring;, c = 17.0877(4) & Aring;, and space group P212121), which is isotypic with Na2Ca6(Si2O7)(SiO4)2 and Na4Sr2Sc2(Si2O7)(SiO4)2 (space group P21/c). Na7Eu2Sc3(Si2O7)2(SiO4)2 crystallizes in a C-centered monoclinic cell (a = 21.7723(3) & Aring;, b = 5.49490(10) & Aring;, c = 17.1881(2) & Aring;, /f = 107.825 (2)degrees, and space group C2/c). Three-dimensional frameworks with tunnels along the a axis in NaEuSc(Si2O7) and Na7Eu2Sc3(Si2O7)2(SiO4)2 and along the b axis in Na4Eu2Sc2(Si2O7)(SiO4)2 were formed by sharing the apical oxygen atoms of [SiO4] tetrahedra and/or [Si2O7] dimers, and [ScO6] octahedra. Na and Eu atoms were located within these tunnels. The fluorescence spectra of the grains used in the single-crystal XRD experiments were measured. The NaEuSc(Si2O7) grains exhibited blue-green fluorescence with an emission peak at 487 nm and a full width at half maximum (FWHM) of 57 nm under excitation at 390 nm. Na4Eu2Sc2(Si2O7)(SiO4)2 showed a yellow-green emission spectrum with a peak at 544 nm and a FWHM of 136 nm under 450 nm excitation. Na7Eu2Sc3(Si2O7)2(SiO4)2 exhibited a green emission spectrum with a peak at 528 nm and a FWHM of 89 nm under 440 nm excitation.

Broadband near-infrared-II (NIR-II) covered phosphors are crucial for LED applications such as biological imaging and optical sensing. In this study, Ca5Ga6-xO14: xCr phosphors, where x represents the molar concentration of Cr4+ substituting Ga sites, were synthesized with x values of 0.0001, 0.0005, 0.005, 0.01, 0.02, 0.03, and 0.04 using a conventional solid-state reaction. X-ray diffraction showed that the main targeted phase was predominantly observed for Cr doping levels below x = 0.001. Optical characterizations revealed that the optimized phosphor (x = 0.0005) exhibited the most intense and broad NIR emission (1150-1600 nm, peak at 1330 nm) by 650 nm excitation, originating from the spin-allowed 3T2 -> 3A2 transition of tetrahedrally coordinated Cr4+ ions. Low-temperature photoluminescence further elucidated the energetic and crystal field environment of Cr4+. These findings demonstrate that Ca5Ga6O14 is an efficient host for Cr4+-doped NIR-II phosphors, offering insights for the design of next-generation luminescent materials.

Narrow-band emitting phosphors are utilized to improve the color purity and expand the color gamut of display. Data-driven approach is an effective technique to search the host material of the narrow-band emitting phosphors. Herein, we discover a new narrow-band emitting phosphor Na5Al3F14:Eu2+ by a local structure similarity. 2D scatter plot of the structural similarity revealed that the local structure of Na1-site of the Na5Al3F14 located to near the Sr-sites of the narrow-band emitting phosphor SrLiAl3N4:Eu2+. A powder of the narrow-band emitting phosphor Na5Al3F14:Eu2+ was successfully synthesized by a solid-state reaction method. The Na5Al3F14:Eu2+ phosphor exhibited a narrow-band emission at 393 nm with a full width at half-maximum of 30 nm (1549 cm-1). The internal and external quantum efficiency was 56.0 % and 45.0 %, respectively. The photoluminescence intensity at 200 degrees C was maintained at 63 % of room temperature. This approach accelerates the discovery of new phosphors with specific luminescence properties.

Single crystal particles of Na2BaZr[SiO4]2 [systematic name: disodium barium zirconium bis(orthosilicate)] and Na2BaHf[SiO4]2 [disodium barium hafnium bis(orthosilicate)] were extracted from grain-grown polycrystals obtained by heating compacts of binary oxide mixtures at 1473 K. Single crystal X-ray diffraction analysis revealed that these are isostructural orthosilicates with a glaserite-type crystal structure, in which all sites of X, Y, M, and T in the general formula XY2[M(TO4)2] are fully occupied by atoms of different elements. The crystal structures of the title compounds were refined in space group P3 under consideration of a two-component twin model. The SiO4 tetrahedra are rotated approximately ±10.2° from the mirror plane of space group P3m around an axis parallel to [001].

Defects in phosphors affect not only luminescence intensity but also emission peak width, decay time, and afterglow. The green phosphor beta-SiAlON:Eu2+ exhibits the green emission of Eu2+ at 520 nm and the blue emission of nitrogen vacancies at 460 nm in time-resolved fluorescence measurements. The decay time of the intrinsic Eu2+ transition is 0.7 mu s, but afterglow is detected from 50 mu s to 0.01 s. This afterglow decay curve is the same for the green emission of Eu2+ and the blue emission of nitrogen vacancies, suggesting that the defect levels of the nitrogen vacancies affect the Eu2+ transition. The afterglow decay curves were analyzed using the formula of the general-order kinetics, (1+t/tau(B))(-n) , where n is the decay power and tau B is the decay time. This equation is generally used when analyzing afterglow on the order of seconds to hours but has not been examined systematically applied in samples with different concentrations of Eu2+ and temperatures on the order of nanoseconds to milliseconds. The decay power n is approximately 1 for all Eu2+ concentrations (x = 0.001-0.3) and undoped beta-SiAlON. The decay time tau B is correlated with the density of the nitrogen vacancies determined by electron spin resonance. Furthermore, the value of n is approximately 1 for 50 mu s to 0.01 s and 0.3 for 1-1000 s. Thus, the luminescence mechanism of Eu2+ can be discussed by comparing n and tau B obtained from the decay curves. In addition, several different Eu2+-doped phosphors, namely SrAl2O4:Eu2+, Dy2+, CaAlSiN3:Eu2+, and CaS:Eu2+, Tm3+, are studied.

Powder ceramic phosphors are fundamental materials for solid-state lighting and have contributed to the progress of general lighting and liquid crystal displays. The development of phosphors for high-color rendering illumination, wide-gamut displays, and high-brightness illumination with laser excitation continues. In recent years, the single-particle diagnostic method, which can determine the crystal structure and composition of the particles in a synthesized mixture, without obtaining a single phase has made it possible to dramatically improve the efficiency of searching for new phosphors. In this study we established a proximity measurement method that can accurately measure the photoluminescence (PL) spectrum of single phosphor particles using a commercially available multichannel photodetector generally used for power evaluation1. In this method, optical fibers were placed at a distance of less than 1 mm from a single particle sample for measurement. The measured intensity was inversely proportional to the square of the distance. PL excitation spectrum can also be measured using a single optical fiber connected to a monochromatized Xe light source. This method allows accurate spectral calibration because no extra optical components are used. Moreover, by focusing the excitation light using an optical fiber with a spherical tip, the measurement efficiency was significantly improved. The measurement intensity was proportional to the cube of the particle diameter, and even particles as small as 1.2 μm could be measured. Furthermore, we applied the proximity method to the quantum efficiency (QE) evaluation for a single particle phosphor2. Fluorescence spectroscopy by goniometric measurement, which can accurately evaluate the quantum efficiency of powdered phosphors, has been standardized by ISO23946:2020, and thus this method was applied to the evaluation of single particle. The apparatus consisted of a sample holder, an excitation fiber, and a detection fiber. A 150 W xenon lamp was used for excitation, and a multichannel photodetector was used for detection. The sample holder was attached to a 3-axis goniometer head. The samples were rotated using a motor. The excitation fiber was placed above the sample, whereas the detection fiber was driven by a motor to change the detection angle. A single optical fiber with a spherical tip was used for the excitation. Consequently, the diameter of the excitation beam was focused to 18 μm. A single optical fiber with a core diameter of 230 μm was used for the measurement. The small hole with a diameter of 0.5 mm in the sample holder was filled with barium sulfate powder as a white standard. A single phosphor particle of the test sample was then placed on it. The distances between the tips of the excitation/detection fiber and the sample were set to 500 and 700 μm, respectively. The zenith angle and the rotation angle intervals were set so that the measurement hemisphere was evenly covered with the detection fiber core diameter. Two steps were involved in the measurement. First, the particle was aligned with the center of rotation, and the light distributions in the scattering and emission spectra of the phosphor were measured. Next, the center of rotation was moved above the barium sulfate without the phosphor particle, and the scattered light distribution of the excitation light was measured. CaSiAlN3:Eu2+ red phosphor was used as the test sample. The particle picked from the powder sample was measured. The QE was calculated from the spatially integrated spectrum. Good agreement was achieved with the value for the powder sample. K. Takahashi, et al, Jpn. J. Appl. Phys., 62, 016510 (2023) K. Takahashi, et al, ECS J. Solid State Sci.Technol., 12, 076002 （2023）

Nitrides and oxynitrides isostructural to α-Si3N4 (M-α-SiAlON, M = Sr, Ca, Li) possess superb thermally stable photoluminescence (PL) properties, making them reliable phosphors for high-power solid-state lighting. However, the synthesis of phase-pure Sr-α-SiAlON still remains a great challenge and has only been reported for Sr below 1.35 at.% as the large size of Sr2+ ions tends to destabilize the α-SiAlON structure. Here, we succeeded to synthesize the single-phase powders of a unique 'Sr-rich' polytypoid α-SiAlON (Sr3Si24Al6N40:Eu2+) phosphor with three distinctive Sr/Eu luminescence sites using a solid-state remixing-reannealing process. The Sr content of this polytypoid structure exceeds those of a few previously reported structures by over 200%. The phase purity, composition, structure, and PL properties of this phosphor were investigated. A single phase can be obtained by firing the stoichiometric mixtures of all-nitride precursors at 2050°C under a 0.92 MPa N2 atmosphere. The Sr3Si24Al6N40:Eu2+ shows an intense orange-yellow emission, with the emission maximum of 590 nm and internal/external quantum efficiency of 66%/52% under 400 nm excitation. It also has a quite small thermal quenching, maintaining 93% emission intensity at 150°C. In comparison to Ca-α-SiAlON:Eu2+, this Sr counterpart shows superior quantum efficiency and thermal stability, enabling it to be an interesting orange-yellow down-conversion luminescent material for white LEDs. The experimental confirmation of the existence of such 'Sr-rich' SiAlON systems, in a single-phase powder form, paves the way for the design and synthesis of novel 'Sr-rich' SiAlON-based phosphor powders with unparalleled properties.

An approach is presented to accelerate the discovery of host compounds for novel Eu2+-activated phosphor materials by integrating systematic data collection, machine learning, and experimental validation. A data set of Eu2+- and Eu3+-activated phosphors has been constructed using systematic data collection methodology from numerous academic articles. A machine-learning classification model has been developed using the collected data set to predict the oxidation states of Eu ions in potential hosts regarding luminescence. The model considers the nonexclusive nature of the divalent and trivalent oxidation states of Eu ions in phosphor applications. A comprehensive exploration of a materials database was conducted to identify host candidates for novel Eu2+-activated phosphor materials, leading to attempts to synthesize them. Photoluminescence analysis revealed the successful synthesis of 12 new Eu2+-activated phosphors, demonstrating the potential of the proposed approach for accelerating material discovery.

New phosphors are consistently in demand for advances in solid-state lighting and displays. Conventional trial-and-error exploration experiments for new phosphors require considerable time. If a phosphor host suitable for the target luminescent property can be proposed using computational science, the speed of development of new phosphors will significantly increase, and unexpected/overlooked compositions could be proposed as candidates. As a more practical approach for developing new phosphors with target luminescent properties, we looked at combining experiments with machine learning on the topics of emission wavelength, full width at half maximum (FWHM) of the emission peak, temperature dependence of the emission spectrum (thermal quenching), new phosphors with new chemical composition or crystal structure, and high-throughput experiments.

Machine learning in conjunction with validation experiments uncovers new Eu2+-activated phosphor materials with a designed green-color luminescence.

White LEDs combined by blue LED chip and yellow YAG: Ce phosphor are the mainstream of the lighting industry. However, due to the lack of red light component, it is difficult to meet the requirements of warm white lighting. The novel needle-like Ba5Si11Al7N25:Eu2+ particle discovered by the single-particle-diagnosis method shows a broad yellow emission band centered at similar to 570 nm and a FWHM of 98 nm. Compared with YAG: Ce phosphor, the novel Ba5Si11Al7N25:Eu2+ phosphor exhibits longer emission wavelength, indicating potential application in warm white LEDs. In this work, the phase-pure powder of Ba5Si11Al7N25:Eu2+ is obtained in a non-stoichiometric ratio by the double-crucible method, and it shows similar luminescent properties as the single particle. Upon excitation with violet light (400 nm), the maximum emission wavelength of the as-prepared Ba5(1-x)Si11Al7N25: 5xEu(2+) phosphors shift from 558 nm to 591 nm with Eu2+ ions concentration increasing, and the full width at half-maximum (FWHM) increase from 87 nm to 100 nm. The broad emission band is attributed to three luminescent centers corresponding to three different Ba2+ ions sites Ba5Si11Al7N25:Eu2+ phosphor, which is demonstrated by time-resolved emission spectra and the decomposition of the emission spectrum at 4 K. The Ba5Si11Al7N25:Eu2+ phosphor shows a small thermal quenching with a quenching temperature as high as 700 K. The as-prepared phosphor can absorb both of UV and blue light effectively and exhibits similar quantum efficiency under the excitation of 400 nm (58.5 %) and 450 nm (54.5 %). The fabricated white LED lamp emitted a warm white light with CIE coordinates of (0.3838, 0.3252) and CCT = 3412 K, implying the potential applications in warm w-LEDs.

Among the various luminescent material series, lanthanide coordination polymer crystals have attracted attention for their characteristic photophysical properties based on the arrangement of lanthanide ions and organic linker ligands. Here, we demonstrate the morphology controllability of the thermostable crystalline europium coordination polymer [Eu(hfa)(3)dpbp](n), [hfa: hexafluoro-acetylacetonate; dpbp: 4,4'-bis(diphenylphosphoryl) biphenyl] to realize transparent phosphors. The obtained spherical Eu3+-coordination particles were characterized using X-ray diffraction, scanning electron microscopy, dynamic light scattering measurements, and thermogravimetric analysis. The knowledge obtained is expected to be widely used as a transparent material development technique for new lanthanide-based coordination phosphors.

A simple method for measuring the luminescence of a single-particle phosphor was developed. Optical fibers for excitation and detection were placed in proximity to the single-particle material to eliminate the requirement for additional optical parts and enable high-precision measurement using commercial instruments. In relation to the background level, the developed method had a sufficient sensitivity margin, allowing phosphor evaluation with a particle size as small as 1.2 μ m.