Oxynitride K2Ba6.72Si16O40-1.5yNy:0.28Eu2+ phosphor with high thermal stability realized by crystal field engineering

Extensive research has gone into modifying the chemical composition of phosphors to achieve desirable optical properties. Here, oxynitride phosphors K2Ba6.72Si16O40-1.5yNy:0.28Eu2+ were synthesized by introducing N3- (y) into a K2Ba6.72Si16O40:0.28Eu2+ lattice. An uneven shrinking of the cell parameters a, b, and c was observed through a combination of X-ray diffraction studies and Rietveld refinements. This shrinking caused a large centroid shift (εc) and splitting of the 5d energy level (εcfs), thus inducing the broadening of the excitation spectra (104 → 127 nm, y = 0 → y = 12) and the red shift of the emission spectra (501 → 543 nm, y = 0 → y = 12). The modified series of samples have a broad excitation spectrum, suitable of use in UV, near-UV, and blue light-emitting LEDs. In addition, the optimal sample, K2Ba6.72Si16O31N6:0.28Eu2+, benefits from an increased activation energy and thermal stability.

Regulation of Local Site Structures to Stabilize Mixed-Valence Eu2+/3+ under a Reducing Atmosphere for Multicolor Photoluminescence

Co-doping mixed-valence Eu^2+/3+ in a single-phase phosphor is an efficient method to realize the emission color regulation, which holds great potential for anticounterfeiting and ratiometric temperature sensing. Here, the mixed-valence Eu-doped Sr_1.95+xLi_1-xSi_1-xAl_xO₄F (0 ≤ x ≤ 0.25) phosphors were designed and prepared under a reducing atmosphere. The correlation of local phase structures and luminescence properties was discussed. Replacing Si⁴⁺-Li⁺ ion pairs with Al³⁺-Sr²⁺ ion pairs compresses the Sr sites occupied by Eu²⁺, and it stabilizes Eu³⁺ in a reducing atmosphere and leads to the coexistence of Eu²⁺ and Eu³⁺ in single-phase Sr_1.95+xLi_1-xSi_1-xAl_xO₄F:0.05Eu (0 ≤ x ≤ 0.25) phosphors. Based on the wavelength-dependent luminescence color behaviors of Sr_1.95+xLi_1-xSi_1-xAl_xO₄F:0.05Eu phosphors, the fluorescent anticounterfeit papers/patterns containing Sr_1.95+xLi_1-xSi_1-xAl_xO₄F:0.05Eu phosphors were the same as ordinary paper under ambient conditions. However, the hidden colors or images can be read out with green-orange luminescence under 365/300 nm light excitation. Benefiting from the diverse thermal response emission behaviors of Eu²⁺ (530 nm) and Eu³⁺ (703 nm), Sr_1.95+xLi_1-xSi_1-xAl_xO₄F:0.05Eu phosphors exhibit temperature sensing performances, with the maximum absolute and relative sensitivity being 0.0294 K^-1 at 573 K and 0.83% K^-1 at 348 K. More importantly, Sr_1.95+xLi_1-xSi_1-xAl_xO₄F:0.05Eu phosphors showed excellent stability in humid, acid, and alkali environments, which contributed to applying mixed-valence Eu^2+/3+-doped Sr_1.95+xLi_1-xSi_1-xAl_xO₄F to the fields of multicolor anticounterfeiting and noncontact optical thermometry.

Consequence of Optimal Bonding on Disordered Structure and Improved Luminescence Properties in T-Phase (Ba,Ca)2SiO4:Eu2+ Phosphor

T-phase (Ba,Ca)₂SiO₄:Eu²⁺, showing excellent luminescent thermal stability, has a positionally disordered structure with the splitting of five atom sites, but until now the reason has remained unclear. Herein, we investigate the coordination environments of each cation site in detail to understand the origins of the atom site splitting. We find that the three cation sites in the split-atom-site model are optimally bonded with ligand O atoms compared to the unsplit-atom-site model. This atom site splitting results in larger room and smaller room for each splitting cation site, which just accommodates larger Ba²⁺ ions and smaller Ca²⁺ ions, respectively, leading to more rigid structure. Based on the X-ray diffraction data refinement, the boundary of the T-phase for (Ba_{1- x}Ca _x)₂SiO₄ is redetermined. The Eu²⁺-doped T-phase (Ba,Ca)₂SiO₄ phosphors show excellent luminescent thermal stability, which can be attributed to optimal bonding and more rigid structure with atom site splitting. These results indicate that T-phase (Ba,Ca)₂SiO₄:Eu²⁺ phosphors have promise for practical applications.

Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives

Advances in solid state white lighting technologies witness the explosive development of phosphor materials (down-conversion luminescent materials). A large amount of evidence has demonstrated the revolutionary role of the emerging nitride phosphors in producing superior white light-emitting diodes for lighting and display applications. The structural and compositional versatility together with the unique local coordination environments enable nitride materials to have compelling luminescent properties such as abundant emission colors, controllable photoluminescence spectra, high conversion efficiency, and small thermal quenching/degradation. Here, we summarize the state-of-art progress on this novel family of luminescent materials and discuss the topics of materials discovery, crystal chemistry, structure-related luminescence, temperature-dependent luminescence, and spectral tailoring. We also overview different types of nitride phosphors and their applications in solid state lighting, including general illumination, backlighting, and laser-driven lighting. Finally, the challenges and outlooks in this type of promising down-conversion materials are highlighted.