Average Bond Length Research Articles

Second‐harmonic generation and structural rearrangements in multicomponent antimonite glasses by electrothermal poling

AbstractElectrothermal poling is shown here to effectively induce second‐order nonlinear effects in heavy‐metal oxide antimonite glasses. In M2O–PbO–WO3–Sb2O3 (M = Li, Na, K) glasses, the poling‐induced second‐harmonic generation intensity is five times larger than in silica (Infrasil) for M = Na, twice as large as in silica for M = Li, and smaller than in silica for M = K. X‐ray photoelectron spectroscopy suggests that antimony ions exist predominantly in the trivalent oxidation state in the studied glass samples. Raman and infrared spectroscopy confirm that the glass network is comprised of SbO3, WO4, WO6, and PbO4 units—with some SiO4 moieties due to leaching from the silica crucible. The WO4 units appear to exist in two distinct sites, as evidenced by comparison of the vibrational spectra of alkali–tungsten–antimonite glasses with those of previously reported crystalline tungstate phases. The alkali type influences the equilibrium between tetrahedral tungstate anions, [WO4]2−, and the isomeric partially polymerized octahedral tungstate units, [WØ4O2]2− (Ø denotes a bridging oxygen). Raman spectroscopy line scans were used to track near‐surface structural changes on the anode side of poled glasses. They reveal that the tungstate equilibrium is also affected by poling. At the anode side, the population of partially polymerized [WØ4O2]2− species increases at the expense of anionic [WO4]2− species. This yields a net increase in the average bond length of the network forming constituents, which is commensurate with poling‐induced structural changes observed in other systems experimentally and computationally.

Iodine capture by ZSM-5 with different Si/Al ratios: An experimental and molecular simulation investigation

In order to capture radioactive iodine from nuclear power waste gas, the performance and mechanism of iodine capture by ZSM-5 molecular sieves were analyzed in this paper by experiments and molecular simulations. The results of the experiment showed that the iodine adsorption capacity increased with the increasing Si/Al ratio of the ZSM-5, and rose and then fell with increasing temperature. After four adsorption-desorption cycles, it could still maintain good morphology and adsorption performance (83.17%–90.61%). The molecular simulation results found that as the Si/Al ratio increased, the average bond length of the I–I bond increased, the adsorption energy increased, the interaction between iodine and molecular sieve increased, and the adsorption capacity of iodine increased, which was consistent with the experimental results. In this study, the capture process and adsorption mechanism of iodine were further understood, and it provides a favorable scheme for the treatment of nuclear waste gas.

Site-specific isotope fractionation during Zn adsorption onto birnessite: Insights from X-ray absorption spectroscopy, density functional theory and surface complexation modeling

Birnessite minerals help control the fate of Zn in surface environments and readily fractionate Zn isotopes through adsorption reactions, yet little is known about the role played by various reactive sites in stable isotopic fractionation. Here we present the Zn isotope fractionation data cause by adsorption on birnessite under different reaction times, pH values, and Zn concentrations. We observe that isotopic equilibrium of Zn is attained after ∼120 h of reaction time at pH 6. At pH 3–5 and Zn concentrations of 0.05–0.3 mM, the isotopic fractionation (Δ66Znadsorbed-aqueous) is around −0.46 ± 0.04‰, and gradually increases to −0.09 ± 0.05‰ at pH 6–8 and Zn concentrations of 0.2 mM. The change in Zn isotopic compositions as a function of pH and Zn concentration is well described using the surface complexation model, where two binding sites are involved: external edge sites and interlayer vacancies. According to this model, two different isotopic fractionation factors of Zn are calculated: Δ66Znadsorbed-aqueous = −0.46 ± 0.04‰ for adsorption on vacancy sites and Δ66Znadsorbed-aqueous = 0.52 ± 0.04‰ for binding to edge sites. Extended X-ray absorption fine structure spectroscopy (EXAFS) demonstrates that Zn forms triple-corner-sharing (TCS) octahedral complex on birnessite vacancies at pH 3 and Zn concentrations of 0.05–0.2 mM, where Zn is coordinated on one side to three oxygen atoms of the Mn vacancy (∼2.03 Å) and to three water molecules on the other side (∼2.15 Å), suggesting the formation of distorted ZnO octahedra (average bond length: ∼2.09 Å). At pH 6 and 8, double-corner-sharing (DCS) complexes on layer edges formed in addition to the TCS octahedral complex on vacancies. Density functional theory (DFT) optimisations suggest that DCS Zn complex exist in tetrahedral coordination. Based on EXAFS spectroscopy, DFT optimisations and surface complexation modeling, the distinct isotopic fractionation of Zn is related to the differences in Zn local structure at different reactive sites of birnessite. Our results provide a molecular-scale understanding of Zn isotopic fractionation in natural birnessite-containing settings, as well as new insights into predicting the links between adsorption and fractionation of other similar metals.

CRYSTAL AND MOLECULAR STRUCTURE OF THE ISLAND TETRANUCLEAR DIOXOMOLYBDENUM(VI) COMPLEX [MoO<sub>2</sub>(<i>L</i><sup>1</sup>)]<sub>4</sub> (H<sub>2</sub><i>L</i><sup>1</sup> IS ACETYLACETONE ISONICOTINOYLHYDRAZONE) WITH LARGE INTRA- AND INTERMOLECULAR CHANNELS

The solvated complex [МоО2(L1)]4dimethylformamide (I) was synthesized. Its structure wasdetermined by X-ray diffraction analysis. The crystal structure is composed of the tetranuclear complexes [МоО2(L1)]4 (Ia) as the structural units lying on crystallographic twofold axes. Both crystallographically independent molybdenum atoms are in a distorted octahedral coordination environment formed by two cis-О(oxo) ligands, two N(L1) atoms of two molecules Ia in trans positions to the О(oxo) ligands, and two О(L1) atoms of one complex molecule in cis positions to О(oxo) and trans to each other. Each (L1)2– ligand is coordinated to two Мо atoms in a tetradentate tridentate-chelating (2О, N) bridging (N) mode. The average bond lengths in complex Iа are as follows: Мо–О(oxo), 1.701 Å; Мо–N(L1), 2.460 (b) and 2.214 Å (c); Мо–О(L1), 1.980 Å. The О(oxo)–МоО–(oxo) bond angle is 105.6°. The ordered dimethylformamide molecule is located in a narrow channel in the structure. The strongly disordered (non-located) solvent molecules (methanol/dimethylformamide/water) occupy wide channels in the structure of I.

Complexation of a Nitrilotriacetate-Derived Triamide Ligand with Trivalent Lanthanides: A Thermodynamic and Crystallographic Study.

Non-heterocyclic N-donor nitrilotriacetate-derived triamide ligands are one of the most promising extractants for the selective extraction separation of trivalent actinides over lanthanides, but the thermodynamics and mechanism of the complexation of this kind of ligand with actinides and lanthanides are still not clear. In this work, the complexation behaviors of N,N,N',N',N″,N″-hexaethylnitrilotriacetamide (NTAamide(Et)) with four representative trivalent lanthanides (La3+, Nd3+, Eu3+, and Lu3+) were systematically investigated by using 1H nuclear magnetic resonance (1H NMR), ultraviolet-visible (UV-vis) and fluorescence spectrophotometry, microcalorimetry, and single-crystal X-ray diffractometry. 1H NMR spectroscopic titration of La3+ and Lu3+ indicates that two species of 1:2 and 1:1 metal-ligand complexes were formed in NO3- and ClO4- media. The stability constants of NTAamide(Et) with Nd3+ and Eu3+ obtained by UV-vis and fluorescence titration show that the complexing strength of NTAamide(Et) with Nd3+ is lower than that with Eu3+ in the same anionic medium, while that of the same lanthanide complex is higher in ClO4- medium than in NO3- medium. Meanwhile, the formation reactions for all metal-ligand complexes are driven by both enthalpy and entropy. The structures of lanthanide complexes in the single ClO4- and NO3- medium and the mixed one were determined to be [LnL2(MeOH)](ClO4)3 (Ln = La, Nd, Eu, and Lu), [LaL2(EtOH)2][La(NO3)6], and [LaL2(NO3)](ClO4)2, separately. The average bond lengths of lanthanide complexes decrease gradually with the decrease in ionic radii of Ln3+, indicating that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, which coincides with the trend of the complexing strength obtained by spectroscopic titration. This work not only reveals the thermodynamics and mechanism of the complexation between NTAamide ligands and lanthanides but also obtains the periodic tendency of complexation between them, which may facilitate the separation of trivalent lanthanides from actinides.

Tuning of magnetic, electronic and electrolytic water properties of silicene supported precious-metal by non-metal doping and vacancy defect

It is well known that atomic adsorption or doping and surface defect can effectively modulate the physical properties and chemical activity of two-dimensional (2D) materials. In this paper, we systematically have explored the stability, electronic and magnetic properties of non-metal (NM = C, N, O, P, S, Se) doped vacancy-defect silicene, precious-metal (PM = Ru, Rh, Pd, Ag, Os, Ir, Pt, Au) doped vacancy-defect silicene, NM/PM co-doped vacancy-defect silicene, and the electrolytic water activity of stable PM single-atom doped systems using first-principles calculations based on density functional theory (DFT). By comparing the binding energy and electron localization functions (ELF) of the studied objects, it can be concluded that both NM-doped single-atomic vacancy (SV) silicene and double-atomic vacancy (DV) silicene have high stability, and the stability of NM-doped vacancy-defect silicene decreases with increasing the average bond length of Si-NM bond; the stability of PM-doped vacancy-defect silicene are stronger than that of the NM/PM co-doped vacancy-defect silicene. By comparing the free energy changes of hydrogen evolution reaction (HER) intermediate and oxygen evolution reaction (OER) intermediates, it can be concluded that Rh and Os-doped SV silicene have excellent HER activity, while Ir-doped DV silicene has good OER performance. Moreover, we also analyzed and discussed the electronic and magnetic properties of NM-doped vacancy-defect silicene, PM-doped vacancy-defect silicene and NM/PM co-doped vacancy-defect silicene, and we found that 109 PM doped systems out of 160 PM doped systems exhibit metallic properties, which is favorable for the development of vacancy-defect silicene supported PM single-atom in electrocatalytic field. Our study can provide theoretical support for the future development of the application value of vacancy-defect silicene.

Uranyl(VI) Mixed-ligand complex synthesis and characterization using 4-acylhydrazone-5-pyrazolone and 4-acylpyrazolone: Covalency, crystal assay, DFT study and Hirshfeld analysis

Newly prepared bidentate HLA and tridentate Bz-HLA ligands were used to synthesize the novel uranyl mixed ligand [UO2(HLA)(Bz-HLA)] complex with pentagonal bipyramidal geometry. HLA ligand was prepared via the common acylation method, and the Bz-HLA ligand was prepared using the Schiff base reaction of HLA with a benzohydrazide molecule. FTIR, 1H and 13C NMR, SCXRD, molar conductance, and TG-DTA were mainly implemented to examine the structure of produced compounds. The combined covalency effect of complex due to both ligands has been studied, which was discovered to be influenced by the average bond length values on the pentagonal plane, uranyl axial bond lengths, ligand coordination, and donor ability of various atoms. To demonstrate that solvents have no impact on the covalency of the synthesised complex, Sinha's covalency parameters were computed using UV–vis data. DFT calculations were performed in order to validate experimental findings in comparison to theoretical findings, and theoretical bond characteristics were also investigated through the recognition of global index values. Hirshfeld surface analysis was used to examine the crystal strength, energy characteristics and non-covalent surface interactions. Analysis using cyclic voltammetry(CV) shows that the complex is irreversibly reducing. Understanding the Schiff base of acylpyrazolones and their covalency in uranyl complexes allow for the investigation of a wide range of chemical properties and applications.

The Local and Electronic Structure Study of LuxGd1−xVO4 (0 ≤ x ≤ 1) Solid Solution Nanocrystals

Rare-earth-doped mixed crystals have demonstrated tunable optical properties, and it is of great importance to study the structural characteristics of the mixed-crystal hosts. Herein, LuxGd1-xVO4 (0 ≤ x ≤ 1) solid solution nanocrystals were synthesized by a modified sol-gel method, with a pure crystalline phase and element composition. The X-ray diffraction (XRD) and Rietveld refinement results showed that LuxGd1-xVO4 nanocrystals are continuous solid solutions with a tetragonal zircon phase (space group I41/amd) and the lattice parameters strictly follow Vegard's law. The detailed local structures were studied by extended X-ray absorption fine structure (EXAFS) spectra, which revealed that the average bond length of Gd-O fluctuates and decreases, while the average bond length of Lu-O gradually decreases with the increase in Lu content. Furthermore, the binding energy differences of core levels indicate that the covalent V-O bond is relatively stable, while the ionicity of the Lu-O bond decreases with the increasing x value, and the ionicity of the Gd-O bond fluctuates with small amplitude. The valence band structures were further confirmed by the first-principles calculations, indicating that the valence band is contributed to by the O 2p nonbonding state, localized Gd 4f and Lu 4f states, and the hybridized states between the bonding O 2p and V 3d. The binding energies of the Lu core and the valence levels tend to decrease gradually with the increase in Lu content. This work provides insight into the structural features of mixed-crystal hosts, which have been developed in recent years to improve laser performance by providing different positions for active ions to obtain inhomogeneous broadening spectra.

Theoretical study on the correlation between the structure, excess energy, surface energy, electronic structure, nitro charge, and friction sensitivity of N,N'-dinitroethylenediamine (EDNA).

The sensitivity of energetic materials along different crystal directions is not the same and is anisotropic. In order to explore the difference in friction sensitivity of different surfaces, we calculated the structure, excess energy, surface energy, electronic structure, and the nitro group along (1 1 1), (1 1 0), (1 0 1), (0 1 1), (0 0 1), (0 1 0), and (1 0 0) surfaces of EDNA based on density functional theory. The analysis results showed that relative to other surfaces, the (0 0 1) surface has the shortest N-N average bond length, largest N-N average bond population, smallest excess energy and surface energy, widest band gap, and the largest nitro group charge value, which indicates that the (0 0 1) surface has the lowest friction sensitivity compared to other surfaces. Furthermore, the conclusions obtained by analyzing the excess energy are consistent with the results of the N-N bond length and bond population, band gap, and nitro charge. Therefore, we conclude that the friction sensitivity of different surfaces of EDNA can be evaluated using excess energy.

First-principles study of torsional single-walled carbon nanotubes

The controllable band gap of single-walled carbon nanotube (SWCNT) has become a research hotspot. This study introduces a torsional model that involves each rotating carbon atom along the axial direction of SWCNT, and a detailed description of the model creation process. Two guidelines for constructing the model are proposed, and the self-consistency of the torsion model is established through first-principles density functional theory. Initially, the band gap map of SWCNTs under torsion is present. As the twist strength increases, the band gap of SWCNT undergoes several phase transitions, including semiconductor-metal transition and metal-semiconductor transition. Moreover, we investigate the variations in the average bond length, average bond angle, and diameter of SWCNT under torsion. Furthermore, this work turns to the analysis of carbon atomic energy statistics, revealing distinct energy changes for different types of single-walled carbon nanotubes under identical torsion intensity. The findings shed light on the controllable band gap of SWCNTs, offering a theoretical foundation for the development of nanoelectronic devices and microintegrated circuits utilizing single-walled carbon nanotubes. In conclusion, this research presents a novel approach for exploring the controllable band gap of single-walled carbon nanotube through torsional manipulation. Theoretical insights into the behavior of SWCNTs under torsion provide valuable contributions to the field and pave the way for potential applications in nanoelectronics and microintegrated circuits.

Simulated synthesis and atomic-level structural characterization of LiNi2O4

LiMn2O4 is a promising cathode material for advancing lithium-ion batteries due to its high-rate capabilities and high operating voltages. However, it suffers capacity fading due to the loss of manganese and lattice instabilities linked to Mn3+ during cycling. The simulated synthesis technique has been used to generate LiNi2O4 models rich in microstructural features that evolve during the crystal growth process. The microstructural features can be linked to the electrochemical performance and properties of LiNi2O4, which will guide the doping of LiMn2O4 spinel with Ni. Substitution of a small amount of manganese with nickel has been proposed as one of the solutions for reducing capacity loss. The LiNi2O4 spinel structure was synthesized successfully with the simulated amorphization and recrystallization technique. The RDF functions indicated the average Ni – O bond length of ~1.925 Å which is comparable to the Ni – O average bond length of ~1.923 Å synthesized by Thomas M.G.SR and co-workers.

Density functional theory studies on graphene/h-boron nitride hybrid nanosheets for supercapacitor electrode applications.

Pristine graphene (C32), hexagonal boron nitride (h-BN), and graphene/h-BN hybrid nanosheets were examined using density functional theory calculations in order to find their suitability as an electrode material for supercapacitor applications. The stability of the structure, charge density, electronic properties, and quantum capacitance of pristine graphene and graphene/h-BN hybrid nanosheets were studied. The structural optimization results reveal that all the nanosheets are stable with zero transverse displacement of atoms along the z-direction. Further, replacing the C-C pair with B-N altered the average bond length and angle, thereby maintaining structural stability. The interaction between graphene and h-BN is higher for C16B8N8 compared to other hybrid nanosheets because of the delocalized distribution of the electron density cloud. The doping of the B-N pair into the graphene nanosheet shifts the Fermi level into either the valence band or the conduction band based on the concentration of the B-N pair. Meanwhile, the effective mass is increased and is relatively high for the hybrid nanosheets with a localized state. The pristine B16N16 nanosheet exhibits a quantum capacitance of 31.539 μF cm-2, while among the hybrid nanosheets, the C4B14N14 nanosheet exhibits a maximum quantum capacitance of 22.518 μF cm-2, and from the outcomes, they are suitable as an electrode for asymmetric supercapacitors.

Machine Learning Diffusion Monte Carlo Forces

Diffusion Monte Carlo (DMC) is one of the most accurate techniques available for calculating the electronic properties of molecules and materials, yet it often remains a challenge to economically compute forces using this technique. As a result, ab initio molecular dynamics simulations and geometry optimizations that employ Diffusion Monte Carlo forces are often out of reach. One potential approach for accelerating the computation of "DMC forces" is to machine learn these forces from DMC energy calculations. In this work, we employ Behler-Parrinello Neural Networks to learn DMC forces from DMC energy calculations for geometry optimization and molecular dynamics simulations of small molecules. We illustrate the unique challenges that stem from learning forces without explicit force data and from noisy energy data by making rigorous comparisons of potential energy surface, dynamics, and optimization predictions among ab initio density functional theory (DFT) simulations and machine-learning models trained on DFT energies with forces, DFT energies without forces, and DMC energies without forces. We show for three small molecules─C2, H2O, and CH3Cl─that machine-learned DMC dynamics can reproduce average bond lengths and angles within a few percent of known experimental results at one hundredth of the typical cost. Our work describes a much-needed means of performing dynamics simulations on high-accuracy, DMC PESs and for generating DMC-quality molecular geometries given current algorithmic constraints.

Effects of aluminum diffusion on the oxide of the FeCrAl alloys surface: A first–principles study

The oxide structure on a FeCrAl alloy containing a mixture of Al 2 O 3 and Cr 2 O 3 or a layer of Al 2 O 3 was observed in experiments. Given that its atomistic mechanisms remain unclear, the formation mechanism of the layer structure of FeCrAl/(Cr 2 O 3, Al 2 O 3 )/Al 2 O 3 was explored based on Al atom diffusion using first–principles calculation. The energy of Al diffusion and the work of separation of the FeCrAl/Cr 2 O 3 interface show that Al atoms tend to diffuse into Cr 2 O 3 and enhance the bonding strength of the interface when the concentration of Al in the Cr atoms layer of Cr 2 O 3 is lower than or equal to 50%. The separation work of the interface is 2.43 J/m 2 and 3.46 J/m 2 for no Al and Al diffuse, respectively. Electronic structure analysis shows that more electrons of Al atoms participate in the formation of Al–O bonds and that reduces the system energy when Al atoms diffuse into Cr 2 O 3 . Hence, the Al atom tends to diffuse into Cr 2 O 3 , and the structure of FeCrAl/(Cr 2 O 3 , Al 2 O 3 ) is formed. When the concentration of Al atoms in the Cr atoms layer of Cr 2 O 3 is more than 50%, Al atoms diffuse close to the surface and the FeCrAl/(Cr 2 O 3 , Al 2 O 3 )/Al 2 O 3 structure may be formed. The crystal orbital Hamilton population (COHP) of Al–O bonds indicates that Al diffusing close to the surface of Cr 2 O 3 can reduce local lattice distortion and restore the strength of the Al–O bond. The average bond length of the Al–O bond changes from 1.923 Å to 1.945 Å, and then returns to 1.926 Å, and the –COHP of the Al–O bond changes from 4.483 to 4.740.

Iron and silicon isotope fractionation in silicate melts using first-principles molecular dynamics

The direct determination of silicate melts iron and silicon isotopes signature remains a major challenge of high-temperature isotope geochemistry. For this reason, melts are often approximated by silicate glasses. Calculation of precise equilibrium Si and Fe isotopes fractionation factors between minerals and melt would indeed allow us to distinguish equilibrium fractionation from diffusion-driven kinetic fractionation involved in the iron and silicon isotopes signatures of Earth and other planets. In this study, we use for the first time, first-principles molecular dynamics based on density functional theory to determine iron and silicon isotope compositions of different silicate melts, namely: iron-rich basalt, iron-depleted basalt, basanite, trachyte and phonolite. The 57Fe/54Fe reduced partition function ratios (β-factors) of the different melts span over a 1.1 ‰ range at 1000 Kelvin (K) while 30Si/28Si β-factors are much less influenced by the melt composition with a 0.5 ‰ fractionation range at the same temperature. The main parameter controlling iron isotope fractionation in silicate melts having similar iron oxidation state is, after temperature, the average Fe-O bond length. The chemical environment around iron (e.g. Fe-Fe distances) is suggested to contribute to Fe isotope fractionation as well. Silicon isotopes fractionation seems also affected, but to a lesser extent, by its local chemical composition with decreasing Si-Fe distances leading to slightly higher Si β-factor in the melt. From these melts Fe and Si β-factors, a new set of equilibrium fractionation factors between a variety of minerals and melts has been calculated. These new Δ57Femin-melt and Δ30Simin-melt sets allow us to discuss whether processes such as fractional crystallization, partial melting and diffusion could be responsible for the documented Fe and Si isotopes variations in igneous rocks. Our results suggest that: 1) fractional crystallization may explain at least part of the Fe and Si isotopic evolution during magmatic differentiation, for values up to δ57Fe = 0.65 ‰ and δ30Si = -0.1 ‰, respectively, 2) partial melting of the upper mantle can produce the Mid-Ocean Ridge Basalts (MORB) iron isotopes signature. Finally, we calculated that olivine at equilibrium with a basaltic melt could display an iron isotope composition down to −0.1 ‰ for δ57Fe. Hence, the lower isotopic compositions (δ57Fe < -0.1 ‰) observed in natural olivines are most likely due to diffusion-driven kinetic fractionation.

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N 5 Site for Enhanced Electrocatalytic Reaction

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N ₅ Site for Enhanced Electrocatalytic Reaction

Theoretical investigation of the stability of A55-nBn nanoalloys (A, B = Al, Cu, Zn, Ag)

Nanoalloys have been investigated for a wide range of applications, however, our atomistic understanding of the physical-chemistry properties is still far from complete as quantum-size effects play an important role for particles with about 1nm diameter. In this work, we employed density functional theory calculations to investigate the structural, energetic and electronic properties of 55-atom A55-nBn nanoalloys, where A, B= Al, Cu, Zn, and Ag. For structure generation, we combined clustering algorithm techniques with design principles, which yields a wide range of conformations. From the analyses of the excess energy results, we found that the CuAl, CuAg, CuZn, and AlAg systems are the most energetically favorable, in particular, the Al42Cu13 and Al42Ag13 compositions (onion-like and core–shell structures, respectively). Through Spearman’s correlation analysis, we found that the structural properties (number of under coordinated atoms, effective coordination number, average bond lengths, and chemical order parameter) are the most important descriptors correlated with the energy stability of the nanoalloys (excess energy). Several properties such as the particle volume, binding energy, and average bond length show a linear dependence as a function of composition.

Effects of Sn-ion irradiation on local structure and flux pinning properties of MgB2 thin films

We studied the effects of tin (Sn) ion irradiation on the local structure and flux pinning properties of 412-nm-thick MgB2 thin films. Ions with 2 MeV incident energy were irradiated onto MgB2 thin films at room temperature and various dose levels from 2 × 1012 to 7 × 1013 atoms/cm2. X-ray diffraction and extended X-ray absorption fine-structure results confirmed extended average bond lengths and increased mean-square relative displacement that correlated with crystalline disorder serving as artificial pinning centers induced by ion irradiation. While the superconducting transition temperature (Tc) exhibited a decreasing trend, the magnetization Jc (H) of the irradiated films measured at both 5 K and 20 K were remarkably enhanced compared to that of pristine films at high applied magnetic fields. The decreases in exponent β from 1.1 and 1.13 to 0.69 and 0.62 after irradiation at 5 K and 20 K, respectively, as the dose increased to 5 × 1013 atoms/cm2 indicates a shift in the pinning mechanism from weak collective pinning to strong plastic pinning according to collective pinning theory. In addition, the scaling behavior of the flux pinning force density (Fp) in the irradiated MgB2 films showed a crossover from surface pinning to normal point pinning. These results suggest that intermediate-energy irradiation by heavy ions such as Sn generates effective pinning sources for improving Jc(H) in MgB2 superconductors.

Synthesis, Covalency Sequence, and Crystal Features of Pentagonal Uranyl Acylpyrazolone Complexes along with DFT Calculation and Hirshfeld Analysis.

Three uranyl acylpyrazolone complexes [UO2(PCBPMP)2(CH3CH2OH)] (complex I), [UO2(PCBMCPMP)2(CH3CH2OH)] (complex II), and [UO2(PCBPTMP)2(CH3CH2OH)] (complex III) were synthesized from σ-donating acypyrazolone ligands to analyze their sequence of covalent characteristics, reactivity, and redox properties (PCBPMP: p-chlorobenzoyl 1-phenyl 3-methyl 5-pyrazolone; PCBMCPMP: p-chlorobenzoyl 1-(m-chlorophenyl) 3-methyl 5-pyrazolone; PCBPTMP: p-chlorobenzoyl 1-(p-tolyl) 3-methyl 5-pyrazolone). An examination of the structure, pentagonal bipyramidal geometry, and composition of these complexes was conducted mainly through their single-crystal X-ray diffraction (XRD) data, 1H nuclear magnetic resonance (NMR) δ-values, plots of thermogravimetric-differential thermal analysis (TG-DTA), significant Fourier transform infrared (FTIR) vibrations, gravimetric estimation, and molar conductivity values. The covalency order was found to be complex II > III > I, which mainly depends on values of stretching frequencies, average bond lengths of axial uranyl bonds, values of average bond lengths on the pentagonal equatorial plane, solvent coordination on the fifth site of a pentagonal plane, and the type of aryl group on the nitrogen of the pyrazolone ring. This was confirmed by FTIR spectroscopy and single-crystal spectral characterization. To verify experimental results by comparison with theoretical results, density functional theory (DFT) calculations were carried out, which further gives evidence for the covalency order through theoretical frequencies and the gap of highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energies. Theoretical bond properties were also examined by the identification of global index parameters. Intermolecular noncovalent surface interactions were studied by the Hirshfeld surface analysis. The irreversible redox behavior of uranyl species was identified through electrochemical cyclic voltammetry-differential pulse voltammetry (CV-DPV) plot analysis.

Equilibrium fractionation of S, Fe, and Ni isotopes in Fe-Ni sulfides: A first-principles investigation

Fe, Ni, and S isotopes are important chemical tracers, and they complement each other to improve our understanding of the Earth's evolution. As common accessory minerals in the Earth's interior and the most important group of ore minerals, sulfide minerals display a wide range of Fe and Ni contents. Using first-principles methods, we investigated the equilibrium fractionation factors of Fe, Ni, and S isotopes of the β-(Ni, Fe)S, FeS2-NiS2, and Fe3S4-Ni3S4 solid solutions, to constrain the effect of cation contents on Fe, Ni, and S isotopic signatures. For the β-(Ni, Fe)S solid solution, the iron β-factors, nickel β-factors, and sulfur β-factors all decrease approximately linearly with the increasing Fe content. For the FeS2-NiS2 solid solution, the iron β-factors, nickel β-factors, and sulfur β-factors show a positive linear relationship with the Fe content. For the Fe3S4-Ni3S4 solid solution, the iron β-factors and nickel β-factors have a strong negative correlation with the Fe content, while the sulfur β-factors have a weak negative correlation with the Fe content. In general, the trend of variation of the β-factors with Fe content is the opposite to that of the average bond lengths with the Fe content. Our results provide an improved understanding of Fe, Ni, and S isotopic fractionation in natural systems that may be used to better constrain physical, chemical, and geo-biological evolution on Earth.