The europium-oxygen interaction in nine different europium(III) oxo-compounds (including C-type EuO) was investigated on the basis of powder reflectance spectra (near-IR/vis/UV) and temperature-dependent magnetic measurements. Computation of the transition energies and of the effective Bohr magneton numbers for Eu in the different ligand fields were performed within the framework of the angular overlap model (AOM) using the computer program BonnMag. These calculations show that all electronic transition energies in the optical spectra, the magnetic susceptibilities as well as their temperature dependence, are very well-accounted for by AOM. BonnMag provides a facile way to perform these calculations. Analysis of the obtained "best fit" AOM parameters e(Eu-O) shows that these are significantly influenced by the further bonding partners of oxygen ("second-sphere ligand-field effect"). An increase of e(Eu-O) from 404 cm (EuPO) to 687 cm (EuSbO), both normalized to d(Eu-O) = 2.38 Å, is found. Correlation of this variation to oxide polarizability and optical basicity of the oxo-compounds is discussed.
The program BonnMag has been developed to calculate the absorption spectra and temperature dependent magnetic susceptibilities of f systems. The computations of the transition energies are performed within the angular overlap model. Using Judd-Ofelt theory BonnMag allows estimation of the relative absorption coefficients of the electronic transitions with reasonable accuracy. A description of the theoretical background of the implemented methods is given. Using Slater-Condon-Shortley parameters and spin-orbit coupling coefficients for free Ln ions from ab initio calculations, the transition energies of all Ln ions are calculated and compared to the results from CASSCF/NEV-PT2 calculations. Splitting due to the ligand field as well as transition energies of all Cs NaLnCl (except Gd and Pm) are calculated using parameters reported in the literature. Based on the comparison between theory and experiment, the potential and limitations of the program BonnMag are shown. © 2017 Wiley Periodicals, Inc.
Optical spectra (powder reflectance, UV/Vis/NIR region), and temperature dependent magnetic behavior (χ, μ/μB) were recorded for the series of anhydrous europium(III) phosphates EuIII3O3(PO4), EuIIIPO4, EuIII2P4O13, lt‐ and ht‐EuIII(PO3)3, and EuIIIP5O14. By modeling within the AOM framework, the experimental data can be related to the ligand‐field splitting experienced by the Eu3+ ions in the various mainly low‐symmetry coordination environments. Our study confirms the well‐established relation eσ(Eu3+–O2–) ~ d(Eu3+–O2–)–7.0 between the AOM parameter and the interatomic distance. In addition it is shown that eσ(Eu3+–O2–) depends strongly on the highly variable polarizability of the oxygen ligator atoms. This polarizability can be related to the optical basicity Λ of the various phosphates.
Detailed experimental data on UPO4Cl comprising single-crystal UV/vis/NIR spectra and temperature-dependent magnetic susceptibilities form the basis for the investigation of the electronic structure of the U(4+) cation in UPO4Cl. For modeling of the observed physical properties the angular overlap model (AOM) was successfully employed. The computations were performed using the newly developed computer program BonnMag. The calculations show that all electronic transitions and the magnetic susceptibility as well as its temperature dependence are well-reproduced within the AOM framework. Using Judd-Ofelt theory BonnMag allows estimation of the relative absorption coefficients of the electronic transitions with reasonable accuracy. Ligand field splitting for states originating from f-electron configurations are determined. Slater-Condon-Shortley parameters and the spin-orbit coupling constant for U(4+) were taken from literature. The good transferability of AOM parameters for U(4+) is confirmed by calculations of the absorption spectra of UP2O7 and (U2O)(PO4)2. The effect of variation of the fit parameters is investigated. AOM parameters for U(4+) (5f) are compared to those of the rare-earth elements (4f) and transition metals (3d).
Emerald-green single crystals of U(PO4)Cl were grown by chemical vapor transport in a temperature gradient (1000 → 900 °C). The crystal structure of U(PO4)Cl (Cmcm, Z = 4, a = 5.2289(7) Å, b = 11.709(2) Å, c = 6.9991(8) Å) consists of a three-dimensional network of [PO4] tetrahedra and bicapped octahedral [U(IV)O6Cl2] groups. Polarized absorption spectra measured for two perpendicular polarization directions show a large number of well-resolved electronic transitions. These transitions can be fully assigned on the basis of a detailed ligand-field treatment within the framework of the angular overlap model. The magnetic behavior predicted on the basis of the spectroscopic data is in agreement with an f (2) system and perfectly matched by the results of temperature-dependent susceptibility measurements.
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