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.
Local structure modification in solid solution is an essential part of photoluminescence tuning of rare earth doped solid state phosphors. Herein we report a new solid solution phosphor of Eu2+-doped xSr2Ca(PO4)2–(1–x)Ca10Li(PO4)7 (0 ≤ x ≤ 1), which share the same β-Ca3(PO4)2 type structure in the full composition range. Depending on the x parameter variation in xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7:Eu2+, the vacancies generated in the M(4) site enable the nonlinear variation of cell parameters and volume, and this increases the magnitude of M(4)O6 polyhedra distortion. The local structure modulation around the Eu2+ ions causes different luminescent behaviors of the two-peak emission and induces the photoluminescence tuning. The shift of the emission peaks in the solid solution phosphors with different compositions has been discussed. It remains invariable at x ≤ 0.5, but the red-shift is observed at x > 0.5 which is attributed to combined effect of the crystal field splitting, Stokes shift, and energy transfer between Eu2+ ions. The temperature-dependent luminescence measurements are also performed, and it is shown that the photoionization process is responsible for the quenching effect.
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.
Recently a lot of Ce3+/Eu2+-activated nitride and oxonitride phosphors have been explored due to potential or practical application for white-light LEDs. In this paper, data of crystal field splitting of the 4f n−15d-levels of Ce3+ and Eu2+ in nitride compounds is collected and analyzed. The relationship between the crystal field splitting and the coordination polyhedron around the Ce3+ and Eu2+ is revealed, showing that crystal field splitting is related to coordination number, polyhedron shape and size, while being irrelevant of the anion types. In addition, the crystal field splitting of Ce3+ and Eu2+ in the nitride compounds is correlated by a multiplication factor 0.76, which is in consistent with those in halides, sulfides and oxides. This paper makes it possible to predict luminescence properties of Ce3+- or Eu2+- doped nitride compounds.
Eu2+ local environments in various crystallographic sites enable the different distributions of the emission and excitation energies and then realize the photoluminescence tuning of the Eu2+ doped solid state phosphors. Herein we report the Eu2+-doped Ca10M(PO4)7 (M = Li, Na, and K) phosphors with β-Ca3(PO4)2-type structure, in which there are five cation crystallographic sites, and the phosphors show a color tuning from bluish-violet to blue and yellow with the variation of M ions. The difference in decay rate monitored at selected wavelengths is related to multiple luminescent centers in Ca10M(PO4)7:Eu2+, and the occupied rates of Eu2+ in Ca(1), Ca(2), Ca(3), Na(4), and Ca(5) sites from Rietveld refinements using synchrotron power diffraction data confirm that Eu2+ enters into four cation sites except for Ca(5). Since the average bond lengths d(Ca–O) remain invariable in the Ca10M(PO4)7:Eu2+, the drastic changes of bond lengths d(M–O) and Eu2+ emission depending on the variation from Li to Na and K can provide insight into the distribution of Eu2+ ions. It is found that the emission band at 410 nm is ascribed to the occupation of Eu2+ in the Ca(1), Ca(2), and Ca(3) sites with similar local environments, while the long-wavelength band (466 or 511 nm) is attributed to Eu2+ at the M(4) site (M = Na and K). We show that the crystal-site engineering approach discussed herein can be applied to probe the luminescence of the dopants and provide a new method for photoluminescence tuning.
Here we report a new phosphor, Ce-doped SrLiAl3N4, which can be effectively excited by green light at ~ 517 nm. A series of synthetic experiments are performed to find an optimal scheme. This phosphor has two emission bands at ~ 545 and ~ 610 nm corresponding to the d-f electronic transition of Ce3+. Large centroid shift of 5d level results in a green light-excitable feature. Compared to other Ce3+-doped nitrides, the crystal field splitting of 5d energy levels for this phosphor, i.e. about 11,300 cm−1, is much smaller due to larger volume and smaller distortion of coordination polyhedron of Ce3+. The phosphor shows an excellent luminescent thermal quenching behavior. At 150 °C, the emission intensity retains about 93% of the initial value at room temperature upon 517 nm excitation. This property can be ascribed to rigid structure and large gap between 5d levels and bottom of conduction band.
Exploring high-performance narrow-band red-emitting phosphor is an important challenge for improving white light LEDs. Here, on the basis of three interesting nitride phosphors with similar vierer rings framework structure, two phosphor series, Eu2+-doped Sr(LiAl)1–xMg2xAl2N4 and Sr(LiAl3)1–y(Mg3Si)yN4 (x, y = 0–1), are successfully synthesized by a solid state reaction. They show narrow-band red emission with tunable emission peaks from 614 to 658 nm and 607 to 663 nm. The varying luminescence behaviors with composition and structure are discussed based on centroid shift, crystal field splitting and Stokes shift. On the basis of experimental data, we construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes of divalent/trivalent lanthanide-doped end-member compounds, and further give thermal quenching mechanism of these series phosphors.
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.
The inductive effect exists widely in inorganic compounds and accounts well for many physicochemical properties. However, until now this effect has not been characterized quantitatively. In this work, we collected and analyzed the structural data of more than 100 nitridosilicates and oxysilicates, whose structures typically consist of [SiN4] or [SiO4] tetrahedra. We introduce a new parameter, the inductive effect factor μΔχ, related to the difference of electronegativity between constituent metal elements and silicon. Then, a linear relationship is established between average length of Si–N/Si–O bonds and the inductive factor with the help of statistical method, that is, l̅ = 1.7313 + 0.0166 μΔχ (Å) with adjusted (adj) R2 = 0.800 for Si–N and l̅ = 1.6221 + 0.0035 μΔχ(Å) with adj R2 = 0.240 for Si–O. Furthermore, our research shows that the distinct positive correlation does exist between the inductive factor and the centroid shift of 5d levels of Ce3+. This work will help us understanding the inductive effect deeply and quantitatively.
Yttrium aluminum-gallium garnets with cerium doped is most widely used as green-yellow phosphor in solid state lighting. Extensive research has been performed on this material concerning the luminescent thermal quenching resistance and persistent luminescence. In this paper we find that a negative correlation exists between temperature-dependent luminescence and persistent luminescence with gallium content varying. The correlation originates from the electronic structures which influence both the thermal quenching of luminescence and persistent luminescence. A detailed crystal-field calculation has been performed to understand the peak shifts. In addition, theoretical calculations reveal that oxygen vacancies provide trap levels which implement the persistent luminescence. This material could be used as potential blue-light excited persistent luminescent material, with the after-glow time up to about 1. h with only cerium as the dopant, which is expected to be prolonged by co-doping other elements. This work may be helpful in guiding the discovery of other after-glow materials.
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