Barycenter of Energy of Lanthanide 4fN-15d Configuration in Inorganic Crystals

The Charge Transfer Band as a Key to Study the Site Selection Preference of Eu3+ in Inorganic Crystals

On account of the strong oxidizing property of the europium(III) ion, its charge transfer band (CTB) can be easily formed in many inorganic compounds. In this work, the Eu³⁺ ions were singly doped into the K₃LuSi₂O₇ compound with a hexagonal structure, and two kinds of Eu³⁺-O^2- CTBs were detected by monitoring at specific wavelengths. The qualitative analysis of Eu³⁺ ion site occupation was illuminated by combining Eu³⁺-O^2- CTBs with the corresponding cell volume. Furthermore, the two kinds of Eu³⁺ sites are eventually assigned to the K(2) and Lu sites, which means that Eu³⁺ ions selectively occupy the site with a low coordination number, according to the calculated CT energy by the dielectric theory of complex crystals and the magnitude of CT energy in the excitation spectra. Meanwhile, at high temperatures, the CTB does not show the traditional thermal quenching like f-f transitions but demonstrates thermal enhancement; thus, by using this opposite variation in excitation spectra, a noninvasive optical thermometer is presented, and this opposite variation tendency is thought to be the difference of thermal stability of disparate excited energy states. When new luminescent phosphors are designed with interesting spectral properties, this work will give us an alternative approach to determine the site occupation preference of Eu³⁺, especially when there are more than two different sites in the compound.

Structural and spectroscopic features of high color purity red-emitting phosphors Sr19Mg2(PO4)14: Re3+ (Re3+= Eu3+, Sm3+, Pr3+)

The discovery of high color purity red-emitting phosphors is a major challenge for solid-state lighting materials. Benefitting from highly condensed and flexible framework structure of β-Ca₃(PO₄)₂-type compounds, we have successfully prepared three different kinds of novel high color purity red-emitting phosphors Sr₁₉Mg₂(PO₄)₁₄: Re³⁺ (Re³⁺= Eu³⁺, Sm³⁺, Pr³⁺) by using traditional sintering method. Rietveld refinement, SEM measurement, absorption spectra, emission/excitation spectra, fluorescence decay analysis and emission spectra in terms of different temperature were investigated and discussed clearly. The matrix optical band gap was calculated to be 4.5 eV by reflection data, which indicated the suitable host for rare earth doping. The single doped Eu³⁺, Sm³⁺ and Pr³⁺ phosphors could respectively exhibit characteristic and strong red emission peaks at 614 nm, 598 nm and 642 nm when excited by (near) ultraviolet radiation. Excitingly, all samples could obtain high color purity with the value of 91.6%, 90.6% and 84.8% for Eu³⁺, Sm³⁺, Pr³⁺ ions, respectively. Moreover, the thermal stability can stay strong which still keep over 75% at 150℃ when comparing with that at atmospheric temperature. The quantum efficiency (QE) is another important parameter for phosphors which were measured to be 46.6% for Eu³⁺, 53.1% for Sm³⁺ and 10.3% for Pr³⁺. The present work indicates that the Sr₁₉Mg₂(PO₄)₁₄: Re³⁺ phosphors are efficient red components with extraordinary color purity and high quantum efficiency for industrial applications as solid-state lighting materials.

Atomistic Simulation of Structural and Mechanical Properties of the AMgF3 (A = K, Rb, and Cs) Compounds Under Hydrostatic Pressure

Structural, mechanical, elastic, and dielectric properties of the AMgF₃ (A = K, Rb, and Cs) compounds were investigated using classical atomistic simulation. A new set of interatomic potentials was developed for these compounds. Lattice parameters and interatomic distances have shown to accurately reproduce all structures, with very close agreement to the experimental data. In all cases, the relative error is below 0.5%. Effect of hydrostatic pressure in the structural, mechanical, elastic, and dielectric properties of these materials were studied from 0 up to 50 GPa. Compounds behavior and stability under pressure were analyzed. KMgF₃ and RbMgF₃ changed from brittle to ductile at approximately 2 GPa. These calculations play an important role in understanding the properties of the AMgF₃ (A = K, Rb, and Cs) compounds under pressure, and open up a new opportunity to study defects in this class of materials. © 2019 Wiley Periodicals, Inc.

Synthesis, Crystal Structure, and Luminescence Properties of a White-Light-Emitting Nitride Phosphor, Ca0.99Eu0.01AlSi4N7

Colorless transparent single crystals of a new europium-doped calcium aluminum silicon nitride, Ca_0.99Eu_0.01AlSi₄N₇, were synthesized by heating a mixture of binary nitrides (Ca₃N₂, EuN, AlN, Si₃N₄, and Mg₃N₂) at 2150 °C under a nitrogen gas pressure of 0.85 MPa. The X-ray diffraction reflections from a single crystal were indexed with orthorhombic cell parameters a = 11.6819(3) Å, b = 21.0193(6) Å, and c = 4.91770(10) Å. The crystal structure was isomorphic to that of SrAlSi₄N₇:Eu (space group Pna2₁). One of the Ca/Eu sites was split into two sites of 4-fold and 8-fold nitrogen coordination. A white-light emission spectrum with two peaks at wavelengths of 498 and 614 nm was observed from the single crystals under 400 nm near-ultraviolet-light irradiation. The CIE1931 chromaticity of the white light was plotted at x = 0.378 and y = 0.429; the mean color-rendering index Ra was 81.

Usage of P–V–L bond theory in studying the structural/property regulation of microwave dielectric ceramics: a review

Combined with bond traits, such as ionicity, susceptibility and lattice energy obtained via P–V–L bond theory, and far-infrared reflectance spectroscopy, intrinsic structural effects on microwave dielectric properties can be specifically evaluated.

Highly Luminous N3--Substituted Li2MSiO4-δN2/3δ:Eu2+ (M = Ca, Sr, and Ba) for White NUV Light-Emitting Diodes

The N^3--substituted Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors were systematically prepared and analyzed. Secondary-ion mass spectroscopy measurements revealed that the average N^3- contents are 0.003 for Ca, 0.009 for Sr, and 0.032 for Ba. Furthermore, the N^3- incorporation in the host lattices was corroborated by infrared and X-ray photoelectron spectroscopies. From the photoluminescence spectra of Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors before and after N^3- doping, it was verified that the enhanced emission intensity of the phosphors is most likely due to the N^3- doping. In Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors, the maximum wavelengths of the emission band were red-shifted in the order Ca < Ba < Sr, which is not consistent with the trend of crystal field splitting: Ba < Sr < Ca. This discrepancy was clearly explained by electron-electron repulsions among polyhedra, LiO₄-MO _n , SiO₄-MO _n , and MO _n -M'O _n associated with structural difference in the host lattices. Therefore, the energy levels associated with the 4f⁶5d energy levels of Eu²⁺ are definitely established in the following order: Li₂CaSiO₄:Eu²⁺ > Li₂BaSiO₄:Eu²⁺ > Li₂SrSiO₄:Eu²⁺. Furthermore, using the Williamson-Hall (W-H) method, the determined structural strains of Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors revealed that the increased compressive strain after N^3- doping induces the enhanced emission intensity of these phosphors. White light-emitting diodes made by three N^3--doped phosphors and a 365 nm emitting InGaN chip showed the (0.333, 0.373) color coordinate and high color-rendering index (R _a = 83). These phosphor materials may provide a platform for development of new efficient phosphors in solid-state lighting field.

Narrow-band green emission of Eu2+ in a rigid tunnel structure: site occupations, barycenter energy calculations and luminescence properties

As a new member of an oxide-based family with a UCr₄C₄-related type structure, CsKNaLi(Li₃SiO₄)₄:Eu²⁺ was synthesized successfully for the first time and its rigid tunnel structure leads to narrow-band green emission, which can be used for white light generation.

Novel Eu2+-activated thiogallate phosphors for white LED applications: structural and spectroscopic analysis

Novel Eu²⁺-activated BaGa₂SiS₆ and Ba₂Ga₈SiS₁₆ thiogallate phosphors were prepared by solid-state reaction route. The BaGa₂SiS₆:Eu²⁺ phosphor generated a green emission upon excitation at 405 nm, whereas the Ba₂Ga₈SiS₁₆:xEu²⁺ phosphor could be tuned from cyan to green range with increasing Eu²⁺ concentration upon excitation at 365 nm. Additionally, the thermal luminescence properties of the thiogallate phosphors were investigated in the temperature range of 25 to 250 °C. A warm-white LED is fabricated using the combination of a 405 nm blue InGaN-based LED chip with the green-emitting BaGa₂SiS₆:0.01Eu²⁺ phosphor, and red-emitting Sr₂Si₅N₈:Eu²⁺ commercial phosphor with the CRI value of ∼88 and the CCT of 4213 K.

BaGa₂SiS₆:Eu²⁺ (a) emits a green light upon excitation at 405 nm, whereas Ba₂Ga₈SiS₁₆:xEu²⁺ (b) can be tuned to emit cyan to green with increasing Eu²⁺ content when excited at 365 nm.

On the character of the optical transitions in closed-shell transition metal oxides doped with Bi3+

The criterion |MMCT(Bi³⁺)–E_A(X)–ΔStokes| is introduced to assign the nature of the optical transitions in transition metal oxides doped with Bi³⁺.

Sr9Mg(1.5)(PO4)7:Eu(2+): A Novel Broadband Orange-Yellow-Emitting Phosphor for Blue Light-Excited Warm White LEDs

A new orange-yellow-emitting Sr9Mg(1.5)(PO4)7:Eu(2+) phosphor was prepared via high-temperature solid-state reaction. The structure and optical properties of it were studied systematically. Sr9Mg(1.5)(PO4)7:Eu(2+) can be well-excited by 460 nm blue InGaN chips and exhibit a wide emission band covering from 470 to 850 nm with two main peaks centered at 523 and 620 nm, respectively, which originate from 5d-4f dipole-allowed transitions of Eu(2+) in different crystallographic sites. The sites attribution, concentration quenching, fluorescence decay analysis, and temperature-dependent luminescence properties were investigated in detail. Furthermore, a warm white LED device was fabricated by combining a 460 nm blue InGaN chip with the optimized orange-yellow-emitting Sr9Mg(1.5)(PO4)7:Eu(2+). The color coordinate, correlated color temperature and color rendering index of the fabricated LED device were (0.393, 0.352), 3437 K, and 86.07, respectively. Sr9Mg(1.5)(PO4)7:Eu(2+) has great potential to serve as an attractive candidate in the application of blue light-excited warm white LEDs.

Effects of changing the M2+ cation on the crystal structure and optical properties of divalent samarium-doped MAl2Si2O8 (M = Ca, Sr, Ba)

The photoluminescence properties and theoretical analysis of Sm²⁺ doped MAl₂Si₂O₈ (M = Ca, Sr, Ba) were reported. The theoretical analysis is in good agreement with our experimental results.

Systematic study of the relationships between energy levels of (Tl+)2, (In+)2, (Ga+)2 dimer centers and the crystalline environment

In this paper, we present the first systematic study of correlations between energy levels of (Tl+)2, (In+)2, and (Ga+)2 dimer centers and the crystalline environment by using dielectric theory of chemical bonding for complex crystals. Our model has successfully built links between the EA′, EB′, and EC′ of these centers and the environmental factor h. The h has linear relationships with EA′, EB′, and EC′. The calculated results are in good agreement with the experimental values. This model can serve as a prediction tool and can be applied to assign the absorption band energies of (Tl+)2, (In+)2, and (Ga+)2 centers.

Photoluminescence of Sr(2)P(2)O(7):Bi(2+) as a red phosphor for additive light generation

We report on photoluminescence of Sr(2)P(2)O(7):Bi(2+) as a potential red-emitting phosphor for multichromatic light sources. If excited with blue light, photoluminescence of this compound spans the spectral range of about 600 to 760nm. Static and dynamic spectral data reveal the presence of two distinct emission centers. Based on bond covalency and emission lifetime, luminescence can clearly be assigned to Bi(2+) ions on Sr(1) and Sr(2) lattice sites, respectively. Energy transfer is observed from Bi(1) to Bi(2). Transfer efficiency, estimated from the lifetimes of the excited states, increases with increasing dopant concentration.

Dependence of Charge Transfer Energy on Crystal Structure and Composition in Eu3+-Doped Compounds

We report a method for estimating the positions of charge transfer (CT) bands in Eu3+-doped complex crystals. The environmental factor (h(e)) influencing the CT energy is presented. h(e) consists of four chemical bond parameters: the covalency, the bond volume polarization, the presented charge of the ligand in the chemical bond, and the coordination number of the central ion. These parameters are calculated with the dielectric theory of complex crystals. The relationship between the experimental CT energies and calculated environmental factors was established by an empirical formula. The calculated values are in good agreement with the experimental results. Such a relationship was confirmed by detailed analysis. In addition, our method is also useful to predict the charge-transfer position of any other rare earth ion.

Photoluminescence Properties and Analysis of Spectral Structure of Eu3+-Doped SrY2O4

The aim of this work is to report on the luminescence properties of SrY2O4 activated by Eu3+ ion. Powder samples were prepared by solid-state reaction. X-ray diffraction powder data, photoluminescence, and high-resolution spectroscopy were carried out. Results revealed that the Eu3+ ions occupied three nonequivalent sites, with one at the Sr site, one at the Y(1) site, and another at the Y(2) site. Their spectra wavelengths for the 7F0-5D0 transition are located at 578.49, 581.86, and 580.63 nm, respectively. The corresponding charge-transfer transitions are located at 248, 257, and 270 nm, respectively, which are also confirmed by theoretical analysis.