Site-Selective Doping and Site-Sensitive Photoluminescence of Eu3+ and Tb3+ in Perovskite-Type LaLuO3

A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Variable Photoluminescence Intensity Ratio with the Excitation Wavelength in Eu3+-Doped Perovskite-Type Alkaline Earth Zirconates─Possibility of a Unique Visualization of Ultraviolet Light

Photoluminescence (PL) spectra arising from 4f-4f dipole transitions of Eu3+ doped at both the A and B sites of perovskite-type alkaline earth zirconates obtained at various excitation wavelengths were evaluated. Changes in the excitation wavelength caused obvious differences in the PL intensity ratio of the induced electric dipole (ED) 5D0 → 7F2 transition to the magnetic dipole (MD) 5D0 → 7F1 transition for Eu3+-doped SrZrO3 and CaZrO3, in which only the B site had a center of symmetry. Two charge transfer (CT) bands associated with the excitation of Eu3+ were observed in the spectra of these oxide samples, which arose from the difference in the electronic structure of the Eu-O coordination at the A and B sites. We conclude that simultaneous Eu3+ substitution at sites with and without a center of symmetry, which have different charge transfer energies, induced the observed novel PL changes with a changing excitation wavelength. Our results may enable the development of a new type of ultraviolet light detector.

Eu3+ Site Distribution and Local Distortion of Photoluminescent Ca3 WO6 :(Eu3+ , K+ ) Double Perovskites as High-Color-Purity Red Phosphors

In this study, Eu3+ and K+ co-doped Ca3 WO6 double perovskite, a high-color-purity red phosphor, is quantitatively investigated using synchrotron X-ray diffraction Rietveld refinement, energy dispersive X-ray spectroscopy, and high-resolution PL spectroscopy. The Eu3+ fluorescence line-narrowing (FLN) results, used to estimate the Eu3+ occupation at given sites (so-called A and B sites) in the host crystal (A2 BMO6 ; A, B = Ca; M = W), reveal that the Eu3+ ions have a twin distribution in both the A and B sites with high asymmetry ratios of ΛA  = 9.7 and ΛB  = 10.7, respectively. More interestingly, at lower Eu3+ doping levels, the ions are predominantly located at the B sites (≈75%), indicating that the high color purity of Ca3 WO6 :(Eu3+ , K+ ) is mainly caused by the high asymmetry ratios of the Eu3+ sites. Rietveld refinement combined with the FLN technique provides more reliable results for increasing the Eu3+ substitution at the A site with Eu3+ and K+ doping concentrations, which lower the lattice energy of Ca3 WO6 :(Eu3+ , K+ ). The structural distortions owing to K+ co-doping in terms of the quadratic elongation and bond-angle variance exhibit good correlations with the Ca(Eu)O12 A-site cuboctahedra and Ca(Eu)O6 B-site octahedra, partially accounting for the higher Λ values.

Achieving excitation wavelength-power-dependent colorful luminescence via multiplexing of dual lanthanides in fluorine oxide particles for multilevel anticounterfeiting

The optical characteristics of multimode luminescent materials like multimode luminescence (photoluminescence, afterglow, thermoluminescence) and a multi-excitation source (light, thermal, mechanical force) play crucial roles in optical data storage and readout, document security and anticounterfeiting. A higher level of advanced anticounterfeiting may rely on multimode anticounterfeiting materials that can realize multicolor luminescence. Here, a highly integrated multimode and multicolor Y7O6F9:Er3+,Eu3+ material is developed through multiplexing of dual lanthanides in fluorine oxide particles. In photoluminescence and photoluminescence/up-conversion luminescence modes, the material Y7O6F9:Er3+,Eu3+ has the characteristic of excitation wavelength and power dependence. In the photoluminescence mode, under excitation at 254 nm and 365 nm, Y7O6F9:Er3+ and Y7O6F9:Eu3+ showed bright red and green emissions, respectively. In the photoluminescence/up-conversion mode, under the increased excitation power from 0.2 to 2.0 W cm-2, the color of luminescence emission can be finely tuned from red to orange, yellow and green. Taking this unique excitation wavelength-power-dependent luminescence property into account, a multilevel anticounterfeiting device with the Lily pattern was designed. The device readily integrates the advantages of the excitation wavelength-dependent photoluminescence emissions and excitation power-dependent photoluminescence emissions in one overall device. These findings offer unique insight for designing highly integrated multimode, multicolor luminescence materials and advanced anticounterfeiting technology toward a wide variety of applications, particularly multilevel anticounterfeiting devices.

Effects of Codoping on Site-Dependent Eu3+ Luminescence in Perovskite-Type Calcium Zirconate and Hafnate

The single doping of Eu³⁺ and codoping with Ga³⁺ or La³⁺ into perovskite (ABO₃)-type CaZrO₃ and CaHfO₃ were carried out to investigate the effect of codoping on the photoluminescence (PL) features of Eu³⁺, attempting the site-selective Eu³⁺ doping at either A or B sites. To comprehend the Eu³⁺ PL features, the accurate locations of Eu³⁺ were examined by X-ray absorption near edge structure (XANES) and the proportions of Eu³⁺ located at A or B sites (Eu³⁺(A) or Eu³⁺(B); Eu_Ca^• or Eu_Zr^'/Eu_Hf^') were analyzed. No considerable differences in the XANES spectra and in the proportions of Eu³⁺(A) and Eu³⁺(B) were observed in the single-doped samples. On the other hand, the shape of the XANES spectra and the Eu³⁺ proportion markedly differed in the codoped samples. Ga³⁺ codoping (Ga_Zr^'/Ga_Hf^') achieved Eu³⁺(A) proportions of more than 90%, and La³⁺ codoping (La_Ca^•) resulted in the largest Eu³⁺(B) proportions of approximately 40%. In both PL and PL excitation spectra, the site-dependent spectral features were changed depending on the proportion of Eu³⁺(A) and Eu³⁺(B), especially the PL peaks at 595 nm were intensified most in the La³⁺ codoped samples. Consequently, the site-dependent Eu³⁺ PL features in CZO and CHO were found to become more conspicuous by the effect of Ga³⁺ or La³⁺ codoping. The results also revealed that the codoped ions work not only to compensate for the charge of aliovalent Eu³⁺ but also to drive Eu³⁺ to the intentional doping sites.

Structural and Optical properties of a new Milarite type Na3 Mg4 LiSi12 O30 :Eu3+ phosphor

A new red-emitting Eu³⁺ (1-17mol%) doped Na₃ Mg₄ LiSi₁₂ O₃₀ (NMLS) phosphor was prepared by conventional solid state reaction method at low temperature. The prepared samples belong to a hexagonal structure with well-matched JCPDS card No.73-0934. The photoluminescence (PL) emission spectrum showed an intense peak at 612nm when excited at 393nm. The variation of electric dipole transition (⁵ D₀ -⁷ F₂ ) emission intensity with increasing Eu³⁺ ion concentration was investigated. The concentration quenching at 11mol% is due to dipole-dipole interaction mechanism in the NMLShost. The optimized NMLS:Eu³⁺ phosphor shows 2.18 times higher luminescence intensity than the (Y,Gd)BO₃ (YGB) commercial phosphor. The NMLS:Eu³⁺ showed an intense red emission having CIE coordinates (0.6280, 0.3691) and color purity of 99.2%. The resulting phosphor exhibits good thermal stability of 80% at 423K. The magnified images of fingerprint showed various minute features such as bridges, sweat pores, ridge end, bifurcation, island and core with better visualization under 393 nm excitation. Furthermore, the optimized NMLS:Eu³⁺ was employed for pc-LED, latent fingerprints and anti-counterfeit applications as a promising red phosphor.

Microstructural origin of peculiar spectra and excellent luminescence properties of Y10Ta4O25:Eu3+ with a fluorite-related structure

Y10Ta4O25:Eu3+ presents peculiar spectra and excellent red luminescence properties due to its special fluorite-related structure. The microstructure was characterized by the site-selective luminescence of Eu3+ activators.

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.

Controllable crystal form transformation and luminescence properties of up-conversion luminescent material K3Sc0.5Lu0.5F6: Er3+, Yb3+ with cryolite structure

In this paper, a novel cryolite-type up-conversion luminescent material K3Sc0.5Lu0.5F6: Er3+, Yb3+ with controllable crystal form was synthesized by a high temperature solid state method. K3Sc0.5Lu0.5F6: Er3+, Yb3+ can crystallize in monoclinic or cubic form at different temperatures. The composition, structure and up-conversion luminescence (UCL) properties of K3Sc0.5Lu0.5F6: Er3+, Yb3+ samples with different crystal form were investigated in detail. It is impressive that both monoclinic and cubic forms of K3Sc0.5Lu0.5F6: Er3+, Yb3+ show green emission (2H11/2/4S3/2→4I15/2). The luminescence intensity of cubic K3Sc0.5Lu0.5F6 is much higher than that of the monoclinic form, and the reasons are also discussed in detail. The results show that the luminescence intensity of up-conversion materials can be effectively tuned by controlling the crystal form. According to the power dependent UCL intensity, the UCL mechanism and electronic transition process were discussed. In addition, the fluorescence decay curves were characterized and the thermal coupling levels (TCLs) of Er3+ (2H11/2/4S3/2 → 4I15/2) in the range of 304-574 k were used to study the optical temperature sensing characteristics. All the results show that K3Sc0.5Lu0.5F6: Er3+, Yb3+ can be used in electronic components and have potential application value in temperature sensing fields.

Luminescence intensity ratio thermometry based on combined ground and excited states absorptions of Tb3+ doped CaWO4

Luminescence intensity ratio (LIR) thermometry is of great interest, because of its wide applications of noninvasive temperature sensing. Here, a LIR thermometry based on combined ground and excited states absorptions is developed using CaWO₄:Tb³⁺. The ratio of single luminescence (⁵D₄-⁷F₅) intensities under 379 and 413 nm excitations with opposite temperature dependences, attributed to the thermal coupling of ground state ⁷F₆ and excited state ⁷F₅, is used to measure temperature. This LIR method achieves a high relative sensitivity of 2.8% K^-1, and can avoid complex spectral splitting by collecting all down-shifting luminescence bands, being a promising accurate luminescence thermometry.

Structural properties and luminescence dynamics of CaZrO3:Eu3+ phosphors

In the present work, the effect of Eu³⁺ doping concentration on the structural and luminescence properties of Eu³⁺ doped CaZrO₃ red phosphors was investigated.

A Sequential Electron Doping for Quadruple Perovskite Oxides ACu3Co4O12 (A = Ca, Y, Ce)

A novel quadruple perovskite oxide CeCu₃Co₄O₁₂ has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the A-site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the ACu₃Co₄O₁₂ series (A = Ca, Y, Ce) from Ca²⁺Cu³⁺₃Co^3.25+₄O₁₂ to Y³⁺Cu³⁺₃Co³⁺₄O₁₂ to Ce⁴⁺Cu^2.67+₃Co³⁺₄O₁₂, where the electrons are doped in the order from B-site (Co^3.25+ → Co³⁺) to A'-site (Cu³⁺ → Cu^2.67+). This electron-doping sequence is in stark contrast to the typical B-site electron doping for simple ABO₃-type perovskite and quadruple perovskites CaCu₃B₄O₁₂ (B = V, Cr, Mn), further differing from the monotonical A'-site electron doping for Na_1-xLa_xMn₃Ti₄O₁₂ and A'- and B-site electron doping for AMn₃V₄O₁₂ (A = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.

Crystal structure, luminescence properties and thermal stability of BaY2−xEuxGe3O10 phosphors with high colour purity for blue-excited pc-LEDs

The luminescence properties of reddish-orange emitting BaY_2−xEu_xGe₃O₁₀ phosphors crystallizing in the monoclinic system (S.G. P2₁/m) have been discussed in detail. The studied germanates are appropriate for applications in high-powered pc-LEDs.

Design, synthesis, crystal structure and luminescent properties of a new tantalum-phosphate K3BiTaP3O13 with negligible concentration quenching and good thermostability

This work reports a new host template, K₃BiTaP₃O₁₃, for both Eu³⁺ and Tb³⁺ luminescence, providing a ‘dimensional obstacle’ to suppress negative concentration quenching with a high density of Eu³⁺ or Tb³⁺.