[en] Full text: The investigation of liquid-crystalline metal complexes ( metallomesogens) is a fast growing branch in coordination chemistry. Metal ions can be complexed with mesomorphic ligands, but the presence of a metal ion can also induce mesomorphism in non-mesomorphic ligands. Whereas the coordination polyhedron around the central metal ion in the first described metallomesogens was a square plane (mainly Pt(II)-, Pd(II)-, Ni(II)- and Cu(II)-complexes), at present metallomesogens with octahedral or other environments around the central metal ion are known. Typical mesogenic ligands are Schiff bases, (-diketonates, phthalocyanines and porphyrins. Not only transition metal ions, but also lanthanide ions can be incorporated in metallomesogens . Lanthanide ions have certain advantages. Because of the high coordination number (CN = 8-9), the coordination polyhedra in lanthanide complexes are different from these observed in transition metal complexes. This opens new possibilities for the investigation of the relation structure-mesomorphism. The strong polarization by the trivalent lanthanide ions stabilizes the mesophase and the lanthanide can induce mesomorphism in non-mesomorphic ligands. Several elements of the lanthanide series show strong luminescence and/or interesting magnetic properties (paramagnetism). In collaboration with Duncan W. Bruce (University of Exeter, UK), I took up the challenge to synthesize new lanthanide-containing liquid crystals and to investigate their physical properties. Types of complexes we are studying at present include, Schiff base complexes, lanthanide soaps (carboxylates of long-chain fatty acids) and complexes with hemicyanine ligands of the type. All these complexes are ionic in nature. The mesophases are characterized by Differential Scanning Calorimetry (DSC) and by optical microscopy. DSC provides us with accurate transition temperatures and enthalpies. The textures observed between crossed polarizers on the heating stage of an optical microscope give an indication of the nature of the mesophase (nematic, smectic or columnar). These ionic liquid crystals show a strong tendency to supercool to glasses, in which the molecular order of the mesophase is retained. Because of the broad scope of the Rare Earth Conference, I will not only discuss my own work, but I will also give an overview of the literature dealing with liquid-crystalline lanthanide complexes and I will discuss the basic principles of mesophases

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[en] Optical absorption spectra of trivalent europium in the rare-earth garnet Y₃Ga₅O₁₂ (YGG) have been recorded between 4600 and 32,000 cm^-1 at 77 and at 293 K. A total of 117 crystal-field transitions has been detected in the spectra. The symmetry of the Eu³⁺ site is D₂, so a total removal of the crystal-field degeneracy of the 4f⁶ configuration can be expected. The energy level scheme of Eu³⁺ in YGG is parametrized in terms of 20 free-ion parameters and nine crystal-field parameters. The crystal field is strong in the garnet host, so J-mixing has to be taken into account for the crystal-field calculation. (author)

[en] The optical absorption spectra and luminescence spectra of TbAl₃(BO₃)₄ have been recorded. The crystal-field transitions were assigned assuming D₃ symmetry. The energy level diagram of the 4f⁸ electronic configuration of Tb³⁺ was parametrized in terms of 21 free-ion and 6 B_q^k crystal-field parameters. A total number of 74 experimental crystal-field levels was used in the fitting procedure. The crystal-field parameters of Tb³⁺ in TbAl₃(BO₃)₄ are compared with those of Eu³⁺ in GdAl₃(BO₃)₄. (orig.)

[en] Although the Ω_λ intensity parameters and especially the Ω₂ parameter can show a considerable spread for a particular lanthanide ion in different host matrices, the values of these parameters for lanthanide ions in glasses are relatively close to each other. Combined with the fact that large errors are inherent to these parameters, we emphasize that one has to be careful when interpreting or comparing intensity parameters for lanthanide ions in glasses. The analysis of a large data-set of intensity parameters reported for glasses indicates that Ω₄≅Ω₆. No correlation was found between the Ω₂ parameter and the two other parameters. (author). Letter-to-the-editor

[en] The polarized absorption spectra of europium bromate enneahydrate Eu(BrO₃)₃·9H₂O have been recorded at ambient temperature and at 77 K between 16,500 and 36,000 cm^-1. The site symmetry is approximated by a D_3h symmetry. 63 crystal field levels have been assigned. The energy level scheme is parametrized in terms of 20 free ion parameters and four crystal field parameters. The crystal field parameters are B₀² = 205 cm^-1, B₀⁴ = -279 cm^-1, B₀⁶ = -557 cm^-1 and B₆⁶ = -872 cm^-1. Because of the weak crystal field felt by the Eu³⁺ ion, J-mixing is not very important in the bromate matrix. A consequence is that most of the observed induced electric dipole transitions starting from the ⁷F₀ level obey the vertical bar ΔJ vertical bar = 2, 4, 6 selection rules of the Judd-Ofelt theory. Comparison with other Eu³⁺ systems enables us to conclude that a tricapped trigonal prism as a coordination polyhedron (C.N.=9) results in weak crystal fields, regardless of the chemical nature of the ligands. (author)

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[en] Full text of publication follows. Recently, the UO_2 lattice parameter has been re-evaluated by Leinders et al. using X-ray diffraction [Ref.1]. The updated unit cell parameter (547.127 ± 0.008 pm) is larger and has been determined with a higher accuracy than the previously accepted value (547.04 ± 0.08 pm) (Ref.2). Today's accepted value for the Cu Kα_1 X-ray wavelength partly explains the difference in cell parameter, but the largest contribution is due to the avoidance of hyper-stoichiometry. Controlling and measuring sample stoichiometry is not straight-forward, as illustrated by the rather large scatter on published lattice parameter values for UO_2 [Ref.1]. The present work discusses important experimental aspects for the production of stoichiometric UO_2, and the accurate measurement of sample stoichiometry. Uranium dioxide powder is known to be sensitive to oxidation, with oxygen taken up at the surface rapidly already at room temperature [Refs.3, 4]. At room temperature, the solubility of oxygen in UO_2 is quite low (O/U ∼2.01), with the precipitation of ordered structures such as U_4O_9_-_y occurring with increasing oxygen content [Ref.5]. At elevated temperatures, a broad range of hyper-stoichiometry exists, and here numerous studies have shown a linear decrease in lattice parameter with increasing oxygen content (Δa = -10*(O/U-2) pm, approximately. Evidently, in order to perform any lattice parameter study on UO_2 precise measurement of the stoichiometry is desirable. The American Society for Testing and Materials (ASTM) has provided guidelines for measuring stoichiometry of uranium dioxide powders and pellets (ASTM C-1453-00). The method is based on the conversion of a sample with unknown stoichiometry UO_2_±_x to U_3O_8. The mass increase corresponding to the oxidation reaction: 3UO_2_±_x + 1/2(2 ± 3x)O_2 → U_3O_8, determines the initial oxygen content x. Comparison of the mass increase measured by in-situ thermogravimetric analysis (TGA) and by ex-situ weighing on an analytical balance revealed an important effect: adsorption of atmospheric species on the sample material and/or the sample crucible. The mass increase as measured ex-situ was consistently larger as compared to the actual in-situ analysis. As a result, sample stoichiometry is underestimated when performing ex-situ weighing. We propose to strictly perform sample retrieval and weighing in an inert atmosphere, or alternatively, to use in-situ TGA when maximum accuracy is required. Where some of the early investigators used the ex-situ weighing approach, samples may have falsely been identified as being stoichiometric, whereas in reality they were hyper stoichiometric. Hence the difference in reported lattice parameter values. References: [1) G. Leinders,T. Cardinaels, K. Binnemans, M. Verwerft, J. Nucl. Mater. 2015, 459, 135. [2) F. Gronvold, J. Inorg. Nucl. Chem. 1955, 1, 357. [3) L. E. J. Roberts, J. Chem. Soc. 1954, 3332. [4) J. S. Anderson, L. E. J. Roberts, E. A. Harper, J. Chem. Soc. 1955, 3946. [5) B. E. Schaner, J. Nucl. Mater. 1960, 2, 110. (authors)

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[en] Liquid-crystalline cationic complexes of lanthanides (Ln = La, Tb, Dy, Gd) with azomethines containing the alkyl sulfate anion as the counterion are synthesized. The structure and chemical composition of prepared complexes are supported by the data of NMR spectroscopy and elementary analysis. It is found that the temperatures of existence of the mesophase of the complexes with the alkyl sulfate anion are by 80-100 deg C lower than those for similar complexes with Cl^- and NO₃^- counterions