Polymorphism and phase transitions in Na2U2O7 from density functional perturbation theory.

Polymorphism and phase transitions in sodium diuranate, Na2U2O7, are investigated with density functional perturbation theory (DFPT). Thermal properties of crystalline α-, ß- and γ-Na2U2O7 polymorphs are predicted from DFPT phonon calculations, i.e., the first time for the high-temperature γ-Na2U2O7 phase (R3̄m symmetry). The standard molar isochoric heat capacities predicted within the quasi-harmonic approximation are for P21/a α-Na2U2O7 and C2/m ß-Na2U2O7, respectively. Gibbs free energy calculations reveal that α-Na2U2O7 (P21/a) and ß-Na2U2O7 (C2/m) are almost energetically degenerate at low temperature, with ß-Na2U2O7 becoming slightly more stable than α-Na2U2O7 as temperature increases. These findings are consistent with XRD data showing a mixture of α and ß phases after cooling of γ-Na2U2O7 to room temperature and the observation of a sluggish α → ß phase transition above ca. 600 K. A recently observed α-Na2U2O7 structure with P21 symmetry is also shown to be metastable at low temperature. Based on Gibbs free energy, no direct ß â†’ γ solid-solid phase transition is predicted at high temperature, although some experiments reported the existence of such phase transition around 1348 K. This, along with recent experiments, suggests the occurrence of a multi-step process consisting of initial ß-phase decomposition, followed by recrystallization into γ-phase as temperature increases.

Crystal and Electronic Structures of A2NaIO6 Periodate Double Perovskites (A = Sr, Ca, Ba): Candidate Wasteforms for I-129 Immobilization.

The synthesis, structure, and thermal stability of the periodate double perovskites A2NaIO6 (A= Ba, Sr, Ca) were investigated in the context of potential application for the immobilization of radioiodine. A combination of X-ray diffraction and neutron diffraction, Raman spectroscopy, and DFT simulations were applied to determine accurate crystal structures of these compounds and understand their relative stability. The compounds were found to exhibit rock-salt ordering of Na and I on the perovskite B-site; Ba2NaIO6 was found to adopt the Fm-3m aristotype structure, whereas Sr2NaIO6 and Ca2NaIO6 adopt the P21/n hettotype structure, characterized by cooperative octahedral tilting. DFT simulations determined the Fm-3m and P21/n structures of Ba2NaIO6 to be energetically degenerate at room temperature, whereas diffraction and spectroscopy data evidence only the presence of the Fm-3m phase at room temperature, which may imply an incipient phase transition for this compound. The periodate double perovskites were found to exhibit remarkable thermal stability, with Ba2NaIO6 only decomposing above 1050 °C in air, which is apparently the highest recorded decomposition temperature so far recorded for any iodine bearing compound. As such, these compounds offer some potential for application in the immobilization of iodine-129, from nuclear fuel reprocessing, with an iodine incorporation rate of 25-40 wt%. The synthesis of these compounds, elaborated here, is also compatible with both current conventional and future advanced processes for iodine recovery from the dissolver off-gas.

Molecular dynamics simulation of zirconium tungstate amorphization and the amorphous-crystalline interface.

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.

Structure-thermodynamics relationship of schoepite from first-principles.

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 C0P = 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. These findings have important implications for the dehydration of layered uranyl corrosion phases and hygroscopic materials.

High-pressure-assisted X-ray-induced damage as a new route for chemical and structural synthesis.

X-ray induced damage has been known for decades and has largely been viewed as a tremendous nuisance. We, on the other hand, harness the highly ionizing and penetrating properties of hard X-rays to initiate novel decomposition and synthetic chemistry. Here, we show that powdered cesium oxalate monohydrate pressurized to ≤0.5 GPa and irradiated with X-rays of energies near the cesium K-edge undergoes molecular and structural transformations with one of the final products exhibiting a new type of bcc crystal structure that has previously not been observed. Additionally, based on cascades of ultrafast electronic relaxation steps triggered by the absorption of one X-ray photon, we propose a model explaining the X-ray induced damage of multitype bounded matter. As X-rays are ubiquitous, these results show promise in the preparation of novel compounds and novel structures that are inaccessible via conventional methods. They may offer insight into the formation of complex organic compounds in outer space.

Phosphorus Dimerization in Gallium Phosphide at High Pressure.

Using combined experimental and computational approaches, we show 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) Šat 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.

First-Principles Structural, Mechanical, and Thermodynamic Calculations of the Negative Thermal Expansion Compound Zr2(WO4)(PO4)2.

The negative thermal expansion (NTE) material Zr2(WO4)(PO4)2 has been investigated for the first time within the framework of the density functional perturbation theory (DFPT). The structural, mechanical, and thermodynamic properties of this material have been predicted using the Perdew, Burke and Ernzerhof for solid (PBEsol) exchange-correlation functional, which showed superior accuracy over standard functionals in previous computational studies of the NTE material α-ZrW2O8. The bulk modulus calculated for Zr2(WO4)(PO4)2 using the Vinet equation of state at room temperature is K 0 = 63.6 GPa, which is in close agreement with the experimental estimate of 61.3(8) at T = 296 K. The computed mean linear coefficient of thermal expansion is -3.1 × 10-6 K-1 in the temperature range ∼0-70 K, in line with the X-ray diffraction measurements. The mean Grüneisen parameter controlling the thermal expansion of Zr2(WO4)(PO4)2 is negative below 205 K, with a minimum of -2.1 at 10 K. The calculated standard molar heat capacity and entropy are C P 0 = 287.6 and S 0 = 321.9 J·mol-1·K-1, respectively. The results reported in this study demonstrate the accuracy of DFPT/PBEsol for assessing or predicting the relationship between structural and thermomechanical properties of NTE materials.

Model representations of kerogen structures: An insight from density functional theory calculations and spectroscopic measurements.

Molecular structures of kerogen control hydrocarbon production in unconventional reservoirs. Significant progress has been made in developing model representations of various kerogen structures. These models have been widely used for the prediction of gas adsorption and migration in shale matrix. However, using density functional perturbation theory (DFPT) calculations and vibrational spectroscopic measurements, we here show that a large gap may still remain between the existing model representations and actual kerogen structures, therefore calling for new model development. Using DFPT, we calculated Fourier transform infrared (FTIR) spectra for six most widely used kerogen structure models. The computed spectra were then systematically compared to the FTIR absorption spectra collected for kerogen samples isolated from Mancos, Woodford and Marcellus formations representing a wide range of kerogen origin and maturation conditions. Limited agreement between the model predictions and the measurements highlights that the existing kerogen models may still miss some key features in structural representation. A combination of DFPT calculations with spectroscopic measurements may provide a useful diagnostic tool for assessing the adequacy of a proposed structural model as well as for future model development. This approach may eventually help develop comprehensive infrared (IR)-fingerprints for tracing kerogen evolution.

Assessing Hubbard-corrected AM05+U and PBEsol+U density functionals for strongly correlated oxides CeO2 and Ce2O3.

The structure-property relationships of bulk CeO2 and Ce2O3 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 CeO2 and Ce2O3 with superior accuracy, especially when +U is chosen close to its value derived by the linear-response approach. 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.

Technetium incorporation in scheelite: insights from first-principles.

Atomistic investigations of crystalline scheelite, CaWO4, and 99Tc-bearing scheelite, CaWO4:Tc, have been carried out using density functional theory. The lattice constants, bulk modulus, and volume compression data of CaWO4 have been calculated and compared with experimental data, with a focus on predictive understanding of 99Tc incorporation in CaWO4. Defect formation energies have been computed for several possible interstitial (I) and substitutional (S) sites of 99Tc in CaWO4. Both I(Oh) and S(W) sites were found to be energetically favourable for Tc doping. X-ray diffraction (XRD) spectra for each 99Tc defect type have been simulated to help interpret the complex experimental XRD patterns. This work on CaWO4:Tc provides insights into materials generated during nuclear weapons testing and useful spectral signatures for nuclear forensics.

Mechanical properties of zirconium alloys and zirconium hydrides predicted from density functional perturbation theory.

The elastic properties and mechanical stability of zirconium alloys and zirconium hydrides have been investigated within the framework of density functional perturbation theory. Results show that the lowest-energy cubic Pn3[combining macron]m polymorph of δ-ZrH1.5 does not satisfy all the Born requirements for mechanical stability, unlike its nearly degenerate tetragonal P42/mcm polymorph. Elastic moduli predicted with the Voigt-Reuss-Hill approximations suggest that mechanical stability of α-Zr, Zr-alloy and Zr-hydride polycrystalline aggregates is limited by the shear modulus. According to both Pugh's and Poisson's ratios, α-Zr, Zr-alloy and Zr-hydride polycrystalline aggregates can be considered ductile. The Debye temperatures predicted for γ-ZrH, δ-ZrH1.5 and ε-ZrH2 are θD = 299.7, 415.6 and 356.9 K, respectively, while θD = 273.6, 284.2, 264.1 and 257.1 K for the α-Zr, Zry-4, ZIRLO and M5 matrices, i.e. suggesting that Zry-4 possesses the highest micro-hardness among Zr matrices.

Time-Resolved Infrared Reflectance Studies of the Dehydration-Induced Transformation of Uranyl Nitrate Hexahydrate to the Trihydrate Form.

Uranyl nitrate is a key species in the nuclear fuel cycle. However, this species is known to exist in different states of hydration, including the hexahydrate ([UO2(NO3)2(H2O)6] often called UNH), the trihydrate [UO2(NO3)2(H2O)3 or UNT], and in very dry environments the dihydrate form [UO2(NO3)2(H2O)2]. Their relative stabilities depend on both water vapor pressure and temperature. In the 1950s and 1960s, the different phases were studied by infrared transmission spectroscopy but were limited both by instrumental resolution and by the ability to prepare the samples for transmission. We have revisited this problem using time-resolved reflectance spectroscopy, which requires no sample preparation and allows dynamic analysis while the sample is exposed to a flow of N2 gas. Samples of known hydration state were prepared and confirmed via X-ray diffraction patterns of known species. In reflectance mode the hexahydrate UO2(NO3)2(H2O)6 has a distinct uranyl asymmetric stretch band at 949.0 cm(-1) that shifts to shorter wavelengths and broadens as the sample desiccates and recrystallizes to the trihydrate, first as a shoulder growing in on the blue edge but ultimately results in a doublet band with reflectance peaks at 966 and 957 cm(-1). The data are consistent with transformation from UNH to UNT as UNT has two inequivalent UO2(2+) sites. The dehydration of UO2(NO3)2(H2O)6 to UO2(NO3)2(H2O)3 is both a structural and morphological change that has the lustrous lime green UO2(NO3)2(H2O)6 crystals changing to the matte greenish yellow of the trihydrate solid. The phase transformation and crystal structures were confirmed by density functional theory calculations and optical microscopy methods, both of which showed a transformation with two distinct sites for the uranyl cation in the trihydrate, with only one in the hexahydrate.

Thermodynamics of technetium: reconciling theory and experiment using density functional perturbation analysis.

The structure, lattice dynamics and thermodynamic properties of bulk technetium were investigated within the framework of density functional theory. The phonon density of states spectrum computed with density functional perturbation theory closely matches inelastic coherent neutron scattering measurements. The thermal properties of technetium were derived from phonon frequencies calculated within the quasi-harmonic approximation (QHA), which introduces a volume dependence of phonon frequencies as a part of the anharmonic effect. The predicted thermal expansion and isobaric heat capacity of technetium are in excellent agreement with available experimental data for temperatures up to ∼1600 K.

Relationship between crystal structure and thermo-mechanical properties of kaolinite clay: beyond standard density functional theory.

The structural, mechanical and thermodynamic properties of 1 : 1 layered dioctahedral kaolinite clay, with ideal Al2Si2O5(OH)4 stoichiometry, were investigated using density functional theory corrected for dispersion interactions (DFT-D2). The bulk moduli of 56.2 and 56.0 GPa predicted at 298 K using the Vinet and Birch-Murnaghan equations of state, respectively, are in good agreement with the recent experimental value of 59.7 GPa reported for well-crystallized samples. The isobaric heat capacity computed for uniaxial deformation of kaolinite along the stacking direction reproduces calorimetric data within 0.7-3.0% from room temperature up to its thermal stability limit.

Layered uranium(VI) hydroxides: structural and thermodynamic properties of dehydrated schoepite α-UO2(OH)2.

The structure of dehydrated schoepite, α-UO2(OH)2, was investigated using computational approaches that go beyond standard density functional theory and include van der Waals dispersion corrections (DFT-D). Thermal properties of α-UO2(OH)2, were also obtained from phonon frequencies calculated with density functional perturbation theory (DFPT) including van der Waals dispersion corrections. While the isobaric heat capacity computed from first-principles reproduces available calorimetric data to within 5% up to 500 K, some entropy estimates based on calorimetric measurements for UO3·0.85H2O were found to overestimate by up to 23% the values computed in this study.

ß-Technetium dichloride: solid-state modulated structure, electronic structure, and physical properties.

A second polymorph of technetium dichloride, ß-TcCl2, has been synthesized from the reaction of Tc metal and chlorine in a sealed tube at 450 °C. The crystallographic structure and physical properties of ß-TcCl2 have been investigated. The structure of ß-TcCl2 consists of infinite chains of face sharing [Tc2Cl8] units; within a chain, the Tc≡Tc vectors of two adjacent [Tc2Cl8] units are ordered in the long-range where perpendicular and/or parallel arrangement of Tc≡Tc vectors yields a modulated structure. Resistivity and Seebeck measurements performed on a ß-TcCl2 single crystal indicate the compound to be a p-type semiconductor while a magnetic susceptibility measurement shows technetium dichloride to be diamagnetic. A band gap of 0.12(2) eV was determined by reflectance spectroscopy measurements. Theoretical calculations at the density functional level were utilized for the investigation of other possible stable forms of TcCl2.

Semiconducting layered technetium dichalcogenides: insights from first-principles.

The structures and properties of layered technetium dichalcogenides TcX2 (X = S, Se, Te) have been investigated using density functional theory. The equilibrium structures of TcSe2 and TcTe2, adopting distorted Cd(OH)2-type unit cells similar to TcS2, are reported for the first time at the atomic level, along with their electronic properties. In contrast to previous X-ray diffraction analyses, calculations reveal that stoichiometric TcTe2 is not isomorphous to the high-temperature monoclinic phase ß-MoTe2. All three compounds are found to be semiconductors with calculated band gaps of 0.9 eV for TcS2, 0.8 eV for TcSe2, and 0.3 eV for TcTe2. The thermal properties of these TcX2 compounds have also been predicted using phonon frequencies calculated with density functional perturbation theory.

Worker exposure for at-reactor management of spent nuclear fuel.

The radiological impact on workers associated with spent nuclear fuel dry storage operations at reactor sites is discussed. The resulting doses to workers exposed to external radiation include the dose during dry storage system loading, unloading and handling activities, the dose associated with independent spent fuel storage installation (ISFSI) operations, maintenance and surveillance activities, and the dose associated with additional ISFSI construction. Comprehensive dose estimates are reported based on previous radiation surveys.

Reactivity of HTcO(4) with methanol in sulfuric acid: Tc-sulfate complexes revealed by XAFS spectroscopy and first principles calculations.

The reaction between HTcO(4) and MeOH in 13 M H(2)SO(4) was investigated by (99)Tc NMR, UV-visible and X-ray absorption fine structure (XAFS) spectroscopy. Experimental results and first principles calculations show the formation of Tc(+5) sulfate complexes. The results expand the fundamental understanding of Tc in high acid solutions.

On the role of strong electron correlations in the surface properties and chemistry of uranium dioxide.

We report density functional calculations of the surface properties and chemistry of UO(2)(111) performed within the generalized gradient approximation corrected with an effective Hubbard parameter (GGA + U within Dudarev's formalism) to account for the strong on-site Coulomb repulsion between U 5f electrons. The variation of the properties of periodic slab models, with collinear ferromagnetic and antiferromagnetic arrangements of the uranium magnetic moments, was investigated while ramping up the effective Hubbard parameter from U(eff) = 0 eV, corresponding to standard density functional theory, up to U(eff) = 4 eV, the value that correctly reproduces the antiferromagnetic ground state of bulk UO(2). The chemical interactions of molecular water, dissociated water, dissociated oxygen and co-adsorbed molecular water and monatomic oxygen with the UO(2)(111) surface were also studied as functions of the U(eff) parameter. Calculations reveal that some of the key electronic and chemical properties controlling the surface reactivity are very sensitive to the value of this strong electron correlation parameter.