Ferromagnetic Quantum Critical Point in CePd2_2P2_2 with Pd →\rightarrow Ni Substitution

An investigation of the structural, thermodynamic, and electronic transport properties of the isoelectronic chemical substitution series Ce(Pd$_{1-x}$Ni$_x$)$_2$P$_2$ is reported, where a possible ferromagnetic quantum critical point is uncovered in the temperature - concentration ($T-x$) phase diagram. This behavior results from the simultaneous contraction of the unit cell volume, which tunes the relative strengths of the Kondo and RKKY interactions, and the introduction of disorder through alloying. Near the critical region at $x_{\rm{cr}}$ $\approx$ 0.7, the rate of contraction of the unit cell volume strengthens, indicating that the cerium $f$-valence crosses over from trivalent to a non-integer value. Consistent with this picture, x-ray absorption spectroscopy measurements reveal that while CePd$_2$P$_2$ has a purely trivalent cerium $f$-state, CeNi$_2$P$_2$ has a small ($<$ 10 \%) tetravalent contribution. In a broad region around $x_{\rm{cr}}$, there is a breakdown of Fermi liquid temperature dependences, signaling the influence of quantum critical fluctuations and disorder effects. Measurements of clean CePd$_2$P$_2$ furthermore show that applied pressure has a similar initial effect to alloying on the ferromagnetic order. From these results, CePd$_2$P$_2$ emerges as a keystone system to test theories such as the Belitz-Kirkpatrick-Vojta model for ferromagnetic quantum criticality, where distinct behaviors are expected in the dirty and clean limits.Comment: 9 pages, 8 figure

Uranium redox transition pathways in acetate-amended sediments

Redox transitions of uranium [from U(VI) to U(IV)] in low-temperature sediments govern the mobility of uranium in the environment and the accumulation of uranium in ore bodies, and inform our understanding of Earth's geochemical history. The molecular-scale mechanistic pathways of these transitions determine the U(IV) products formed, thus influencing uranium isotope fractionation, reoxidation, and transport in sediments. Studies that improve our understanding of these pathways have the potential to substantially advance process understanding across a number of earth sciences disciplines. Detailed mechanistic information regarding uranium redox transitions in field sediments is largely nonexistent, owing to the difficulty of directly observing molecular-scale processes in the subsurface and the compositional/physical complexity of subsurface systems. Here, we present results from an in situ study of uranium redox transitions occurring in aquifer sediments under sulfate-reducing conditions. Based on molecular-scale spectroscopic, pore-scale geochemical, and macroscale aqueous evidence, we propose a biotic-abiotic transition pathway in which biomass-hosted mackinawite (FeS) is an electron source to reduce U(VI) to U(IV), which subsequently reacts with biomass to produce monomeric U(IV) species. A species resembling nanoscale uraninite is also present, implying the operation of at least two redox transition pathways. The presence of multiple pathways in low-temperature sediments unifies apparently contrasting prior observations and helps to explain sustained uranium reduction under disparate biogeochemical conditions. These findings have direct implications for our understanding of uranium bioremediation, ore formation, and global geochemical processes

Uranium speciation and stability after reductive immobilization in sediments

It has generally been assumed that the bioreduction of hexavalent uranium in groundwater systems will result in the precipitation of immobile uraninite (UO2). In order to explore the form and stability of uranium immobilized under these conditions, we introduced lactate (15 mM for 3 months) into flow-through columns containing sediments derived from a former uranium-processing site at Old Rifle, CO. This resulted in metal-reducing conditions as evidenced by concurrent uranium uptake and iron release. Despite initial augmentation with Shewanella oneidensis, bacteria belonging to the phylum Firmicutes dominated the biostimulated columns. The immobilization of uranium (similar to 1 mmol U per kg sediment) enabled analysis by Xray absorption spectroscopy (XAS). Tetravalent uranium associated with these sediments did not have spectroscopic signatures representative of U-U shells or crystalline UO2. Analysis by microfocused XAS revealed concentrated micrometer regions of solid U(IV) that had spectroscopic signatures consistent with bulk analyses and a poor proximal correlation (mu m scale resolution) between U and Fe. A plausible explanation, supported by biogeochemical conditions and spectral interpretations, is uranium association with phosphoryl moieties found in biomass; hence implicating direct enzymatic uranium reduction. After the immobilization phase, two months of in situ exposure to oxic influent did not result in substantial uranium remobilization. Ex situ flow-through experiments demonstrated more rapid uranium mobilization than observed in column oxidation studies and indicated that sediment-associated U(IV) is more mobile than biogenic UO2. This work suggests that in situ uranium bioimmobilization studies and subsurface modeling parameters should be expanded to account for non-uraninite U(IV) species associated with biomass. (C) 2011 Elsevier Ltd. All rights reserved

Zur Theorie verallgemeinerter Koethescher Folgen- und Funktionenraeume

SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Repraesentation von positionellem Schachwissen mit Techniken der kuenstlichen Intelligenz

TIB: RN 3437 (111)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Effect of Ca²⁺ and Zn²⁺ on UO₂ Dissolution Rates

The dissolution of UO₂ in a continuously stirred tank reactor (CSTR) in the presence of Ca²⁺ and Zn²⁺ was investigated under experimental conditions relevant to contaminated groundwater systems. Complementary experiments were performed to investigate the effect of adsorption and precipitation reactions on UO₂ dissolution. The experiments were performed under anoxic and oxic conditions. Zn²⁺ had a much greater inhibitory effect on UO₂ dissolution than did Ca²⁺. This inhibition was most substantial under oxic conditions, where the experimental rate of UO₂ dissolution was 7 times lower in the presence of Ca²⁺ and 1450 times lower in the presence of Zn²⁺ than in water free of divalent cations. EXAFS and solution chemistry analyses of UO₂ solids recovered from a Ca experiment suggest that a Ca–U­(VI) phase precipitated. The Zn carbonate hydrozincite [Zn₅(CO₃)₂(OH)₆] or a structurally similar phase precipitated on the UO₂ solids recovered from experiments performed in the presence of Zn. These precipitated Ca and Zn phases can coat the UO₂ surface, inhibiting the oxidative dissolution of UO₂. Interactions with divalent groundwater cations have implications for the longevity of UO₂ and the mobilization of U­(VI) from these solids in remediated subsurface environments, waste disposal sites, and natural uranium ores

Ferromagnetic quantum critical point in CePd2 P2 with Pd → Ni substitution

An investigation of the structural, thermodynamic, and electronic transport properties of the isoelectronic chemical substitution series Ce(Pd1-xNix)2P2 is reported, where a possible ferromagnetic quantum critical point is uncovered in the temperature-concentration (T-x) phase diagram. This behavior results from the simultaneous contraction of the unit cell volume, which tunes the relative strengths of the Kondo and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, and the introduction of disorder through alloying. Near the critical region at xcr≈ 0.7, the rate of contraction of the unit cell volume strengthens, indicating that the cerium f valence crosses over from trivalent to a noninteger value. Consistent with this picture, x-ray absorption spectroscopy measurements reveal that while CePd2P2 has a purely trivalent cerium f state, CeNi2P2 has a small (&lt;10 %) tetravalent contribution. In a broad region around xcr, there is a breakdown of Fermi-liquid temperature dependences, signaling the influence of quantum critical fluctuations and disorder effects. Measurements of clean CePd2P2 furthermore show that applied pressure has an initial effect similar to alloying on the ferromagnetic order. From these results, CePd2P2 emerges as a keystone system to test theories such as the Belitz-Kirkpatrick-Vojta model for ferromagnetic quantum criticality, where distinct behaviors are expected in the dirty and clean limits

Examining the Effects of Ligand Variation on the Electronic Structure of Uranium Bis(imido) Species

Arylazide and diazene activation by highly reduced uranium­(IV) complexes bearing trianionic redox-active pyridine­(diimine) ligands, [Cp^PU­(^MesPDI^Me)]₂ (<b>1-Cp</b>^<b>P</b>), Cp*U­(^MesPDI^Me)­(THF) (<b>1-Cp*</b>) (Cp^P = 1-(7,7-dimethylbenzyl)­cyclopentadienide; Cp* = η⁵-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U­(^<i>t</i>Bu-^MesPDI^Me) (THF) (<b>1-</b>^{<i><b>t</b></i>}<b>Bu</b>) (2,6-((Mes)­NCMe)­2-<i>p</i>-R-C₅H₂N, Mes = 2,4,6-trimethylphenyl; R = H, ^MesPDI^Me; R = C­(CH₃)₃, ^<i>t</i>Bu-^MesPDI^Me), has been investigated. While <b>1-Cp*</b> and <b>1-Cp</b>^<b>P</b> readily reduce N₃R (R = Ph, <i>p</i>-tolyl) to form <i>trans</i>-bis­(imido) species, Cp^PU­(NAr)₂(^MesPDI^Me) (Ar = Ph, <b>2-Cp</b>^<b>P</b>; Ar = <i>p</i>-Tol, <b>3-Cp</b>^<b>P</b>) and Cp*U­(NPh)₂(^MesPDI^Me) (<b>2-Cp*</b>), only <b>1-Cp*</b> can cleave diazene NN double bonds to form the same product. Complexes <b>2-Cp*</b>, <b>2-Cp</b>^<b>P</b>, and <b>3-Cp</b>^<b>P</b> are uranium­(V) <i>trans</i>-bis­(imido) species supported by neutral [^MesPDI^Me]⁰ ligands formed by complete oxidation of [^MesPDI^Me]^3– ligands of <b>1-Cp</b>^<b>P</b> and <b>1-Cp*</b>. Variation of the arylimido substituent in <b>2-Cp*</b> from phenyl to <i>p</i>-tolyl, forming Cp*U­(NTol)₂(^MesPDI^Me) (<b>3-Cp*</b>), changes the electronic structure, generating a uranium­(VI) ion with a monoanionic pyridine­(diimine) radical. The <i>tert</i>-butyl-substituted analogue, Cp*U­(NTol)₂(^<i>t</i>Bu-^MesPDI^Me) (<b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>), displays the same electronic structure. Oxidation of the ligand radical in <b>3-Cp*</b> and <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b> by Ag­(I) forms cationic uranium­(VI) [Cp*U­(NTol)₂(^MesPDI^Me)]­[SbF₆] (<b>4-Cp*</b>) and [Cp*U­(NTol)₂(^<i>t</i>Bu-^MesPDI^Me)]­[SbF₆] (<b>4-</b>^{<i><b>t</b></i>}<b>Bu</b>), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent <b>2-Cp*</b> with AgSbF₆ affords cationic [Cp*U­(NPh)₂(^MesPDI^Me)]­[SbF₆] (<b>5-Cp*</b>) from a metal-based U­(V)/U­(VI) oxidation. All complexes have been characterized by multidimensional NMR spectroscopy with assignments confirmed by electronic absorption spectroscopy. The effective nuclear charge at uranium has been probed using X-ray absorption spectroscopy, while structural parameters of <b>1-Cp</b>^<b>P</b>, <b>3-Cp*</b>, <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>, <b>4-Cp*</b>, <b>4-</b>^{<i><b>t</b></i>}<b>Bu</b>, and <b>5-Cp*</b> have been elucidated by X-ray crystallography