Trivalent Eu Research Articles

Sorption of trivalent europium Eu(III) by Na+-substituted bentonite: Mechanistic insight and stabilization effect

The sorption of Eu(III) by Na+-substituted bentonite (Na-bentonite) was investigated as a function of pH and NaNO3 concentration ([NaNO3]0). At pH < ∼7.5, Eu(III) sorption decreased with the increasing [NaNO3]0, whereas at pH > ∼7.5, it remained nearly complete, independent of [NaNO3]0. Our thermodynamic model indicated that the sorption at pH < ∼7.5 occurred via a combination of cation exchange and surface complexation, with the former diminishing as [NaNO3]0 increased. Meanwhile, the sorption at pH > ∼7.5 was primarily due to surface complexation. By X-ray diffraction, the incorporation of hydrated Eu(III) in the interlayers of montmorillonite increased its lattice spacing and crystallinity along the c-axis, where cation exchange was predominant. Also, Eu LIII-edge X-ray absorption spectroscopy revealed that the sample dominated by cation exchange had an absorption edge energy and coordination structure similar to aqueous Eu(III), indicating outersphere complex formation. In other cases, Eu(III) sorption was characterized by innersphere complexation, as indicated by increased covalency and the presence of more pronounced second coordination shells. Importantly, the analysis of dissolved Si and Al suggested that the increased stability of Na-bentonite was likely due to the surface complexation of Eu(III) with aluminol groups at the edges, especially at higher surface coverages. Given its high sorption capacity for Eu(III) and stabilization effect mediated by sorption, Na-bentonite could be serve as an effective backfill material and migration barrier for containing actinides in nuclear waste repositories.

Intense red emission from trivalent Eu3+ doped Ca9La(VO4)7 nanophosphor for lighting and latent fingerprinting applications

Intense red emission from trivalent Eu3+ doped Ca9La(VO4)7 nanophosphor for lighting and latent fingerprinting applications

Theoretical and experimental characterization of trivalent lanthanide (La-Eu) aquacomplexes with losartan

In this work, losartan compounds with trivalent ions La(III), Ce(III), Pr(III), Nd(III), Sm(III), and Eu(III) were synthesized in the solid-state. The thermal and spectroscopic study of the compounds was performed using TG-DSC, FTIR, DRX, and evolved gas analysis (EGA) by TG-DSC/FTIR. The theoretical and experimental spectroscopic data suggested the possible modes of coordination of the ligand with the metal ions by the tetrazole ring. Thermogravimetric analyses provide information on the thermal stability, water coordination, and stoichiometry of the compounds. The minimum formulae of the compounds is [Ln(Los)3(H2O)x], where Ln=La and Ce/x = 7; and Pr, Nd, Sm, Eu/x = 6. TG-DSC coupled to FTIR in an air atmosphere suggested the release of H2O, CO2, CO, MeOH, NH3, and -NCO during the thermal decomposition of losartan from lanthanides. Amorphous compounds were obtained, and their poor solubility suggests their potential use as prodrugs in the solid-state.

Complexation of trivalent Eu and Am ions with phenanthroline-derived ligands tuned by rotating thiazole/oxazole substituents

The reprocessing of spent nuclear fuels is greatly challenging, as the current separation technique of lanthanide (Ln) and actinide (An) suffers too many difficulties. In this paper, selective ligands of phenanthroline derivatives modified by thiazole/oxazole were designed by relativistic density functional theory calculations. Their complexation of Eu3+ and Am3+ and the mechanism of selective separation were comprehensively examined. The spatial rotation against the CC bond between phenanthroline and thiazole/oxazole has access to six ligands. It is shown that Looo with oxazole substituents and their oxygens being opposite is the best electron donor for metal ions. The results of WBIs and MBO unravel that Am-L bond has more covalency than respective Eu-L bond. From topological analyses, it is found that these metal–ligand bonds are dominated by electrostatic interaction and largely of ionic bond character. Thermodynamically, all ligands prefer binding Eu3+ to Am3+.

Experimental and Theoretical Insight into the Ionic Liquid-Mediated Complexation of Trivalent Lanthanides with β-Diketone and Its Fluorinated Analogue.

A multitechnique approach with theoretical insights has been employed to understand the complexation of trivalent lanthanides with two β-diketones, viz. 1-phenyl-1,3-butanedione (L1) and 4,4,4-trifluoro-1-phenyl-1,3-butanedione (L2), in an ionic liquid (C6mim·NTf2). UV-vis spectral analysis of complexation using Nd3+ revealed the predominance of ML2+ and ML4- species. The stability constants for the PB complexes were higher (β2 ∼ 10.45 ± 0.05, β4 ∼ 15.51 ± 0.05) than those for the TPB (β2 ∼ 7.56 ± 0.05, β4 ∼ 13.19 ± 0.06). The photoluminescence titration using Eu3+ corroborated the same observations with slightly higher stability constants, probably due to the higher ionic potential of Eu3+. The more asymmetric (AL2ML4 ∼ 5.2) Eu-L2 complex was found to contain one water molecule in the primary coordination sphere of Eu3+ with more covalency of the Eu3+-O bond (Ω2L1 = 8.5 × 10-20, Ω4L1 = 1.3 × 10-20) compared to the less asymmetric Eu-L1 complex (AL1ML4 ∼ 3.5) with two water molecules having less Eu-O covalency (Judd-Offelt parameters: Ω2L1 = 7.3 × 10-20, Ω4L1 = 1.0 × 10-20). Liquid-liquid extraction studies involving Nd3+ and Eu3+ revealed the formation of the ML4- complex following an 'anion exchange' mechanism. The shift of the enol peak from 1176 to 1138 cm-1 on the complexation of the β-diketones with Eu3+ was confirmed from the FTIR spectra. 1H NMR titration of the β-diketones with La(NTf2)3 evidenced the participation of α-H of the β-diketones and protons at C2, C4, and C5 positions of the methylimidazolium ring. For the ML2 complex, 4 donor O atoms are suggested to coordinate to the trivalent lanthanides with bond distances of 2.3297-2.411 Å for La-O, 2.206-2.236 Å for Eu-O, and 2.217-2.268 Å for Nd-O, respectively, while for the ML4 complex, 8 donor O atoms were coordinated with bond lengths of 2.506-2.559 Å for La-O, 2.367-2.447 Å for Eu-O, and 2.408-2.476 Å for Nd-O. The Nd3+ ion was higher by 9.7 kcal·mol-1 than that of the La3+ ion for the 1:4 complex. The complexation energy with L1 was quite higher than that with L2 for both 1:2 and 1:4 complexes. Using cyclic voltammetry, the redox behavior of trivalent lanthanides Eu and Gd with β-diketonate in ionic liquid medium was probed and their redox energetic and kinetic parameters were determined.

Harnessing reaction media as charge compensator to improve red emission from CaWO4:Eu3+ a new strategy

This is probably the first report wherein we have correlated the optical properties of red emitting doped scheelite (CaWO4:Eu3+) synthesized by solid state (SS) reaction and sodium tungstate employed ethylene glycol (EG) method. The presence of sodium in EG sample was confirmed using neutron activation analysis whereas defect formation was probed using positron annihilation lifetime spectroscopy (PALS). The positron lifetimes suggested formation of calcium vacancies in CaWO4:Eu3+synthesized by SS method whereas Na from Na2WO4 in EG method enters the lattice and charge compensates for trivalent Eu3+ doping at divalent Ca2+ sites and plays pivotal role in removal of the calcium vacancies. Due to the removal of Ca vacancies, Eu doped sample prepared by EG method demonstrated higher internal quantum efficiency (IQE) and longer excited state lifetime with lower non-radiative channels and improved branching ratio compared to SS. The comparison with externally added sodium ion in SS samples with EG showed slightly higher IQE in former but at the cost of branching ratio and asymmetry factor. This work paved a new strategic pathway for research community working in the area of phosphor converted light emitting diodes to employ a alkali rich precursor to compensate for aliovalent doping and in lowering the non-radiative channels to improve the luminescence output of the phosphor.

Epitaxial thin films of binary Eu-compounds close to a valence transition

Intermetallic binary compounds of europium reveal a variety of interesting phenomena due to the interconnection between two different magnetic and 4f electronic (valence) states, which are particularly close in energy. The valence states, or magnetic properties are thus particularly sensitive to strain tuning in these materials. Consequently, we grew epitaxial EuPd2 (Eu2+, high magnetic moment) and EuPd3 (Eu3+, vanishing magnetic moment) thin films on MgO(001) substrates using molecular beam epitaxy. Ambient X-ray diffraction confirms an epitaxial relationship of cubic Laves-type (C15) EuPd2 with an [111]-out-of-plane orientation, whereby eight distinct in-plane crystallographic domains develop. For simple cubic EuPd3 two different out-of-plane orientations can be obtained by changing the substrate annealing temperature under ultra-high vacuum conditions from 873K to 1273K for one hour. A small resistance minimum evolves for EuPd3 thin films grown with low temperature substrate annealing, which was previously found even in single crystals of EuPd3 and might be attributed to a Kondo or weak localization effect. Absence of influence of applied magnetic fields and magnetotransport measurements suggest a purely trivalent Eu valence for EuPd3 thin films, as found in EuPd3 single crystals. For EuPd2 magnetic ordering below ∼72K is observed, quite similar to single crystal behavior. Field dependent measurements of the magnetoresistance and the Hall effect show hysteresis effects below ∼0.4T and an anomalous Hall effect below ∼70K, which saturates around 1.4T, thus proving a ferromagnetic ground state of the divalent Eu.

Europium(II/III) coordination chemistry toward applications.

Europium is an f-block metal with two easily accessible oxidation states (+2 and +3) that have vastly different magnetic and optical properties from each other. These properties are tunable using coordination chemistry and are useful in a variety of applications, including magnetic resonance imaging, luminescence, and catalysis. This review describes important aspects of coordination chemistry of Eu from the Allen Research Group and others, how ligand design has tuned the properties of Eu ions, and how those properties are relevant to specific applications. The review begins with an introduction to the coordination chemistry of divalent and trivalent Eu followed by examples of how the coordination chemistry of Eu has made contributions to magnetic resonance imaging, luminescence, catalysis, and separations. The article concludes with a brief outlook on future opportunities in the field.

Deferasirox Derivatives: Ligands for the Lanthanide Series.

Deferasirox is an FDA-approved iron chelator used in the treatment of iron toxicity. In this work, we report the use of several deferasirox derivatives as lanthanide chelators. Solid-state structural studies of three representative trivalent lanthanide cations, La(III), Eu(III), and Lu(III), revealed the formation of 2:2 complexes in the solid state. A 1:1 stoichiometry dominates in DMSO solution, with Ka values of 472 ± 14, 477 ± 11, and 496 ± 15 M-1 being obtained in the case of these three cations, respectively. Under the conditions of competitive precipitation in the presence of triethylamine, high selectivity (up to 80%) for lutetium(III) was observed in competition with La(III), Ce(III), and Eu(III). Theoretical calculations provided support for the observed selective crystallization.

Stagnant pore-blocked supports for improved extraction chromatography

Prior work has demonstrated that the complex pore structure of the polymeric supports employed for the preparation of commercial extraction chromatographic resins is a major contributor to their poor column efficiency and susceptibility to pronounced peak tailing. Particularly problematic is the presence of a number of small, relatively inaccessible (“stagnant”) pores. By blocking these pores with an appropriate inert filler, the extractant can be confined to the more readily accessible intermediate and large pores. The result is improved column efficiency (i.e., up to a 40% increase in N), reduced peak tailing and asymmetry (i.e., up to a 34% reduction in As), and in some instances, higher metal ion uptake capacity (up to a 64% increase) versus a commercial resin. The utility of the improved chromatographic performance arising from this “stagnant pore blocking” is illustrated in the separation of selected trivalent lanthanide ions, specifically Nd(III) and Eu(III), whose resolution has been reported to be unsatisfactory using conventional extraction chromatographic materials.

Accurate Evaluation of Dispersion Energies at Coupled Cluster Level to Understand the Substituent Effects in Am(III) and Eu(III) Complexes.

The effect of cyclic and aromatic substituents on the complexation behavior of phosphine oxide ligands with Am(III) and Eu(III) was investigated at density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster (DLPNO-CC) levels. Combining DFT with accurate coupled cluster methods, we have evaluated the dispersion energy contributions to the complexation energies for trivalent Am and Eu complexes for the first time. Irrespective of the nature of substituents on the P atom, the electronic structure of the P═O group remains identical in all of the ligands. The study reveals the importance of dispersion interactions during complexation and is estimated to be more significant for Am(III) than for Eu(III) complexes.

Tailoring oxygen vacancies in ThO2 for improved light emission and ORR electrocatalysis

The role of defects in multifunctional materials and defect engineering are fascinating to scientific community. To understand the role of oxygen vacancies in photoluminescence (PL) and oxygen reduction reaction (ORR), thoria samples were prepared with Eu3+ doping and Nb5+ codoping. The positron annihilation lifetime spectroscpy (PALS) showed the increase in the oxygen vacancies in thoria upon trivalent Eu3+ doping and their removal upon codoping with Nb5+. This is in concordance with increase in nearest oxygen neighbours to Eu3+ and Th4+ upon Nb5+ codoping as revealed by Extended X-ray Absorption Fine Structure (EXAFS). Oxygen vacancy removal on Nb5+ codoping manifested in restoration of symmetry around Eu3+ and enhanced photoluminescence (PL) intensities giving very bright red emission while the oxygen vacancy rich thoria samples aided the electrochemical ORR. The studies showed that the concentration of the oxygen vacancies can be tuned upon the judicious choice and concentration of the aliovalent dopant and codopant depending on the envisaged functionality of the material.

Boosting up color tunability of Sr2Ga2GeO7 by energy transfer between Dy3+ and Eu3+ ions

A series of Sr2Ga2GeO7 (SGGO) phosphors doped with Dy3+ and/or Eu3+ were synthesized using solid-phase reaction. The structure and luminescence properties of phosphors were studied. The XRD, FT-IR, SEM and XPS analysis show that the doping with trivalent Dy and Eu have no effect on the structure of the phosphor, and the feature emission of Dy3+ or Eu3+ was observed by photoluminescence investigation. Based on the spectral overlapping of the Dy3+ emission and Eu3+ excitation, there is an energy transfer from Dy3+ to Eu3+ in the Sr2Ga2GeO7. Under excitation at 344 nm, with the increment of Eu3+ concentration, the Dy3+ emission intensity was decreased and the emission lifetime was shortened, whereas the lifetime of Eu3+ was increased, confirming the energy transfer from Dy3+ to Eu3+. The resulting tunable orange emission has potential applications in plant growth.

Pressure evolution of electronic and crystal structure of noncentrosymmetric EuCoGe3

We report on the pressure evolution of the electronic and crystal structures of the noncentrosymmetric antiferromagnet EuCoGe3. Using a diamond anvil cell, we performed high pressure fluorescence detected near-edge x-ray absorption spectroscopy at the Eu L3, Co K, and Ge K edges and synchrotron powder x-ray diffraction. In the Eu L3 spectrum, both divalent and trivalent Eu peaks are observed from the lowest pressure measurement (~2 GPa). By increasing pressure, the relative intensity of the trivalent Eu peak increases, and an average Eu valence continuously increases from 2.2 at 2 GPa to 2.31 at~50 GPa. On the other hand, no discernible changes are observed in the Co K and Ge K spectra as a function of pressure. With the increase in pressure, lattice parameters continuously decrease without changing I4mm symmetry. Our study revealed a robust divalent Eu state and an unchanged crystal symmetry of EuCoGe3 against pressure.

Trivalent europium – a scarce case in intermetallics

Abstract In most intermetallic europium compounds, the Eu atoms exhibit a divalent oxidation state with a high effective magnetic moment since Eu2+ is isoelectronic with Gd3+. Trivalent intermetallic Eu compounds, in contrast, are extremely scarce and under 20 examples are known to literature. This mini-review summarizes the known binary and ternary examples along with their crystal-chemical peculiarities as well as their magnetic and 151Eu Mössbauer spectroscopic behavior. Additionally, compounds that exhibit valence phase transitions are summarized.

Direct Separation of UO2 2+ by Coordination Sieve Effect via Spherical Coordination Traps.

Molecule sieve effect (MSE) can enable direct separation of target, thus overcoming two major scientific and industrial separation problems in traditional separation, coadsorption, and desorption. Inspired by this, herein, the concept of coordination sieve effect (CSE) for direct separation of UO2 2+ , different from the previously established two-step separation method, adsorption plus desorption is reported. The used adsorbent, polyhedron-based hydrogen-bond framework (P-HOF-1), made from a metal-organic framework (MOF) precursor through a two-step postmodification approach, afforded high uptake capacity (close to theoretical value) towards monovalent Cs+ , divalent Sr2+ , trivalent Eu3+ , and tetravalent Th4+ ions, but completely excluded UO2 2+ ion, suggesting excellent CSE. Direct separation of UO2 2+ can be achieved from a mixed solution containing Cs+ , Sr2+ , Eu3+ , Th4+ , and UO2 2+ ions, giving >99.9% removal efficiency for Cs+ , Sr2+ , Eu3+ , and Th4+ ions, but <1.2% removal efficiency for UO2 2+ , affording benchmark reverse selectivity (SM/U ) of >83 and direct generation of high purity UO2 2+ (>99.9%). The mechanism for such direct separation via CSE, as unveiled by both single crystal X-ray diffraction and density-functional theory (DFT) calculation, is due to the spherical coordination trap in P-HOF-1 that can exactly accommodate the spherical coordination ions of Cs+ , Sr2+ , Eu3+ , and Th4+ , but excludes the planar coordination UO2 2+ ion.

Microscopic analysis of the valence transition in tetragonal EuPd2Si2

Under temperature or pressure tuning, tetragonal EuPd$_2$Si$_2$ is known to undergo a valence transition from nearly divalent to nearly trivalent Eu accompanied by a volume reduction. Albeit intensive work, its microscopic origin is still being discussed. Here, we investigate the mechanism of the valence transition under volume compression by $ab~initio$ density functional theory (DFT) calculations. Our analysis of the electronic and magnetic properties of EuPd$_2$Si$_2$ when approaching the valence transition shows an enhanced $c$-$f$ hybridization between localized Eu 4$f$ states and itinerant conduction states (Eu 5$d$, Pd 4$d$, and Si 3$p$) where an electronic charge redistribution takes place. We observe that the change in the electronic structure is intimately related to the volume reduction where Eu-Pd(Si) bond lengths shorten and, for the transition to happen, we trace the delicate balance between electronic bandwidth, crystal field splitting, Coulomb repulsion, Hund's coupling and spin-orbit coupling. In a next step we compare and benchmark our DFT results to surface-sensitive photoemission data in which the mixed-valent properties of EuPd$_2$Si$_2$ are reflected in a simultaneous observation of divalent and trivalent signals from the Eu $4f$ shell. The study serves as well to explore the limits of density functional theory and the choice of exchange correlation functionals to describe such a phenomenon as a valence transition.

First-principles method justifying the Dieke diagram and beyond

We present a method to determine the model Hamiltonians to treat rare-earth multiplets in solids from the results of the quasiparticle self-consistent GW (QSGW) method. We apply the method to trivalent Eu compounds ${\mathrm{EuCl}}_{3}$, EuN, and Eu-doped GaN after examining free rare-earth ions. We solve the model Hamiltonian by the exact diagonalization. Our results justify applying the Dieke diagram to ions in solid, while its limitation is clarified. In particular, we show that the crystal fields cause sizable breaking of the Russell-Saunders coupling.

High performance solution-processed red phosphorescent organic light-emitting diodes by co-doping europium complex as sensitizer

In this work, efficient red phosphorescent OLEDs (PHOLEDs) were designed and fabricated by solution processing method. Based on well optimized doping concentrations of emitter and ratio of mixed host materials, Eu(DBM) 3 Phen (DBM = 1,3-diphenylpropane-1,3-dion, Phen = 1,10-phenonthroline) was doped into the light-emitting layer (EML) which plays the role of sensitizer. Experimental results indicated that the incorporation of Eu(DBM) 3 Phen molecules can effectively trap the superfluous electrons within EML, which is helpful for balancing charges’ distribution and widening the recombination zone. Besides, the presence of Eu(DBM) 3 Phen molecules can accelerate the transfer of triplet energy from hosts to red emitters, therefore remarkably enhances device performance. By optimizing the doping concentrations of emitter and sensitizer, ratio of co-hosts and the functional layer thickness, recombination center was accurately adjusted to further broaden recombination zone, therefore causing decreased excitons density and suppressed excitons quenching. Eventually, the optimal solution-processed red PHOLED obtained the maximum current efficiency (47.12 cd/A) and external quantum efficiency (27.3%). High performance solution-processed red phosphorescent organic light-emitting diodes by incorporating trivalent rare earth europium complex Eu(DBM) 3 Phen (DBM = 1,3-diphenylpropane-1,3-dion, Phen = 1,10-phenonthroline) into light-emitting layer as sensitizer. • High performance solution-processed red phosphorescent OLED was designed by utilizing trivalent rare earth europium complex Eu(DBM) 3 Phen into light-emitting layer as sensitizer. • Co-doped sensitizer molecules function as electron trappers, which is helpful in balancing carriers' distribution and broadening recombination zone. • The optimal solution-processed red PHOLED obtained the maximum current efficiency up to 47.12 cd/A and external quantum efficiency up to 27.3%. • This work firstly demonstrated the efficacy of utilizing rare earth complex as sensitizer in realizing high performance solution-processed OLEDs.

Pressure-induced valence fluctuation in CsEuF3: From divalent Eu valence to trivalent Eu valence state

The ternary rare-earth metal fluoride CsEuF3 adopts an ideal cubic perovskite structure [ABX3] under ambient conditions. The B-cation site is occupied by the rare-earth Eu ion and EuF6 octahedra are formed. In the present study, magnetic susceptibility and synchrotron X-ray absorption spectroscopy (XAS) analysis at the Eu-L3 edge confirmed that Eu is in a divalent oxidation state under ambient conditions. Temperature-dependent magnetic susceptibility data revealed that the average Eu valence increased below 20 K due to a partial transition from the Eu2+ state to the Eu3+ state, thereby resulting in a mixed valence state with an average valence of +2.23. Direct evidence for valence fluctuation by the Eu ions in CsEuF3 was obtained using the high pressure high energy resolution fluorescence detection-XAS technique, where continuous changes in valence were observed from 2.15+ at ambient pressure up to 2.5+ at 10.5 GPa. These findings indicate the possibility of discovering interesting physical properties associated with valence instabilities by rare-earth metals in similar systems.