Tailoring the optical properties of lanthanide phosphors: prediction and characterization of the luminescence of Pr(3+)-doped LiYF4

Influence of symmetry on the magneto-optical properties of a bifunctional macrocyclic DyIII complex

In this work, a novel complex, [Dy(LPr)(NO3)2]·(H2O)·(NO3) (1), containing a highly distorted macrocyclic ligand (LPr) and weak axial anions (NO3-), was synthesized and characterized. Even though this coordination environment is not ideal for maximizing the magnetic anisotropy of a DyIII ion, a magneto-structural analysis reveals that the high distortion of the macrocycle promotes a disposition of the hard plane and easy axis opposite to the expected one. This results in a quite symmetrical environment which allows obtaining a field induced SMM behaviour. The magnetic relaxation properties of this complex were rationalized with the aid of ab initio multireference calculations. Moreover, 1 showed the characteristic emission bands of DyIII ion, indicating that the macrocyclic ligand acts as an efficient sensitizer in the energy transfer process to the emissive state of the DyIII ion. Due to the symmetric environment of 1, the Y/B intensity ratio (0.61) results in CIE coordinates (0.278; 0.314), close to those of the white light region. To gain further insight into the mechanism leading to the luminescence properties, ab initio calculations were performed to elucidate the key factors controlling the Y/B intensity ratio in this bifunctional complex.

Investigation of thermoluminescence and photoluminescence properties of Tb3+, Eu3+, and Dy3+ doped NaYF4 phosphors for dosimetric applications

Hexagonal phase sodium yttrium fluoride activated with lanthanide ions; Tb³⁺, Eu³⁺ and Dy³⁺ doped NaYF₄ phosphors were synthesized using a simplistic hydrothermal method. The photoluminescence studies demonstrated green, red and blue emission lines corresponding to ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3), ⁵D₀ → ⁷F_J (J = 1, 2 and 4) and ⁴F_9/2 to ⁶H_J (J = 15/2 and 13/2) transitions, which are characteristic of Tb³⁺, Eu³⁺ and Dy³⁺ ions, respectively. The as-synthesized samples were subjected to annealing at varying temperatures from 500 °C to 800 °C primarily for the optimization of the thermoluminescence glow curve. Meanwhile, we studied the influence of thermal annealing treatment on the crystal phase, morphological features, and photoluminescence properties of the phosphors. The spherical-like morphology of NaYF₄:Tb³⁺ phosphor changed to micro block-like structures and the hexagonal phase of NaYF₄ transforms into a cubic phase at a higher annealing temperature of ∼800 °C. The photoluminescence emission intensity also varied at different annealing temperatures. The systematic study using different dopants and annealing temperatures was carried out to accomplish efficient thermoluminescence (TL) properties. The most suitable TL dosimetric glow peak was attained for NaYF₄:0.5%Tb³⁺ phosphor, which was annealed at a temperature of 800 °C, located at 194 °C. The NaYF₄:Tb³⁺ phosphor exhibited TL response fairly linear in the dose range from 1 kGy to 25 kGy of gamma radiation. The phosphor showed TL response at higher doses, which stipulates that the NaYF₄:Tb³⁺ is reasonably well suitable for high dose measurements and respective applications. The phosphor exhibited negligible fading and good reproducibility features. Trap-level analysis and experimental determination of activation energy were performed using the T_m-T_stop technique and initial rise method (IRM). The trapping parameters of the TL glow curve, such as activation energy (E), order of kinetics (b), and frequency factor (s) were estimated by Chen's glow peak shape (PS) method and glow curve deconvolution (GCD) method. The trapping parameters obtained using the IRM, PS and GCD methods are in good accordance with each other. Henceforth, along with efficient photoluminescence properties, the NaYF₄:Tb³⁺ phosphor exhibited favorable thermoluminescence dosimetric properties. Consequently, this study provides new opportunities for utilizing these phosphors in the area of radiation dosimetry applications such as environmental and food monitoring, space dosimetry etc.

Probing a variation of the inverse-trans-influence in americium and lanthanide tribromide tris(tricyclohexylphosphine oxide) complexes

The synthesis, characterization, and theoretical analysis of meridional americium tribromide tris(tricyclohexylphosphine oxide), mer-AmBr3(OPcy3)3, has been achieved and is compared with its early lanthanide (La to Nd) analogs. The data show that homo trans ligands display significantly shorter bonds than the cis or hetero trans ligands. This is particularly pronounced in the americium compound. DFT along with multiconfigurational CASSCF calculations show that the contraction of the bonds relates qualitatively with overall covalency, i.e. americium shows the most covalent interactions compared to lanthanides. However, the involvement of the 5p and 6p shells in bonding follows a different order, namely cerium > neodymium ∼ americium. This study provides further insight into the mechanisms by which ITI operates in low-valent f-block complexes.

Study of electronic structure in the L-edge spectroscopy of actinide materials: UO2 as an example

While the electronic structure calculation for actinide materials, using ligand-field phenomenology in conjunction with density functional theory (LFDFT) treating configurations with single or two open-shells 5f and 6d electrons, is well established and currently practiced, the consideration of the three open-shells electron configurations for LFDFT treatment is a challenging task addressed in the present work. Herein, we report the first-principles method, developed for the first time on the basis of LFDFT, to evaluate the uranium L₃-edge X-ray absorption near-edge structure (XANES), which requires non-equivalent active electrons within the 2p, 5f and 6d orbitals of the uranium ion. The theoretical results, when compared with the experimental XANES data measured from uranium dioxide fresh fuel pellets and rector-exposed spent fuel materials, show good agreement with the experimental findings elucidating the local oxidation in the spent fuel materials. This report is relevant for the commonly used L-edge spectroscopy of actinide isotopes and important for understanding the structural, optical and electronic properties of actinide-based materials.

On The Density Functional Theory Treatment of Lanthanide Coordination Compounds: A Comparative Study in a Series of Cu-Ln (Ln = Gd, Tb, Lu) Binuclear Complexes

The nontrivial aspects of electron structure in lanthanide complexes, considering ligand field (LF) and exchange coupling effects, have been investigated by means of density functional theory (DFT) calculations, taking as a prototypic case study a series of binuclear complexes [LCu(O₂COMe)Ln(thd)₂], where L^2- = N,N'-2,2-dimethyl-propylene-di(3-methoxy-salicylidene-iminato) and Ln = Tb, Lu, and Gd. Particular attention has been devoted to the Cu-Tb complex, which shows a quasi-degenerate nonrelativistic ground state. Challenging the limits of density functional theory (DFT), we devised a practical route to obtain different convergent solutions, permuting the starting guess orbitals in a manner resembling the run of the β electron formally originating from the f⁸ configuration of the Tb(III) over seven molecular orbitals (MOs) with predominant f-type character. Although the obtained states cannot be claimed as the DFT computed split of the ⁷F multiplet, the results are yet interesting numeric experiments, relevant for the ligand field effects. We also performed broken symmetry (BS) DFT estimation of exchange coupling in the Cu-Gd system, using different settings, with Gaussian-type and plane-wave bases, finding a good match with the coupling parameter from experimental data. We also caught BS-type states for each of the mentioned series of different states emulated for the Cu-Tb complex, finding almost equal exchange coupling parameters throughout the seven LF-like configurations, the magnitude of the J parameter being comparable with those of the Cu-Gd system.

Electronic fine structure calculation of metal complexes with three-open-shell s, d, and p configurations

The ligand field density functional theory (LFDFT) algorithm is extended to treat the electronic structure and properties of systems with three-open-shell electron configurations, exemplified in this work by the calculation of the core and semi-core 1s, 2s, and 3s one-electron excitations in compounds containing transition metal ions. The work presents a model to non-empirically resolve the multiplet energy levels arising from the three-open-shell systems of non-equivalent ns, 3d, and 4p electrons and to calculate the oscillator strengths corresponding to the electric-dipole 3d ^m  → ns ¹3d ^m 4p ¹ transitions, with n = 1, 2, 3 and m = 0, 1, 2, …, 10 involved in the s electron excitation process. Using the concept of ligand field, the Slater-Condon integrals, the spin-orbit coupling constants, and the parameters of the ligand field potential are determined from density functional theory (DFT). Therefore, a theoretical procedure using LFDFT is established illustrating the spectroscopic details at the atomic scale that can be valuable in the analysis and characterization of the electronic spectra obtained from X-ray absorption fine structure or electron energy loss spectroscopies.

A non-empirical calculation of 2p core-electron excitation in compounds with 3d transition metal ions using ligand-field and density functional theory (LFDFT)

It is shown that LFDFT can be used to simulate the optical spectrum of 2p core-electron excitation in compounds with 3d transition metal ions.

On the calculation of multiplet energies of three-open-shell 4f135fn6d1electron configuration by LFDFT: modeling the optical spectra of 4f core-electron excitation in actinide compounds

My presentation relates the modeling of X-ray absorption spectra of actinides, exemplified here by the study of U⁴⁺ion with configuration 4f¹³5f²6d¹.

Core electron excitations in U(4+): modelling of the nd(10)5f(2)→nd(9)5f(3) transitions with n = 3, 4 and 5 by ligand field tools and density functional theory

Ligand field density functional theory (LFDFT) calculations have been used to model the uranium M4,5, N4,5 and O4,5-edge X-ray absorption near edge structure (XANES) in UO2, characterized by the promotion of one electron from the core and the semi-core 3d, 4d and 5d orbitals of U(4+) to the valence 5f. The model describes the procedure to resolve non-empirically the multiplet energy levels originating from the two-open-shell system with d and f electrons and to calculate the oscillator strengths corresponding to the dipole allowed d(10)f(2)→ d(9)f(3) transitions appropriate to represent the d electron excitation process. In the first step, the energy and UO2 unit-cell volume corresponding to the minimum structures are determined using the Hubbard model (DFT+U) approach. The model of the optical properties due to the uranium nd(10)5f(2)→nd(9)5f(3) transitions, with n = 3, 4 and 5, has been tackled by means of electronic structure calculations based on the ligand field concept emulating the Slater-Condon integrals, the spin-orbit coupling constants and the parameters of the ligand field potential needed by the ligand field Hamiltonian from Density Functional Theory. A deep-rooted theoretical procedure using the LFDFT approach has been established for actinide-bearing systems that can be valuable to compute targeted results, such as spectroscopic details at the electronic scale. As a case study, uranium dioxide has been considered because it is a nuclear fuel material, and both atomic and electronic structure calculations are indispensable for a deeper understanding of irradiation driven microstructural changes occurring in this material.

Photoluminescence properties of Yb2+ ions doped in the perovskites CsCaX3 and CsSrX3 (X = Cl, Br, and I) – a comparative study

Photoluminescence spectra of Yb²⁺ ions doped into CsCaX₃ and CsSrX₃ (X = Cl, Br, and I) depict a manifold of transitions in high resolution, which allows a detailed understanding of the optical properties of divalent lanthanide ions in perovskite host lattices.

Development and applications of the LFDFT: the non-empirical account of ligand field and the simulation of the f-d transitions by density functional theory

Ligand field density functional theory (LFDFT) is a methodology consisting of non-standard handling of DFT calculations and post-computation analysis, emulating the ligand field parameters in a non-empirical way. Recently, the procedure was extended for two-open-shell systems, with relevance for inter-shell transitions in lanthanides, of utmost importance in understanding the optical and magnetic properties of rare-earth materials. Here, we expand the model to the calculation of intensities of f → d transitions, enabling the simulation of spectral profiles. We focus on Eu(2+)-based systems: this lanthanide ion undergoes many dipole-allowed transitions from the initial 4f(7)((8)S7/2) state to the final 4f(6)5d(1) ones, considering the free ion and doped materials. The relativistic calculations showed a good agreement with experimental data for a gaseous Eu(2+) ion, producing reliable Slater-Condon and spin-orbit coupling parameters. The Eu(2+) ion-doped fluorite-type lattices, CaF2:Eu(2+) and SrCl2:Eu(2+), in sites with octahedral symmetry, are studied in detail. The related Slater-Condon and spin-orbit coupling parameters from the doped materials are compared to those for the free ion, revealing small changes for the 4f shell side and relatively important shifts for those associated with the 5d shell. The ligand field scheme, in Wybourne parameterization, shows a good agreement with the phenomenological interpretation of the experiment. The non-empirical computed parameters are used to calculate the energy and intensity of the 4f(7)-4f(6)5d(1) transitions, rendering a realistic convoluted spectrum.

Prospecting Lighting Applications with Ligand Field Tools and Density Functional Theory: A First-Principles Account of the 4f(7)-4f(6)5d(1) Luminescence of CsMgBr3:Eu(2+)

The most efficient way to provide domestic lighting nowadays is by light-emitting diodes (LEDs) technology combined with phosphors shifting the blue and UV emission toward a desirable sunlight spectrum. A route in the quest for warm-white light goes toward the discovery and tuning of the lanthanide-based phosphors, a difficult task, in experimental and technical respects. A proper theoretical approach, which is also complicated at the conceptual level and in computing efforts, is however a profitable complement, offering valuable structure-property rationale as a guideline in the search of the best materials. The Eu(2+)-based systems are the prototypes for ideal phosphors, exhibiting a wide range of visible light emission. Using the ligand field concepts in conjunction with density functional theory (DFT), conducted in nonroutine manner, we develop a nonempirical procedure to investigate the 4f(7)-4f(6)5d(1) luminescence of Eu(2+) in the environment of arbitrary ligands, applied here on the CsMgBr3:Eu(2+)-doped material. Providing a salient methodology for the extraction of the relevant ligand field and related parameters from DFT calculations and encompassing the bottleneck of handling large matrices in a model Hamiltonian based on the whole set of 33,462 states, we obtained an excellent match with the experimental spectrum, from first-principles, without any fit or adjustment. This proves that the ligand field density functional theory methodology can be used in the assessment of new materials and rational property design.