Molecular-dynamics simulations and first-principles calculations are employed to understand vibra... more Molecular-dynamics simulations and first-principles calculations are employed to understand vibrational spectroscopy and molecular and electronic structure of the encaged hydrogen molecules in hydrogen clathrate hydrate. The molecular-dynamics simulations, using empirical potentials, are performed to generate collections of the clathrate water cages with different hydrogen occupancies. The first-principles calculations, using Density Functional Theory with B3LYP hybrid density functionals for exchange and correlation, are carried out to optimize the structures and to calculate the Raman shift and activity of the stretching mode of the encaged hydrogen molecules. The Raman spectra are computed by a weighted moving average over a number of different structural configurations for different hydrogen occupancies. The results show that experimentally observed Raman peaks around 4120-4125 cm -1 are from small cages with single H 2 occupancy and peaks around 4125-4150 cm -1 from those in the large cages with one to four H 2 molecules. The Raman peaks of hydrogen molecules in the doubly occupied small cages are expected to be around or above the gas phase frequency 4155 cm -1 . Molecular structural analysis shows that the single hydrogen molecule in the small cages and single to quadruple hydrogen molecules in the large cage are encaged in loose cages, while double hydrogen molecules in the small cage are confined in a tight cage. Normal-mode analysis shows that there is limited vibrational coupling for H 2 molecules in doubly to quadruply occupied large cages while a strong vibrational coupling is observed in the doubly occupied small cage. The isovalue maps of total electron density and electrostatic potential suggest significant electron sharing between hydrogen molecules and water molecules, and important interaction between hydrogen and water oxygen atoms for confining the hydrogen clusters. The results help explain experimentally observed Raman spectra of hydrogen clathrates and provide new insights into the confinement effect by the water host framework on vibrational, molecular, and electronic properties of hydrogen molecules in the cages of clathrate hydrates.
Determination of the local configuration of interacting defects in a crystalline, periodic solid ... more Determination of the local configuration of interacting defects in a crystalline, periodic solid is problematic because defects typically do not have a long-range periodicity. Uranium dioxide, the primary fuel for fission reactors, exists in hyperstoichiometric form, UO 21x . Those excess oxygen atoms occur as interstitial defects, and these defects are not random but rather partially ordered. The widely-accepted model to date, the Willis cluster based on neutron diffraction, cannot be reconciled with the first-principles molecular dynamics simulations present here. We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration. The simulations show that the average structure of UO 21x involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the fast diffusion of the defect cluster. The results provide new insights in differentiating the average structure from the local configuration of defects in a solid and the transport properties of UO 21x . ranium dioxide is the principal fuel of nuclear reactors. One of the unique properties of UO 2 is its ability to accommodate a variable stoichiometry, depending on temperature and oxygen pressure 1,2 . The excess oxygen atoms in hyperstoichiometric uranium dioxide (UO 21x ) occur as interstitial defects [3][4][5][6][7][8][9] . Positions and dynamics of these excess oxygen atoms control many important properties, such as thermal conductivity , fission-product accommodation and transport 15 , micro-structure evolution , and corrosion behavior . These properties are closely related to the performance of the fuel in a reactor and its behavior in a geologic disposal. For UO 21x at low x values, the interstitial O atoms occur as isolated point defects. As x increases, individual defects interact with each other increasingly and form clusters [3][4][5] . Various experimental and theoretical studies have shown that these clusters are not random but rather structured with well-defined configurations [3][4][5][6] . However, the defect structures are difficult to quantify experimentally using diffraction techniques because the clusters are local structures and lack long-range periodicity. Based on early neutrondiffraction studies, a defect cluster model, the so-called 25252 Willis type, was proposed for UO 2.11-2.13 over fifty years ago 3 . Since then, this has remained the dominant conceptual model in the literature. Although slightly modified later by the original author and the collaborators 4,5 , the proposed oxygen configuration remains the same 4,5 . The acceptance of the model is largely based on the neutron diffraction data. However, recent firstprinciples calculations and empirical potential molecular dynamics show that the Willis cluster is not stable . Upon optimization, it spontaneously relaxes to a split di-interstitial or tri-oxygen cluster sharing a vacancy (V3O'') . The essential question is whether the Willis defect is an appropriate model for UO 21x or whether it is a limitation of static first-principles calculations being performed at the athermal limit that cannot account for finite temperature effects on the defect structure. In order to overcome the limitations related to zero temperature and the accuracy of empirical potentials in the previous theoretical calculations , first-principles moleculardynamics simulations at high temperatures are employed here. These results provide a self-consistent explanation of the Willis cluster model that is based on neutron diffraction data and the atomistic-scale structure that is derived from recent theoretical calculations. The simulations also improve the understanding of different defect types and reveal the role of the Willis defect model in the transport of the oxygen defect clusters in UO 21x . Uranium dioxide has the isometric fluorite structure (Fm3m). The 4a site is occupied by uranium, the 8c site by oxygen, and 4b site is empty in UO 2 . The Willis 25252 defect cluster consists of two vacancies (V o ) at the O 8c site, two O interstitials displaced in ,110. directions (O') from the 4b site, and two other O interstitials displaced in ,111. (O'') from the 4b site (Figure ) [3][4][5] . Starting with this configuration without constraints, the cluster was
High-pressure and high-temperature phases show unusual physical and chemical properties, but they... more High-pressure and high-temperature phases show unusual physical and chemical properties, but they are often difficult to 'quench' to ambient conditions 1 . Here, we present a new approach, using bombardment with very high-energy, heavy ions accelerated to relativistic velocities, to stabilize a high-pressure phase. In this case, Gd 2 Zr 2 O 7 , pressurized in a diamond-anvil cell up to 40 GPa, was irradiated with 20 GeV xenon or 45 GeV uranium ions, and the (previously unquenchable) cubic high-pressure phase was recovered after release of pressure. Transmission electron microscopy revealed a radiation-induced, nanocrystalline texture. Quantummechanical calculations confirm that the surface energy at the nanoscale is the cause of the remarkable stabilization of the high-pressure phase. The combined use of high pressure and high-energy ion irradiation 2,3 provides a new means for manipulating and stabilizing new materials to ambient conditions that otherwise could not be recovered. Investigations of the properties of materials at high pressures have been essential to understanding the internal structure of the Earth 4,5 . Most recently, materials scientists have begun to take advantage of high pressures to fabricate materials with unique properties 1,6-8 . For example, super-hard materials have been discovered 9,10 and elementary high-temperature superconductors have been produced under pressures as high as 200 GPa (ref. 11). The development of next-generation high-pressure devices , coupled with state-of-the-art analytical techniques, will extend the possibilities for the synthesis and in situ characterization of unique solids under extreme conditions 1 . A serious drawback of these efforts is that many of the unusual materials are stable only at high pressure, that is, they rapidly transform to the low-pressure phase on the release of pressure and cannot be examined ex situ or used in special applications. Here, we describe a new strategy for the recovery of such high-pressure phases to ambient conditions. The approach is based on a recently developed method 2,3 using swift heavy ions to manipulate directly the structure of a solid at high pressures and to induce nanoscale modifications that result in stabilization of the high-pressure phase. The irradiation is carried out with relativistic projectiles (v/c ∼ 0.5) produced by one of the world's largest ion accelerators. The high velocity (kinetic energy) of the ion beam is required because it first has to traverse the millimetre-thick diamond anvil of the high-pressure cell before reaching the pressurized sample. As the projectiles slow down along their trajectory, they induce intense electronic excitations and ionizations (electronic energy loss, dE/dx), which finally trigger complex processes within the solid. A cylindrical zone of ∼10 nm diameter is formed in which the deposited energy
The compressibility, phase stability, and vibrational properties of coffinite (USiO 4 ) were stud... more The compressibility, phase stability, and vibrational properties of coffinite (USiO 4 ) were studied by in situ X-ray diffraction and infrared (IR) measurements at high pressures. An irreversible phase transition from the zircon-type to scheelite-type structure was found to occur at 14-17 GPa. Accompanying the structural transition, partial amorphization was also evident in the XRD analysis. The predicted transition pressure calculated by density functional theory is in good agreement with the experimental results. IR spectra also suggest that water is incorporated into the coffinite structure, and a pressure-induced electron transfer (U 4+ → U 5+ ) may also occur.
Metastable vaterite crystals were synthesized by increasing the pH and consequently the saturatio... more Metastable vaterite crystals were synthesized by increasing the pH and consequently the saturation states of Ca 2+ -and CO 3 2--containing solutions using an ammonia diffusion method. SEM and TEM analyses indicate that vaterite grains produced by this method are polycrystalline aggregates with a final morphology that has a sixfold-symmetry. The primary structure develops within an hour and is almost a spherical assemblage of nanoparticles (5-10 nm) with random orientation, followed by the formation of hexagonal platelets (1-2 µm), which are first composed of nanoparticles and that develop further into single crystals. As determined using transmission electron microscopy, these hexagonal crystallites are terminated by (001) surfaces and are bounded by {110} edges. The hexagonal crystals subsequently stack to form the "petals" (20 µm wide, 1 µm thick) of the final "flower-like" vaterite morphology. The large flakes gradually tilt toward the center as growth progresses so that their positions become more and more vertical, which eventually leads to a depression in the center. Since this sequence encompasses several morphologies observed in previous studies (spheres, hexagons, flowers etc.), they may actually represent different stages of growth rather than equilibrium morphologies for specific growth conditions.
Pressure-induced structural changes in Gd 2 Zr 2 O 7 pyrochlore have been investigated at pressur... more Pressure-induced structural changes in Gd 2 Zr 2 O 7 pyrochlore have been investigated at pressures up to 43 GPa by synchrotron x-ray diffraction and Raman scattering measurements. With increasing pressure, the ordered pyrochlore begins to transform to a disordered defect-fluorite-type ͑cubic͒ structure up to 15 GPa. Above 15 GPa, a high-pressure ͑HP͒ phase forms that has a distorted defect-fluorite-structure of lower symmetry. Upon release of pressure, the HP phase is not stable and gradually transforms back to the cubic defect-fluorite structure.
The structural transitions of the pyrochlore, Cd 2 Nb 2 O 7 , at pressures up to 32.5 GPa have be... more The structural transitions of the pyrochlore, Cd 2 Nb 2 O 7 , at pressures up to 32.5 GPa have been investigated by in situ Raman scattering and angle-dispersive x-ray diffraction ͑ADXRD͒ methods. The x-ray diffraction results reveal that small amounts ͑ϳ7%͒ of metallic cadmium form by chemical decomposition at pressures greater than 4 GPa. Both Raman and XRD results indicate that a pressure-induced structural distortion from pyrochlore to defect fluorite occurs in pyrochlore Cd 2 Nb 2 O 7 at pressures of 12-14 GPa. Subsequently, a new high-pressure phase formed and the phase transition was complete at ϳ27 GPa. The high-pressure phase is either monoclinic or orthorhombic and transforms to either the pyrochlore ͑or defect fluorite͒ structure or the amorphous state when quenched to ambient conditions. Energy dispersive spectroscopy ͑EDS͒ analysis and high-resolution transmission electron microscopy ͑HRTEM͒ observation of the quenched sample confirmed the Cd loss and resulting mixture of ordered pyrochlore, defect fluorite, high-pressure phase, as well as amorphous domains.
To evaluate the stability, potential reactivity, and relaxation mechanisms on different uraninite... more To evaluate the stability, potential reactivity, and relaxation mechanisms on different uraninite surfaces, surface energy values were calculated and structural relaxation was determined for the (111), (110), and (100) crystallographic faces of uranium dioxide (UO 2 ) using quantum mechanical (density functional theory) and empirical potential computational methods. Quantum mechanical results are compared with empirical potential results, which use surface slab models with two different geometries, as well as various different empirical force fi elds. The strengths and weaknesses of the different approaches are evaluated, and surface stabilizing mechanisms such as relaxation, charge redistribution, and electronic stabilization are investigated. Quantum mechanical (q.m.) surface energy results are in agreement with the relative surface energy trends resulting from calculations using three different empirical potential sets for uranium and oxygen (two from Catlow 1977; one from Meis and Gale 1998), and with empirical force-fi eld values published in the literature . The (111) surface consistently has the lowest surface energy (0.461 J/m 2 from q.m. calculations) and the smallest amount of surface relaxation, followed by the ( ) surface (0.846 J/m 2 ; q.m.), and the (100) surface (1.194 J/m 2 ; q.m.) (quantum mechanical surface energy values in parentheses are for surface slabs with a thickness of four UO 2 units). Differences exist, however, in the absolute values of surface energies calculated as a function of potential set used. Quantum mechanical values are consistently lower than values calculated using empirical potential methods. A fourth potential set is presented that is derived from fi tting electrostatic and short-range repulsive parameters to experimental bulk properties and surface energies and relaxations from quantum mechanical calculations.
Oil extraction efficiency strongly depends on the wettability status (oil vs. water-wet) of reser... more Oil extraction efficiency strongly depends on the wettability status (oil vs. water-wet) of reservoir 9 rocks during oil recovery. Aromatic compounds with polar functional groups in crude oil have a significant 10 influence on binding hydrophobic molecules to mineral surfaces. Most of these compounds are in the 11 asphaltene fraction of crude oil. This study focuses on the hydroxyl functional group, an identified 12 functional group in asphaltenes, to understand how the interactions between hydroxyl groups in asphaltenes 13 and mineral surfaces begin. Phenol and 1-naphthol are used as asphaltene surrogates to model the simplest 14 version of asphaltenes. Adsorption of oil molecules on the calcite {10 4} surface is described using static 1 15 quantum-mechanical Density Functional Theory (DFT) calculations and classical Molecular Dynamics 16 (MD) simulations. DFT calculations indicate that adsorption of phenol and 1-naphthol occurs preferentially 17 between their hydroxyl group and calcite step edges. 1-naphthol adsorbs more strongly than phenol, with 18 different adsorption geometries due to the larger hydrophobic part of 1-naphthol. MD simulations show 19 that phenol can behave as an agent to separate oil from water phase, and to bind oil phase to the calcite 20 surface in water/oil mixture. In the presence of phenol, partial separation of water/oil with an incomplete 21 lining of phenol at the water/oil boundary is observed after 0.2 ns. After 1 ns, perfect separation of water/oil 22 with a complete lining of phenol at the water/oil boundary is observed, and the calcite surface become oil-23 wet. Phenol molecules enclose decane molecules at the water/decane boundary preventing water from 24 repelling decane molecules from the calcite surface, and facilitate further accumulation of hydrocarbons 25 near the surface rendering the surface oil-wet. This study indicates phenol and 1-naphthol being good
There are many studies describing the influence of parameters such as pH, pCO 2 , and complexing ... more There are many studies describing the influence of parameters such as pH, pCO 2 , and complexing ligands on the sorption of the aqueous uranyl species onto mineral surfaces. However, few of these studies describe the reduction reaction mechanisms and the factors that influence the rate of reduction, despite the fact that the oxidation state of uranium is the most important factor controlling the mobility of uranium. In this study, the energetics and kinetics of the U(VI) reduction half-reaction on pyrite, hematite, and magnetite were investigated by electrochemical methods using a powder microelectrode (PME) as the working electrode. Anodic and cathodic peaks corresponding to the 1 e -redox couple, U(VI)/U(V), were identified in cyclic voltammograms of pyrite, hematite, and magnetite at pH 4.5. A second oxidation peak, corresponding to the oxidation of U(IV), was identified and provides evidence for the formation of reduced uranium phase(s) on the mineral surfaces. In addition, uranium-containing precipitates were identified on pyrite surfaces after polarization in a PME. This study identifies the disproportionation of U(V) species on the surface as a possible rate-limiting step in the two-step U(VI) reduction mechanism: (1) charge transfer to form U(V) followed by, (2) a disproportionation reaction that forms U(IV) and U(VI). The Tafel slope (i.e., the derivative of the electrode potential with respect to log [current]) was used to evaluate electrochemical mechanisms. High Tafel slopes (>220 mV/(log unit of current) on all minerals evaluated) suggest that uranyl reduction is mediated by insulating (hydr)oxide layers that are present on the semiconducting mineral surfaces. The onset potential for uranyl reduction was determined for pyrite (>+0.1 V vs. Ag/AgCl), and hematite and magnetite (betweenÀ0.02 andÀ0.1 V vs. Ag/AgCl). The onset potential values establish a baseline kinetic parameter that can be used to evaluate how solution conditions (e.g., dissolved reductants, complexing ligands, and polarizing ions) affect the kinetics of uranyl reduction. The results of this study demonstrate the potential for using PMEs to evaluate redox potentials and mechanisms for U(VI) reduction by Fe-oxides and sulfides under more complex solution conditions as well as other environmentally-relevant mineral-analyte systems. However, it should be noted that the determination of redox kinetics using Butler-Volmer theory has limitations when applied to semiconductor mineral electrodes. Charge depletion in semiconductor surface states can affect the kinetic values obtained for redox reactions on the surface. These limitations and a discussion of the flat band potential are considered in the interpretation of U redox kinetics in this study.
nano-colloids by epitaxial distortion on mineral surfaces. Environ. Sci. Technol. 45, 2698 (2011)... more nano-colloids by epitaxial distortion on mineral surfaces. Environ. Sci. Technol. 45, 2698 (2011).] deduced the heteroepitaxial growth of a bcc Pu 4 O 7 phase when sorbed onto goethite from d-spacing measurements obtained from selected-area electron diffraction (SAED) patterns. The structural and/or chemical modification of Pu(IV) oxide (PO) nanocolloids upon sorption to goethite, in turn, affects colloidal-transport of Pu in the subsurface. In this study, molecular simulations were applied to investigate mechanisms affecting the formation of non-fcc PO phases and to understand the influence of goethite in stabilizing the non-fcc PO phase. Analyses of the structure, chemistry, and formation energetics for several bulk PuO 2 and PuO 2-x phases, using ab initio methods, show that the formation of a non-fcc PO can occur from the lattice distortion (LD) of fcc PuO 2 upon sorption and formation of a PO-goethite interface. To strain and non-uniformly distort the PuO 2 lattice to match that of the goethite substrate at ambient conditions would require 88 kJ/mol Pu 4 O 8 . The formation of a hypostoichiometric PuO 2-x phase, such as the experimentally-deduced bcc, Ia3̅ Pu 4 O 7 phase, requires more O-poor conditions and/or high energetic inputs ( > +365 kJ/mol Pu 4 O 7 at O-rich conditions). Empirical methods were also applied to study the effect of lattice distortion on sorption energetics and adsorbate particle growth using simple heterointerfaces between cubic salts, where KCl clusters (notated as KCl LD ) of varying size and lattice mismatch (LM) were sorbed to a NaCl cluster. When the lattice of a KCl LD cluster has < 15% LM with that of a NaCl
Despite previous studies investigating selenium (Se) redox reactions in the presence of semicondu... more Despite previous studies investigating selenium (Se) redox reactions in the presence of semiconducting minerals, Se redox reactions mediated by galena (PbS) are poorly understood. In this study, the redox chemistry of Se on galena is investigated over a range of environmentally relevant Eh and pH conditions (+0.3 to -0.6 V vs. standard hydrogen electrode, SHE; pH 4.6) using a combination of electrochemical, spectroscopic, and computational approaches. Cyclic voltammetry (CV) measurements reveal one anodic/cathodic peak pair at a midpoint potential of +30 mV (vs. SHE) that represents reduction and oxidation between HSeO 3 -and H 2 Se/HSe -. Two peak pairs with midpoint potentials of -400 and -520 mV represent the redox transformation from Se(0) to HSe -and H 2 Se species, respectively. The changes in Gibbs free energies of adsorption of Se species on galena surfaces as a function of Se oxidation state were modeled using quantum-mechanical calculations and the resulting electrochemical peak shifts are (-0.17 eV for HSeO 3-/H 2 Se, -0.07 eV for HSeO 3-/HSe -, 0.15 eV for Se(0)/HSe -, and -0.15 eV for Se(0)/H 2 Se). These shifts explain deviation between Nernstian equilibrium redox potentials and observed midpoint potentials. X-ray photoelectron spectroscopy (XPS) analysis reveals the formation of Se(0) potentials below -100 mV and Se(0) and Se(-II) species at potentials below -400 mV.
Strain-Induced Segmentation of Magnesian Calcite Thin Films Growing on a Calcite Substrate
Crystal Growth & Design, Aug 27, 2010
ABSTRACT In crystal growth of mineral species or different compositional members of a solid solut... more ABSTRACT In crystal growth of mineral species or different compositional members of a solid solution on one another, the degree of lattice mismatch at their interface affects the growth pattern of the precipitating mineral phase. Fast layer-by-layer growth of magnesian calcite on pure calcite (101̅4) substrates has been observed at Mg2+/Ca2+ ratios of 2−7 using in situ atomic force microscopy. Under solution conditions of calcite saturation states starting from Ω ≈ 33, depending on Mg2+/Ca2+ ratios and carbonate content, bulging in the epitaxial magnesian calcite thin film led to the formation of networks of ridges along the [4̅41], [481̅], and [421̅] directions. Eventually, spreading of monolayers stopped at the ridges and formed stationary multilayer steps, resulting in separate and individually growing crystal segments. Molecular dynamics computational modeling suggests that relaxation of strain energy, caused by the interfacial lattice mismatch between pure calcite and the isostructural magnesium-containing phase with smaller lattice constants, leads to a semicoherent interface and disordered linear zones cutting through the thin film. As a consequence, the surface bulges up in a way similar to our laboratory observations. This strain-induced segmentation produces aggregates of aligned microcrystals and increase knowledge of the behavior of strained thin films in general.
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