Optical Spectra and Magnetic Behavior of a Wide Range of Europium(III) Oxo-Compounds: Analysis of the Ligand-Field Effects

The europium–oxygen interaction in nine different europium­(III) oxo-compounds (including <i>C</i>-type Eu₂O₃) 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 Eu³⁺ in the different ligand fields were performed within the framework of the <i>angular overlap model</i> (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 <i>e</i>_σ(Eu^III–O) shows that these are significantly influenced by the further bonding partners of oxygen (“second-sphere ligand-field effect”). An increase of <i>e</i>_σ, max(Eu^III–O) from 404 cm^–1 (EuPO₄) to 687 cm^–1 (EuSbO₄), both normalized to <i>d</i>(Eu^III–O) = 2.38 Å, is found. Correlation of this variation to oxide polarizability and optical basicity of the oxo-compounds is discussed

The Electronic States of U⁴⁺ in U(PO₄)Cl: An Example for Angular Overlap Modeling of 5f^<i>n</i> Systems

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 U⁴⁺ cation in UPO₄Cl. For modeling of the observed physical properties the <i>angular overlap model</i> (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 U⁴⁺ were taken from literature. The good transferability of AOM parameters for U⁴⁺ 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 U⁴⁺ (5f) are compared to those of the rare-earth elements (4f) and transition metals (3d)

(Bis(terpyridine))copper(II) Tetraphenylborate: A Complex Example for the Jahn–Teller Effect

The surprisingly complicated crystal structure of (bis­(terpyridine))­copper­(II) tetraphenylborate [Cu­(tpy)₂]­(BPh₄)₂ (tpy = 2,2′:6′,2″-terpyridine) consists of six crystallographically independent [Cu­(tpy)₂]²⁺ complexes. At ambient temperature, five out of six [Cu^IIN₆] chromophores appear to be compressed octahedra, while at 100 K, four exhibit elongated and only two compressed octahedral geometry. Temperature dependent single crystal UV/vis (100, 298 K) and EPR measurements (20, 100, 298 K) as well as AOM calculations suggest that the octahedra which show apparently compressed octahedral geometry (XRD) result from dynamic Jahn–Teller behavior of elongated octahedra [Cu^IIN₆]. The detailed correlation of structural and spectroscopic data allows an understanding of the strongly solvent-dependent structures of the [Cu­(tpy)₂]²⁺ complex in solution

Comprehensive Characterization of the Electronic Structure of U⁴⁺ in Uranium(IV) Phosphate Chloride

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 (<i>Cmcm</i>, <i>Z</i> = 4, <i>a</i> = 5.2289(7) Å, <i>b</i> = 11.709(2) Å, <i>c</i> = 6.9991(8) Å) consists of a three-dimensional network of [PO₄] tetrahedra and bicapped octahedral [U^IVO₆Cl₂] 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 <i>angular overlap model</i>. 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

Statistical Analysis of Coordination Environments in Oxides

Coordination or local environments (e.g., tetrahedra and octahedra) are powerful descriptors of the crystalline structure of materials. These structural descriptors are essential to the understanding of crystal chemistry and the design of new materials. However, extensive statistics on the occurrence of local environment are not available even on common chemistries such as oxides. Here, we present the first large-scale statistical analysis of the coordination environments of cations in oxides using a large set of experimentally observed compounds (about 8000). Using a newly developed method, we provide the distribution of local environment for each cation in oxides. We discuss our results highlighting previously known trends and unexpected coordination environments, as well as compounds presenting very rare coordinations. Our work complements the know-how of the solid state chemist with a statistically sound analysis and paves the way for further data mining efforts linking, for instance, coordination environments to materials properties