Crystal Structure and Site Symmetry of Undoped and Eu3+ Doped Ba2 LaSbO6 and BaLaMSbO6 Compounds (M=Mg,Ca): Tuning Europium Site Occupancy to Develop Orange and Red Phosphor

Concentration quenching inhibition and fluorescence enhancement in Eu3+-doped molybdate red phosphors with two-phase mixing

Red phosphor plays a crucial role in improving the quality of white light illumination and backlight displays. However, significant challenges remain to enhance red emission intensity in different matrix materials. Herein, a class of two-phase mixing red phosphors of NaIn1-x(MoO4)2:xEu3+ (NIMO:xEu3+) has been successfully prepared by the traditional high-temperature solid-state reaction method. The coordination environment, phase structure, excitation and emission spectra, fluorescence kinetics, and temperature-dependent luminescence properties of the system have been studied comprehensively. It is worth mentioning that the red emission intensity continues to increase with the increased Eu3+ doping concentration, and the fluorescence lifetimes remain unchanged. These extraordinary phenomena mainly stem from the special concentration quenching mechanism in such two-phase mixing material, namely, the increased lattice interface barriers from Eu six-coordinated units and Eu eight-coordinated units can effectively block the non-radiation by enlarging the average distance between luminescent centers. The improved fluorescence thermal stability and suppressed non-radiative transition rate in NIMO:40%Eu3+ sample are further proving regulatory role of lattice interface barriers. In addition, a warm white light-emitting diode (LED) is successfully fabricated, exhibiting Commission Internationale de l'Eclairage (CIE) coordinates of (0.343, 0.335), a color rendering index (CRI) of 92.1, and a correlated color temperature (CCT) of 5013 K, showing significant application prospects for high-quality lighting devices.

Tailoring defect structure and dopant composition and the generation of various color characteristics in Eu3+ and Tb3+ doped MgF2 phosphors

A novel approach to generate a wide range of color characteristics such as near white, yellow, orange and red in MgF₂, by proper tailoring of the defect structure and varying the composition of Eu³⁺ and Tb³⁺ dopant ions have been presented here. It has been observed from positron annihilation lifetime spectroscopy (PALS) study that various defect centers such as mono vacancies and their cluster forms exist in the system, whose amount varies upon varying the dopant ion's composition. The experimentally observed positron lifetime values of the defect centers also matched well with the theoretically calculated lifetime values using the MIKA-DOPPLER package. It has been found that a few vacancies or defect centers act as color centers, while the cluster vacancies change the local symmetry of the rare earth ion by inducing more distortion surrounding them thereby resulting in different emission characteristics in the photoluminescence (PL) study. The defect-related host emission in combination with the green and red emission from Tb³⁺ and Eu³⁺ ions generated near-white-light in some of the compounds, while other compounds showed a variety of other color characteristics due to the Tb³⁺ → Eu³⁺energy transfer dynamics. The various defect-related emissions, the role of the defect-related trap state in the decay kinetics and the energy-transfer dynamics were also understood by analyzing the electronic structure using HSE06 hybrid functional calculation.

Unraveling U6+, Am3+&Eu3+ ion's distribution in Ca10(PO4)6F2for radioactive waste immobilization and the associated U6+→ Eu3+energy transfer dynamics for tunable emission characteristics

A combined photoluminescence (PL) and theoretical study has been performed on Ca₁₀(PO₄)₆F₂:U⁶⁺ and Ca₁₀(PO₄)₆F₂:U⁶⁺,Eu³⁺ compounds in order to explore Ca₁₀(PO₄)₆F₂ as potential host for radioactive waste immobilization by understanding the distribution U⁶⁺, Eu³⁺ and Am³⁺ ions among the lattice sites and the related radiation stability. DFT based calculations on various structures with different distribution of U⁶⁺, Eu³⁺ and Am³⁺ ions showed that Eu³⁺ and Am³⁺ ions prefer to occupy the Ca2 sites while the highly charged U⁶⁺ ions prefer Ca1 site. This is also supported by the PL lifetime study, which provided two lifetime components with different contribution for both U⁶⁺ and Eu³⁺ ions present at two different lattice sites. The PL study of U⁶⁺ doped compounds confirmed the existence of U in the UO₂²⁺ form, which makes it as a pure green emitter. Upon co-doping Eu³⁺ ion, the compounds were transformed to red emitter. Further, there is an energy transfer process from U⁶⁺to Eu³⁺, which shifted the CIE color coordinates towards pure red region while increasing doping level of U⁶⁺. This proves U⁶⁺ as a good sensitizer for Eu³⁺ ion. PL study on gamma irradiated U⁶⁺ doped Ca₁₀(PO₄)₆F₂ compound also showed excellent radiation stability at Ca2 site.

Rapid annealing: minutes to enhance the green emission of the Tb3+-doped ZnGa2O4 nanophosphor with restricted grain growth

Rapid annealing boosted the green emission of the Tb3+:ZnGa2O4 nanophosphor within minutes.

Unraveling the site-specific energy transfer driven tunable emission characteristics of Eu3+ & Tb3+ co-doped Ca10(PO4)6F2 phosphors

In this study we have explored Ca₁₀(PO₄)₆F₂ as host to develop a variety of phosphor materials with tunable emission and lifetime characteristics based on Eu³⁺ and Tb³⁺ as co-dopant ions and the energy transfer process involved with them. The energy transfer from the excited state of Tb³⁺ ion to the ⁵D₀ state of Eu³⁺ makes it possible to tune the colour characteristics from yellow to orange to red. Further, such energy transfer process is highly dependent on the concentration of Eu³⁺ and Tb³⁺ ions and their site-selective distribution among the two different Ca-sites (CaO₉ and CaO₆F) available. We have carried out DFT based theoretical calculation for both Eu³⁺ and Tb³⁺ ions in order to understand their distribution. It was observed that in cases of co-doped sample, Tb³⁺ ions prefer to occupy the Ca2 site in the CaO₆F network while Eu³⁺ ions prefer Ca1 site in the CaO₉ network. This distribution has significant impact on the lifetime values and the energy transfer process as observed in the experimental photoluminescence lifetime values. We have observed that for the 1^st series of compounds, wherein the concentration Tb³⁺ ions are fixed, the energy transfer from Tb³⁺ ion at Ca2 site to Eu³⁺ ion at Ca1 site is dominating (Tb³⁺@Ca2 → Eu³⁺@Ca1). However, for the 2^nd series of compounds, wherein the concentration Eu³⁺ ions are fixed, the energy transfer process was found to occur from the excited Tb³⁺ ion at Ca1 site to Eu³⁺ ions at both Ca1 and Ca2 (Tb³⁺@Ca1 → Eu³⁺@Ca1 and Tb³⁺@Ca1 → Eu³⁺@Ca2). This is the first reports of its kind on site-specific energy transfer driven colour tunable emission characteristics in Eu³⁺ and Tb³⁺ co-doped Ca₁₀(PO₄)₆F₂ phosphor and it will pave the way for the future development of effective colour tunable phosphor materials based on a single host and same co-dopant ions.

Various site specific energy transfer (ET) process such as Tb³⁺@Ca2 → Eu³⁺@Ca1, Tb³⁺@Ca1 → Eu³⁺@Ca2 and Tb³⁺@Ca1 → Eu³⁺@Ca1 were explored in Eu³⁺ and Tb³⁺ co-doped Ca₁₀(PO₄)₆F₂ phosphor, which are responsible for tunable colour characteristics.

Multifunctional Ca10(PO4)6F2 as a host for radioactive waste immobilization: Am3+/Eu3+ ions distribution, phosphor characteristics and radiation induced changes

Na⁺₂Eu³⁺₂:Ca₆(PO₄)₆F₂ is explored as a potential host for radioactive waste immobilization. Since Eu³⁺ ion is a surrogate of highly radioactive Am³⁺ ion, the photoluminescence (PL) characteristics of Eu³⁺ ion helped to investigate the possible distribution of hazardous and radioactive Am³⁺ ion among the two lattice sites in the matrix. It was observed that Am³⁺ will prefer to occupy the Ca2-site lattice which has a direct linkage to F atom. From DFT calculation we have found that both Eu³⁺ and Am³⁺ ions are following similar trend of distribution into the Ca2-site compared to Ca1-site which has no F atom linkage. The radiation stability of the compound was also investigated by PL study after irradiating it with a ⁶⁰Co gamma source with different doses starting from 2 kGy to as high as 1000 kGy. It was observed that radiation induced changes were more surrounding the Ca1-site than in Ca2-site.Considering all the experimental and theoretical observations it is concluded that from radioactive waste immobilization point of view it is more preferable to dope the Am³⁺ ion into the Ca2 site. The Eu³⁺ doped compound was also found to be red color emitting phosphor materials with color purity of 95.24%.

Exploring color tunable emission characteristics of Eu3+-doped La2(MoO4)3 phosphors in the glass-ceramic form

The glass–ceramic form of phosphor materials can overcome the many serious issues of phosphor/silicone composite in commercial phosphor-converted LEDs and are considered as new-generation color converters. In this report, we have shown a novel approach of developing inorganic red phosphor [Eu³⁺:La₂(MoO₄)₃] in the glass–ceramic form based on lanthanum molybdate system. The ceramic form of the compound was found to have a glass transition temperature of 1002 °C, as confirmed by TGA and DSC studies. Further, XRD, FTIR and Raman studies also confirmed that the compounds prepared at 1050 °C are in glass–ceramic form, while those prepared at 750 °C are in ceramic form. Photoluminescence studies showed that both the ceramic and glass–ceramic forms of the phosphor are red color-emitting materials. However, the glass–ceramic forms have better color purity and more radiation transition probabilities. Further, the decay kinetics of both ceramic and glass–ceramic forms confirmed that only those Eu³⁺ ions which exist in the grain boundaries of the ceramics go inside the glass network structure upon heating the compound at or above the glass transition temperature. On the other hand, Eu³⁺ ions which exist at the La-site in the bulk of the particles are retained in the ceramic form in the glass–ceramic mixture.

The glass–ceramic Eu-LMO-1050 is more suitable as a red-color-emitting phosphor material than the ceramic Eu-LMO-750. Further, Eu-LMO-1050 can overcome the problems related to the phosphor/silicone composite in commercial LEDs.

Engineering defect clusters in distorted NaMgF3 perovskite and their important roles in tuning the emission characteristics of Eu3+ dopant ion

An attempt has been made to explore various new defect clusters in distorted NaMgF3 perovskite and their important role in tuning optical properties. We have tried to tailor the defect clusters and to understand the impact on the luminescence of the lanthanide, for example the Eu3+ ion. Defect engineering has been carried out by doping aliovalent dopant ions to create a charge imbalance in the matrix, which in turn led to the creation of various mono-, di- and new cluster vacancies. Such vacancies have been characterized by Electron Para-magnetic Resonance (EPR), Positron Annihilation Lifetime Spectroscopy (PALS) and Photoluminescence (PL) studies. The PALS data of both undoped and Eu3+ doped compounds confirmed that in addition to Mg mono vacancies, cluster vacancies with different configurations comprising Mg, Na and F atom vacancies also exist in the matrix. The PL study revealed that depending on the surrounding defect structure, three different types of Eu3+ components can be created. The position of the Eu3+ ion with respect to these cluster vacancies determines the respective emission profiles and the decay kinetics. It has been found that when Li+ ions are co-doped with Eu3+, there is a sudden change in the decay kinetics and the emission profiles. The PALS study revealed that Li+ co-doping modified the configuration of the vacancy clusters, which in turn changes the emission characteristics. The EPR study confirmed the presence of different types of F-centers (F, F2, etc.) which are responsible for the host emission. Overall, this new study will be very helpful for a detailed understanding of the defect structures, in particular the cluster vacancies in distorted NaMgF3 perovskite, which have a direct or indirect impact on many physical properties.

Simultaneous tuning of optical and electrical properties in a multifunctional LiNbO3 matrix upon doping with Eu3+ ions

Combined photoluminescence (PL) and dielectric studies have been carried out on both undoped and Eu³⁺ doped LiNbO₃ compounds for their potential application in optical–electrical integration for the first time. Special focus has been given to simultaneously tuning both these physical properties. A PL study reveals that the blank compound is a blue emitting material, while upon doping with Eu³⁺ ions, the emitting color can be tuned from blue to red upon changing the excitation wavelength. Interestingly, the electrical property measurement of this ferroelectric compound showed that upon doping with Eu³⁺ ions, the remnant polarization was increased significantly. Density Functional Theory (DFT) based calculations were carried out to explain both the optical and electrical properties. It has been found that different defect centers are responsible for the bluish host emission while Eu³⁺ ions are energetically preferred to occupy the Nb site and gives rise to red emission. The DFT based results also showed that Eu³⁺ ions induced more distortion into the nearby Nb-site, which is responsible for enhancement of the remnant polarization. Stark-splitting patterns in the PL study also showed that the point symmetry of LiNbO₃ upon Eu³⁺ doping changes from C_6v to D₃, which indicates that the structure becomes less symmetric. Overall, the study presents a novel approach to designing multifunctional materials for optical–electrical integration application and to tuning their physical properties simultaneously in the desired range.

PL and dielectric studies have been carried out on LiNbO₃ and Eu³⁺:LiNbO₃ compounds with a special focus on simultaneous tuning of optical and electrical properties for their potential application in optical–electrical integration.