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  1. Solubility of U(VI) in chloride solutions. III. The stable oxides/hydroxides in MgCl 2 systems: Pitzer activity model for the system UO 2 2+–Na +–K +–Mg 2+–H +–OH –Cl –H 2O(l)

    Here we have developed new chemical, thermodynamic and activity models for the system UO 2 2+–Na +–K +–Mg 2+–H +–OH –Cl –H 2O(l) within the Pitzer approach. The new thermodynamic model is based on previously reported data treated within the SIT approach for NaCl and KCl systems, as well as on new experimental data determined in this work for the MgCl 2 system. The solubility of uranium(VI) was studied in 0.01–5.15 mol·kg w –1 MgCl 2 solutions at pH m = 4.1–9.7 (with pH m = –log [H +]). Experiments were performed under Ar atmosphere at T = (22 ±more » 2) °C. In all investigated systems, the solubility of U(VI) is controlled by metaschoepite, UO 3·2H 2O(cr). In contrast to previously investigated NaCl and KCl systems, no ternary Mg–U(VI)–OH(s) solid phases formed in alkaline MgCl 2 solutions within the timeframe of this study (≤ 200 days). A very significant increase in the solubility (up to 3 log 10-units) is observed in acidic to near-neutral pH m conditions when increasing MgCl 2 concentration from 0.01 to 5.15 mol·kg w –1, which reflects the strong ion interaction processes taking place in concentrated MgCl 2 brines. The solubility of UO 3·2H 2O(cr) in the investigated NaCl, KCl and MgCl 2 solutions is well described with the solubility and hydrolysis constants recommended by Altmaier et al., (2017) and NEA–TDB, and a Pitzer activity model derived in the present work. The latter model considers experimental data reported in the present study and available in the literature for NaCl, KCl and MgCl 2 systems (solubility, potentiometric and spectroscopic data), in combination with well-stablished estimation methods and correlations with SIT coefficients. Finally, chemical, thermodynamic and Pitzer activity models provided in this work for the system UO 2 2+–Na +–K +–Mg 2+–H +–OH –Cl –H 2O(l) accurately describe all evaluated datasets, and represent an adequate tool for the calculation of U(VI) solubility and aqueous speciation in a variety of geochemical conditions including concentrated brine systems of relevance in salt-based repositories for nuclear waste disposal.« less
  2. Both NaCl and H 2O 2 Long-Term Stresses Affect Basal Cytosolic Ca 2+ Levels but Only NaCl Alters Cytosolic Ca 2+ Signatures in Arabidopsis

    Salinity is one of the formidable environmental factors that affect plant growth and development and constrain agricultural productivity. Experimentally imposed short-term NaCl treatment triggers a transient increase in cytosolic free Ca 2+ concentration ([Ca 2+] i) via Ca 2+ influx across the plasma membrane. Salinity stress, as well as other stresses, induces the production of reactive oxygen species (ROS), such as H 2O 2. It is well established that short-term H 2O 2 treatment also triggers a transient increase in [Ca 2+] i. However, whether and how long-term NaCl and H 2O 2 treatments affect the basal levels of [Camore » 2+] i as well as plant responses to additional NaCl and H 2O 2 stresses remain poorly understood. Using an aequorin-based Ca 2+ imaging assay, we found that the long-term treatment of Arabidopsis seedlings with both moderate NaCl and H 2O 2 in the growth media reduced the basal [Ca 2+] i levels. Interestingly, we found that the long-term treatment with NaCl, but not H 2O 2, affected the responses of plants to additional NaCl stress, and remarkably the roots displayed enhanced responses while the leaves showed reduced responses. Furthermore, these findings suggest that plants adapt to the long-term NaCl stress, while H 2O 2 might be an integrator of many stresses.« less
  3. Neutron diffraction study on magnetic structures and transitions in Sr 2 Cr 3 As 2 O 2

    Sr 2Cr 3As 2O 2 is composed of alternating square-lattice CrO 2 and Cr 2As 2 stacking layers, where CrO 2 is isostructural to the CuO 2 building-block of cuprate high- T c superconductors and Cr 2As 2 to Fe 2As 2 of Fe-based superconductors. Current interest in this material is raised by theoretic prediction of possible superconductivity. In this neutron powder diffraction study, we discovered that magnetic moments of Cr(II) ions in the Cr 2As 2 sublattice develop a C-type antiferromagnetic structure below 590 K, and the moments of Cr(I) in the CrO 2 sublattice form the La 2CuOmore » 4-like antiferromagnetic order below 291 K. The staggered magnetic moment 2.19(4)μ B/Cr(II) in the more itinerant Cr 2As 2 layer is smaller than 3.10(6)μ B/Cr(I) in the more localized CrO 2 layer. Furthermore, different from previous expectations, a spin-flop transition of the Cr(II) magnetic order observed at 291 K indicates a strong coupling between the CrO 2 and Cr 2As 2 magnetic subsystems.« less
  4. Photoelectron spectroscopy of solvated dicarboxylate and alkali metal ion clusters, M + [O 2 C(CH 2 ) 2 CO 2 ] 2− [H 2 O] n (M = Na, K; n = 1–6)

    We present results of combined experimental photoelectron spectroscopy and theoretical modeling studies of solvated dicarboxylate species ( O 2 C(CH 2 ) 2 CO 2 ) in complex with Na + and K + metal cations.
  5. Direct air capture of CO 2– topological analysis of the experimental electron density (QTAIM) of the highly insoluble carbonate salt of a 2,6-pyridine-bis(iminoguanidine), (PyBIGH 2)(CO 3)(H 2O) 4

    Chemical bonding and all intermolecular interactions in the highly insoluble carbonate salt of a 2,6-pyridine-bis(iminoguanidine), (PyBIGH 2 )(CO 3 )(H 2 O) 4 , recently employed in the direct air capture of CO 2 via crystallization, have been analyzed within the framework of the quantum theory of atoms in molecules (QTAIM) based on the experimental electron density derived from X-ray diffraction data obtained at 20 K. Accurate hydrogen positions were included based on an analogous neutron diffraction study at 100 K. Topological features of the covalent bonds demonstrate the presence of multiple bonds of various orders within the PyBIGH 2 2+ cation.more » Strong hydrogen bonds define ribbons comprising carbonate anions and water molecules. These ribbons are linked to stacks of essentially planar dications via hydrogen bonds from the guanidinium moieties and an additional one to the pyridine nitrogen. The linking hydrogen bonds are approximately perpendicular to the anion–water ribbons. The observation of these putative interactions provided motivation to characterize them by topological analysis of the total electron density. Thus, all hydrogen bonds have been characterized by the properties of their (3,−1) bond critical points. Weaker interactions between the PyBIGH 2 2+ cations have similarly been characterized. Integrated atomic charges are also reported. A small amount of cocrystallized hydroxide ion (∼2%) was also detected in both the X-ray and neutron data, and included in the multipole model for the electron-density refinement. The small amount of additional H + required for charge balance was not detected in either the X-ray or the neutron data. The results are discussed in the context of the unusually low aqueous solubility of (PyBIGH 2 )(CO 3 )(H 2 O) 4 and its ability to sequester atmospheric CO 2 .« less
  6. Direct air capture of CO 2– topological analysis of the experimental electron density (QTAIM) of the highly insoluble carbonate salt of a 2,6-pyridine-bis(iminoguanidine), (PyBIGH 2)(CO 3)(H 2O) 4

    Chemical bonding and all intermolecular interactions in the highly insoluble carbonate salt of a 2,6-pyridine-bis(iminoguanidine), (PyBIGH 2)(CO 3)(H 2O) 4, recently employed in the direct air capture of CO 2via crystallization, have been analyzed within the framework of the quantum theory of atoms in molecules (QTAIM) based on the experimental electron density derived from X-ray diffraction data obtained at 20 K. Accurate hydrogen positions were included based on an analogous neutron diffraction study at 100 K. Topological features of the covalent bonds demonstrate the presence of multiple bonds of various orders within the PyBIGH 2 2+cation. Strong hydrogen bonds definemore » ribbons comprising carbonate anions and water molecules. These ribbons are linked to stacks of essentially planar dications via hydrogen bonds from the guanidinium moieties and an additional one to the pyridine nitrogen. The linking hydrogen bonds are approximately perpendicular to the anion–water ribbons. The observation of these putative interactions provided motivation to characterize them by topological analysis of the total electron density. Thus, all hydrogen bonds have been characterized by the properties of their (3,-1) bond critical points. Weaker interactions between the PyBIGH 2 2+cations have similarly been characterized. Integrated atomic charges are also reported. A small amount of cocrystallized hydroxide ion (~2%) was also detected in both the X-ray and neutron data, and included in the multipole model for the electron-density refinement. The small amount of additional H +required for charge balance was not detected in either the X-ray or the neutron data. Here, the results are discussed in the context of the unusually low aqueous solubility of (PyBIGH 2)(CO 3)(H 2O) 4and its ability to sequester atmospheric CO 2.« less
  7. Catalytic Conversion of Bamboo Sawdust over ZrO 2–CeO 2/γ-Al 2O 3 to Produce Ketonic Hydrocarbon Precursors and Furans

    Here, a series of metal oxides including ZrO 2/γ-Al 2O 3, CeO 2/γ-Al 2O 3, and ZrO 2–CeO 2/γ-Al 2O 3 were synthesized and employed to conduct catalytic fast pyrolysis of bamboo sawdust via an analytical pyrolyzer/gas chromatograph/mass spectrometer (Py-GC/MS) to produce hydrocarbon precursors such as ketones and furans. Experimental results illustrated that the use of metal oxides enhanced the formation of ketones and monoaromatic hydrocarbons compared to catalyst-free trial. Among the metal oxides, ZrO 2–CeO 2/γ-Al 2O 3 exhibited the most significant ketonization and aldol condensation activities yielding the highest concentration of linear and cyclic ketones. In addition, nonacidicmore » oxygenates such as furfural, acetol, butanedial, 2,3-dihydrobenzofuran, and methyl acetate, were efficiently converted into hydrocarbon precursors over metal oxides. A dual catalytic bed system integrating ZrO 2–CeO 2/γ-Al 2O 3 and HZSM-5 significantly facilitated the production of aromatic hydrocarbons compared to pure HZSM-5 catalytic run. The use of ZrO 2–CeO 2/γ-Al 2O 3 mixed with HZSM-5 mode maximized the formation of xylenes, while ZrO 2–CeO 2/γ-Al 2O 3 mixed with bamboo sawdust catalytic trial increased the concentration of benzene, toluene, and alkylbenzenes.« less
  8. Local-Ising-type magnetic order and metamagnetism in the rare-earth pyrogermanate Er 2 Ge 2 O 7

    The recent discoveries of proximate quantum spin-liquid compounds and their potential application in quantum computing informs the search for new candidate materials for quantum spin-ice and spin-liquid physics. While the majority of such work has centered on members of the pyrochlore family due to their inherently frustrated linked tetrahedral structure, the rare-earth pyrogermanates also show promise for possible frustrated magnetic behavior. With the familiar stoichiometry R 2Ge 2O 7, these compounds generally have tetragonal symmetry with a rare-earth sublattice built of a spiral of alternating edge and corner-sharing rare-earth site triangles. Studies on Dy 2Ge 2O 7 and Ho 2Gemore » 2O 7 have shown tunable low temperature antiferromagnetic order, a high frustration index, and spin-ice-like dynamics. Here we use neutron diffraction to study magnetic order in Er 2Ge 2O 7 (space group P4 12 12) and find the lowest yet Neél temperature in the pyrogermanates of 1.15 K. Using neutron powder diffraction, we find the magnetic structure to order with k=(0,0,0) ordering vector, magnetic space group symmetry P4' 12 12', and a refined Er moment of m=8.1μ B near the expected value for the Er 3+ free ion. Provocatively, the magnetic structure exhibits similar “local Ising” behavior to that seen in the pyrocholres where the Er moment points up or down along the short Er-Er bond. Upon applying a magnetic field, we find a first-order metamagnetic transition at ~0.35T to a lower symmetry P2' 12' 12 structure. This magnetic transition involves an inversion of Er moments aligned antiparallel to the applied field describing a class I spin-flip-type transition, indicating a strong local anisotropy at the Er site—reminiscent of that seen in the spin-ice pyrochlores.« less
  9. Antiferromagnetism in semiconducting SrMn 2 Sb 2 and BaMn 2 Sb 2 single crystals

    Here, crystals of SrMn 2Sb 2 and BaMn 2Sb 2 were grown using Sn flux and characterized by powder and single-crystal x-ray diffraction, respectively, and by single-crystal electrical resistivity ρ, heat capacity C p, and magnetic susceptibility χ measurements versus temperature T, and magnetization versus field M(H) isotherm measurements. SrMn 2Sb 2 adopts the trigonal CaAl 2Si 2-type structure, whereas BaMn 2Sb 2 crystallizes in the tetragonal ThCr 2Si 2-type structure. The ρ(T) data indicate semiconducting behaviors for both compounds with activation energies of ≳0.35 eV for SrMn 2Sb 2 and 0.16 eV for BaMn 2Sb 2. The χ(T) andmore » C p(T) data reveal antiferromagnetic (AFM) ordering at T N = 110 K for SrMn 2Sb 2 and 450 K for BaMn 2Sb 2. The anisotropic χ(T≤T N) data also show that the ordered moments in SrMn 2Sb 2 are aligned in the hexagonal ab plane, whereas the ordered moments in BaMn 2Sb 2 are aligned collinearly along the tetragonal c axis. The ab-plane M(H) data for SrMn 2Sb 2 exhibit a continuous metamagnetic transition at low fields 02Sb 2 exhibits no metamagnetic transitions up to 5.5 T. The χ(T) and C p(T) data for both SrMn 2Sb 2 and BaMn 2Sb 2 indicate strong dynamic short-range AFM correlations above their respective T N up to at least 900 K within a local-moment picture, corresponding to quasi-two-dimensional magnetic behavior. The present results and a survey of the literature for Mn pnictides with the CaAl 2Si 2 and ThCr 2Si 2 crystal structures show that the T N values for the CaAl 2Si 2-type compounds are much smaller than those for the ThCr 2Si 2-type materials.« less
  10. Synthesis and control the morphology of Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+}/Eu{sup 2+},Mn{sup 2+} phosphors by precipitation method

    Highlights: • Use facile and mild precipitation method obtained Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+}/Eu{sup 2+},Mn{sup 2+} phosphors. • Through controlling the morphology to tune the photoluminescence properties. • The columnar structure Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+} phosphor shows better emission intensity. • Self-assembled phenomenon appears by changing the Mn{sup 2+} concentration of the Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+},Mn{sup 2+} phosphor. - Abstract: Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+}/Eu{sup 2+},Mn{sup 2+} phosphors have been synthesized by the facile and mild precipitation method and they are characterized by X-ray powder diffraction, scanning electron microscopy, photoluminescence spectra. The SEM results indicate the obtained Sr{submore » 2}P{sub 2}O{sub 7}:Eu{sup 2+} phosphors have the columnar structure and the Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+},Mn{sup 2+} phosphors is the sphere like. The effects of the stirring and sintering temperature on the morphology of Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+}/Eu{sup 2+},Mn{sup 2+} phosphors have been investigated. The photoluminescence spectra of Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+} and Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+},Mn{sup 2+} phosphors showed that the Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+} phosphors can be excited by 345 nm and exhibit blue emission around 421 nm and the Sr{sub 2}P{sub 2}O{sub 7}:Eu{sup 2+},Mn{sup 2+} phosphors can be excited by 355 nm and exhibited the emission around 420 nm and 578 nm showing the white light.« less
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