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.

Exploring Defect-Induced Emission in ZnAl2O4: An Exceptional Color-Tunable Phosphor Material with Diverse Lifetimes.

Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol-gel combustion route, which can be used both as a blue-emitting phosphor material and a white-emitting phosphor material, depending on the annealing temperature during synthesis. The material also has the potential to be used in optical thermometry. These fascinating color-tunable emission characteristics can be linked with the various defect centers present inside the matrix and their changes upon thermal annealing. Various defect centers, such as anionic vacancy, cationic vacancy, antisite defect, etc., create different electronic states inside the band gap, which are responsible for the multicolor emission. The color components are isolated from the complex emission spectra using time-resolved emission spectroscopy (TRES) study. Interestingly, the lifetime values of the various defect centers were found to change significantly from milliseconds to microseconds upon thermal annealing, which makes the phosphors more diverse (i.e., either long-persistent blue-emitting phosphors or short-persistent white-emitting phosphors). Fourier transform infrared (FTIR) and diffuse reflectance spectroscopy (DRS) confirmed the presence of antisite defect centers such as AlZn+ or ZnAl- in the matrix. X-ray absorption fine structure (EXAFS) study showed that the spinel structure was more disordered in nature for low-temperature-annealed compounds. Electron paramagnetic resonance (EPR) and positron annihilation lifetime spectroscopy (PALS) studies were also carried out in order to characterize various anionic and cationic vacancies and their clusters present in the compounds. Antisite defect centers such as AlZn+ or ZnAl-, which act as an electron or hole trap, were found to be responsible for the diverse lifetime behavior. To gain insight about the electronic states inside the band gap, density functional theory (DFT)-based calculations were performed for both pure and various vacancy-introduced spinel structures. Finally, based on the theoretical and experimental results, for the first time, a detailed investigation of various defect-induced emission behavior in ZA is presented, which also explains the mechanism of color tunability and dynamic lifetimes.

Defect induced ferromagnetism in MgO and its exceptional enhancement upon thermal annealing: a case of transformation of various defect states.

MgO particles of few micron size are synthesized through a sol-gel method at different annealing temperatures such as 600 °C (MgO-600), 800 °C (MgO-800) and 1000 °C (MgO-1000). EDX and ICP-AES studies confirmed a near total purity of the sample with respect to paramagnetic metal ion impurities. Magnetic measurements showed a low temperature weak ferromagnetic ordering with a TC (Curie temperature) around 65 K (±5 K). Unexpectedly, the saturation magnetization (Ms) was found to be increased with increasing annealing temperature during synthesis. It was observed that with J = 1 or 3/2 or S = 1 or 3/2, the experimental points are fitted well with the Brillouin function of weak ferromagnetic ordering. A positron annihilation lifetime measurement study indicated the presence of a divacancy (2VMg + 2VO) cluster in the case of the low temperature annealed compound, which underwent dissociations into isolated monovacancies of Mg and O at higher annealing temperatures. An EPR study showed that both singly charged Mg vacancies and oxygen vacancies are responsible for ferromagnetic ordering. It also showed that at lower annealing temperatures the contribution from was very low while at higher annealing temperatures, it increased significantly. A PL study showed that most of the F+ centers were present in their dimer form, i.e. as centers. DFT calculation implied that this dimer form has a higher magnetic moment than the monomer. After a careful consideration of all these observations, which have been reported for the first time, this thermally tunable unusual magnetism phenomenon was attributed to a transformation mechanism of one kind of cluster vacancy to another.

Luminescence Properties of SrZrO3/Tb(3+) Perovskite: Host-Dopant Energy-Transfer Dynamics and Local Structure of Tb(3+).

SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematically using X-ray diffraction and time-resolved photoluminescence techniques. A detailed analysis of the optical properties of Tb(3+) ions in SrZrO3 was performed to correlate them with the local environment of the lanthanide ions in this perovskite. Photoluminescence (PL) spectroscopy showed that emission spectrum consists of host as well as Tb(3+) emission indicating the absence of complete host-dopant energy transfer. On the basis of emission spectrum and PL decay study it was also observed that Tb(3+) is not homogeneously distributed in SrZrO3 perovskite; rather, it is occupying two different sites. It is corroborated using extended X-ray absorption fine structure studies that Tb(3+) is stabilized on both six-coordinated Zr(4+) and eight-coordinated Sr(2) site. The energies calculated using density functional theory (DFT) indicates that Tb occupation in Sr site is energetically more favorable than Zr site. The analysis of valence charge distribution also substantiated our structural stability analysis of site-selective Tb doping in SrZrO3. Time-resolved emission spectroscopy is employed to elucidate the difference in the spectral feature of Tb(3+) ion at Sr(2+) and Zr(4+) site. DFT-calculated density of states analysis showed that energy mismatch of Tb-d states with Zr-d and O-p states of SZP makes the energy transfer from host SZP to Tb(3+) ion difficult.

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.

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.

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%.

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 1st 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 2nd 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.

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 D 3, 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.

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.

Double perovskite antimonates of the type BaLaMSbO6 (M=Mg, Ca) were synthesized by a standard solid-state route. The compounds were characterized by X-ray crystallography and the structures were refined using Rietveld method. BaLaMgSbO6 and BaLaCaSbO6 crystallized in monoclinic space groups (I2/m) and (P21 /n), respectively. In both compounds, La occupied the A-site of perovskite, which is 12-coordinated as compared to Ba2 LaSbO6 where La ion shifts to the B-site octahedral coordination due to the larger size of Ba as compared with Mg and Ca. The samples were further characterized using FTIR and the frequency of the octahedral vibration is correlated to the electronegativity of the B-site ions. Photoluminescence study of the title compounds and Ba2 LaSbO6 was carried out upon doping with 2 atom% Eu3+ ion, which confirmed that Eu3+ occupies distorted 12-coordinated A-site in BaLaMSbO6 (M=Mg, Ca) and symmetrical octahedral B-site in Ba2 LaSbO6 . Furthermore, the emission spectrum corresponding to each Eu3+ ion at different crystal site was successfully isolated through a TRES study. This site selective occupancy of Eu3+ ion also has a direct impact on the light emission, which was found to change from orange to red in a dark room in the order Ba2 LaSbO6 : Eu→BaLaCaSbO6 : Eu→BaLaMgSbO6 : Eu. Such an outcome will have high impact in designing new commercial Eu3+ ion doped phosphor materials and tailoring of their optical properties.

Investigations on local structures in new Bi2-2xLa2xUO6 (x = 0-0.05) solid solutions: a combined XRD, EXAFS, PL and EPR study.

La doped Bi2UO6 solid solutions of the general formula Bi2-2xLa2xUO6 (x = 0-0.05) were prepared by the solid state reaction of Bi2O3, La(OH)3 and U3O8 in a stoichiometric ratio. These solid solutions were characterized by X-ray diffraction, Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Spectroscopy (XANES) studies. It was found that La goes to the Bi sites of the Bi2UO6 lattice during the formation of these solid solutions. For further confirmation, a photoluminescence (PL) study on an iso-structural Bi1.96Eu0.04UO6 was performed, and this supports the above observation. The solid solutions were also probed by EPR studies. The PL and EPR studies suggest that there are doping induced oxygen vacancies in these solid solutions.

Photoluminescence and EPR studies on Fe³âº doped ZnAl2O4: an evidence for local site swapping of Fe³âº and formation of inverse and normal phase.

Considering that ZnAl2O4 spinel has two different sites (octahedral and tetrahedral) and its properties change with dopant ion distribution among these two sites; ZnAl2O4 doped with varied concentrations of Fe(3+) was synthesized by a low temperature sol-gel combustion method. Phase purity and structural investigations were carried out using Rietveld refined X-ray diffraction which shows a decrease in the value of cell parameters at higher doping levels. Photoluminescence (PL) and electron paramagnetic resonance (EPR) studies have shown that on doping, Fe(3+) ions were distributed in both tetrahedral and octahedral sites. At octahedral sites, Fe(3+) exhibited a broad red emission around 745 nm while at tetrahedral sites it exhibited well-defined vibronic sidebands at 665, 674, 684 and 693 nm along with a broad blue band with a maxima at 445 nm at room temperature. EPR studies have shown a broad spectrum at g ≈ 2.2 which corresponds to the Fe(3+) in octahedral sites, while the broad signal at g ≈ 4.2 belongs to Fe(3+) in tetrahedral sites. It was also inferred from these studies that Fe(3+) prefers to occupy octahedral sites at higher concentrations and at higher annealing temperatures. The PL decay behavior of Fe(3+) in ZnAl2O4 has also shown that two different types of Fe(3+) ions were present in this matrix. The first type was a long lived species (τ ≈ 170 µs) present at octahedral sites and the other was a short lived species (τ ≈ 40 µs) present at the tetrahedral sites; the fraction of the long lived species predominate at higher concentrations. Thus the present work is mainly focused on understanding the tuning of local site occupancy of the dopant ion among those sites with varying concentration and annealing temperature, using the dopant ion itself as a spectroscopic probe, which further helps in understanding the phase (inverse and normal) of the spinel.