High-energy Structure Research Articles

Size Dependence of Lattice Parameter and Electronic Structure in CeO2 Nanoparticles.

Intrinsic properties of a compound (e.g., electronic structure, crystallographic structure, optical and magnetic properties) define notably its chemical and physical behavior. In the case of nanomaterials, these fundamental properties depend on the occurrence of quantum mechanical size effects and on the considerable increase of the surface to bulk ratio. Here, we explore the size dependence of both crystal and electronic properties of CeO2 nanoparticles (NPs) with different sizes by state-of-the art spectroscopic techniques. X-ray diffraction, X-ray photoelectron spectroscopy, and high-energy resolution fluorescence-detection hard X-ray absorption near-edge structure (HERFD-XANES) spectroscopy demonstrate that the as-synthesized NPs crystallize in the fluorite structure and they are predominantly composed of CeIV ions. The strong dependence of the lattice parameter with the NPs size was attributed to the presence of adsorbed species at the NPs surface thanks to Fourier transform infrared spectroscopy and thermogravimetric analysis measurements. In addition, the size dependence of the t2g states in the Ce LIII XANES spectra was experimentally observed by HERFD-XANES and confirmed by theoretical calculations.

Intersections of two stacking faults in zincblende GaN

The structure and energetics of an isolated {111} stacking fault and the interactions of two non-coplanar {111} stacking faults in zincblende GaN are investigated using an empirical potential of the Tersoff-Brenner type. For a single stacking fault, a metastable configuration is found only when the fault is created on the {111} plane in the glide set, which results in local transformation into a more stable wurtzite structure. This energetically favorable configuration is separated from the unfaulted crystal by a large energy barrier. Interactions between two stacking faults on non-coplanar {111} planes, where one fault corresponds to the metastable configuration created in the glide set and the second fault is created on a different {111} plane, lead to a reduction of the aforementioned energy barrier and an increase of the energy of the second metastable fault. The intersection of the two faults results in a significant reconstruction of atomic positions around the line common to both faults. Apart from the wurtzite stacking, the structure of this intersection shows a partial transformation into the rocksalt structure that is normally stable only at high pressures. The presence of this high-energy rocksalt structure is avoided if the second fault is non-planar. In this case, four different structures of the intersection exist. We demonstrate that one of these structures agrees well with TEM observations.

Thermal-pressure treatment for tuning the atomic structure of metallic glass Cu-Zr

Both thermal and pressure treatments are effective to obtain rejuvenated metallic glass (MG) with high energy state and densely packed structure. In this study, we studied the changes of potential energy and atomic volume under various cooling rate and pressure during quenching stage for Cu-Zr MGs by atomic structure analysis. It is found that: (1) Increasing pressure leads to monotonous increasing potential energy but decreasing and then increasing atomic volume; (2) The potential energy becomes higher, and the densest atomic structure is obtained at a higher pressure with more Cu content; (3) Higher energy state obtained by increasing pressure is corresponding to more full icosahedron and 1551 bond type. This work provides a profound understanding of rejuvenated MG with high energy and unique atomic structure by thermal-pressure treatments.

The structure of the oxidized state of cytochrome c oxidase - experiments and theory compared

Cytochrome c oxidase (CcO), the terminal enzyme in the respiratory chain, reduces molecular oxygen to water. Experimental data on the midpoint potentials of the heme iron/copper active site cofactors do not match the overall reaction energetics, and are also in conflict with the observed efficiency of energy conservation in CcO. Therefore it has been postulated that the ferric/cupric intermediate (the oxidized state) exists in two forms. One form, labelled OH, is presumably involved during catalytic turnover, and should have a high CuB midpoint potential due to a metastable high energy structure. When no more electrons are supplied, the OH state supposedly relaxes to the resting form, labelled O, with a lower energy and a lower midpoint potential. It has been suggested that there is a pure geometrical difference between the OH and O states, obtained by moving a water molecule inside the active site. It is shown here that the difference between the two forms of the oxidized state must be of a more chemical nature. The reason is that all types of geometrically relaxed structures of the oxidized intermediate have similar energies, all with a high proton coupled reduction potential in accordance with the postulated OH state. One hypothesized chemical modification of the OH state is the transfer of an extra proton, possibly internal, into the active site. Such a protonated state has several properties that agree with experimental data on the relaxed oxidized state, including a decreased midpoint potential.

Probing Dopant Locations in Silicon Nanocrystals via High Energy X-ray Diffraction and Reverse Monte Carlo Simulation.

Understanding the locations of dopant atoms in ensembles of nanocrystals is crucial to controlling the dopants' function. While electron microscopy and atom probe tomography methods allow investigation of dopant location for small numbers of nanocrystals, assessing large ensembles has remained a challenge. Here, we are using high energy X-ray diffraction (HE-XRD) and structure reconstruction via reverse Monte Carlo simulation to characterize nanocrystal structure and dopant locations in ensembles of highly boron and phosphorus doped silicon nanocrystals (Si NCs). These plasma-synthesized NCs are a particularly intriguing test system for such an investigation, as elemental analysis suggests that Si NCs can be "hyperdoped" beyond the thermodynamic solubility limit in bulk silicon. Yet, free carrier densities derived from local surface plasmon resonances suggest that only a fraction of dopants are active. We demonstrate that the structural characteristics garnered from HE-XRD and structure reconstruction elucidate dopant location and doping efficacy for doped Si NCs from an atomic-scale perspective.

Competitive Reaction of Neptunium(V) and Uranium(VI) in Potassium-Sodium Carbonate-Rich Aqueous Media: Speciation Study with a Focus on High-Resolution X-ray Spectroscopy.

Neptunium(V) and uranium(VI) are precipitated from an aqueous potassium-sodium-containing carbonate-rich solution, and the solid phases are investigated. U/Np M4,5-edge high-energy resolution X-ray absorption near edge structure (HR-XANES) spectroscopy and Np 3d4f resonant inelastic X-ray scattering (3d4f RIXS) are applied in combination with thermodynamic calculations, U/Np L3-edge XANES, and extended X-ray absorption fine structure (EXAFS) studies to analyze the local atomic coordination and oxidation states of uranium and neptunium. The XANES/HR-XANES analyses are supported by ab initio quantum-chemical computations with the finite difference method near-edge structure code (FDMNES). The solid precipitates are also investigated with powder X-ray diffraction, scanning electron microscopy-energy dispersive X-ray spectroscopy, and Raman spectroscopy. The results strongly suggest that K[NpVO2CO3](cr), K3[NpVO2(CO3)2](cr), and K3Na[UVIO2(CO3)3](cr) are the predominant neptunium and uranium solid phases formed. Despite the 100 times lower initial neptunium(V) concentration at pH 10.5 and oxic conditions, neptunium(V)-rich phases predominately precipitate. The prevailing formation of neptunium(V) over uranium(VI) solids demonstrates the high structural stability of neptunium(V) carbonates containing potassium. It is illustrated that the Np M5-edge HR-XANES spectra are sensitive to changes of the Np-O axial bond length for neptunyl(V/VI).

Tris(butadiene) Metal Complexes of the First-Row Transition Metals versus Coupling of Butadiene to Eight- and Twelve-Carbon Hydrocarbon Chains.

The role that zerovalent nickel plays in catalyzing the trimerization of butadiene to 1,5,9-cyclododecatriene conveys interest in the properties of the tris(butadiene)metal complexes (C4H6)3M. In this connection the complexes (C4H6)3M (M = Ti-Ni) of the first-row transition metals have been investigated by density functional theory. The intermediate C12H18Ni which has been isolated in the nickel-catalyzed trimerization of butadiene but is too unstable for X-ray crystallography is suggested here to have an open-chain hexahapto η3,3-C12H18 ligand rather than the octahapto such ligand suggested by some investigators. The lowest energy (C4H6)3M structures of the other first-row transition metals from vanadium to cobalt are found to have related structures with open-chain C12H18 ligands having hapticities ranging from four to eight with hexahapto structures being most common. The nickel and cobalt (C12H18)M derivatives favor low-spin singlet and doublet spin states, respectively, whereas the manganese derivative (C12H18)Mn favors the high-spin sextet state corresponding to the half-filled d5 shell of Mn(II). A (C4H6)3Cr structure with three separate tetrahapto butadiene ligands analogous to the very stable (η4-C4H6)3M (M = Mo, W) with the favored 18-electron metal configuration is found to be a very high energy structure relative to isomers containing an open-chain C12H18 ligand.

Torsional behaviors in Zr-based bulk metallic glass with high stored energy structure

To overcome brittleness of bulk metallic glasses, great efforts have been devoted to optimizing BMGs′ plasticity under compressive, tensional or bending load. Yet, it is hardly found any work done to enhance torsional plasticity of BMGs. Herein, based on the different stored energy of BMGs determining diverse properties, we introduce the energy into Zr54.46Al9.9Ni4.95Cu29.7Pd0.99 BMG by a high rheological rate forming method, leading to the higher microstructural heterogeneity, which is conducive to increase not only the plasticity (from 0.2% up to 1.56%) but also the ultimate strength from (990 MPa up to 1110 MPa) on the torsional deformation effectively. This work provides a new orientation to break through the brittleness of BMGs.

Coherent Structure of Flow Based on Denoised Signals in T-junction Ducts with Vertical Blades.

The skin friction consumes some of the energy when a train is running, and the coherent structure plays an important role in the skin friction. In this paper, we focus on the coherent structure generated near the vent of a train. The intention is to investigate the effect of the vent on the generation of coherent structures. The ventilation system of a high-speed train is reasonably simplified as a T-junction duct with vertical blades. The velocity signal of the cross duct was measured in three different sections (upstream, mid-center and downstream), and then the coherent structure of the denoised signals was analyzed by continuous wavelet transform (CWT). The analysis indicates that the coherent structure frequencies become abundant and the energy peak decreases with the increase of the velocity ratio. As a result, we conclude that a higher velocity ratio is preferable to reduce the skin friction of the train. Besides, with the increase of velocity ratio, the dimensionless frequency St of the high-energy coherent structure does not change obviously and St = 3.09 × 10−4–4.51 × 10−4.

Dynamic mode decomposition of cavitating flow around ALE 15 hydrofoil

The Dynamic Mode Decomposition (DMD) and Proper Orthogonal Decomposition (POD) are employed to analyze the coherent structure of cavitating flow around ALE 15 hydrofoil. The snapshot data sequence is obtained from the numerical simulations by means of LES approach and modified Schnerr-Sauer cavitation model. Under cavitation number σ = 2.3, the cavitating flow around ALE 15 hydrofoil sheds at the short side while it almost remains stable at the long side. The eigenvalue distribution of DMD is symmetrical along the real axis of the plane, and the eigenvalues appear as conjugate pairs. The DMD method can accurately extract the frequency characteristics, and results show that the decomposed Mode at St = 2.224 is related to the shedding frequency of cloud cavitation, which agrees well with the shedding frequency of 2.208–2.805 in experimental measurement. The POD method can effectively analyze the major structure of high energy, in which the first four modes contain over 60% total energy of flow field. In comparison of POD, the DMD is more effective to decompose the complex flow field into uncoupled coherent structures with specific dynamic modes and corresponding frequencies.

Sp3 bonded 2-dimensional allotrope of carbon: A first-principles prediction

2-dimensional allotropes of carbon have lately attracted significant research interests owing to their unique electronic and structural properties. Such sp and sp2 bonded atomic systems, i.e. graphdiyne and graphene are well represented in the forefront of theoretical and experimental chemistry. However, no stable sp3 bonded 2-dimensional all-carbon atomic structure has been identified yet. To this end, using state-of-the-art theoretical calculations, we considered cyclobutane motifs, and investigated whether a sp3 bonded 2-dimensional carbon allotrope could be achieved by assembling ladderane chains. Energetic and dynamic stability studies yielded two such promising structures: one with 4-coordinated carbon atoms and a relatively more stable structure with a combination of 3 and 4-coordinated carbon atoms; both having puckered geometries and partially sp3 C-C bonds. However thermal stability investigations indicated only the lower energy configuration could be stable at ambient temperature and pressure. The higher energy structure was found to be metastable and was observed to suffer phase-transitions towards the lower energy structure or other stable 2-dimensional allotropes of carbon under exposure to ambient conditions. Investigation of electronic properties of these proposed materials revealed them to be direct-gap semiconductors with small bandgaps, and their tunability by applying external strain implied wide scope of applications.

Thermal-pressure effects on energy state of metallic glass Cu50Zr50

Combined application of thermal and pressure are effective to obtain structure excited metallic glass with high energy state. This work studied the thermal-pressure effects on energy state, atomic level structure and tensile behavior during each separate fabrication stage (heating, equilibrating, and quenching) of Cu50Zr50 metallic glasses by dynamic simulation. It is found that quenching under pressure is the most effective approach obtaining high energy state Cu50Zr50 metallic glass among 4 designed methods. For the effective approach, the potential energy increases and the atomic volume decreases with pressure for Cu50Zr50 metallic glass when the pressure is below 30 GPa, and the latter reaches its minimum at 30 GPa. Moreover, Cu-centered full icosahedron <0, 0, 12, 0> plays a dominant role in the formation of high energy structure, and Cu50Zr50 metallic glass fabricated by method 3 under 30 GPa shows the most improved plasticity.

Braided bioresorbable cardiovascular stents mechanically reinforced by axial runners

Polymeric bioresorbable stents (BRSs) can eliminate the long-term stent restenosis by degrading after vascular remolding and have been recommended for the congenital heart disease treatment. However, the mechanical weakness remains one of main inferiorities of their applications. So, the aim of this study was to develop mechanically reinforced bioresorbable stents (MRBSs) based on poly(p-dioxanone) (PPDO) monofilaments and braiding technology. Axial runners were introduced and MRBSs showed greatly higher compression force and relatively lower viscous performance, as well as longer mechanical stability during degradation, compared with controls. Besides, stent compression behaviors were analyzed experimentally and numerically to investigate their deformation mechanisms. The results showed increased contacting points and friction force among yarns in MRBSs. Also, the skeleton formed in MRBSs attributed to higher yarn bending degree, strain energy and better structure stability during compression. Combined with the non-linear PPDO material stress-strain ratio and thermodynamic theory, yarn based stent compression modes were discussed. In addition, the autocatalysis and nonrandom chain scission degradation behaviors of MRBSs were revealed.

Uptake of actinides by calcium silicate hydrate (C-S-H) phases

The sorption of actinides (Th, U – Am) was studied in dependence of the solid-to-liquid (S/L) ratio (0.5–20.0 g/L) and the calcium-to-silicon (C:S) ratio. The C:S ratio was varied between 1.80 and 0.70 to simulate the changing composition of the C-S-H phases during cement degradation from high to low C:S ratios. The decrease of the calcium content in the C-S-H phases by time is accompanied by a decrease in pH in the corresponding suspensions from 12.6 to 10.2. X-ray photoelectron spectroscopy (XPS) of the C-S-H phases showed an increasing depletion of Ca on the surface with increasing C:S ratio in comparison to the composition of the solid phase as a whole.The sorption experiments were performed with the redox stable species Am(III), Th(IV) and U(VI), as well as the redox sensitive Np(V) and Pu(III). The average distribution coefficients Rd for all investigated actinides are around 105 L/kg. The oxidation state of Pu retained by the C-S-H phases was investigated with high-energy resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy. Samples with C:S ratios of 0.75 and 1.65 showed that the initially added Pu(III) was oxidized to Pu(IV) in the course of the experiment.

CdO-CuO-TiO2 ternary dielectric systems: Subsolidus phase diagram and the effects of Cu segregation

Valence of Cu cations and Cu segregation of CdCu3Ti4O12 ceramics are initially examined by establishing the subsolidus phase diagram of Cd1-xCu3+x+2yTi4-yO12 (−7 ≤ x ≤ 0.07, −1.333 ≤ y ≤ 2.91) systematically. Different from CaO-CuO-TiO2 system, the co-existence of hexagonal and orthorhombic CdTiO3 phases is observed in the line between CdTiO3 and CuO. Basing on the obtained phase diagram, three typical ceramics with Cu-equal (CdCu3Ti4O12), Cu-rich (Cd0.930Cu3.070Ti4O12), and Cu-poor (Cd1.137Cu2.863Ti4O12), showing high dielectric constant, are chosen as target samples to investigate the role of Cu valence and segregation in dielectric behaviors. Cu3+ ions, rather than Cu+ ones as reported previously, forms in the grain to act as the hopping charge carriers as demonstrated by X-ray photoelectron spectroscopy and High-energy X-ray absorption Near Edge Structure. This work would give a hint in tuning the stoichiometry for achieving more fascinating structures and properties in new ceramic material design.

Enhanced water resistance and energy performance of core–shell aluminum nanoparticles via in situ grafting of energetic glycidyl azide polymer

The application of aluminum nanoparticle is limited due to its passivation Al2O3 layer even though it owns extreme specific surface and high reaction activity. In this work, the aluminum nanoparticles were modified via in situ grafting onto energetic glycidyl azide polymer (GAP) to improve the stability and energy-releasing performance. The results showed that GAP was grafted on the nano-aluminum surface with chemical bonds of –O–(CO–NH)– formed, and the thickness of shell layer of GAP could be tuned by changing the relative ratio of reactants. Furthermore, modified nanoparticles show hydrophobicity with static water contact angle changing from 20.2° to 142.4°. Significant increasing stability of aluminum nanoparticles is obtained in hot water, which is evidenced by that around 10 wt% of modified aluminum is reacted after 210 min. Based on the core–shell configurations, (Al@GAP)/fluorine composites were prepared. The violent combustion phenomenon and high release rate profiles revealed high energy-releasing performance with the assistance of GAP. This synthetic strategy may provide an effective approach to prepare other metal nanoparticles, and possess potential application value in the fields of metallized explosives and high-energy structure with energy release.

Revealing the structure of isolated peptides: IR-IR predissociation spectroscopy of protonated triglycine isomers

We report an isomer specific IR-IR double resonance study of the mass-selected protonated triglycine peptide. Comparison of experimental spectra with calculations reveals the presence of two isomers, with protonation occurring at either the terminal amine site or one of the amide oxygen sites. The amine protonated isomer identified in our experiment contains an atypical cis amide configuration as well as a more typical trans amide. The amide protonated peptide, on the other hand, contains two trans amide moieties. Both isomers are found to be the lowest energy structures for their respective protonation site, but it is unclear, from experiments and calculations, which one is the global minimum. The presence of both in our experiments likely points to kinetic trapping of a higher energy structure. Finally, the observed frequencies of the NH and OH stretch vibrations are used to estimate the hydrogen-bond strengths present in each isomer, accounting for the relative stabilities of these structures.

High-energy gravitational scattering and the general relativistic two-body problem

A technique for translating the classical scattering function of two gravitationally interacting bodies into a corresponding (effective one-body) Hamiltonian description has been recently introduced [Phys.\ Rev.\ D {\bf 94}, 104015 (2016)]. Using this technique, we derive, for the first time, to second-order in Newton's constant (i.e. one classical loop) the Hamiltonian of two point masses having an arbitrary (possibly relativistic) relative velocity. The resulting (second post-Minkowskian) Hamiltonian is found to have a tame high-energy structure which we relate both to gravitational self-force studies of large mass-ratio binary systems, and to the ultra high-energy quantum scattering results of Amati, Ciafaloni and Veneziano. We derive several consequences of our second post-Minkowskian Hamiltonian: (i) the need to use special phase-space gauges to get a tame high-energy limit; and (ii) predictions about a (rest-mass independent) linear Regge trajectory behavior of high-angular-momenta, high-energy circular orbits. Ways of testing these predictions by dedicated numerical simulations are indicated. We finally indicate a way to connect our classical results to the quantum gravitational scattering amplitude of two particles, and we urge amplitude experts to use their novel techniques to compute the 2-loop scattering amplitude of scalar masses, from which one could deduce the third post-Minkowskian effective one-body Hamiltonian.

Analysis of the higher-energy structure in strong-field ionization with inhomogeneous electric fields

Recently, the emergence of a higher energy structure (HES) due to a spatial inhomogeneity in the laser electric field, as is typically found close to a nano tip, was reported in Phys.~Rev. Letter {\bf 119}, 053204 (2017). For practical applications, such as the characterization of near-fields or the creation of localized sources of monoenergetic electron beams with tunable energies, further insight into the nature of this higher energy structure is needed. Here, we give a closed form analytical approximation to describe the movement of the electron in the inhomogeneous electric field. In particular, we derive a simple scaling law for the location of the HES peak and give a scheme to analytically tune the width of the peak, both of which will prove useful in optimizing the nanostructure size or geometry for creating the HES in experimental settings.

Dehydration of the Uranyl Peroxide Studtite, [UO2(η2-O2)(H2O)2]·2H2O, Affords a Drastic Change in the Electronic Structure: A Combined X-ray Spectroscopic and Theoretical Analysis.

The minerals studtite, [UO2(η2-O2)(H2O)2]·2H2O, and metastudtite, [UO2(η2-O2)(H2O)2], are uranyl peroxide minerals that are major oxidative alteration phases of UO2 under conditions of geological storage. The dehydration of studtite has been studied using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy. XPS of the U 4f region shows small but significant differences between studtite and metastudtite, with the 4f binding energy of studtite being the highest reported for a uranyl mineral studied by this technique. Further information about the changes in the electronic structure was elucidated using U M4-edge high-energy resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy, which directly probes f orbital states. The transition from the 3d to 5fσ* orbital is sensitive to variations in the U═Oaxial bond length and to changes in the bond covalency. We report evidence that the covalence in the uranyl fragment decreases upon dehydration. Photoluminescence spectroscopy at near-liquid helium temperatures reveals significant spectral differences between the two materials, correlating with the X-ray spectroscopy results. A theoretical investigation has been conducted on the structures of both studtite and metastudtite and benchmarked to the HR-XANES spectra. These illustrate the sensitivity of the 3d to 5f σ* transition toward U═Oaxial bond variation. Small structural changes upon dehydration have been shown to have an important electronic effect on the uranyl fragment.