Probing site-selective doping and charge compensating defects in KMgF3: insights from a hybrid DFT study.

Design of optoelectronic materials with tunable properties using activators and defect clusters has become one of the prime interests of current research. In this study, detailed Density Functional Theory based calculations have been presented to investigate the geometries and electronic structures of various possible defect clusters using Eu-KMgF3 as a probe which has numerous technological and industrial applications. Using a more reliable hybrid density functional, we have calculated defect formation energies and thermodynamic transition levels to get knowledge about the site selectivity of Eu. It has been observed that the electronic structure of Eu-KMgF3 is not only dependent on the site of doping but also on the oxidation state of Eu (2+/3+). The present study also investigates the relative stability of different kinds of defects and defect clusters under various synthetic growth conditions. The ultimate aim is to find out the microscopic origin of the fundamental optical properties of Eu-KMgF3 and provide an unambiguous explanation of available experimental results. Thus, it has been revealed that doping with Eu results in the spontaneous formation of intrinsic defects, which contribute to the observed optical behaviour. We have also extended our study to investigate the role of codoping with Li in determining the geometry and electronic structure of Eu-KMgF3 aiming to explain its impact on the optical properties. Thus, a complete presentation of the influence of the activator in the absence and presence of lattice defects on the optical properties of KMgF3 has been accomplished in the current study. We strongly believe that the present study will be helpful in designing tunable phosphor materials by a defect-controlled synthesis strategy.

Harvesting Light from BaHfO3/Eu3+ through Ultraviolet, X-ray, and Heat Stimulation: An Optically Multifunctional Perovskite.

Materials with optical multifunctionality such as photoluminescence (PL), radioluminescence, and thermoluminescence (TL) are a boon for a sustainable society. BaHfO3 (barium hafnium oxide [BHO]) under UV irradiation demonstrated visible PL endowed by oxygen vacancies (OVs). Eu3+ doping in BHO (BHOE) introduces f-state impurity levels just below the conduction band for both Eu@Ba and Eu@Hf sites, causing efficient host-to-dopant energy transfer, generating intense 5D0 → 7F1 magnetic dipole transitions (MDT) with internal quantum yield of ∼70%. X-ray photoelectron spectroscopy and electron paramagnetic resonance showed the formation of OVs in both BHO and BHOE samples with more vacancies in the doped sample. The positron lifetime measurements suggested that Eu3+ ions are distributed at both Ba2+ and Hf4+ sites. The association of OVs with Hf4+ and Eu3+ ions due to high charge/radius ratio is considered to be responsible for lowering the symmetry around Eu3+ ions to C 4v in BHOE. Density functional theory studies of defect formation energy justified the same. Time-resolved emission spectroscopy showed distinct spectra for Eu@Ba and Eu@Hf sites corresponding to symmetric and asymmetric environments, respectively. This could be highly relevant in designing color tunable phosphor by forcing dopant ions at one specific site because Eu@Ba displayed orange emission whereas Eu@Hf displayed red emission. We could further harness BHOE for X-ray scintillator application by designing a thin film, which showed efficient conversion of high-energy X-ray into visible light. Under beta irradiation; both BHO and BHOE showed distinct TL glow curves as shallow traps were formed in the former and deep traps in the latter, which could have long-term implications in harnessing this material for persistent luminescence. We believe that BHO/BHOE demonstrated an extraordinary credential as a perovskite for multifunctional applications in the area of defect-induced light emission, UV phosphor, X-ray scintillator, and TL crystals.

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.

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.

Exploring the role of vacancy defects in the optical properties of LiMgPO4.

Linearity in dose response up to very high radiation doses and sufficient sensitivity to even low radiation doses are extremely important for the measurement of radiation dose in the field of radiation technology, ranging from medical to industrial applications. Olivine type LiMgPO4 has been shown immense interest as a phosphor material in the fields of thermoluminescence and optically stimulated luminescence dosimetry. In the present study, we have explored the role of different vacancy defects in the optical properties of LiMgPO4 aiming at enhancing its sensitivity for the measurement of radiation dose. For this purpose, we have systematically investigated the electronic structure of LiMgPO4 in the absence and presence of various vacancy defects using density functional theory as a tool. The present study considers all possible vacancy defects including neutral, charged and mixed lattice vacancy defects in LiMgPO4. To find the most energetically favourable vacancy defect, we have compared the defect formation energy of all the vacancy defects. We have also calculated vacancy formation energy in different chemical environments to investigate how the formation of different types of vacancy defect can be controlled by tuning the chemical environment. Finally, the origin of the different optical properties of LiMgPO4 has been explained by using a possible mechanism based on our detailed electronic structure calculations. Thus, the present study is believed to provide valuable insight for the development of materials with improved features for the measurement of radiation dose.