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March 1, 2013
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February 11, 2013
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The conditioning of radioactive waste from nuclear power plants and in some countries even of weapons plutonium is an important issue for science and society. Therefore the research on appropriate matrices for the immobilization of fission products and actinides is of great interest. Beyond the widely used borosilicate glasses, ceramics are promising materials for the conditioning of actinides like U, Np, Pu, Am, and Cm. Monazite-type ceramics with general composition Ln PO 4 ( Ln = La to Gd) and solid solutions of monazite with cheralite or huttonite represent important materials in this field. Monazite appears to be a promising candidate material, especially because of its outstanding properties regarding radiation resistance and chemical durability. This article summarizes the most recent results concerning the characterization of monazite and respective solid solutions and the study of their chemical, thermal, physical and structural properties. The aim is to demonstrate the suitability of monazite as a secure and reliable waste form for actinides.
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October 29, 2012
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Recently, the removal of carbon dioxide from gas mixtures containing methane or nitrogen has received considerable attention, as it could help to reduce global carbon dioxide emissions. Both natural and synthetic zeolites could find use in adsorption-based carbon dioxide removal. In this contribution, the interaction of carbon dioxide, methane, and nitrogen with alkali-exchanged chabazites is studied computationally, using dispersion-corrected density-functional theory. All alkali metals from lithium to cesium are considered. Because the focus lies on a study of the interaction with a single cation and the surrounding framework, a very high Si/Al ratio is assumed in the model system used. Having determined the preferred cation site for each cation species, the interaction energies and equilibrium geometries for systems with one molecule äadsorbed at the cation are analysed. The relative contriäbutions of electrostatic and dispersion interactions are evaluated. Due to the complex interplay between cation-guest and framework-guest interactions, the evolution of the interaction energy on increasing atomic number of the cation is not monotonic. While the selectivity towards carbon dioxide cannot be inferred directly from the computations, estimations based on the difference in interaction energy reveal that K- and Rb-exchanged systems are expected to be most promising. The results are discussed in detail, establishing correlations with experimental results where possible.
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February 11, 2013
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The crystal structure and composition of the mineral creaseyite from Iquique, Chile has been studied using X-ray single crystal diffraction and electron microprobe analysis. The needle like, pale green crystals are elongated parallel [001]. An idealized composition Pb 2 Cu 2 Fe 2 3+ (Si 4.667 Al 0.333 )O 15.333 (OH) 3 · H 2 O has been obtained by crystal structure analysis. Creaseyite is orthorhombic, space group Cmcm , a = 12.502(8), b = 21.402(11), c = 7.303(6) Å. It exhibits substantial disorder of Si, Al and their oxygen ligands along [001]. Pb 2+ occurs in a strongly asymmetric coordination, imposed by its lone pair electrons. Creaseyite is nanoporous, forming open channels of elliptical cross section along [001]. In the untreated crystals the channels are occupied by water molecules in an off-centred position. The lone pair electrons of Pb 2+ extend into the channels. It is expected that the lone pair electrons are accessible to small acceptor molecules or polarizable atoms substituting for water as the extra framework constituent.
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February 11, 2013
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CaFeGe 2 O 6 , the germanium-analogue to the mineral Hedenbergite, has been synthesized at 1273 K in evacuated SiO 2 -glass-tubes. Powder neutron diffraction data collected between 4 K and 300 K were used to evaluate the magnetic spin as well as the nuclear crystal structure and its T -evolution. CaFeGe 2 O 6 is monoclinic, C 2/ c , a = 10.1778(5) Å, b = 9.0545(4) Å, c = 5.4319(3) Å, β = 104.263(3)°, Z = 4 at room temperature. No change of symmetry was observed down to 4 K. Below 43 K, additional magnetic Bragg reflections appear, which can be indexed on the basis of a commensurate magnetic propagation vector k [1, 0, 0]. The successful description of the magnetic spin structure reveals a ferromagnetic spin coupling within the Fe 2+ O 6 M1 chains, while the coupling between the chains is antiferromagnetic. Spins are oriented collinearly within the a – c plane and form an angle of ∼60° with the crystallographic a -axis. The magnetic moment at 4 K amounts to about 4.4 μ B. The observed magnetic structure is similar to that of other Ca-clinopyroxenes. The present data are put into context with the structural and magnetic properties of other pyroxenes – among them magnetoelectric and multiferroic pyroxene-type compounds.
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February 25, 2013
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The structure of synthetic murataite-3C intended for long-term immobilization of high-level radioactive waste has been solved using crystals prepared by melting in an electric furnace at 1500 °C. The material is cubic, F -43 m , a = 14.676(15) Å, V = 3161.31(57) Å 3 . The structure is based upon a three-dimensional framework consisting of α-Keggin [ Al [4] Ti 12 [6] O 40 ] clusters linked by sharing the O5 atoms. The Keggin-cluster-framework interpenetrates with the metal-oxide substructure that can be considered as a derivative of the fluorite structure. The crystal chemical formula of synthetic murataite-3C derived from the obtained structure model can be written as [8] Ca 6 [8] Ca 4 [6] Ti 12 [5] Ti 4 [4] Al O 42 . Its comparison with the natural murataite shows that the synthetic material has a noticeably less number of vacancies in the cation substructure and contains five instead of four symmetrically independent cation positions. The presence of the additional site essentially increases the capacity of synthetic murataite with respect to large heavy cations such as actinides, rare earth and alkaline earth metals in comparison with the material of natural origin.
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February 25, 2013
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Hackmanite (Na 8 [AlSiO 4 ] 6 Cl 2– y – z (S x 2– ) y □ z ) is a rare photochrome variety of sodalite (Na 8 Al 6 Si 6 O 24 Cl 2 ) that shows orange colored photoluminescence in UV light, exhibits a purple-red color subsequently to UV irradiation or after cracking, bleaches within minutes to hours in visible light, and usually recovers color in darkness. Part of the Cl – anion is replaced by S in some hackmanites described in the literature and in all of our samples from Afghanistan, Canada, and Russia. Up to about 10% of Cl is replaced by S for the Russian hackmanites. Sulfur has been identified as sulfate and as reduced sulfur (most probably polysulfide radical ions) from S K X-ray absorption near edge structure (XANES) spectroscopy in hackmanites from Afghanistan and Canada. The Russian hackmanites from different sites of Lovozero massif (Kola peninsula) are free or almost free of sulfate. The spectral features of reduced sulfur in hackmanite significantly differ from that of other reduced sulfur containing minerals and compounds with sodalite structure like lazurite and ultramarine by a sharp pre-edge peak at 2465.2 eV and a shift of XANES features (singlet and triplet) by 2.52 eV towards higher energies. The pre-edge peak is related to sulfides donating electrons, triggered by UV radiation, to vacant sodalite cages creating sulfide radicals and color centers ( F -centers). Its increase during X-ray irradiation favors radicals involved in the process in the way that X-rays are both, activator and probe of the partly- or un-occupied electronic states of the radicals. Prolongated exposure to X-rays changes the electronic configuration of hackmanite significantly indicated by two new pre-edge peaks.
