[en] The speciation of Tc(VII) in 12 M sulfuric acid was studied by NMR, UV-visible and XAFS spectroscopy, experimental results were supported by DFT calculation and were in agreement with the formation of TcO₃OH(H₂O)₂. In summary, the speciation of heptvalent technetium has been investigated in sulfuric acid. In 12 M H₂SO₄, a yellow solution is observed, and its ⁹⁹Tc NMR spectrum is consistent with a heptavalent complex. The yellow solution was further characterized by EXAFS spectroscopy, and results are consistent with the formation of TcO₃(OH)(H₂O)₂. No technetium heptoxide or sulfato- complexes were detected in these conditions. The molecular structure of TcO₃(OH)(H₂O)₂ has been optimized by DFT techniques, and the structural parameters are well in accordance with those found by XAFS spectroscopy. The experimental electronic spectra exhibit ligand-to-metal charge transfer transitions that have been assigned using TDDFT methods. Calculations demonstrate the theoretical electronic spectrum of TcO₃(OH)(H₂O)₂ to be in very good agreement with the experimental one. Recent experiments in 12 M H₂SO₄ show the yellow solution to be very reactive in presence of reducing agents presumably forming low valent Tc species. Current spectroscopic works focus on the speciation of these species.

[en] The structure–property relationships of bulk CeO_2 and Ce_2O_3 have been investigated using AM05 and PBEsol exchange–correlation functionals within the frameworks of Hubbard-corrected density functional theory (DFT+U) and density functional perturbation theory (DFPT+U). Compared with conventional PBE+U, RPBE+U, PW91+U and LDA+U functionals, AM05+U and PBEsol+U describe experimental crystalline parameters and properties of CeO_2 and Ce_2O_3 with superior accuracy, especially when +U is chosen close to its value derived by the linear-response approach. Lastly, the present findings call for a reexamination of some of the problematic oxide materials featuring strong f- and d-electron correlation using AM05+U and PBEsol+U.

[en] The adsorption of atomic and gaseous chlorine on Zr (0001) surface has been investigated using state-of-the-art density functional theory calculations as part of an effort to gain fundamental understanding of chlorination processes occurring in fuel cladding materials. Zirconium alloys, including Zircaloy-4, Zircaloy-2 and Zr-Nb alloys (Zirlo^TM and Zr-2.5Nb) are the most common fuel cladding materials in nuclear light water reactors (LWRs) due to their favorable mechanical properties, corrosion resistance, low thermal-neutron capture cross section and criticality. The United States produces a large amount of cladding material from used nuclear fuel, which could approach 1,000 MT/year in the next 50 years. In nuclear fuel recycling, spent nuclear fuel assemblies are disassembled and the cladding hulls are separated from the U oxide spent fuel. Zr can be recovered from Zr cladding using the chlorination method. However the presence of impurities in the recovered ZrCl₄ was reported, including Sn, Cr and Fe. Thus, further efforts are necessary to improve the purification process significantly. This requires fundamental understanding of chlorination processes in cladding materials. Most of the material properties are expected to be similar between the pure zirconium and its alloys due to the fact that the alloy composition consists of more than 95 weight-percent zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals. Therefore, while the overarching goal is to understand the chlorination process occurring in complex fuel cladding materials, chlorination of pure zirconium was used as an initial system for the modeling and simulation efforts in this study

[en] We have performed a high pressure synchrotron x-ray diffraction study of the ionic salt, cesium fluoride (CsF), up to 120 GPa. We observed the B1 → B2 phase transition near 5 GPa as previously reported. Beyond this pressure, no phase transitions were determined to have occurred up to the highest pressure studied. Unit cell data were calculated from the known B2 (CsCl) structure for all of the pressures studied above 5 GPa, and an equation of state (EOS) was fit to the data using a 3rd order Birch–Murnaghan equation in this phase. Density Functional Theory (DFT) was also employed to compute an EOS for comparison purposes. Our experimental results agreed very well with both sets of the predicted EOS. (author)

[en] In this report, we present a thermodynamic-based model of hydride precipitation in Zr-based claddings. The model considers the state of the cladding immediately following drying, after removal from cooling-pools, and presents the evolution of precipitate formation upon cooling as follows: The pilgering process used to form Zr-based cladding imparts strong crystallographic and grain shape texture, with the basal plane of the hexagonal α-Zr grains being strongly aligned in the rolling-direction and the grains are elongated with grain size being approximately twice as long parallel to the rolling direction, which is also the long axis of the tubular cladding, as it is in the orthogonal directions.

[en] The relationship between the structure and thermodynamic properties of schoepite, an important uranyl phase with formula [(UO₂)₈O₂(OH)₁₂]·12H₂O formed upon corrosion of UO₂, has been investigated within the framework of density functional perturbation theory (DFPT). Experimental crystallographic lattice parameters are well reproduced in this study using standard DFT. Phonon calculations within the quasi-harmonic approximation predict standard molar entropy and isobaric heat capacity of S⁰ = 179.60 J mol^-1 K^-1 and C⁰_P = 157.4 J mol^-1 K^-1 at 298.15 K, i.e., ~6% and ~4% larger than existing DFPT-D2 calculations. The computed variation of the standard molar isobaric heat capacity with water content from schoepite (UO₃·xH₂O, x = 2.25) to dehydrated schoepite (x = 1) is predicted to be essentially linear along isotherms ranging from 100 to 500 K. Finally, these findings have important implications for the dehydration of layered uranyl corrosion phases and hygroscopic materials.

[en] Classical molecular dynamics (MD) simulations were performed to provide a conceptual understanding of the amorphous-crystalline interface for a candidate negative thermal expansion (NTE) material, ZrW₂O₈. Simulations of pressure-induced amorphization at 300 K indicate that an amorphous phase forms at pressures of 10 GPa and greater, and this phase persists when the pressure is subsequently decreased to 1 bar. However, the crystalline phase is recovered when the slightly distorted 5 GPa phase is relaxed to 1 bar. Simulations were also performed on a two-phase model consisting of the high-pressure amorphous phase in direct contact with the crystalline phase. Upon equilibration at 300 K and 1 bar, the crystalline phase remains unchanged beyond a thin layer of disrupted structure at the crystalline-amorphous interface. Differences in local atomic structure at the interface are quantified from the simulation trajectories. (paper)

[en] Zirconium alloys have been extensively used as fuel cladding materials in nuclear light water reactors (LWRs) due to their favorable mechanical properties, corrosion resistance, low thermal neutron capture cross section and criticality. Because the main functions of these structural materials are to maintain the integrity of UO₂ fuel rods and prevent the release of fission products into the biosphere, a detailed knowledge of their structural stability is of critical importance to ensure the safe operation of nuclear reactors, and the safety of storage, transportation or disposition of used nuclear fuel assemblies. While the Zr-H phase diagram has been known since the 1950's and has remained largely unchanged, the structures, formation mechanisms, and stability of Zr-hydride phases in pure α-Zr or Zr alloys matrices have been subjects of much debate over the last few decades. The formation of hydride phases in the Zr matrix has been linked in previous studies to the degradation of mechanical properties of Zr/Zr-alloys materials. Therefore, an accurate knowledge of the interplay between the structures, stability and mechanical properties of Zr/Zr-alloys and Zr hydrides is of paramount significance to ensure materials reliability during their in- service lifetime. Despite few first-principles studies have investigated the mechanical stability of Zr hydrides polymorphs proposed so far, no ab initio studies have been carried out, to the best of our knowledge, on the mechanical properties of modern nuclear-grade zirconium alloys such as Zircaloy-4 (Zry-4; 1.2-1.7% Sn), Zirconium low oxidation alloy (ZIRLO; 0.7-1.0% Sn and ∼1.0% Nb), or M5 alloy (0.8-1.2% Nb). Therefore, it is important to carry out a series of first-principles calculations systematically to investigate the structure-property relationship of zirconium hydrides and zirconium alloys, that can help explain differences in elastic properties between high-purity zirconium and nuclear-grade zirconium alloys. (authors)

[en] For this research, the crystal structure, lattice dynamics and themomechanical properties of bulk monoclinic zirconium tetrachloride (ZrCl₄) have been investigated using zero-damping dispersion-corrected density functional theory [DFT-D3(zero)]. Phonon analysis reveals that ZrCl₄(cr) undergoes negative thermal expansion (NTE) near T≈10 K, with a coefficient of thermal expansion of α=-1.2 ppm K^-1 and a Grüneisen parameter of γ=-1.1. The bulk modulus is predicted to vary from K₀=8.7 to 7.0 GPa in the temperature range 0–550 K. Lastly, the isobaric molar heat capacity derived from phonon calculations within the quasi-harmonic approximation is in fair agreement with existing calorimetric data.

[en] Using combined experimental and computational approaches, we show in this paper that at 43 GPa and 1300 K gallium phosphide adopts the super-Cmcm structure, here indicated with its Pearson notation oS24. First-principles enthalpy calculations demonstrate that this structure is more thermodynamically stable above ~20 GPa than previously proposed polymorphs. In contrast to other polymorphs, the oS24 phase shows a strong bonding differentiation and distorted fivefold coordination geometries of both P atoms. The shortest bond of the phase is a single covalent P–P bond measuring 2.171(11) Å at synthesis pressure. Phosphorus dimerization in GaP sheds light on the nature of the super-Cmcm phase and provides critical new insights into the high-pressure polymorphism of octet semiconductors. Bond directionality and anisotropy explain the relatively low symmetry of this high-pressure phase.