Cerium Substitution in Yttrium Iron Garnet: Valence State, Structure, and Energetics
Abstract not provided
Guo, Xiaofeng↗
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Covalency-Driven Differences in the Hydrogenation Chemistry of Lanthanide- and Actinide-Based Frustrated Lewis Pairs
The electronic organization of Frustrated Lewis Pairs (FLPs) allows them to activate strong bonds in mechanisms that are usually free of redox events at the Lewis acidic site. The unique 6d/5f manifold of uranium could serve as an interesting FLP acceptor site, but to date FLP-like catalysis with actinide ions is unknown. In this paper, the catalytic, FLP-like hydrogenation reactivity of trivalent uranium complexes is explored in the presence of base-stabilized silylenes. Comparison to isoelectronic, isostructural lanthanide and thorium complexes lends insight into the electronic factors governing dihydrogen activation. Mechanistic studies of the uranium- and lanthanide-catalyzed hydrogenations are presented, including discussion of likely intermediates. Computational modeling of the f-element complexes, combined with experimental comparison to p-block Lewis acids, elucidates the relevance of steric hindrance to productive reactivity with dihydrogen. As a result, consideration of the complete experimental and theoretical evidence provides a clear picture of the electronic and steric factors governing dihydrogen activation by these FLPs.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
The Significance of Multivalent Bonding Motifs and “Bond Order” in DNA-Directed Nanoparticle Crystallization
Multivalent oligonucleotide-based bonding elements have been synthesized and studied for the assembly and crystallization of gold nanoparticles. Through the use of organic branching points, divalent and trivalent DNA linkers were readily incorporated into the oligonucleotide shells that define DNA-nanoparticles and compared to monovalent linker systems. These multivalent bonding motifs enable the change of "bond strength" between particles and therefore modulate the effective "bond order." In addition, the improved accessibility of strands between neighboring particles, either due to multivalency or modifications to increase strand flexibility, gives rise to superlattices with less strain in the crystallites compared to traditional designs. Furthermore, the increased availability and number of binding modes also provide a new variable that allows previously unobserved crystal structures to be synthesized, as evidenced by the formation of a thorium phosphide superlattice.
Thaner, Ryan V.↗
Transient Triamidoamine Neptunium(V)–Mono(Imido) Complexes: C–H Activations and Hydrogen Atom Transfer Driven by Effective Nuclear Charge
Metal-mono(imido) linkages have been known for seven decades, and they are found in transition metal, main group, lanthanide, thorium, and uranium complexes. However, transuranium-mono(imido) complexes remain unknown in any scenario. Here, we present evidence for transient neptunium(V)–mono(imido) complexes. Treatment of [Np III (Tren TIPS )] (1, Tren TIPS = {N(CH 2 CH 2 NSiPr i 3 ) 3 } 3– ) with N 3 R (R = SiMe 3 ; 1-adamantyl, Ad) results in N 2 evolution and dark purple solutions consistent with the formation of [Np V (Tren TIPS )(NR)] (3NpNR). However, solutions of 3NpNR rapidly turn orange, where for R = SiMe 3 the isolated 1:1 products are [Np IV (Tren TIPS ){N(H)SiMe 3 }] (4a) and [Np IV (Tren TIPS-2H ){N(H)SiMe 3 }] (4b, Tren TIPS-2H = {N(CH 2 CH 2 NS i Pri 3 ) 2 (NCH 2 CH 2 NSiPr i 2 C[Me]=CH 2 )} 3– ). The latter contains a dehydrogenated-Pr i vinyl functionality accounting for the source of the two amido H atoms. The reaction for R = Ad proceeds similarly, but only [Np IV (Tren TIPS ){N(H)Ad}] (5a) could be unequivocally confirmed, though its isolation suggests generality of the imido-to-amido functional group transformation. Complexes 4a/4b exhibit slow relaxation of their magnetization, adding to the small number of transuranium single ion magnets. Experimental and computational analysis suggests that the amido products are formed by C–H activation and two sequential hydrogen atom transfer reactions involving a three-step proton-coupled electron-transfer sequence of H • radical abstraction, electron transfer, then another H • radical abstraction step. In contrast to transient 3NpNR, the 5f 2 uranium(IV)-imido complex [K(2.2.2-cryptand)][U IV (Tren TIPS )(NSiMe 3 )] (8UNSiMe 3 ) is robust, even in boiling THF, suggesting the transience of 5f 2 3NpNR is not due to the 5f n -count but the increased effective nuclear charge of neptunium vs uranium. This work highlights divergence of uranium- and neptunium-imido stabilities, emphasizing that the latter is an inherently challenging synthetic target.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Continental degassing of 4He by surficial discharge of deep groundwater
Radiogenic He-4 is produced by the decay of uranium and thorium in the Earths mantle and crust. From here, it is degassed to the atmosphere(1-5) and eventually escapes to space(1,5,6). Assuming that all of the He-4 produced is degassed, about 70% of the total He-4 degassed from Earth comes from the continental crust(2,-5,7). However, the outgoing flux of crustal He-4 has not been directly measured at the Earths surface(2) and the migration pathways are poorly understood(2-4,7,8). Here we present measurements of helium isotopes and the long-lived cosmogenic radio-isotope Kr-81 in the deep, continental-scale Guarani aquifer in Brazil and show that crustal He-4 reaches the atmosphere primarily by the surficial discharge of deep groundwater. We estimate that He-4 in Guarani groundwater discharge accounts for about 20% of the assumed global flux from continental crust, and that other large aquifers may account for about 33%. Old groundwater ages suggest that He-4 in the Guarani aquifer accumulates over half- to one-million-year timescales. We conclude that He-4 degassing from the continents is regulated by groundwater discharge, rather than episodic tectonic events, and suggest that the assumed steady state between crustal production and degassing of He-4, and its resulting atmospheric residence time, should be re-examined
54 ENVIRONMENTAL SCIENCES↗
Machine learning potential assisted exploration of complex defect potential energy surfaces
Abstract Atomic-scale defects generated in materials under both equilibrium and irradiation conditions can significantly impact their physical and mechanical properties. Unraveling the energetically most favorable ground-state configurations of these defects is an important step towards the fundamental understanding of their influence on the performance of materials ranging from photovoltaics to advanced nuclear fuels. Here, using fluorite-structured thorium dioxide (ThO 2 ) as an exemplar, we demonstrate how density functional theory and machine learning interatomic potential can be synergistically combined into a powerful tool that enables exhaustive exploration of the large configuration spaces of small point defect clusters. Our study leads to several unexpected discoveries, including defect polymorphism and ground-state structures that defy our physical intuitions. Possible physical origins of these unexpected findings are elucidated using a local cluster expansion model developed in this work.
36 MATERIALS SCIENCE↗
Isolation and characterization of a californium metallocene
Californium (Cf) is currently the heaviest element accessible above microgram quantities. Cf isotopes impose severe experimental challenges due to their scarcity and radiological hazards. Consequently, chemical secrets ranging from the accessibility of 5f/6d valence orbitals to engage in bonding, the role of spin–orbit coupling in electronic structure, and reactivity patterns compared to other f elements, remain locked. Organometallic molecules were foundational in elucidating periodicity and bonding trends across the periodic table, with a twenty-first-century renaissance of organometallic thorium (Th) through plutonium (Pu) chemistry, and to a smaller extent americium (Am), transforming chemical understanding. Yet, analogous curium (Cm) to Cf chemistry has lain dormant since the 1970s. Here, we revive air-/moisture-sensitive Cf chemistry through the synthesis and characterization of [Cf(C 5 Me 4 H) 2 Cl 2 K(OEt 2 )] n from two milligrams of 249 Cf. This bent metallocene motif, not previously structurally authenticated beyond uranium (U), contains the first crystallographically characterized Cf–C bond. Analysis suggests the Cf–C bond is largely ionic with a small covalent contribution. Lowered Cf 5f orbital energy versus dysprosium (Dy) 4f in the colourless, isoelectronic and isostructural [Dy(C 5 Me 4 H) 2 Cl 2 K(OEt 2 )] n results in an orange Cf compound, contrasting with the light-green colour typically associated with Cf compounds.
Chemical bonding↗
Investigation of a tellurium-packed column for isolation of astatine-211 from irradiated bismuth targets and demonstration of a semi-automated system
Astatine-211 is an attractive radionuclide for use in targeted alpha therapy of blood-borne diseases and micrometastatic diseases. Efficient isolation methods that can be adapted to robust automated 211 At isolation systems are of high interest for improving the availability of 211 At. Based on the early studies of Bochvarova and co-workers involving isolation of 211 At from irradiated thorium targets, we developed a method for 211 At isolation from bismuth targets using tellurium-packed columns. Dissolution of irradiated bismuth targets is accomplished using HNO 3 ; however, 211 At is not captured on the Te column material in this matrix. Our method involves slow addition of aqueous NH 2 OH·HCl to the Bi target dissolved in HNO 3 to convert to a HCl matrix. The amount of NH 2 OH·HCl was optimized because (1) the quantity of NH 2 OH·HCl used appears to affect the radiolabeling yield of phenethyl- closo -decaborate(2-) (B10)-conjugated antibodies and (2) reducing the volume of NH 2 OH·HCl solution can effectively shorten the overall isolation time. A proof-of-concept semi-automated process has been demonstrated using targets containing ~0.96 GBq (~26 mCi) of 211 At. High isolation yields (88–95%) were obtained. Radiochemical purity of the isolated 211 At was assessed by radio-HPLC. Concentrations of Bi and Te contaminants in the 211 At and the astatinated antibodies were evaluated using ICP-MS.
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
Stability of aqueous neodymium complexes in carbonate-bearing solutions from 100–600 °C
Rare earth element exploration requires a quantitative understanding of factors governing their mobilization and economic concentration. However, the behavior of rare earth elements in carbonate- bearing hydrothermal fluids associated with carbonatite-hosted deposits is poorly understood, and conflicting mechanisms of rare earth transport by anionic ligands and alkali behavior have been described. Here, we report quantitative data to characterize the role of carbonate-bearing solutions in the hydrothermal mobilization of neodymium. Solubility studies of neodymium phosphate were performed at temperatures ranging from 100 to 600 °C in carbonate-bearing solutions. The thermodynamic data determined for the predominant complex were used to model the separation of neodymium from thorium in a simple flow-through system based on fluid and mineral compositions characteristic of carbonatite deposits. Our data suggest that neodymium transport is controlled by the stability of the carbonate species NdCO 3 OH o , and at temperatures of 500–600 °C, the concentrations of neodymium in solutions can reach ~1000 ppm.
58 GEOSCIENCES↗
Phenylsilane as a safe, versatile alternative to hydrogen for the synthesis of actinide hydrides
The thorium and uranium dihydride dimer complexes [(C 5 Me 5 ) 2 An(H)(μ-H)] 2 (An = Th, U) have been easily prepared using phenylsilane, which is an efficient and safer alternative to hydrogen gas. We demonstrated the synthetic utility of this new hydriding method by the preparation of a variety of organometallic complexes, including, for the first time, (C 5 Me 5 ) 2 U(SMe) 2 , (C 5 Me 5 ) 2 Th(C 4 Ph 4 ), (C 5 Me 5 ) 2 U(C 4 Ph 4 ), (C 5 Me 5 ) 2 ThS 5 , and (C 5 Me 5 ) 2 U(bipy) using [(C 5 Me 5 ) 2 An(H)(μ-H)] 2 (An = Th, U) as multi-electron reductants.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Thermodynamic calculations of oxygen self-diffusion in mixed-oxide nuclear fuels
Mixed-oxide fuels containing uranium with thorium and/or plutonium may play an important part in future nuclear fuel cycles. There are, however, significantly less data available for these materials than conventional uranium dioxide fuel. In the present study, we employ molecular dynamics calculations to simulate the elastic properties and thermal expansivity of a range of mixed oxide compositions. These are then used to support equations of state and oxygen self-diffusion models to provide a self-consistent prediction of the behaviour of these mixed oxide fuels at arbitrary compositions.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
Synthesis, structure and bonding of actinide disulphide dications in the gas phase
Actinide disulphide dications, AnS 2 2+ , were produced in the gas phase for An = Th and Np by reaction of An 2+ cations with the sulfur-atom donor COS, in a sequential abstraction process of two sulfur atoms, as examined by FTICR mass spectrometry. For An = Pu and Am, An 2+ ions were unreactive with COS and did not yield any sulphide species. High level multiconfigurational (CASPT2) calculations were performed to assess the structures and bonding of the new AnS 2 2+ species obtained for An = Th, Np, as well as for An = Pu to examine trends along the An series, and for An = U to compare with a previous experimental study and DFT computational scrutiny of US 2 2+ . The CASPT2 results showed that, like in the case of uranium, the new AnS 2 2+ ions have ground states with triangular geometries, corresponding to the presence of a persulphide in the case of thorium that formally leads to a stable Th IV S 2 2+ species, while a supersulphide appears to be present in the case of U, Np and Pu, formally leading to a An III S 2 2+ species. Finally, the computations also revealed that linear thioactinyl structures are higher in energy, with a difference that increases fourfold upon moving from U to Pu, apparently indicating that it will be even more pronounced for Am.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Actinide tetra-N-heterocyclic carbene ‘sandwiches’
Highly-symmetrical, thorium and uranium octakis-carbene ‘sandwich’ complexes have been prepared by ‘sandwiching’ the An( iv ) cations between two anionic macrocyclic tetra-NHC ligands, one with sixteen atoms and the other with eighteen atoms.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Structural transformation of metal oxo species within UiO-66 type metal–organic frameworks
Studying the coordination of actinide-based metal oxo clusters can provide valuable insights for nuclear energy technologies and radioactive waste containment. Metal–organic frameworks serve as a platform to directly interrogate the structure and properties of understudied actinide elements, including thorium. Examples of structural evolutions within Th oxo species within MOFs are rare yet relevant for nuclear waste speciation in solution. Herein, we report the serendipitous discovery of the structural evolution of Th-UiO-66 containing a hexanuclear Th node to a mononuclear Th(bdc) 2 (dmf) 2 upon the evaporation of solvent from the reaction. We observe a partial reversal of Th(bdc) 2 (dmf) 2 back to Th-UiO-66 upon hydrothermal treatment, indicating the complex dynamics of Th oxo species in solution. As a result, we report that isolated Ce-UiO-66 similarly transforms to a newly isolated 1D Ce III carboxylate chain MOF named NU-351 under the same conditions as Th-UiO-66, while Zr-UiO-66 and Hf-UiO-66 retain their structures.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
A versatile strategy for the formation of hydride-bridged actinide–iridium multimetallics
Uranium- and thorium-iridium multimetallic species with unprecedented actinide–iridium interactions are prepared via salt-elimination reactions between U/Th halides and K[IrCp*H 3 ].
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
The inorganic chemist's guide to actinide radiation chemistry: a review
This review aims to provide an overview of the current state of radiation chemistry with respect to the actinide elements, thorium through californium. Despite the inherent radioactivity of the actinides, only a few studies explore the effects of ionizing radiation on their redox chemistry and surrounding environment. This fundamental knowledge gap, coupled with the current renaissance in actinide-based technologies such as nuclear power, space exploration, and medicine, underscores the importance of research in this interdisciplinary area. This review will focus on the interactions between reactive species formed by radiolysis with actinides and their complexes, offering an inorganic chemist's perspective on research in radiation chemistry. In addition, a thorough discussion of our current understanding of radiation-induced changes in actinide speciation in both aqueous solution and the solid-state will be provided, focusing on changes in oxidation state distribution, complexation, and secondary coordination effects within inorganic materials. Finally, this review will discuss challenges and opportunities for inorganic chemists to explore this unique intersection of fields.
Actinides↗
Comparison of Ce( iv )/Th( iv )-alkynyl complexes and observation of a trans -influence ligand series for Ce( iv )
Organometallic cerium(iv) complexes have been challenging to isolate and characterize due to the strongly oxidizing nature of the cerium(iv) cation. Herein, we report two cerium(iv) alkynyl complexes, [Ce(TriNOx)(C[triple bond, length as m-dash]C-SiMe3)] (1-CeTMS) and [Ce(TriNOx)(C[triple bond, length as m-dash]C-Ph)] (1-CePh) (TriNOx3- = tris(2-tert-butylhydroxylaminato)benzylamine), that include terminal alkyne moieties. The isostructural thorium analogue [Th(TriNOx)(C[triple bond, length as m-dash]C-SiMe3)] (1-ThTMS) was also synthesized and compared with 1-CeTMS in bond distance, 13C-NMR spectra, vibrational spectra and electronic structure. The Ce-C bond distances were 2.501(3) Å for 1-CePh and 2.513(5) Å for 1-CeTMS on the shorter end of the few reported CeIV-C single bonds (2.478(3)-2.705(2) Å), possibly indicating significant Ce 5d- and 4f-orbital involvement. 13C-NMR spectroscopy was also consistent with Ce-C covalency, with significantly deshielded resonances ranging from 185-213 ppm. Such 13C-NMR shifts demonstrate a strong influence from spin-orbit coupling (SOC) effects, corroborated by computational studies. Raman analysis showed ν C[triple bond, length as m-dash]C stretching frequencies of 2000 cm-1 (1-CeTMS) and 2052 cm-1 (1-CePh), indicating the cerium(iv)-alkynyl interaction, compared to the parent HC[triple bond, length as m-dash]CPh (IR = 2105 cm-1 and Raman = 2104 cm-1). L3-edge X-ray absorption measurements revealed a predominant Ce(iv) electronic configuration, and magnetic measurements revealed temperature-independent paramagnetism. Electrochemical studies similarly revealed the electron donating ability of the alkynyl ligands, stronger than either fluoride or imido ligands for the Ce(iv)(TriNOx)-framework, with a cerium(iv/iii) reduction potential of E pc = -1.58 to -1.66 V vs. Fc/Fc+. Evidence for a trans-influence has been observed by evaluating a series including previously reported [CeIV(TriNOx)X]+/0 complexes with axial ligands X = THF, I-, Br-, Cl-, F-, -C[triple bond, length as m-dash]C-Ph, -C[triple bond, length as m-dash]C-SiMe3, -NH(3,5-(CF3)2-Ar), -OSiPh3, -N(M(L))(3,5-(CF3)2-Ar) [M(L) = Li(TMEDA), K(DME)2 or Cs(2,2,2-crypt)]. These data stand in contrast with previous reports of an inverse trans-influence at cerium(iv) and point to differences in involvement of cerium 4f- versus 5d-orbitals in the electronic structures of the complexes.
Yang, Qiaomu↗
Helium refrigeration considerations for cryomodule design
Many of the present day accelerators are based on superconducting radio frequency (SRF) cavities, packaged in cryo-modules (CM), which depend on helium refrigeration at sub-atmospheric pressures, nominally 2 K. These specialized helium refrigeration systems are quite cost intensive to produce and operate. Particularly as there is typically no work extraction below the 4.5-K supply, it is important that the exergy loss between this temperature level and the CM load temperature(s) be minimized by the process configuration choices. This paper will present, compare and discuss several possible helium distribution process arrangements to support the CM loads.
43 PARTICLE ACCELERATORS↗