Understanding the abnormal lack of spectral shift with cation substitution in highly efficient phosphor La3Si6N11:Ce3+

Phys. Chem. Chem. Phys., 2020,22, 14162-14168. https://doi.org/10.1039/D0CP01445A

Cation substitution is a common strategy to tune the luminescence by modulating the cell parameter, polyhedral volume and bond length in solid-solution-type phosphors. Generally a close correlation between their cationic composition and spectral peak shifts can be observed. In certain compounds, however, luminescence tuning by cationic modification is almost invalid. This work is devoted to providing a reasonable explanation for the anomaly in Ce3+ doped La3Si6N11, which demonstrates unshifted excitation peaks with various cation substitutions. By simplifying the local coordination polyhedron that accommodates Ce3+ to a truncated square pyramid model, the quantitative crystal-field calculations are conducted to demonstrate the influences of the coordination environment on energy levels. The results show that the crystal-field levels become insensitive to this special type of ligand environment, leading to imperceptible peak shifts. Therefore, the relationship between the cationic composition and luminescence is determined not only by the ionic radii but also by the type of coordination polyhedron. This work shows that studying the coordination environment is helpful for achieving effective luminescence tuning.

Enhanced persistent luminescence via Si4+ co-doping in Y3Al2Ga3O12:Ce3+, Yb3+, B3+

J. Lumin., 2020, 222, 117190. https://doi.org/10.1016/j.jlumin.2020.117190.

Persistent luminescence phosphors with long duration and high emitting intensity have attracted considerable attention for applications in safety signage and energy storage. Herein, we successfully introduce non-equivalent ions Si4+ into Al3+ sites in the garnet phosphor Y3Al2Ga3O12:Ce3+,Yb3+,B3+ by conventional solid-state reaction. The persistent luminescence duration has been dramatically enhanced over 40 h at the 0.32 mcd/m2 threshold value after visible light radiation, almost twice longer than the sample without Si4+. Moreover, the afterglow emission intensity of the persistent luminescence is also improved. We confirm that the synthesized phosphors possess not only deeper trap depth but also wider trap distribution and higher trap density after the cooperation of Si4+. The initial rise approach is used by performing a series of thermoluminescence analyses at various temperatures after 432 nm excitation, which demonstrates the exact trap distribution from 0.47 to 1.11 eV. At the end, the mechanism of the persistent luminescence is depicted using a schematic energy diagram of the vacuum referred binding energy of Y3Al2Ga3O12.

Crystal-field splitting of Ce3+ in narrow-band phosphor SrLiAl3N4

J. Rare Earths., 2021, 39(4), 386-389https://www.sciencedirect.com/science/article/abs/pii/S1002072120300053

As a promising narrow-band phosphor, SrLiAl3N4 has a seemingly ultra-small total crystal-field splitting of only 2400 cm−1 with Ce3+ as dopant ions. This paper is devoted to unravel this anomalous phenomenon based on semi-quantitative crystal-field calculations. The results show that there may exist undetected excitation peaks immersed in the host excitation band, and the calibrated crystal-field splitting is 27000 cm−1, comparable to those of other Ce3+ doped phosphors. In the end the effect of polyhedral deformation on energy level is briefly discussed.

Structure and photoluminescence properties of Ca0.99−xSrxAlSiN3:0.01Ce3+ solid solutions

J. Am. Ceram. Soc., 2019, 102(8), 4648-4658. https://ceramics.onlinelibrary.wiley.com/doi/abs/10.1111/jace.16337

Nitride phosphors of Ca0.99−xSrxAl1.01Si0.99N3:0.01Ce3+ (0 ≤ ≤ 0.9) were synthesized by conventional solid-state method. XRD data analysis shows that all samples are single phase with CaAlSiN3-type structure. Under blue or near-ultraviolet (~400 nm) light excitation, the emission peak can be tuned largely from 615 to 568 nm by increasing Sr content, and the emission intensity is maximal at = 0.8. With the Sr content increase, the emission band blue-shifts due to the decreased crystal field splitting and the reduced centroid shift; while the thermal luminescence quenching resistance is almost unchanged. The quenching temperature (T50) is well above 500 K for all samples, which satisfies the requirement of commercial application. The quenching process is mainly attributed to the radiationless transitions by thermally activated crossover from the 5d excited state to 4f ground state in the configurational coordinate diagram. The luminescence properties show that the (Ca,Sr)AlSiN3:Ce3+ phosphors are very promising for use in blue and near-ultraviolet light excited white-light-emitting diodes.

Structural Indicator to Characterize the Crystal-Field Splitting of Ce3+ in Garnets

J. Phys. Chem. C., 2020, 124, 1, 870–873. https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.9b09322

The structure–property relationship is a hot research topic in both chemistry and material science. A structural descriptor has significant applications in machine learning and high-throughput screening for rapid estimations of material properties. In this work, we report a simple structural indicator to characterize the crystal-field splitting of Ce3+ ions in the garnet structure. The indicator is the interplanar distance of the octahedron, which is calculated as 2/√3 | x + y + z | · L, where L stands for the cell parameter and (x, y, z) are the oxygen coordinates in a specific form. The indicator value is correlated to the crystal-field splitting of Ce3+ in the lanthanide aluminum/gallium garnets and is able to reproduce the reverse garnet effect. By inspecting the polyhedral competition effect in the garnet structure, this indicator is found to be correlated to the tetragonal distortion of the dodecahedron, which determines the crystal-field splitting.

Effects of Neighboring Polyhedron Competition on the 5d Level of Ce3+ in Lanthanide Garnets


Abstract Image

Herein, we provide a direct explanation for the reverse garnet effect based on polyhedron competition. On multisite substitution, the deformation of the dodecahedron which will accommodate Ce3+ meets suppression from a neighboring octahedron and tetrahedron. This makes the dodecahedral deformation nonisotropic. Further, it is found that the lowest 5d state of Ce3+ in garnet is closely related to the tetragonal distortion of the dodecahedron, which is characterized by a simplified cuboid model. Crystal-field calculations reveal how the edge-ratio of the cuboid affects energy levels. This model gives a satisfactory explanation for the reverse garnet effect and is helpful for seeking novel garnet-based luminescent materials.

Enhanced Persistence Properties through Modifying the Trap Depth and Density in Y3Al2Ga3O12:Ce3+,Yb3+ Phosphor by Co-doping B3+

Journal of Alloys and Compounds, Volume 770, 5 January 2019, Pages 1069-1077

To fabricate white-light-emitting diodes (white LEDs) with high color-rendering index or full light spectrum emission, the discovery of more efficient deep-red emitting phosphor materials is essential. In this paper, we have synthesized a series of Sr2-2xEu2xSi5N8 (0 ≤ x ≤ 1) solid-solution compounds, and have systemically investigated effects of full-range Eu concentration on their luminescence. Their emission band maximum can be largely tuned from 610 to 725 nm by increasing Eu content. Reabsorption at low Eu2+ concentration while both the energy transfer and Stocks shift at high Eu2+ concentration account for this large spectral red-shift. Luminescent thermal quenching performance gets worse with Eu2+ concentration increasing. The compound with x = 0.15 possesses the best crystallinity and the highest luminescence intensity with the peak position around 660 nm, and still maintains 88.5% room-temperature intensity at 400 K, indicating that great potential for the application as a deep-red phosphor.

Effect of polyhedron deformation on the 5d energy level of Ce3+ in lanthanide aluminum perovskites

 Physical Chemistry Chemical Physics, January 2019, 21(5)

Graphical abstract: Effect of polyhedron deformation on the 5d energy level of Ce3+ in lanthanide aluminum perovskites

The crystal-field levels of Ce3+ in a series of lanthanide aluminum perovskites have been investigated with reference to polyhedron deformation. For each compound, the corresponding ideal cuboctahedron is derived through a least-square procedure. The virtual energy levels of Ce3+ in these ideal polyhedrons are then obtained considering both crystal-field splitting and spin–orbit coupling. From comparison to real levels, we have a clear understanding of how polyhedron deformation affects the energy levels of Ce3+ in the perovskites.

5d-level centroid shift and coordination number of Ce3+ in nitride compounds

Journal of Luminescence 200 (2018) 35–42

Energy of 5d-levels of Ce3+ in numerous nitrides has become available due to the development of nitride phosphors recently. In this work, we have collected data on 5d-levels of Ce3+ and reconsidered the 5d centroid shift of Ce3+ in nitrides. The uniform standard, derived from the bond valence theory and the requirement for the high stability of the coordination polyhedron, has been proposed to determine the coordination number. The relationship between the 5d centroid shift of Ce3+, the polarizability of the anions and the electronegativity of the cations is revealed. The anion polarizability is linearly related to the inverse square of the average electronegativity of the cations; and the 5d centroid shift of Ce3+ can be well predicted by virtue of crystallographic data. This paper provides a feasibility to predict luminescence properties of Ce3+-doped nitrides.

Ce3+-Doped garnet phosphors: composition modification, luminescence properties and applications

Chem. Soc. Rev., 2017,46, 275-299. https://doi.org/10.1039/C6CS00551A

Garnets have the general formula of A3B2C3O12 and form a wide range of inorganic compounds, occurring both naturally (gemstones) and synthetically. Their physical and chemical properties are closely related to the structure and composition. In particular, Ce3+-doped garnet phosphors have a long history and are widely applied, ranging from flying spot cameras, lasers and phosphors in fluorescent tubes to more recent applications in white light LEDs, as afterglow materials and scintillators for medical imaging. Garnet phosphors are unique in their tunability of the luminescence properties through variations in the {A}, [B] and (C) cation sublattice. The flexibility in phosphor composition and the tunable luminescence properties rely on design and synthesis strategies for new garnet compositions with tailor-made luminescence properties. It is the aim of this review to discuss the variation in luminescence properties of Ce3+-doped garnet materials in relation to the applications. This review will provide insight into the relation between crystal chemistry and luminescence for the important class of Ce3+-doped garnet phosphors. It will summarize previous research on the structural design and optical properties of garnet phosphors and also discuss future research opportunities in this field.