Charge Transfer, Local Structure, and the Inductive Effect in Rare-Earth-Doped Inorganic Solids

Inorg. Chem. 2018, 57, 19, 12376–12383

The charge transfer (CT) process is widely present in inorganic compounds. However, the explanation of this process accounting the inductive effect was not reported. In this work, through the analysis of local structure about the second nearest cations (SNCs) in some compounds doped with trivalent lanthanide, we verify successfully the important role of the inductive effect in the CT process. By introducing electronegativity factor ∑iχi(Ai)/N – x(M), and ionic radius factor ∑iri(Ai)/N, the semiquantitative model is proposed. Strong positive correlation between the electronegativity factor and CT energy and strong negative correlation between the ionic radius factor and CT energy are given. At last, the interrelationship among these two inductive factors, the CT process, the change of local coordination structure, and the chemical composition is revealed. This work will facilitate our understanding of the CT process and the delicate role of the local structure and the inductive effect.

Enhanced performance of Sr2Si5N8: Eu2+ red afterglow phosphor by co-doping with boron and oxygen

Journal of Luminescence 204 (2018) 36–40

Herein we report the improvement in persistent luminescence of Eu2+ doped Sr2Si5N8 nitride phosphor. A systematic experiment has been performed to investigate the influence of co-doping B3+, O2-, Tm3+ in Sr2Si5N8: Eu2+. It is found that B3+ and O2- co-doped Sr2Si5N8: Eu2+ phosphor exhibits better afterglow properties with higher afterglow intensity, which can be attributed to larger trap density. The afterglow luminescence mechanism in Sr2Si5N8: Eu2+, B3+, O2- is discussed on the basis of the host referred binding energy (HRBE) scheme of Sr2Si5N8, the relative energy level positions of 5d and 4 f electron of Eu2+ and the trap depth in the host lattice.

Next‐Generation Narrow‐Band Green‐Emitting RbLi(Li3SiO4)2:Eu2+ Phosphor for Backlight Display Application

Adv. Mater. 2018, 30, 1802489.

The discovery of high efficiency narrow-band green-emitting phosphors is a major challenge in backlighting light-emitting diodes (LEDs). Benefitting from highly condensed and rigid framework structure of UCr4C4-type compounds, a next-generation narrow green emitter, RbLi(Li3SiO4)2:Eu2+ (RLSO:Eu2+), has emerged in the oxide-based family with superior luminescence properties. RLSO:Eu2+ phosphor can be efficiently excited by GaN-based blue LEDs, and shows green emission at 530 nm with a narrow full width at half maximum of 42 nm, and very low thermal quenching (103%@150 °C of the integrated emission intensity at 20 °C), however its chemical stability needs to be improved later. The white LED backlight using optimized RLSO:8%Eu2+ phosphor demon-strates a high luminous efficacy of 97.28 lm W−1 and a wide color gamut (107% National Television System Committee standard (NTSC) in Commission Internationale de L’Eclairage (CIE) 1931 color space), suggesting its great poten-tial for industrial applications as liquid crystal display (LCD) backlighting.

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.

Synthesis, structure and luminescence of SrLiAl3N4:Ce3+ phosphor

Journal of Luminescence 199 (2018) 271–277

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.

The Inductive Effect in Nitridosilicates and Oxysilicates and Its Effects on 5d Energy Levels of Ce3+

Inorg. Chem. 2018, 57, 4, 2320–2331

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.

Control of Luminescence in Eu2+-Doped Orthosilicate-Orthophosphate Phosphors by Chainlike Polyhedra and Electronic Structures

Inorg. Chem. 2018, 57, 2, 609–616

A series of Eu2+-doped orthosilicate-orthophosphate solid-solution phosphors, KxBa1.97–x(Si1–xPx)O4:0.03Eu2+, have been synthesized via the conventional solid-state reaction. Using varying compositions, the lowest-energy excitation can be tuned from 470 to 405 nm, with an emission from 515 to 423 nm. We determined how chainlike cation polyhedra controlled excitation- and emission-band features by introducing in-chain characteristic length d22 and outside-chain characteristic length d12 and that there was a nearly linear relationship between the lowest-energy-excitation position and the ratio of d22 to d12. This influence of chainlike polyhedra on luminescence can be understood through the inductive effect. Luminescent thermal properties are improved remarkably by the cosubstitution of K+ and P5+ ions for Ba2+ and Si4+ ions with a T1/2 over 200 °C. We have established the host-referred-binding-energy (HRBE) and vacuum-referred-binding-energy (VRBE) schemes for the electronic structure of the series of lanthanide-doped phosphors according to the Dorenbos model and given a thermal-quenching mechanism for this series of phosphors.

A series of Eu2+-doped orthosilicateorthophosphate solid-solution phosphors, KxBa1.97–x(Si1–xPx)O4:0.03Eu2+, have been synthesized via the conventional solid-state reaction. Using varying compositions, the lowest-energy excitation can be tuned from 470 to 405 nm, with an emission from 515 to 423 nm. We determined how chainlike cation polyhedra controlled excitation- and emission-band features by introducing in-chain characteristic length d22 and outside-chain characteristic length d12 and that there was a nearly linear relationship between the lowest-energy-excitation position and the ratio of d22 to d12. This influence of chainlike polyhedra on luminescence can be understood through the inductive effect. Luminescent thermal properties are improved remarkably by the cosubstitution of K+ and P5+ ions for Ba2+ and Si4+ ions with a T1/2 over 200 °C. We have established the host-referred-binding-energy (HRBE) and vacuum-referred-binding-energy (VRBE) schemes for the electronic structure of the series of lanthanide-doped phosphors according to the Dorenbos model and given a thermal-quenching mechanism for this series of phosphors.

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

Inorg. Chem. 2018, 57, 7, 4146–4154

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 (Ba1–xCax)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.

Insight into the Relationship between Crystal Structure and Crystal-Field Splitting of Ce3+ Doped Garnet Compounds

J. Phys. Chem. C 2018, 122, 6, 3567–3574
jp-2017-12826z_0006

The common understanding of the negative relationship between bond lengths and crystal-field splitting (CFS) is renewed by Ce3+ doped garnets in this work. We represent the contradictory relationship between structure data and spectroscopic crystal-field splitting in detail. A satisfactory explanation is given by expressing crystal-field splitting in terms of crystal-field parameters, on the basis of structural data. The results show that not only the bond length, but also the geometrical configuration have influence on the magnitude of crystal-field splitting. Also it is found that the ligand oxygen behaves differently with regard to multiple site substitution in garnet structure.