Simple Overlap Model Research Articles

Theoretical investigation of the 7F1 level splitting in a series of Eu3+ doped oxides matrixes

The point charge electrostatic model (PCEM) and the simple overlap model (SOM) are applied to a series of oxide crystals (Gd2O3, Y2O3, Lu2O3, In2O3 and Sc2O3) and a silicate glass (Al2O3–SiO2) doped with the Eu3+ ion. The SOM factor ρ(2β)k+1 is input as the shielding factor for all Bqk expressions, which enters in the calculations of the crystal-field strength parameter, NV. The maximum splitting ΔE of the 7F1 manifold of the Eu3+ ion is then obtained as a function of NV. It has been developed another way to calculate alpha, which is an expansion factor in the ΔE expression. For the glass, as the mean metal-ligating ions distances are larger than for the crystals, NV and ΔE are smaller, as expected. The prediction of the PCEM shows a linear dependence between ΔE and NV, even though the known mismatch in respect to the experimental splitting is kept. In the case of the SOM, two situations have been analyzed: firstly the charge factor varies in order to reproduce the experimental splitting (a phenomenological procedure); secondly the charge factor is the valence of the oxygen ions. The agreement between the experimental results and theoretical predictions for all investigated systems is very satisfactory in respect to both the linearity between ΔE and NV and the ΔE splitting.

On the use of theoretical tools in the study of photophysical properties of the new Eu(fod) 3 complex with diphenbipy

The synthesis, characterization, spectroscopic, and fluorescence properties of Eu(fod) 3 · 2H 2O and Eu(fod) 3 · diphenbipy (fod = 6,6,7,7,8,8, 8-heptafluoro-2,2-dimethyl-3,5-octadionate, diphenbipy = 4,4′-diphenyl-2,2′-dipyridyl) are described. New spectroscopic studies of these complexes are presented based on theoretical and experimental analyses of the intensity parameters, Ω 2 and Ω 4, and on the maximum splitting of the 7F 1. The geometries calculated by Sparkle/AM1 model was used to calculate the 7F 1 manifold splitting (Δ E 0–1) and the intensity parameters Ω 2, Ω 4 based on the Simple Overlap Model (SOM). The reconciliation among the data from the theoretical 7F 1 manifold splitting, and the intensity parameter certifies the reliability of the models used in the theoretical calculations.

Spectroscopic properties of the Eu(fod) 3Phen–NO incorporated carboxylate glass

The new composite lithium and sodium acetate (2:1 mol%) glass doped with Eu(fod) 3Phen-NO (fod=6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadionate) was prepared and characterized by differential scanning calorimetric and thermogravimetric techniques and electronic emission. The melting has not affected the complex structure because the typical 5D 0→ 7F J ( J = 0 – 4 ) transitions of the Eu 3+ ion in the complex have not changed. The Sparkle model was used to predict the ground state geometry of the complex. The simple overlap model was used to calculate the crystal field and intensity parameters, the 5D 0→ 7F 1 transition splitting and the spontaneous emission coefficient. The mean relative difference between the experimental results and the theoretical predictions was below 13%.

Synthesis, sparkle model and spectroscopic studies of the Eu(hfc) 3·bipyO 2 complex

The synthesis and spectroscopic study of the Eu(hfc) 3·2H 2O complex (hfc: 4,4,4-trifluoro-butyryl (+) canfore) with bipyridinedioxide (bipyO 2) is described. The coordination of bipyO 2 to the Eu(III) ion in the complex is evidenced due to the presence of a strong band at 1241 cm −1 shifted from 1252 cm −1 in the bipyO 2 spectrum and attributed to the stretch of its N-oxide group. The complex Eu(hfc) 3·bipyO 2 (hfc: 6,6,7,7,8,8,8-heptafluoro-2,2′-dimethyl 1,3,5-hydroxymethyllene (+) canfore) has a lifetime of 652 μs which is higher than that of the Eu(hfc) 3·2H 2O complex as well as that of the similar complex with β-diketone with CF 3 group. This lifetime value confirms that the Eu(III) luminescence in the complex Eu(hfc) 3·bipyO 2 is stronger than in both complexes above cited. The geometry of this complex has been previously optimized using the sparkle model and the results have been used to perform theoretical predictions of the Judd–Ofelt intensity parameters Ω λ ( λ=2.4) using the simple overlap model (SOM) and correlated to experimental data obtained by means of the emission spectra. The experimental value of the area of the 5D 0→ 7F 2 transition is 20 times larger than that of the 5D 0→ 7F 1 transition. This reflects the hypersensitivity of the 5D 0→ 7F 2 transition. A good agreement between theoretical and experimental UV absorption spectra of the complex has been obtained. This result increased our confidence in the predicted structure.

Synthesis, sparkle model, intensity parameters and spectroscopic studies of the new Eu(fod)3phen-NO complex

We report the synthesis, characterization, spectroscopic properties, structure prediction and intensity parameters of the Eu(fod)3phen-NO (fod=6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadionate and phen-NO=1,10-phenanthroline N-oxide). Both the elementary analysis and the IR vibrational data are consistent with the formula of the cited complex. The absorption spectrum in ethanol shows a maximum at 272nm, which is shifted from 293nm in relation to Eu(fod)3. This is an indication of the coordination of phen-NO. The emission spectra at room temperature and at 77K show a very high intensity for the hypersensitive 5D0→7F2 transition, pointing to a highly polarizable chemical environment around the Eu3+ ion. The complex has a decay time, τ, lower than the Eu(fod)32H2O. It can be attributed to a considerable resonance of the ligand triplet with the excited levels of the lanthanide ion. The sparkle and INDO/S-CI models were used to calculate the structure and electronic spectrum of this complex. Good agreement between theoretical and experimental UV absorption spectra has been obtained. Structural data were used to perform theoretical predictions of the Judd–Ofelt intensity parameters (Ωλ,λ=2,4), the 5D0→7F0/5D0→7F2 intensity ratio and 5D0→7F1 transition splitting using the simple overlap model. The satisfactory results obtained are an indication that the models used can lead to reliable prediction of the structure and 4f–4f intensities.

Crystal-field analysis of Eu3+ energy levels in the new rare-earth R BiY1−xRxGeO5 oxide

Pale colored BiY1−xRxGeO5 (R=rare-earth from Pr to Yb) polycrystalline samples exhibit a crystalline phase isostructural with the orthorhombic Pbca (No. 61) structure-type established for BiYGeO5 and BiYbGeO5. R occupies a single point site in the host, with the lowest C1 symmetry. While for Pr and Nd x must be ⩽0.35, for smaller R ions, Sm to Yb, the phase appears for any x content. Detailed crystallographic data for BiErGeO5 have been determined from the structure refinement of its neutron diffraction profile at room temperature. Optical absorption and photoluminescence measurements at 10K have been performed for BiEuGeO5. An initial approach to the parametrization of crystal-field effects on this new host has been provided by results of the semi-empirical Simple Overlap Model, which considers the crystallographic positions of the nearest neighbors around R. Furthermore, the strongly reduced 7FJM set of levels of the 4f6 configuration has been taken into account for a trustworthy phenomenological determination of crystal-field parameters of the observed optical center for the Eu3+ sample. In spite of difficulties imposed by the low symmetry of Eu3+, a very good root mean squares deviation σ=5.6cm−1 between experimental and simulated energy level schemes has been obtained considering the approximate C2(Cs) symmetry for the Eu3+ in the host.

Spectroscopic properties of Yb-doped scandium based compounds Yb:CaSc2O4, Yb:SrSc2O4 and Yb:Sc2SiO5

Abstract Three Yb3+-doped scandium based materials are elaborated and their optical properties investigated. Scandium materials exhibit large energy levels splitting and favorable thermal properties. The energy level diagrams for Yb:CaSc2O4, Yb:SrSc2O4, and Yb:Sc2SiO5 are determined through optical spectroscopic analysis and semi-empirical crystal-field calculations using the simple overlap model. Laser parameters for Yb:Sc2SiO5 are compared to Yb:Y2SiO5 ones. Yb:Sc2SiO5 looks promising since it melts congruently and exhibits larger overall splitting of the 2F7/2Yb3+ ground state and broader absorption line width.

Overlap polarizability of a chemical bond: a scale of covalency and application to lanthanide compounds

We introduce the concept of overlap polarizability of a single directional chemical bond and propose an expression for this quantity. The concept of ionic specific valence is also introduced, as a consequence of a relation, involving the bond force constant, between the overlap polarizability and the square of the overlap charge. It is proposed that the overlap polarizability can be used to define an ordinal scale of covalency. When applied to lanthanide compounds, each pair lanthanide-ligating atom being regarded as a diatomic-like molecule, these concepts lead to a remarkable result in which an expression for the charge factors, appearing in the simple overlap model for the ligand field, can be obtained when these factors are identified with the ionic specific valences of the ligating atoms. An ordinal scale of covalency for a few representative lanthanide compounds correlates well with the nephelauxetic effect.

Crystal field analysis of Nd3+ energy levels in monoclinicNdAl3(BO3)4 laser

The energies of 135 Kramers doublets extending up to the 2H111/2multiplet for Nd3+ in a monoclinic C2/c space group (No 15) NdAl3(BO3)4 (NAB) single crystal laser have been determined frompolarized optical absorption and photoluminescence measurements at 7 K. Thestrongly polarized character of the Nd spectra has been discussed under theassumption of a local D3 symmetry, higher than the C2symmetry of NAB, and the observed energy levels have been labelled with theadequate crystal quantum numbers and irreducible representations. A detailedHamiltonian of 21 parameters has been used in the simulation of the energylevels and associated wavefunctions of the 4f3 configuration of Nd3+. The diagonalized complete energy matrix combines simultaneously thefree-ion and single-particle crystal field interactions. Starting Bkq CFparameters were calculated from the semi-empirical simple overlap model SOM. Acomparative simulation considering the C2 symmetry of NAB isprovided. Moreover, two-electron CF interactions as well as an empiricalcorrection have been tested in calculating the anomalous splitting of the2H211/2 levels. A final fit in D3 symmetry produces avery good adjustment with a low rms deviation σ = 15.3 cm-1between observed and calculated energy levels.

Simulation of optical and magnetic properties of neodymium (4f 3) in the solid state: extreme sensitivity to the wave vector’s composition

The optical absorption spectra of neodymium at 4 K are reproduced with a very good agreement between calculation and experiment (15<rms<20 cm −1). The wavefunctions which are obtained as a result of the energy calculation can be used to compute other physical properties. Among them we consider the paramagnetic susceptibility and the electron paramagnetic resonance. We report the results of these calculations concerning six compounds (Nd 2O 2S, A-Nd 2O 3, NdOCl, NdOF, Nd 3+:LaCl 3 and NdF 3) for which the optical absorption spectra were thoroughly interpreted. We notice that these properties, depending on the point symmetry of the neodymium, are very sensitive to the values of the coefficients of the kets in the wavefunctions. These wavefunctions are extremely dependent on the crystal field parameters (cfps). A regard on the magnetic properties is very useful to determine if a set of crystal field parameters which fits the overall Stark levels is also convenient for the ground state ( 4I 9/2). A comparison is also made between cfps determined from atomic positions in the structure (Simple Overlap Model) and phenomenological cfps. We also consider the effect of the spin-correlated crystal field (SCCF) through the action of its radial contribution represented by the c k parameters ( k=2,4,6).

Sparkle model and intensity parameters of the Eu(3-amino-2-carboxypyridine-N-oxide)33H2O complex

The sparkle model for structural calculation of lanthanide complexes (SMLC) was applied to the Eu(3-aminopyridine-2-carboxylic acid N-oxide)3H2O, a new lanthanide complex recently synthesized. For the calculation of the electronic spectra of the complexes, the sparkle is replaced by a point charge with the ligands held in the positions as determined by the SMLC, and uses an INDO/S-CI model. A good agreement between theoretical and experimental UV absorption spectra of the complex has been obtained. This result increased our confidence in the predicted structure. The optimized spherical coordinates of the ligands were then used to perform theoretical predictions of the Judd–Ofelt intensity parameters (Ωλ,λ=2and4), the 5D0→7F0/5D0→7F2 intensity ratio and the 5D0→7F1 transition splitting using the simple overlap model (SOM). The satisfactory results obtained in this system are an indication that both SMLC and SOM can lead to reliable prediction of complex structure and 4f–4f intensities.

On the charge factors of the simple overlap model for the ligand field in lanthanide coordination compounds

In this Letter, we propose a semi-empirical procedure through which the charge factors, appearing in the simple overlap model (SOM) for the ligand field in lanthanide compounds, can be obtained. The idea is based on the concept of bond valence and bond strength introduced by Pauling in the 1920's. The charge factors thus obtained are used in the calculation of the so-called ligand field parameters, B q k 's, and, subsequently, in the prediction of the Stark levels of the 7 F J manifolds ( J=1,2,3 and 4) of the Eu 3+ ion in coordination compounds with mixed ligands. Comparison with experiment shows that the results are quite satisfactory.

Optical spectroscopy and crystal-field effects on the paramagnetic susceptibility of rare-earth germanates GaRGe 2O 7, R=Pr, Nd

Optical absorption and photoluminescence measurements at 9 K were performed on stoichiometric GaRGe 2O 7 and on R-doped LaGaGe 2O 7, R=Pr 3+, Nd 3+, polycrystalline samples. In this monoclinic matrix, space group P2 1/c (no. 14), the lanthanide ion occupies a single crystallographic point site with symmetry C 1. From the crystal-field analysis of the optical spectra, energy level schemes and an expression of the associated wavefunctions for the 4f 2 and 4f 3 configurations were derived. In both cases, the fitting of experimental Stark level energies and the phenomenological calculation of free-ion as well as crystal-field parameters (CFPs) were performed for the approximate C s (C 2) symmetry. Despite the low symmetry, very satisfactory correlations between experimental and simulated energy level schemes were obtained, with root mean square deviations σ=10.1 and 14.8 cm −1 for Pr 3+ and Nd 3+, respectively. Energy levels and the composition of their wavefunctions were checked for both configurations through a calculation of the thermal evolution of the paramagnetic susceptibility χ, according to the Van Vleck formula. The same calculation was accomplished with CFPs resulting from the semi-empirical simple overlap model (SOM), considering the real point symmetry of R and C 1. Over the whole measured range, 1.7–300 K, a very good reproduction of the observed χ vs. T is yielded by each of the phenomenological and SOM calculated collections of CFPs.

Measurement and simulation of the energy levels of R=Pr 3+ and Nd 3+ in GaRGe 2O 7

The optical absorption spectra of GaRGe 2O 7, R=Pr, Nd, have been measured, on polycrystalline powders, at 9 K. These compounds are monoclinic, space group P2 1/c , and in them R 3+ occupies a single crystallographic position with C 1 point symmetry. From the crystal field analysis of the spectra, energy level schemes and an expression of the associated wavefunctions for the 4f 2 and 4f 3 configurations have been derived. The semi-empirical simple overlap model is used to calculate an initial set of crystal-field parameters from the crystal structure. The fitting of experimental Stark level energies and the phenomenological calculation of crystal-field parameters were performed for the approximate C s(C 2) symmetry. Very satisfactory correlations were obtained between calculated and experimental levels, with root mean square deviations σ=14.1 and 15.7, respectively, and despite the very low symmetry of R 3+ in the matrix, the similarity of both sets of crystal-field parameters indicates the consistence of these calculations.

Simple overlap model and crystal field analysis of Cs 2ZnCl 4:Co 2+. Correlation with optical and magnetic properties

The optical (energy level scheme) and magnetic properties (paramagnetic susceptibility as a function of temperature and g values) of Co 2+ in a Cs 2ZnCl 4 single crystal were reproduced simultaneously according to the crystal field theory involving a set of free ions and B k q crystal field parameters. The Co 2+ are placed in a weak tetrahedral ligand field. The crystal field parameters calculated from the structure are in good agreement with the experimental values. The correlation between the g values and B k q parameters is discussed.

Simulation of the spectroscopic and magnetic properties of RE(III) ions in RE oxychlorides based on exact crystal structure from Rietveld refinements

The procedures to calculate the 4f N energy level schemes — together with the related properties such as the paramagnetic susceptibility — of the trivalent rare earth ions (RE 3+) in the tetragonal RE oxychlorides (REOCl, RE=La–Nd, Sm–Ho and Y) from structural data are presented. The methods include the determination of coherent and reliable structural parameters from X-ray powder diffraction measurements using Rietveld analyses. The measurement of the absorption and luminescence spectra of the RE 3+ ions from the IR to UV region yield the experimental 4f N energy level schemes which are then simulated with a phenomenological model involving the parametrization of the inter- and intra-ionic interactions. Parallel calculations — already proven to be successful for ionic systems such as RE oxyfluorides — are based on the modified point charge model (PCEM). The more covalent simple overlap (SOM) model is also applied and compared to the other results. Finally, the paramagnetic susceptibility of the pure oxychlorides as a function of temperature was calculated using the eigenfunctions and eigenvalues from the phenomenological simulation of the experimental 4f N energy level schemes.

Spectroscopic properties of Y 2BaZnO 5:Er 3+

Absorption, luminescence, relaxation time measurements and selective-site excitation in the visible region of Er 3+ in Y 2BaZnO 5 at 1.8 K were analyzed. The results allowed the assignment of transition lines involving the 4I 15/2 state for the two sites in the compound. Crystal field modelling using the simple overlap model enabled crystallographic site and spectroscopic result correlation.

L-Histidine-europium(III) complex: a spectroscopical study

Chemical characterization as well as spectroscopical study of the l-histidine-europium(III) complex were developed both experimental and theoretically. Molecular mechanics (MM) simulation was performed in order to have indication of the compound structure and the Eu3+ chemical environment. The Simple Overlap Model (SOM) was applied to predict spectroscopic quantities as 5D0→7F0/5D0→7F2 intensity ratio, 5D0→7F1 transition splitting and the intensity Ωλ parameters (λ=2 and 4). Satisfactory results are obtained with 0.1 and 2/3 as the effective charges of the nitrogen (gN) and oxygen (gO) respectively, and their polarizabilities (α) depend on the distance.

Relationship between phenomenological crystal field parameters and the crystal structure: The simple overlap model

The simple overlap model is used to calculate crystal field parameters from the crystal structure. It is applied to 25 rare earth and some actinide and 3d transition element compounds for which crystal field parameters have been previously determined experimentally. The model is based on effective charges, located around the middle of the rare earth ion–ligand distance, which are assumed to be proportional to the magnitude of the overlap, ρ, between the rare earth ion and ligand wavefunctions. Good agreement between theoretical and experimental crystal field parameters is found for most of the cases studied. The best predictions are obtained when the overlap varies slightly from one rare earth compound to another (0.05<ρ<0.08), in agreement with the general assumption that the f orbitals have a small part in the chemical bonding. For the two examples involving the 5f elements, the overlaps with the ligand have, curiously, the same magnitude as in the case of the rare earth elements. As expected, the 3d element wavefunctions show a stronger mixing with those of the ligands (0.10<ρ<0.25).

Paramagnetic susceptibility simulations from crystal field effects on Nd3+ in magnesium borate MgNd(BO2)5

Abstract From the crystal field analysis of optical absorption spectra on monoclinic S.G. P21/c MgNd(BO2)5, where Nd3+ occupies a single crystallographic position with no symmetry elements, energy level schemes and an expression of the associated wavefunctions for the 4f3 configuration of Nd3+ have been derived, considering approximate C2v and Cs (C2) symmetries. Despite of the low symmetry of Nd3+, resulting rms deviations between calculated and experimental levels are very satisfactory. The composition of the crystal field wavefunctions from both sets of phenomenological free-ion and crystal field parameters has been checked through a calculation of the thermal evolution of the paramagnetic susceptibility χ, according to the Van Vleck formula. The same simulation was performed with crystal field parameters (CFPs) resulting from the ab initio simple overlap model (SOM), considering the Nd3+ real point symmetry, C1. Over the whole measured range, 1.7–375 K, and especially at low temperatures, a very good reproduction of the experimental χ vs. T is yielded by each of the experimental and SOM calculated collections of CFPs. The oversimplified SOM model seems thus to be useful providing reasonable estimations of crystal field strength parameters.