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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Covalency in Actinide Compounds

Covalency in actinides has emerged as a resounding research topic on account of the technological importance in separating minor actinides from lanthanides for spent nuclear fuel processing, and utilization of their distinct bonding properties has been realized as a route towards overcoming this challenge. Because of the limited radial extent of the 4f orbitals, there is almost no 4f electron participation in bonding in lanthanides; this is not the case for the actinides, which have extended 5f orbitals that are capable of overlapping with ligand orbitals, although not to the degree of overlap as in the d orbitals of transition metals. In this concept paper, we provide a general description of covalency in actinide compounds. After introducing two main approaches to enhance covalency, either by exploiting increased orbital overlap or decreasing energy differences between the orbitals causing orbital energy degeneracy, we show the current state of the field using several examples from the recent literature. Here, we will conclude by proposing the use of actinide chalcogenides as a convenient auxiliary tool to study covalency in actinide compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Synthesis and Characterization of Novel Actinide Compounds (LDRD External Report Summary)

It is important to gain new understanding of actinides in order to understand the fate of these elements in the environment and the factors that contribute to actinide-lanthanide extraction selectivity. Detailed structural, magnetic, and electronic studies of novel actinide compounds can provide useful information on bonding behavior to guide the development of novel separation and environmental remediation strategies. Development of new actinide compounds remains a relatively open area to study due to the numerous challenges faced when performing chemical synthesis with radioactive materials. In this LDRD, we sought to synthesize and characterize single crystals of novel polyoxometalates that selectively encapsulate plutonium and uranium. We were successful in preparation of [U III Pd II 12 (AsPh) 8 O 32 ] 5⁻ ; the first actinide-based polyoxopalladate.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2019

Charge densities in actinide compounds: strategies for data reduction and model building

The data quality requirements for charge density studies on actinide compounds are extreme. Important steps in data collection and reduction required to obtain such data are summarized and evaluated. The steps involved in building an augmented Hansen–Coppens multipole model for an actinide pseudo-atom are provided. The number and choice of radial functions, in particular the definition of the core, valence and pseudo-valence terms are discussed. The conclusions in this paper are based on a re-examination and improvement of a previously reported study on [PPh 4 ][UF 6 ]. Topological analysis of the total electron density shows remarkable agreement between experiment and theory; however, there are significant differences in the Laplacian distribution close to the uranium atoms which may be due to the effective core potential employed for the theoretical calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

High-resolution X-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds

Applying the high-energy resolution fluorescence-detection (HERFD) mode of X-ray absorption spectroscopy (XAS), we were able to probe, for the first time to our knowledge, the crystalline electric field (CEF) splittings of the 5f shell directly in the HERFD-XAS spectra of actinides. Using ThO 2 as an example, data measured at the Th 3d edge were interpreted within the framework of the Anderson impurity model. Because the charge-transfer satellites were also resolved in the HERFD-XAS spectra, the analysis of these satellites revealed that ThO 2 is not an ionic compound as previously believed. The Th 6d occupancy in the ground state was estimated to be twice that of the Th 5f states. Here, we demonstrate that HERFD-XAS allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Novel Relativistic Electronic Structure Theories for Actinide-Containing Compounds

Actinides of importance to basic energy sciences contain electrons moving at speed comparable to the speed of light. Reliable computational simulation of these electrons and hence actinide chemistry requires accurate description of relativistic effects. The present project advances computational actinide chemistry with development of new methodologies, algorithms, and computer programs in relativistic quantum chemistry, as well as applications to actinide chemistry and spectroscopy. A new “electrons-only” exact two-component approach has been developed to provide efficient treatments of relativistic effects, while maintaining chemical accuracy. New computational algorithms developed here extend the applicability of relativistic electron-correlation methods to larger molecules. The method-development work in this project also features the first implementation of analytic gradient technique for relativistic electron-correlation methods, which provides significantly enhanced ability to compute properties for molecules containing actinides. The applicability and usefulness of these new methods and computer programs have been demonstrated in calculations of actinide-containing molecules to facilitate understanding of actinide chemistry and spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Experimental Electron Density Distribution in Actinide Compounds – an Experimental Atoms in Molecules (AIM) Approach

Understanding the basic chemistry of the actinides, in particular of uranium, is essential to address issues related to nuclear fuels, nuclear waste and environmental concerns. Atoms in general are chemically bound to their neighbors via the distribution of electrons within the intervening space. However, this only represents a small proportion of the total electrons, the others are tightly bound to the atomic nuclei and do not participate in chemical bonding. The distribution of electrons in crystalline materials may be examined using the scattering of X-rays. As all electrons scatter X-rays, obtaining the information about the bonding electrons in the presence of a large excess of “irrelevant” electrons requires extremely accurate scattering measurements. Indeed, until recently this was considered impossible for elements as heavy as uranium. In our current project we have made significant improvements in the data quality obtainable for uranium containing compounds. In order to determine the electron density distribution we fit a mathematical model describing the scattering effect to the observed scattering data. This mathematical model has a number of variable parameters that describe the radial and angular distribution of electrons around an atomic center. We have developed such a model to be particularly useful for elements as heavy as uranium. Given the best fit parameters from this model, we are able to map the electron distribution around all atoms in a chemical compound. The topology of this distribution provides us with important characteristics with regard to the chemical bonds. We have now applied these methods to several uranium compounds. In particular we have studied the bonds made by uranium to oxygen, fluorine and chlorine in a number of different environments. The experimental protocols and data analysis protocols developed during this project will permit others in the field to explore the chemical bonding of other heavy element systems. From the few compounds studied to date, we are already discovering the parameters that are characteristic of bonding in such systems, and providing essential experimental benchmarks for theoreticians.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Electrodeposition of actinide compounds from an aqueous ammonium acetate matrix. Experimental development and optimization

Electrodeposition is a technique that is routinely employed in nuclear research for the preparation of thin solid films of actinide materials which can be used in accelerator beam bombardments, irradiation studies, or as radioactive sources. The present study investigates the deposition of both lanthanides and actinides from an aqueous ammonium acetate electrolyte matrix. Electrodepositions were performed primarily on stainless steel disks; with yield analysis evaluated using -spectroscopy. Experimental parameters were studied and modified in order to optimize the uniformity and adherence of the deposition while maximizing the yield. The initial development utilized samarium as the plating material, with and without a radioactive tracer. As a result, surface characterization studies were performed by scanning electron microscopy, electron microprobe analysis, radiographic imaging, and x-ray diffraction.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2023

The Potential Interactions of Novec 1230 and Its Thermal Degradation Products with Actinide Metals and Oxides

The environmentally friendly and mainly chemically inert fire suppressant, Novec 1230, is being increasingly used worldwide. However, no published study has considered using Novec 1230 with fires involving radioactive material including actinides; here, therefore, this research note focuses on the possible interactions between Novec 1230 and its thermal degradation products (TDPs) with some actinide compounds (e.g., actinide metal and oxides) commonly present in a radiological facility in the event of a fire. Previous studies of halogenated fire suppressants used in actinide metal fires indicate a possibility for Novec 1230 or its TDPs to chemically interact with actinide compounds at elevated temperatures; however, these reactions are highly unlikely to cause either runaway exothermic reactions or gaseous actinide release.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Machine learning study of magnetism in uranium-based compounds

Actinide and lanthanide-based materials display exotic properties that originate from the presence of itinerant or localized f electrons and include unconventional superconductivity and magnetism, hidden order, and heavy-fermion behavior. Due to the strongly correlated nature of the 5f electrons, magnetic properties of these compounds depend sensitively on applied magnetic field and pressure, as well as on chemical doping. However, precise connection between the structure and magnetism in actinide-based materials is currently unclear. In this investigation, we established such structure-property links by assembling and mining two datasets that aggregate, respectively, the results of high-throughput density functional theory simulations and experimental measurements for the families of uranium- and neptunium-based binary compounds. Various regression algorithms were utilized to identify correlations among accessible attributes (features or descriptors) of the material systems and predict their cation magnetic moments and general forms of magnetic ordering. Descriptors representing compound structural parameters and cation f-subshell occupation numbers were identified as most important for accurate predictions. The best machine learning model developed employs the random forest regression algorithm. It can predict both spin and orbit moment size with root-mean-square error of 0.17 μ B and 0.19 μ B , respectively. Lastly, the random forest classification algorithm is used to predict the ordering (paramagnetic, ferromagnetic, and antiferromagnetic) of such systems with 76% accuracy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ligand induced shape transformation of thorium dioxide nanocrystals

Nanocrystals (NCs) with size and shape dependent properties are a thriving research field. Significant progress has been made in the controlled synthesis of NCs of stable elements in the past two decades; however, the knowledge of the NCs of actinide compounds has been considerably limited due the difficulties in handling them both experimentally and theoretically. Actinide compounds, especially actinide oxides, play a critical role in many stages of the nuclear fuel cycle. Recently, a non-aqueous surfactant assisted approach has been developed for the synthesis of actinide oxide NCs with different morphologies, but an understanding of its control factors is still missing to date. Herein we present a comprehensive study on the low index surfaces of thorium dioxide (ThO 2 ) and their interactions with relevant surfactant ligands using density functional calculations. A systematic picture on the thermodynamic stability of ThO 2 NCs of different sizes and shapes is obtained employing empirical models based on the calculated surface energies. It is found that bare ThO 2 NCs prefer the octahedral shape terminated by (111) surfaces. Oleic acid displays selective adsorption on the (110) surface, leading to the shape transformation from octahedrons to nanorods. Other ligands such as acetylacetone, oleylamine, and trioctylphosphine oxide do not modify the equilibrium shape of ThO 2 NCs. This research provides atomic level insights into the anisotropic growth of ThO2 NCs that was recently observed in experiments, and thus may contribute to the controlled synthesis of actinide oxide NCs with well-defined size and shape for future applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Analysis of the thermodynamic stability of ThO 2 and UO 2 surfaces in the presence of surfactant ligands

Nanocrystals (NCs) with size and shape dependent properties are a thriving research field. Remarkable progress has been made in the controlled synthesis of NCs of stable elements in the past two decades; however, the knowledge of the NCs of actinide compounds has been considerably limited due the difficulties in handling them both experimentally and theoretically. Actinide compounds, especially actinide oxides, play a critical role in many stages of the nuclear fuel cycle. Recently, a non-aqueous surfactant assisted approach has been developed for the synthesis of actinide oxide NCs with different morphologies, but an understanding of its control factors is still missing to date. Herein we present a comprehensive study on the low index surfaces of thorium dioxide (ThO 2 ) and their interactions with relevant surfactant ligands using density functional calculations. A systematic picture on the thermodynamic stability of ThO 2 NCs of different sizes and shapes is obtained employing empirical models based on the calculated surface energies. It is found that bare ThO 2 NCs prefer the octahedral shape terminated by (111) surfaces. Oleic acid displays selective adsorption on the (110) surface, leading to the shape transformation from octahedrons to nanorods. Other ligands such as acetylacetone, oleylamine, and trioctylphosphine oxide do not modify the equilibrium shape of ThO 2 NCs. This work provides atomic level insights into the anisotropic growth of ThO 2 NCs that was recently observed in experiments, and thus may contribute to the controlled synthesis of actinide oxide NCs with well-defined size and shape for future applications.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

High Pressure X-ray Absorption Studies on Correlated-Electron Systems

This project used high pressure to alter the electron-electron and electron-lattice interactions in rare earth and actinide compounds. Knowledge of these properties is the starting points for a first-principles understanding of electronic and electronically related macroscopic properties. The research focused on a systematic study of x-ray absorption measurements on rare earth and actinide compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2012

JOWOG 22/2 - Actinide Chemical Technology (July 9-13, 2012)

The Plutonium Science and Manufacturing Directorate provides world-class, safe, secure, and reliable special nuclear material research, process development, technology demonstration, and manufacturing capabilities that support the nation's defense, energy, and environmental needs. We safely and efficiently process plutonium, uranium, and other actinide materials to meet national program requirements, while expanding the scientific and engineering basis of nuclear weapons-based manufacturing, and while producing the next generation of nuclear engineers and scientists. Actinide Process Chemistry (NCO-2) safely and efficiently processes plutonium and other actinide compounds to meet the nation's nuclear defense program needs. All of our processing activities are done in a world class and highly regulated nuclear facility. NCO-2's plutonium processing activities consist of direct oxide reduction, metal chlorination, americium extraction, and electrorefining. In addition, NCO-2 uses hydrochloric and nitric acid dissolutions for both plutonium processing and reduction of hazardous components in the waste streams. Finally, NCO-2 is a key team member in the processing of plutonium oxide from disassembled pits and the subsequent stabilization of plutonium oxide for safe and stable long-term storage.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Evaluation of f -element borate chemistry

The synthesis, structure elucidation, and spectroscopic measurements of a myriad of new, f-element borates has revealed the unusual effects that this electron-rich oxoanion has on the electronic properties of lanthanides and actinides. One purpose of these studies was to provide models for actinide compounds that may exist in either vitrified nuclear waste or in repositories located within salt deposits that have significant borate content. In addition, the radiation-damage resilience of polyborate networks positions these materials as suitable candidates for probing coordination chemistry and physical properties much deeper into the actinide series than is normally possible, and compounds with actinides up to californium (Z = 98) have been successfully prepared and characterized in exquisite detail. Structural determination of these materials show that both the lanthanide and actinide series display previously unknown coordination chemistry, but, more importantly, that the two series have little overlap in terms of structure and composition, and have few parallels. In addition, some of these compounds display unique physico-chemical properties, one example of which is the selective trapping of radionuclides. The foremost discovery first identified in actinide borates is that the chemistry of californium represents an onset of unprecedented chemical behavior that compares better with high-oxidation state, early transition metal complexes than it does with earlier f-elements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

A Tetrapositive Metal Ion in the Gas Phase: Thorium(IV) Coordinated by Neutral Tridentate Ligands

ESI of 1:1 mixtures of Th(ClO₄)₄ and ligand TMOGA in acetonitrile resulted in the observation of the TMOGA supported tetracation, Th(L)₃⁴⁺, in the gas phase. Three TMOGA ligands are necessary to stabilize the tetrapositive thorium ion; no Th(L)₂⁴⁺ or Th(L)₄⁴⁺ was observed. Theoretical calculations reveal that the Th(L)₃⁴⁺ complex possesses C₃ symmetry with the thorium center coordinated by nine oxygen atoms from three ligands, which forms a twisted TPP geometry. Actinide compounds with such a geometry feature a nine-coordinate chiral actinide center. The Th-L binding energy and bond orders of Th(L)n⁴⁺ decrease as the coordination number increases, consistent with the trend of concurrently increasing Th-O distances. The Th-O bonding is mainly electrostatic in nature, but the covalent interactions are not negligible. CID of the Th(L)₃⁴⁺ complex mainly resulted in charge reduction to form Th(L)₂(L-86)³⁺oss of neutral TMOGA was not observed. The protic ligand methanol stabilized only tri- and dications of ligated thorium. The intensity of the Th(L)₃⁴⁺ peak was reduced as the percentage of water increased in the Th(ClO₄)₄/TMOGA solution.

Actinides, Density Functional Theory, Gas-Phase Ch↗