Average Interatomic Distance Research Articles

Unraveling the synergistic mechanisms of dual-atom catalysts on BeN4 substrates for enhanced hydrogen evolution reaction: A machine learning-assisted first-principles study

Synergistic effects have been widely studied in both homogeneous and heterogeneous bimetallic catalysis. However, the exact mechanisms underlying the synergistic effects between bimetallic atoms remain elusive. This study, using density functional theory calculations, systematically explores a dual-atom catalyst (DAC) on a BeN4 substrate, where two beryllium atoms are substituted with transition metals. The results demonstrate that the dual-atom catalysis TMTM’@BeN4 exhibits synergistic effects. Notably, five DAC systems, particularly those co-doped with V and Cr, significantly reduce the Gibbs free energy (ΔGH∗) for the hydrogen evolution reaction (HER). We further utilized machine learning to explore how the dual-atom catalysts exhibit synergistic effects. Machine learning analysis indicates that changes in the average interatomic distance and the Fermi level of the catalyst system are the main factors influencing the Gibbs free energy variation in dual-atom catalysts, jointly determining the hydrogen evolution catalytic activity. This work provides profound insights into understanding the synergistic mechanisms of dual-atom catalysts.

Direct Determination of POSS Cage Structure in a Self-Assembled Ionic Bridged Silsesquioxane by Using the Total Pair Distribution Function G(r) and Density Functional Theory.

In the present study, both short-range and long-range structural features of an ionic bridged silsesquioxane, specifically one containing the 1,4-diazoniabicyclo[2.2.2]octane chloride group (ISSQ), were elucidated. This ionic silsesquioxane was synthesized via direct polycondensation of a bridged organosilane precursor, without any additional functionalization step. Si-O-Si cage structures typical of Polyhedral Oligomeric Silsesquioxanes (POSS) were identified. The average interatomic distances of the POSS cages, including the open T8 cage and the T12 cage for the ISSQ, as well as the T8 cage for a commercially available pendant POSS were determined. It is the first report of the interatomic distance determination of POSS cage; achieved by using total pair distribution function G(r) values obtained through high-resolution synchrotron X-ray diffraction combined with density functional theory (DFT) calculations. The application of DFT was crucial for accurately assigning X-ray peaks and verifying structural details. Furthermore, the analysis of X-ray diffraction peaks and the examination of crystalline domains via transmission electron microscopy enabled the proposal of a hexagonal arrangement of Si-O-Si cages over long ranges within the ionic bridged silsesquioxane. This proposed arrangement highlights a distinctive structural organization that could impact the material's properties and applications.

Analyzing the atomic pair distribution function of BMT-based nanopowders via X-ray diffraction

In this study, we investigate the three-dimensional atomic structure of 0.65[Ba(Mg1/3, Ti2/3)]−0.35[BaTiO3] (BMT-BT) using x-ray diffraction (XRD) with molybdenum radiation (MoKα 1), Rietveld refinement, and the atomic pair distribution function (PDF) technique. We provide an analysis of the advantages of the PDF method for studying the unit cell of electroceramics. The XRD results show that BMT has a bulk crystal structure with a cubic perovskite structure. The new structural information enhances our understanding of the dielectric properties of BMT. The PDF analysis reveals that the average interatomic bond distance in the octahedral TiO6 atomic bond is around 2.8 Å, and the first peak at r = 1.39 Å corresponds to the nearest neighbor M-O distance (M = Ti, Mg, Nb). Additionally, the peak at r = 2.4 Å corresponds to the Ba-O and O-O distances in the MO6 octahedron. Peaks in the PDF data below 1 Å are ignored as they have no physical implications.

Mechanical properties and thermal stability of ZrCuAlx thin film metallic glasses: Experiments and first-principle calculations

In this work, we provide a holistic picture about the relationship between atomic structure, mechanical properties, and thermal stability of ZrCuAlx thin film metallic glasses (TFMGs) varying the Al content from 0 to 12 at.%, carrying out a broad characterization involving experiments and ab initio molecular dynamic simulations (AIMD). We show that the addition of Al resulted in a change of average interatomic distances by ∼10 pm with the formation of shorter bonds (Al-Zr and Al-Cu), influencing the mechanical response (shear/elastic moduli and hardness) which increases by ∼15% for 12 at.% Al. Moreover, tensile tests on polymer substrate revealed a maximum value for the crack initiation strain of 2.1% for ZrCuAl9, while the strain-to-failure rapidly decreases at higher Al contents. The observed reduction in damage tolerance is correlated to a transition in atomic configuration. Specifically, a maximum in density of full and defective icosahedral cluster population is observed at 9 at.% Al, inducing a more shear-resistant behavior to the material. Thermal stability is investigated by high-energy and conventional x-ray diffraction and electrical resistivity measurements as a function of the temperature. Glass transition (Tg) and crystallization (Tx) temperature increase by Al addition reaching 450 and 500 °C, respectively for ZrCuAl12. The increase in thermal stability is related to the reduction in atomic mobility due to the formation of shorter chemical bonds, inhibiting atomic reconfiguration during crystallization. In conclusion, we provide guidelines to the design of compositional-tailored ZrCuAlx TFMGs with tuned mechanical properties and thermal stability with potential impact on industrial applications.

Mn2+ Luminescence in Ca9Zn1-xMnxNa(PO4)7 Solid Solution, 0 ≤ x ≤ 1.

The solid solution Ca9Zn1-xMnxNa(PO4)7 (0 ≤ x ≤ 1.0) was obtained by solid-phase reactions under the control of a reducing atmosphere. It was demonstrated that Mn2+-doped phosphors can be obtained using activated carbon in a closed chamber, which is a simple and robust method. The crystal structure of Ca9Zn1-xMnxNa(PO4)7 corresponds to the non-centrosymmetric β-Ca3(PO4)2 type (space group R3c), as confirmed by powder X-ray diffraction (PXRD) and optical second-harmonic generation methods. The luminescence spectra in visible area consist of a broad red emission peak centered at 650 nm under 406 nm of excitation. This band is attributed to the 4T1 → 6A1 electron transition of Mn2+ ions in the β-Ca3(PO4)2-type host. The absence of transitions corresponding to Mn4+ ions confirms the success of the reduction synthesis. The intensity of the Mn2+ emission band in Ca9Zn1-xMnxNa(PO4)7 rising linearly with increasing of x at 0.05 ≤ x ≤ 0.5. However, a negative deviation of the luminescence intensity was observed at x = 0.7. This trend is associated with the beginning of a concentration quenching. At higher x values, the intensity of luminescence continues to increase but at a slower rate. PXRD analysis of the samples with x = 0.2 and x = 0.5 showed that Mn2+ and Zn2+ ions replace calcium in the M5 (octahedral) sites in the β-Ca3(PO4)2 crystal structure. According to Rietveld refinement, Mn2+ and Zn2+ ions jointly occupy the M5 site, which remains the only one for all manganese atoms within the range of 0.05 ≤ x ≤ 0.5. The deviation of the mean interatomic distance (∆l) was calculated and the strongest bond length asymmetry, ∆l = 0.393 Å, corresponds to x = 1.0. The large average interatomic distances between Mn2+ ions in the neighboring M5 sites are responsible for the lack of concentration quenching of luminescence below x = 0.5.

Correlating the diffusion of water to performance in model reverse osmosis polyamides with controlled crosslink densities

We systematically reduce the crosslink density of a PA network based on m-phenylene diamine by substituting a fraction of the trifunctional trimesoyl chloride crosslinking agent with a difunctional isophthaloyl analog that promotes chain extension, in order to elucidate robust design cues for improving the polyamide (PA) separation layer in reverse osmosis (RO) membranes for desalination. Thin films of these model PA networks are fully integrated into a composite membrane and evaluated in terms of their water flux and salt rejection. By incorporating 15 mol % of the difunctional chain extender, we reduce the crosslink density of the network by a factor of two, which leads to an 80% increase in the free or unreacted amine content. The resulting swelling of the PA network in liquid water increases by a factor of two accompanied by a 30% increase in the salt passage through the membrane. Surprisingly, this leads to a 30% decrease in the overall permeance of water through the membrane. This conundrum is resolved by quantifying the microscopic diffusion coefficient of water inside the PA network with quasi-elastic neutron scattering. In the highest and lowest crosslink density networks, water shows strong signatures of confined diffusion. At short length scales, the water exhibits translational diffusion that is consistent with the jump-diffusion mechanism. This translational diffusion coefficient is approximately five times slower in the lowest crosslinked density network, consistent with the reduced water permeance. This is interpreted as water molecules interacting more strongly with the increased free amine content. Over longer length scales the water diffusion is confined, exhibiting mobility that is independent of length scale. The length scales of confinement from the quasi-elastic neutron scattering experiments at which this transition from confined to translational diffusion occurs is on the order of (5–6) Å, consistent with complementary X-ray scattering, small angle neutron scattering, and positron annihilation lifetime spectroscopy measurements. The confinement appears to come from heterogeneities in the average inter-atomic distances, suggesting that diffusion occurs by water bouncing between chains and occasionally sticking to the polar functional groups. The results obtained here are compared with similar studies of water diffusion through both rigid porous silicates and ion exchange membranes, revealing robust design cues for engineering high-performance RO membranes.

Micro-scale tribological study of a Ni-Cr-Fe-Ti-Al-V high entropy alloy thin film by magnetron co-sputtering of Inconel-718 and Ti-6Al-4V

In this study, we fabricated high entropy alloy (HEA) thin films by RF magnetron co-sputtering of Inconel 718 and Ti-6Al-4V targets. Their coefficient of friction (CoF) and specific wear rate (K0) were investigated to evaluate their potential as a material for MEMS moving parts. By placing different number of Ti-6Al-4V pellets (samples abbreviated as IT2, IT4, IT6) on an Inconel 718 target (abbreviated as IT0), Ni-Cr-Fe-Ti-Al-V HEA thin films with configurational entropy of mixing (∆Sconf) >1.5R were successfully deposited. Electron microscopy and diffraction confirms that IT0 possessed a mixed nanocrysalline/amorphous structure, while the Ti-added IT2, IT4 and IT6 films were amorphous. They also have a significantly reduced root-mean-square roughness. In order to evaluate their micro-scale wear behavior, various tribological studies including indentation, scratch test and scanning wear test were conducted. From IT0 to IT6, film hardness H increases while reduced modulus Er remains similar. Plastic criterion Rw is also reduced, showing that the films IT2, IT4 and IT6 are more prone to elastic deformation. Scratch test also reveals that IT6 can afford the highest normal load before the transition of wear mode occurs. Lastly, a scanning wear test was conducted to evaluate the specific wear rate (K0) of each HEA film. It was found that IT6 had the lowest value of K0 = 2.26 × 10−4 mm3 N−1 m−1, which was also a lot lower than Si-to-Si (K0 = 0.0183 mm3 N−1 m−1) wear in other studies. The excellence of IT6 can be attributed to the fact that alloying of Ti effectively reduced the average interatomic distance in the HEA films, resulting in a densely packed, very elastic microstructure. Therefore, IT6 is expected to have longer service life than silicon if it were made into MEMS moving parts.

Structural features and optical-luminescent properties of the Pb-containing germanate and silicate oxyfluoride glasses

The XRD, IR transmission, EPR, optical absorption, luminescence spectra and decay kinetics in new un-doped 0.5PbO–0.3GeO2–0.2AlF3 and 0.53PbO–0.226SiO2–0.244AlF3 glasses were investigated. The XRD patterns were analysed in order to obtain the radial distribution functions and define the average interatomic distances and coordination numbers in the network of investigated glasses. The IR transmission, EPR, and UV–vis optical absorption spectra of the studied glasses were interpreted. Optical band gap and Urbach energy were obtained from analysis of the fundamental absorption edge. Under UV photoexcitation the 0.5PbO–0.3GeO2–0.2AlF3 and 0.53PbO–0.226SiO2–0.244AlF3 glasses demonstrate broad emission band at 493 nm and 499 nm, respectively. Fast (band-to-band recombination) and slow (recombination with a contribution of defects levels) mechanisms of intrinsic luminescence have been proposed from a detailed analysis of luminescence spectra and decay kinetics curves. The CIE chromaticity diagram shows a variation in emission colour of studied glasses with the change in their composition.

Suppressed phase segregation for triple-junction perovskite solar cells

The tunable bandgaps and facile fabrication of perovskites make them attractive for multi-junction photovoltaics1,2. However, light-induced phase segregation limits their efficiency and stability3-5: this occurs in wide-bandgap (>1.65 electronvolts) iodide/bromide mixed perovskite absorbers, and becomes even more acute in the top cells of triple-junction solar photovoltaics that require a fully 2.0-electron-volt bandgap absorber2,6. Here we report that lattice distortion in iodide/bromide mixed perovskites is correlated with the suppression of phase segregation, generating an increased ion-migration energy barrier arising from the decreased average interatomic distance between the A-site cation and iodide. Using an approximately 2.0-electron-volt rubidium/caesium mixed-cation inorganic perovskite with large lattice distortion in the top subcell, we fabricated all-perovskite triple-junction solar cells and achieved an efficiency of 24.3 per cent (23.3 per cent certified quasi-steady-state efficiency) with an open-circuit voltage of 3.21 volts. This is, to our knowledge, the first reported certified efficiency for perovskite-based triple-junction solar cells. The triple-junction devices retain 80 per cent of their initial efficiency following 420 hours of operation at the maximum power point.

ИССЛЕДОВАНИЕ ИСКАЖЕНИЙ КРИСТАЛЛИЧЕСКОЙ РЕШЁТКИ В ТВЁРДОМ РАСТВОРЕ АЛЮМИНИЕВОГО СПЛАВА Д16 (АА2024) ПОСЛЕ ОТЖИГА И СТАРЕНИЯ

Annealing, quenching, natural and artificial aging at 100 and 200°С during 15 and 30 minutes of Al-Cu-Mg alloy D16 (AA2024) are provided. Chemical composition of solid solution is found using scanning electron microscopy and micro-X-ray-spectral analysis. Based on results of chemical analysis the calculation of average interatomic distance changing in the solid solution is found, which shows the value of crystal lattice distortion. Also crystal lattice distortion and residual stresses in the solid solution are studied using X-ray analysis. It was found, annealed state and artificial aging at 200°С during 30 minutes lead to similar content of alloying elements in the solid solution, mass percentage of Cu 15…16 times and Mg 4…5 times higher than should be in the solid solution at equilibrium state in accordance with state diagram. Exposition of 15 minutes for artificial aging at temperature of 100 and 200°С does not lead to significant difference in the level of residual stresses calculated and measured. High level of residual stress in the solid solution of alloy D16 at quenched state and after short exposition before 30 minutes allows receiving the high strength, but may give less exfoliation corrosion resistance in water-salt environment.

Structural, morphological and photocatalytic properties of TiO2 obtained by thermolytic decomposition of the [Ti(OH2)6]3+•3Cl¯ aquacomplex

Thermolytic decomposition in a chloride-acid medium of a titanium aquacomplex precursor solution [Ti(OH2)6]3+•3Cl¯ as a result of its boiling at a temperature of (110-114) 0С is accompanied by precursor hydrolysis and the formation of hydrate molecules Ti(OH)3Cl•2Н2О, which, as a result of condensation, provide the formation of rutile ТіО2 with rod-shaped particles (30-80) nm long and (8-20) nm in diameter, combined into flower-shaped associates with a diameter of (200-280) nm. IR spectroscopic studies of the precursor hydrolysis products and the synthesized oxide material show that the geometric parameters of ТіО5Cl octahedra of Ti(OH)3Cl •2Н2О hydrate molecules are close to those of ТіО6 octahedra of ТіО2 rutile. The average Ti-O interatomic distance of octahedra of titanium-containing hydrate molecules is a kind of template that directs the process of ТіО2 crystallization towards the formation of rutile. Rutile ТіО2 nanopowder synthesized in this way is an effective photocatalyst for the photooxidation of organic dyes in an aqueous medium by UV radiation. The photocatalytic activity of the synthesized rutile ТіО2 was determined by neutralization of Congo Red (CR) dye dissolved in an aqueous medium. The effectiveness of TiO2 was studied by examining the effect of TiO2 amount and Н2О2 concentration. It has been established that complete bleaching of the dye is achieved in just 10 minutes of UV irradiation at a photocatalyst concentration of 1.5 g/L, an Н2О2 concentration of 5 mM, and an initial CR dye concentration of 5 mg/L, at reaction rate constant of 2.1959 min-1.

Compositional effects in the liquid Fe–Ni–C system at high pressure

We performed molecular dynamics simulations based on density functional theory to systematically investigate the Fe–Ni–C system including (1) pure Fe and Ni; (2) binary Fe–Ni, Fe–C, and Ni–C; and (3) ternary Fe–Ni–C liquid compositions at 3000 K and three simulation volumes corresponding to pressure (P) up to 83 GPa. Liquid structural properties, including coordination numbers, are analyzed using partial radial distribution functions. Self-diffusion coefficients are determined based on the atomic trajectories and the asymptotic slope of the time-dependent mean-square displacement. The results indicate that the average interatomic distance between two Fe atoms (rFe–Fe) decreases with P and is sensitive to Ni (XNi) and C (XC) concentration, although the effects are opposite: rFe–Fe decreases with increasing XNi, but increases with increasing XC. Average rFe–C and rNi–C values also decrease with increasing XNi and generally remain constant between the two lowest P points, corresponding to a coordination change of carbon from ~ 6.8 to ~ 8.0, and then decrease with additional P once the coordination change is complete. Carbon clustering occurs in both binary (especially Ni–C) and ternary compositions with short-range rC-C values (~ 1.29 to ~ 1.57 Å), typical for rC-C in diamond and graphite. The self-diffusion results are generally consistent with high-P diffusion data extrapolated from experiments conducted at lower temperature (T). A subset of additional simulations was conducted at 1675 and 2350 K to estimate the effect of T on diffusion, yielding an activation enthalpy of ~ 53 kJ/mol and activation volume of ~ 0.5 cm3/mol.

Spectroscopy and photoluminescence of complex lead-silicate glass doped with copper

The EPR, XRD, IR transmission, optical absorption, photoluminescence and its decay kinetics in lead-silicate glass with complex 0.521Pb3O4-0.371SiO2-0.068ZnO-0.027K2O-0.013BaO composition, doped with CuO (or PbSiZnKBaO:CuO) were studied. The observed characteristic EPR spectrum of the Cu2+ (3d9, 2D5/2) paramagnetic ions were adequately described using the spin Hamiltonian formalism. Average interatomic distances and coordination numbers for main structural units, SiO4 and PbOn (n = 4–6), in the network of studied glass were determined from radial distribution functions. Broad absorption band with a peak about 870 nm was ascribed to the 2B1g → 2B2g transition of the Cu2+ centers. Optical band gap and Urbach energy of the studied glass were evaluated. Photoluminescence spectra and decay curve of the Cu+ (3d10, 1S0) centers in PbSiZnKBaO:CuO glass were registered and interpreted. The PbSiZnKBaO:CuO glass upon UV excitation demonstrates intense emission band at 574 nm with fast kinetics belonging to intrinsic recombination luminescence.

Structural and optical investigations on amorphous SnSe[formula omitted] alloy

The structural and optical properties of an amorphous SnSe9 alloy made using mechanical alloying were investigated through Extended X-ray Absorption Fine Structure and photoacoustic spectroscopy techniques. Average interatomic distances, average coordination numbers, disorder and asymmetry of the gij(r) functions, and also optical parameters such as Urbach and weak absorption tail energies and the optical gap were obtained. At 300 K, we found rSe–Se=2.388±0.002 Å and NSe–Se=1.96±0.04, for Se–Se pairs, and rSe–Sn=2.65±0.02 Å and NSe–Sn=0.35±0.04 for Se–Sn pairs. We determined a direct gap at Eg=1.51±0.02eV for SnSe9. Results indicated that the structure of SnSe9 is very different from those found in crystalline Sn–Se alloys, and the optical gap is the largest one considering amorphous alloys, being even larger than those found in some crystalline alloys of Sn–Se system.

Evaluation of the Effect of Macrocyclic Ring Size on [203Pb]Pb(II) Complex Stability in Pyridyl-Containing Chelators.

As an element-equivalent theranostic pair, lead-203 (203Pb, 100% EC, half-life = 51.92 h) and lead-212 (212Pb, 100% β-, half-life = 10.64 h), through the emission of γ rays and an α particle in its decay chain, respectively, can aid in the development of personalized targeted radionuclide treatment for advanced and currently untreatable cancers. With these isotopes currently being used in clinical trials, an understanding of the relationship between the chelator structure, ability to incorporate the radiometal, and metal-complex stability is needed to help design appropriate chelators for clinical use. Herein, we report an investigation into the effect of ring size in macrocyclic chelators where pyridine, an intermediate Lewis base, acts as an electron donor toward lead. Crown-4Py (4,7,13,16-tetrakis(pyridin-2-ylmethyl)-1,10-dioxa-4,7,13,16-tetraazacyclooctadecane), cyclen-4Py (1,4,7,10-tetrakis(pyridin-2-ylmethyl)-1,4,7,10-tetraazacyclododecane), and NOON-2Py (7,16-bis(pyridin-2-ylmethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) were synthesized and analyzed for their ability to coordinate Pb2+. Metal complex stability was investigated via [203Pb]Pb2+ radiolabeling studies, 1H NMR spectroscopy, X-ray crystallography, and potentiometry. With the smallest macrocyclic backbone, cyclen-4Py had the highest radiochemical yield, while, in descending order, crown-4Py and NOON-2Py had the lowest. Thermodynamic stability constants (log KML) of 19.95(3), 13.29(5), and 11.67 for [Pb(Cyclen-4Py)]2+, [Pb(Crown-4Py)]2+, and [Pb(NOON-2Py)]2+, respectively, correlated with their radiochemical yields. The X-ray crystal structure of the least stable complexes [Pb(NOON-2Py)]2+ revealed a hemidirected Pb2+ center, as reflected by a void within the coordination sphere, and [Pb(Crown-4Py)]2+ showed an average Pb-N pyridine interatomic distance of >3 Å. By contrast, the crystal structure of [Pb(Cyclen-4Py)]2+ showed shorter Pb-N pyridine interactions, and in solution, only one highly symmetric isomer existed for this complex, whereas conformational flexibility was observed for both [Pb(Crown-4Py)]2+ and [Pb(NOON-2Py)]2+ at the NMR timescale. This study illustrates the importance of the macrocyclic backbone size when incorporating bulky electron-donor groups into the design of a macrocyclic chelator as it affects the accessibility of lead to the donor arms. Our results show that cyclen-4Py is a promising chelator for future studies with this theranostic pair.

Influence of coating thickness on microstructure, mechanical and LBE corrosion performance of amorphous AlCrFeTiNb high-entropy alloy coatings

The AlCrFeTiNb HEA coatings with different thickness were specially designed and successfully deposited on F/M steel tube using magnetron sputtering method. The dependence of microstructure, mechanical properties and LBE corrosion resistance on the coating thickness was systematically investigated. The increase in coating thickness makes the average inter-atomic distance of the amorphous AlCrFeTiNb coatings become larger, and the atomic arrangement become looser. With increasing coating thickness, the increase in nanohardness H, H/E and H3/E2 values were observed. The LBE corrosion test results indicated the increased parabolic oxide growth rate constant Kp (i.e. the increased corrosion oxidation rate) and the raised crack density with increasing coating thickness. The crack generation induced LBE penetration and internal oxidation of the coating, and the relevant crack-assisted internal oxidation mechanism is discussed intensively.

Zn stable isotope fractionation during adsorption onto todorokite: A molecular perspective from X-ray absorption spectroscopy and density functional theory

Mineral-solution interface reactions control the mobility and fate of trace metals (e.g., Zn) and may drive their associated isotopic fractionation in the natural environment, but understanding the coupling between interfacial reactions and isotopic behavior requires a molecular-level understanding of these processes. In this study, we investigate Zn stable isotope fractionation during adsorption to todorokite as a function of reaction time, pH and Zn concentrations. We show that Zn stable isotope fractionation reaches equilibrium at ∼12 h reaction time at pH 6 and Zn concentrations of 0.05 mM. For Zn concentrations of 0.05 and 0.2 mM, Zn isotopic fractionation between adsorbed and aqueous Zn (Δ66Znadsorbed-aqueous) is approximately −0.1 ± 0.04‰ at pH 3–5, and Δ66Znadsorbed-aqueous gradually increases from −0.1 ± 0.04‰ to 0.05 ± 0.05‰ at higher pH 6–8. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows that Zn adsorbs to the todorokite surface as an outer-sphere octahedral complex with an average ZnO interatomic distance of 2.06 Å at pH 3. In contrast, Zn is predominantly present as a tetrahedral coordinated structure with a shorter average ZnO interatomic distance of ∼2.00–2.05 Å at pH 6 and 8, suggesting the presence of a mixture of octahedral outer-sphere and tetrahedral inner-sphere surface complexes. Density functional theory calculations suggest that outer-sphere surface complexes exist in the center of the structural tunnels of todorokite, yielding a theoretical Zn isotopic fractionation (Δ66Znadsorbed-aqueous) of −0.2‰ to −0.3‰, whereas the tetrahedral inner-sphere surface complex results in a large Δ66Znadsorbed-aqueous of +0.5‰ to +0.8‰. Combined laboratory experiments and theoretical calculations demonstrate that different magnitudes of Zn isotopic fractionation are controlled by structural changes (e.g., coordination and bond distance) in the Zn surface complexes formed on todorokite relative to its aqueous form (i.e., aqua Zn(H2O)62+). These results provide important new constraints for understanding Zn isotope signatures in natural Mn-rich sediments and lead to a more complete understanding of Zn isotopes in the ocean.

Structural properties of HeN4 ( N=2–10 ) clusters for different potential models at the physical point and at unitarity

Since the $^{4}\mathrm{He}$ dimer supports only one weakly bound state with an average interatomic distance much larger than the van der Waals length and no deeply bound states, $^{4}\mathrm{He}_{N}$ clusters with $N>2$ are a paradigmatic model system with which to explore foundational concepts such as large $s$-wave scattering length universality, van der Waals universality, Efimov physics, and effective field theories. This work presents structural properties such as the pair and triple distribution functions, the hyper-radial density, the probability to find the $N\mathrm{th}$ particle at a given distance from the center of mass of the other $N\ensuremath{-}1$ atoms, and selected contacts. The kinetic energy release, which can be measured via Coulomb explosion in dedicated size-selected molecular beam experiments---at least for small $N$ -- is also presented. The structural properties are determined for three different realistic $^{4}\mathrm{He}\ensuremath{-}^{4}\mathrm{He}$ interaction potentials and contrasted with those for an effective low-energy potential model from the literature that reproduces the energies of $^{4}\mathrm{He}_{N}$ clusters in the ground state for $N=2$ to $N=\ensuremath{\infty}$ at the $\ensuremath{\gtrsim}$95% level with just four input parameters. The study is extended to unitarity (infinite $s$-wave scattering length) by artificially weakening the interaction potentials. In addition to contributing to the characterization of small bosonic helium quantum droplets, our study provides insights into the effective low-energy theory's predictability of various structural properties.

Local structure, spectroscopic properties and intrinsic photoluminescence of the un-doped lead-containing glasses of different composition

The X-ray diffraction, infrared transmission, optical absorption, photoluminescence spectra and decay kinetics of a series of un-doped Pb-containing glasses with compositions of 0.4PbO–0.6GeO2, 0.6PbO–0.4SiO2, 0.7PbO–0.3B2O3, and 0.54PbO–0.34B2O3–0.12GeO2 were studied. Average interatomic distances and coordination numbers for main structural units GeO4, SiO4, BO4, BO3, and PbOn (n = 3 – 6) in all studied glasses were derived from radial distribution functions. The 0.4PbO–0.6GeO2 and 0.54PbO–0.34B2O3–0.12GeO2 glasses upon UV photoexcitation show intense broad emission bands with maxima at 478 and 543 nm, respectively, and weaker broad band with peak about 610 nm. Based on luminescence decay kinetics analysis were proposed fast (band-to-band recombination) and slow (recombination with participation of point defects) mechanisms of intrinsic luminescence in the 0.4PbO–0.6GeO2 and 0.54PbO–0.34B2O3–0.12GeO2 glasses. The 0.6PbO–0.4SiO2 and 0.7PbO–0.3B2O3 glasses under UV – violet photoexcitation reveal only broad emission band with a maximum at 599 and 593 nm, respectively, caused by the band-to-band fast recombination mechanism.

Local structure and dynamics of tungsten oxide-based glasses: insights from concurrent neutron diffraction and Compton scattering

In this work, following our previous work on molybdate glasses, we employ a combination of neutron diffraction and neutron Compton scattering, augmented by ab initio harmonic lattice dynamics and Reverse Monte Carlo modelling to characterise the force-constant disorder in the tungsten oxide-based glasses. Specifically, we discuss the correlations between the average interatomic force constant magnitudes inferred from neutron Compton scattering and the glass formation ability, measured in terms of the value of the glass transition temperature, as well as the average bond-lengths and interatomic distances obtained from diffraction data analysis. Moreover, we provide a comparative analysis of the widths of force-constant distributions of individual atomic species in glasses and their precursor metal oxides based on the distributions of the widths of nuclear momentum distributions. Furthermore, we assess the degree of softening of atom-projected vibrational densities of states induced by the force-constant disorder in the glasses.