87 Search Results

A new quaternary 3D chalcogenide, Na ₂ Si ₆ B ₁₂ Se ₁₈ , was synthesized via solid-state methods and features a unique framework containing the rare icosahedral [B ₁₂ ] ¹⁰⁺ cation. The tunnels in this 3D framework structure are occupied by Na ⁺ ions.

The crystal chemistry of americium-containing extended structures was investigated, and several classes of americium-containing solid-state oxide materials were obtained in single-crystal form via high-temperature flux crystal growth. This enabled the structural characterization of rare examples of ternary, quaternary, and penternary americium-containing silicates K₃Am (Si₂O₇) and Cs₆Am₂Si₂₁O₄₈, phosphates Na₃Am (PO₄)₂ and K₃Am (PO₄)₂, borates Ba₃Am₂(BO₃)₄ and AmBO₃, borate halides Ca₅Am(BO₃)₄Cl, molybdates Li_0.5Am_0.5MoO₄, and fluorides CsAm₂F₇. Using these crystallographic data, the ionic radii of Am³⁺ with coordination numbers of six (0.975 Å), seven (1.052 Å), and nine (1.162 Å) were established. A maximum entropy method (MEM) analysis was performed on the single-crystalmore » X-ray diffraction data that were collected for K₃Nd(PO₄)₂/K₃Am(PO₄)₂, K₃NdSi₂O₇/K₃AmSi₂O₇, and NdBO₃/AmBO₃, to qualitatively compare the ionicities of the Nd–O and Am–O bonds. In conclusion, Raman spectroscopy data were collected on single crystals of K₃Am(PO₄)₂ and compared to the calculated Raman spectrum of K₃Am(PO₄)₂ obtained from DFT calculations.« less

In nature, many complex multi-physics coupling problems exhibit strong diffusivity inhomogeneity. For instance, in the context of radionuclide absorption by porous wasteform materials within a flowing waste stream, the difference of species’ diffusivity in solid and liquid phases spans by 3~8 orders of magnitude. To solve the diffusion equations with strongly inhomogeneous diffusivity, traditional discretization-based methods, such as the Finite Difference Method (FDM), require infinitesimally small time steps (<10^-10) as high spatial resolutions are employed in most microstructure evolution processes, leading to prohibitively high computational costs. Here, this work developed an integrated numerical approach (FDiRW: Finite Difference informed Random Walk)more » to tackle this challenge. The idea is that utilizing the Random Walk concept, the fast diffusion is modeled as a superposition of point source’s solution for a concentration distribution while FDM is used to obtain the point source’s solution at each node. A mesh-coarsening algorithm is developed to generate an exclusive coarse mesh for FDiRW approach to maximize its efficiency. The effectiveness of the coarse mesh-based FDiRW approach is validated by benchmarking Finite Difference solutions. Numerical results demonstrated that FDiRW achieves a remarkable 1000x computational efficiency improvement over FDM while preserving desired accuracy for a medium-sized model of 192 × 192 × 192 grids. Finally, as models scale up, a floating-point operations (PLOPs) analysis of the FDiRW algorithm reveals that its computational complexity grows quadratically in terms of the number of nodes employed in computation.« less

Few plutonium containing silicates have been reported in the literature and only one other tetravalent plutonium silicate structure has been reported using the flux growth method. With the goal of understanding actinide incorporation into silicates, we describe herein two new plutonium (IV) silicates, Rb₂PuSi₆O₁₅ and K₂PuSi₂O₇, synthesized using a mixed alkali chloride/alkali fluoride flux growth. Both structures crystallize in the C2/c space group with lattice parameters a=24.2980(10) Å, b=7.1466(3) Å, c=17.2025(8) Å and $$\blacksquare$$96.6560(10)° for Rb₂PuSi₆O₁₅, and a=9.9478(2) Å, b=5.59820 (10) Å, c=13.1848(2) Å, and $$\blacksquare$$105.7400(10)° for K₂PuSi₂O₇. Comparisons to the synthesis of other reported alkali actinide silicates (Ce, U,more » and Th) are made. Raman spectra and DFT calculations of formation enthalpies and vibrational modes provide confirmation of structure refinements and enhanced product characterization for comparison with other materials.« less

In actinide systems, the 5f electrons experience a uniquely delicate balance of effects and interactions having similar energy scales, which are often difficult to properly disentangle. This interplay of factors such as the dual nature of 5f-states, strong electronic correlations, and strong spin–orbit coupling results in electronically unusual and intriguing behavior such as multi-k antiferromagnetic ordering, multipolar ordering, Mott-physics, mixed valence configurations, and more. Despite the inherent allure of their exotic properties, the exploratory science of even the more basic, binary systems like the actinide oxides has been limited due to their toxicity, radioactivity, and reactivity. In this article, wemore » provide an overview of the available synthesis techniques for selected binary actinide oxides, including the actinide dioxides, sesquioxides, and a selection of higher oxides. For these oxides, here we also review and evaluate the current state of knowledge of their crystal structures and magnetic properties. In many aspects, substantial knowledge gaps exist in the current body of research on actinide oxides related to understanding their electronic ground states. Bridging these gaps is vital for improving not only a fundamental understanding of these systems but also of future nuclear technologies. To this end, we note the experimental techniques and necessary future investigations which may aid in better elucidating the nature of these fascinating systems.« less

Investigation of solvation effects emphasizes the importance of including explicit and implicit solvent for accurate DFT predictions on ion exchange.

Mixed anion compounds have exhibited interesting structures and properties that differ from compounds only incorporating a single anion in their composition. Unfortunately, difficulties in the synthetic methods used to obtain these materials has slowed the evolution of this field, prompting investigations into alternate synthetic pathways to these materials. Our recently establish Boron-Chalcogen Mixture (BCM) method, which was originally developed for the synthesis of pure actinide chalcogenides from oxides, has been adapted to achieve the partial oxide to sulfide conversion of Ba₂MUO₆ (M = Co, Zn) which resulted in two new uranium (IV/V) oxysulfide compounds, Ba₆Co₆U_0·91S_13·5O_0.5 and Ba_5·47K_0·53Zn₆US_13·5O_0.5 These compounds crystallizemore » in the tetragonal crystal system adopting the space group I4/mcm. Finally, their syntheses, crystal structures, and trends observed in the pursuit of these new mixed anion compounds are reported.« less

K_0.72Fe_0.72Ti_1.28O₄, a layered mixed-metal oxide belonging to the lepidocrocite structure type, was grown out of a molten KCl-KF mixture at 800 °C. The compound crystallizes in orthorhombic space group Cmcm and exhibits two-dimensional layers, composed of mixed Fe/Ti octahedra, separated by K ions that occupy the interlayer layer space and maintain charge balance. The targeted flux synthesis of the Ni, Cu, and Zn analogs was carried out after density functional theory (DFT) calculations predicted their formation to be energetically favorable, yielding K_0.77Ni_0.38Ti_1.62O₄, K_0.77Cu_0.38Ti_1.62O₄, and K_0.73Zn_0.36Ti_1.64O₄. Finally, all four compositions can be prepared via conventional solid-state technique by mixing and heatingmore » stoichiometric amounts of KNO₃, appropriate transition metal oxide precursors, and TiO₂.« less

Two polytypic heterolayered salt-inclusion chalcogenides, o-[Na₂Cl]GaQ₂ and t-[Na₂Cl]GaQ₂, were obtained via a NaCl/NaI flux-assisted synthesis, as part of an investigation of the Na–Ga–Q (Q = S and Se) system. The use of a different flux, NaBr/NaI, in the Na–Ga–Se system did not lead to the formation of salt-inclusion phases, but instead the novel Na₂GaSe₃ and Na₄Ga₂Se₅ phases were obtained. Thermal and electronic properties of [Na₂Cl]GaS₂ materials were investigated with differential scanning calorimetry, post-quenching ex situ powder X-ray diffraction (PXRD), high-temperature PXRD, and enthalpy and electronic structure calculations via density functional theory. Those studies determined the absence of any temperature-induced phasemore » transition between the o-[Na₂Cl]GaS₂ and t-[Na₂Cl]GaS₂ polytypic compounds. Moreover, herein we probed the suitability of the heterolayered [Na₂Cl]GaS₂ single crystals as a sorbent for UO₂²⁺ uptake and monitored this process by energy-dispersive and infrared spectroscopies and PXRD, which revealed that the [Na₂Cl]+ insert could be exchanged with UO₂²⁺ on the surface while the UO₂²⁺ intercalation decomposes the [Na₂Cl]GaS₂ structure.« less