Atomic Order Research Articles

Electrochemical Synthesis of Nanostructured Ordered Intermetallic Materials under Ambient Conditions.

ConspectusThe enhanced catalytic properties of alloy nanostructures have made them a focus of extensive research in the field of catalysis. Alloy nanostructures can be classified into two types: disordered alloys (also known as solid solutions) and ordered intermetallics. The latter are of particular interest as they possess long-range atomic scale ordering, which leads to well-defined active sites that can be used to accurately assess structure-property relationships and their impact on (electro)catalytic performance.While many ordered intermetallics (OICs) have been synthesized and evaluated as electrocatalysts, there is still a lack of understanding on how the local structure of atoms controls their catalytic performance. Ordered intermetallics are difficult to synthesize and often require high-temperature annealing for the atoms to equilibrate into ordered structures. High temperature processing results in aggregated structures (usually >30 nm) and/or contamination from the support, which can decrease their performance and preclude these materials from being used as model systems for elucidating insight into structure and electrochemical properties. Therefore, alternative methods are required to enable more efficient atomic ordering while maintaining some level of morphological control.This Account delves into the potential of electrochemical methods as a practical alternative for synthesizing ordered intermetallics at lower temperatures. Specifically, it explores the viability of electrochemical dealloying and electrochemical deposition to synthesize Pd-Bi and Cu-Zn intermetallics at room temperature and atmospheric pressure. These methods have proven useful in synthesizing phases that are typically inaccessible under ambient conditions. The high homologous temperatures at which these materials are synthesized provide the necessary atomic mobility required for equilibration and formation of ordered phases, thus making the direct synthesis of ordered intermetallic materials at room temperature by electrochemical means a reality.Beyond synthesis, the electrocatalytic performance of these intermetallics was assessed for the oxygen reduction reaction (ORR), which is an important process employed in fuel cells. The OICs displayed increased performance with respect to commercial Pd/C and Pt/C benchmarks because of lower coverages of spectator species. Furthermore, these materials exhibited improved methanol tolerance.This Account provides valuable insights into the electrochemical synthesis of ordered intermetallics and their potential use as highly effective catalysts for electrocatalytic reactions. By using electrochemical methods, it is possible to obtain ordered intermetallics with unique atomic arrangements and tailored properties, which can be optimized for specific catalytic applications. With further research, electrochemical synthesis methods may enable the development of new and improved ordered intermetallics with even higher catalytic activity and selectivity, making them ideal candidates for use in a wide range of industrial processes. Furthermore, the ability to access intermetallics under milder conditions may accelerate the ability to use these materials as model systems for revealing fundamental insight into electrocatalyst structure and function.

Phase-Controlled Pd-Sn Nanostructures via Co-Digestive Ripening: Catalytic Performance for Base-Free Oxidation of Benzyl Alcohol.

Diluting the precious metal to be used for catalysis with an abundant and non-precious metal to form alloy/intermetallic nanostructures is of great interest owing to the cost-effectiveness of the catalyst. The physicochemical properties of such bimetallic nanostructures are influenced by the atomic ordering of different atoms in the system, generally enhancing the catalytic activity, selectivity, and durability compared to their monometallic counterparts. To understand the relationship between the structure and the activity of the catalyst, phase-controlled synthesis of alloy/intermetallic nanostructures is crucial. Easy and scalable synthesis of such nanostructures with phase control presents a true challenge. We exploited a colloidal-based synthetic route termed 'co-digestive ripening' to prepare Pd-Sn alloy/intermetallic nanostructures. Oleylamine capped Pd and Sn colloids were utilized to obtain network-like Pd3 Sn and grape-like Pd2 Sn nanostructures. Temperature and the stoichiometric ratio between Pd and Sn played significant roles in achieving phase control. The mixture of ligands (oleylamine and trioctylphosphine) in the synthetic procedure resulted in the formation of well-separated nanoparticles (2.9±0.5 nm) in the case of Pd3 Sn and few nm-sized particles along with aggregates in the case of Pd2 Sn. Pd-Sn nanostructures showed enhanced activity and selectivity as compared to their monometallic counterparts for the catalytic performance towards oxidation of benzyl alcohol.

Control of atomic ordering of LiNiO2 thin films on a sapphire substrate from rock-salt to layered structure using uniaxial compression

Atomic ordering in oxide thin films influences electrical and catalytic properties of the materials. Herein, we investigated the atomic arrangement of LiNiO2 thin films on sapphire substrates treated by uniaxial compression (UAC) and annealing at 600∘C. Thin films with a rock-salt structure were grown using a pulsed laser deposition method at room temperature. We found that the as-grown thin films treated by UAC at 600∘C transformed from a rock-salt to a layered structure. The layer thickness of the later structure increased with increasing UAC pressure. UAC offers a useful tool in oxide thin film research that will contribute to development of new functional oxide materials and adjustment of the atomic ordering of cation materials.

Atomic Ordering Kinetics in a Cu–56 at % Au Alloy at a Temperature of 250°C

Atomic Ordering Kinetics in a Cu–56 at % Au Alloy at a Temperature of 250°C

Temperature dependence of the bulk and surface properties of GaZn liquid alloys

The temperature dependency of the bulk and surface properties of GaZn molten alloys has been theoretically explained. The bulk properties i.e. the thermodynamic functions namely the excess free energy of mixing, free energy of mixing, entropy of mixing, heat of mixing, activity and microscopic functions including concentration fluctuations along with short-range order parameter for GaZn molten alloys at 650 K, 750 K, 850 K and 950 K have been analyzed in the light of self association model. At first, the model parameters have been estimated for GaZn system at 750 K. For the authentication of model parameters, the theoretical data for the excess Gibbs free energy of mixing, Gibbs free energy of mixing, entropy of mixing, heat of mixing, activity together with concentration fluctuations have been matched with the experimental data. Again, the investigations have been extended for the temperatures i.e. 650 K, 850 K and 950 K. The theoretical data for the bulk properties namely the excess Gibbs free energy of mixing, Gibbs free energy of mixing, thermodynamic activity and concentration fluctuations of GaZn liquid alloys in the temperature range 650 – 950 K have been correlated with the available data in literature which are in well agreement. For quantification of the degree of atomic order in the liquid GaZn alloys, the short-range parameter is also determined for the temperature range 650 – 950 K. Further, the surface properties such as surface tension, surface concentration as well as surface concentration fluctuations of molten GaZn system in the temperature range 650 – 950 K are analyzed by Butler model in the light of self association model. The computed data matched with the available data in literature show a good harmony. For the explanation of the arrangement of atoms in the liquid solution, the transport property such as diffusivity of liquid GaZn alloys at 650 K, 750 K, 850 K as well as 950 K have been computed. The results reveal that the segregating character of molten GaZn alloys degrades with the elevation in the temperature from 650 K to 950 K.

Structural, magnetic and electronic properties of L2[formula omitted]-ordered Ru2–xMn1+xAl Heusler alloy thin films

The cubic Heusler alloy Ru2–xMn1+xAl is grown in thin film form on MgO and MgAl2O4 substrates. It is a highly spin-polarised ferrimagnetic metal, with weak magnetocrystalline anisotropy. Although structurally and chemically similar to Mn2RuxGa, it does not exhibit ferrimagnetic compensation, or large magneto galvanic effects. The differences are attributed to a combination of atomic order and the hybridisation with the group 13 element Al or Ga. The spin polarisation is around 50% to 60%. There is a gap in the density of states just above the Fermi level in fully ordered compounds.

Twinned single crystal structure of Li4P2S6

Abstract Yellow needles-like single crystals of Li4P2S6 were obtained serendipitously during the preparation of Li7P3S10O. The twinned crystal structure of Li4P2S6 was determined from single-crystal X-ray diffraction data [wR(F 2) = 0.069, 716 reflections, 40 variables]. Li4P2S6 crystallizes in the trigonal system, space group P 3 ‾ m 1 $P\overline{3}m1$ (N° 164), a = 10.5042(8) Å, c = 6.5837(6) Å, V = 629.11(9) Å3 and Z = 2. The lithium octahedra form a [Li4S6]8− honeycomb-like structure within which diphosphate units are located. The comparison of our crystal structure to those of P63/mcm-, P 3 ‾ 1 m $P\overline{3}1m$ -, and P321-Li4P2S6 demonstrated group-subgroup relationships and associated the disorder or order of the phosphorus atoms within the identical [Li4S6]8− 3d-frameworks to the choice of the unit cell (the subcell with a ∼ 6.07 Å vs. the supercell with a ∼ 10.5 Å).

Correlation of phase composition, magnetic properties and hyperthermia efficiency of silica-coated FeCo nanoparticles for therapeutic applications

The local atomic order, magnetic properties, hyperthermia efficiency, drug loading capacity and in-vitro cytotoxicity are investigated for the core–shell (Fe25Co75)x(SiO2)100-x (70 ≤ x ≤ 100, wt.%) nanoparticles (NPs) sintered by the metals co-precipitation followed with SiO2 TEOS deposition. Electron microscopy reveal the formation of nearly spherical structures with medium diameter from 80 nm to 220 nm containing 20–50 nm cores identified by XRD as α-FeCo bcc alloy. Mössbauer spectra and magnetometry reflects the formation of magnetically-interacting net of single-domain FeCo cores with broad size distribution. Observed decrease in magnetization (MS) and effective magnetic anisotropy (Keff) for NPs with × is the consequence of the increasing thickness of the SiO2 shells. Calculations indicate that there is an optimal thickness δ of SiO2 shells ranging between 90 ÷ 120 nm for the highest value of specific absorption rate (SAR) originating from their effect on Keff values. Experimentally proved that SAR has the highest value for (FeCo)80(SiO2)20 NPs in correlation with the Linear Response Theory approach. High drug loading capacity (up to 98%) and low in-vitro cytotoxicity (IC50 index below 230 µg/ml) experimentally proved for (FeCo)x(SiO2)100-x NPs make them perfect candidates for designing smart drug delivery systems.

Racemic Benzenesulfonohydrazide Enantioconvergent Tertiary C(sp3 )-H Amination Catalyzed by Copper and Chiral Phosphoric Acid: Mechanistic Insights and Computational Prediction.

Density functional theory computations reveal mechanistic insights into Cu and chiral phosphoric acid (CPA) catalyzed enantioconvergent amination of racemic benzenesulfonohydrazide. The O-O bond homolysis of tert-butyl 4-phenylbutaneperoxoate was found to be the turnover-limiting step with a total free energy barrier of 19.1 kcal/mol. The enantioconvergent amination is realized to obtain the same intermediate through prochiral carbon atom. The order and mode of hydrogen atom transferred by CPA and tert-butyloxy have a significant influence on the enantioselectivity and energy barriers. The olefinic side product generated by β-hydride elimination is 9.9 kcal/mol thermodynamically less favourable. A series of phosphoric acids are predicted as promising co-catalysts with lower barriers for O-O bond homolysis.

Phase interface manipulation by adjusting atomic ordering in metal-organic framework to facilitate microwave absorption

Metal-organic frames (MOFs) have been a suitable template for high-performance microwave absorbing materials. However, there has been a lack of effective solutions for the agglomeration and inadequate graphitization transformation of metal nanoparticles after annealing of MOFs. In this work, a simple and safe metal ion exchange method for the experimental process is proposed to etch the surface of Fe MOF (MIL-88) template using nickel metal salts, and the subsequent heat treatment to synthesize Fe-Ni/C nanorods. These fine and uniform metal particles and carefully designed phase engineering have excellent electromagnetic attenuation and good impedance matching. In particular, suitable defects and rich heterogeneous interfaces cause polarization loss to increase, and more obviously, hysteresis loss and natural resonance increase magnetic loss. The experimental results show that the materials have good broadband absorption (6.13 GHz) and high-efficiency loss (−35 dB) microwave absorbing (MA) performance. This work provides a generalized phase interface engineering approach for polymetallic nanorods and provides a useful reference for microwave absorbing materials.

Thermal Annealing Driven Enhancement of Perpendicular Magnetic Anisotropy and the Interfacial Dzyaloshinskii–Moriya Interaction in Ultrathin Ru/Co/W/Ru Films

Spin-related effects discovered in ultrathin magnetic films generally have an interfacial nature. The quality of interfaces between a heavy metal and a ferromagnet is the main driver of surface anisotropy, antisymmetric exchange, chiral damping, spin Hall effect, and spin–orbit torques. To understand the physics and usage of these phenomena for future atomic-scale devices, one has to study systems with disordered interfaces and search there for novel practically desirable effects, which lie beyond the atomic order and surface flatness. Here, we report the interface-related effects in Ru/Co/W/Ru ultrathin films treated by thermal annealing in vacuum. The surface roughness and degree of atomic intermixing at Ru/Co and Co/W interfaces change with increasing annealing temperature up to 240 °C, promoting an enhancement of perpendicular magnetic anisotropy and the interfacial Dzyaloshinskii–Moriya interaction (i-DMI). Further annealing at higher temperatures brings the interface deterioration and, consequently, a drastic degradation of the magnetic properties. Our first-principles calculations qualitatively support the experimental findings, providing an understanding of the i-DMI enhancement nature.

Molecular dynamics simulation of the nano-cutting mechanism of a high-phosphorus NiP coating

Electroless nickel-phosphorus workpieces undergo a variety of surface/subsurface deformations and internal organizational changes throughout the nano-cutting process that have a major impact on the machining quality of microstructured functional surfaces. In this paper, molecular dynamics simulations were performed for nano-cutting of electroless nickel-phosphorus (Ni–P) workpieces obtained at different cooling rates (1e11 K/s, 1e12 K/s, 1e13 K/s). To research the mechanisms of material removal and surface formation during nano-cutting, the impacts of various cutting depths on atomic ordering, cutting force, cutting temperature, shear strain, material pile-up, and surface shape were examined. The simulations showed that material removal of NiP coatings was mainly based on nanoscale extrusion rather than macroscale shear. The radial distribution function of NiP coatings was dominated by the Ni–P pair, the cutting force of the nano-cutting process was more stable, the thrust force was smaller, and the machined surface roughness Ra was smaller when the cooling rate was higher. However, the number of atoms on the upper surface of the workpiece increased more, and the actual machined surface contour line deviated from the ideal surface contour when the cooling rate was faster. To increase the forming accuracy of the machined surface on NiP without destroying the machined surface roughness, the cooling rate must be slowed appropriately for the NiP alloy to form a tiny amount of crystalline organization or to minimize the nano-cutting depth.

Multiscale electric-field imaging of polarization vortex structures in PbTiO3/SrTiO3 superlattices

In ferroelectric heterostructures, the interaction between intrinsic polarization and the electric field generates a rich set of localized electrical properties. The local electric field is determined by several connected factors, including the charge distribution of individual unit cells, the interfacial electromechanical boundary conditions, and chemical composition of the interfaces. However, especially in ferroelectric perovskites, a complete description of the local electric field across micro-, nano-, and atomic-length scales is missing. Here, by applying four-dimensional scanning transmission electron microscopy (4D STEM) with multiple probe sizes matching the size of structural features, we directly image the electric field of polarization vortices in (PbTiO3)16/(SrTiO3)16 superlattices and reveal different electric field configurations corresponding to the atomic scale electronic ordering and the nanoscale boundary conditions. The separability of two different fields probed by 4D STEM offers the possibility to reveal how each contributes to the electronic properties of the film.

Atomic Ordering and Interfacial Interaction at Liquid-Mg/SiC{0 0 0 1} Interfaces: An Ab Initio Molecular Dynamics Study

We present the results of ab initio molecular dynamics investigations on the atomic ordering and chemical interactions at the interfaces between liquid Mg and SiC{0 0 0 1} interfaces. The simulations reveal distinct borders between the SiC substrates and liquid Mg. The liquid Mg atoms adjacent to the substrates are bonded to the outmost C/Si atoms and are positively charged. The terminating Mg layers contain a variety of atomic vacancies, being topologically rough. The liquid Mg atoms adjacent to the substrates display unusual prenucleation phenomenon with strong layering but weak in-plane ordering. The obtained information here is helpful to get insight into the formation and interfacial interactions in the SiC joined nano-sized magnesium matrix composites and the role of SiC particles as potential nucleation sites during solidification, and further helps understand interfacial interactions at the grain boundaries in ceramic/metal composites and welded parts, etc. in general.

Direct evidence of the ultramicroporous structure of carbon molecular sieves

We utilize gas sorption experiments, neutron pair distribution function, and small-angle X-ray scattering studies on carbon molecular sieves (CMSs) to gain new insight into their ultramicroporous structure. CMS materials derived from the pyrolysis of PIM-1 (PIM = polymer of intrinsic microporosity) under an inert atmosphere (PIM-1-CMS) and H2 atmosphere (4% H2-PIM-1-CMS) were studied. Neutron total scattering studies of these materials reveal these CMS materials to be mainly graphene-like ribbons with short-range atomic ordering. Small-angle X-ray scattering and low angle diffraction peaks corroborate the presence of well-defined ultramicroporosity observed in carbon dioxide and cryogenic neon adsorption studies suggesting, expectedly, that these materials are non-graphitizing carbon structures. Our findings provide microscopic evidence of the non-graphitizing and ribbon-like structure of these CMS materials and suggest a possible hypothetical microporous structure.

Novel borides of the boron filled β-Mn-type structure

The existence of a boron-filled β-Mn-type compound has been observed for the first time in the Pd-Cu-B system. The crystal structure of this phase, its homogeneity field and phase equilibria in the relevant compositional range have been examined by electron probe microanalysis, and single crystal and powder X-ray diffraction of alloys annealed at 650 °C for 240 h. The B-filled β-Mn-type structure has been found to exist within ∼50 at% - ∼57 at% Pd at 60–80% boron population in [Pd6] octahedral voids (B in 4a). Metal atom ordering (Mo3Al2C-type structure, Pd in 12d, Cu in 8c, B in 4a) has been proved from single crystal data in Pd3Cu2B0.8 at 80% boron population of the 4a boron atom sites. The structures of the B-filled β-Mn-type samples in the Pd-Cu-B system exhibit preferential occupation of the 12d Wyckoff position by the more electronegative element with larger atomic radius, i.e. Pd. Metallic behavior of this phase was ascertained by measurements of electrical resistivity. No superconductivity has been observed from electrical resistivity measurements down to 0.3 K. Ab initio electronic structure calculations indicate a rather low electronic density of states at the Fermi level for Pd3Cu2B, with the palladium d-states contributing almost 50% to the total density. The band structure of Pd3Cu2B exhibits multiple bands intersecting the Fermi energy.

The effect of spin-polarization, atomic ordering and charge transfer on the stability of CoCrNi medium entropy alloy

The aim of this paper is to determine the effects of spin-polarization and atomic ordering on the phase stability of the medium entropy alloy CoCrNi by employing density functional theory calculations. We have found that high Cr ordering causes stabilisation of both the fcc and hcp structures compared to fully disordered alloy, while high Ni ordering leads to destabilisation of the hcp structure but has no effect on the fcc structure. The differences in the mutual stability of fcc and hcp structures for alloys with different levels of ordering are accompanied by differences in the distribution of local magnetic moments. However, their analysis revealed a generally increasing trend in the magnitude of the magnetic moment with the increasing number of Cr atoms in the 1st nearest neighbours (NN) shell for all Co, Cr, and Ni elements. Moreover, the charge analysis revealed a charge transfer from Cr to the other two elements in the alloy and also its dependence on the number of Cr atoms in the 1st NN shell. Thus, we have described the clear indirect dependence of the magnetic moment of all elements in this alloy on the ongoing charge transfer. This also points to the possibility that the charge transfer has the more important effect on stabilisation of particular atomic ordering rather than the spin-polarization as the same behaviour of charge transfer was observed also for results obtained without spin-polarization.

Endothelial Cell Behavior and Nitric Oxide Production on a-C:H:SiOx-Coated Ti-6Al-4V Substrate.

This paper focuses on the surface modification of the Ti-6Al-4V alloy substrate via a-C:H:SiOx coating deposition. Research results concern the a-C:H:SiOx coating structure, investigated using transmission electron microscopy and in vitro endothelization to study the coating. Based on the analysis of the atomic radial distribution function, a model is proposed for the atomic short-range order structure of the a-C:H:SiOx coating, and chemical bonds (C-O, C-C, Si-C, Si-O, and Si-Si) are identified. It is shown that the a-C:H:SiOx coating does not possess prolonged cytotoxicity in relation to EA.hy926 endothelial cells. In vitro investigations showed that the adhesion, cell number, and nitric oxide production by EA.hy926 endothelial cells on the a-C:H:SiOx-coated Ti-6Al-4V substrate are significantly lower than those on the uncoated surface. The findings suggest that the a-C:H:SiOx coating can reduce the risk of endothelial cell hyperproliferation on implants and medical devices, including mechanical prosthetic heart valves, endovascular stents, and mechanical circulatory support devices.

Artificial Theory of Mind in contextual automated negotiations within peer-to-peer markets

With the advent of the sharing economy (eBay, Airbnb, Uber) and peer-to-peer electricity markets, negotiation agents become key to automate trading between prosumers. To be competitive, agents need to develop models of their opponents’ strategic behavior depending on contextual circumstances. However, existent approaches do not account for others’ ways of reasoning about how the context influences opponents’ strategies. In this work, negotiation agents are endowed with artificial Theory of Mind (aToM) to represent mental states of others. Those states include their strategies, beliefs, the context influencing beliefs and the beliefs they may have about the beliefs others have about themselves depending on the context. From prior knowledge that relates private and contextual states with opponent’s plausible strategies, agents build models of different orders of aToM using weighted Gaussian Processes with heteroscedastic noise and adapt their strategies. During a negotiation episode, each agent selects the order of aToM that best accounts for its opponent’s counteroffers based on Bayesian surprise. To assess the proposed approach, a benchmark Smart Grid is used, where energy prosumers negotiate on a peer-to-peer basis to compensate for their energy deficits and sell their surpluses. Results obtained highlight that low orders of aToM increase social welfare at least in a 54% despite the selfish nature of prosumers. Also, more sophisticated aToM abilities increase the individual competitiveness of agents. This is the first work in the related literature that adds aToM abilities to negotiation agents while considering the negotiation context.

On the Possible Half-Metallic Properties of Fe2RhZ (Z = Al, Si, Ga, Ge, In, Sn) Ferromagnetic Heusler Alloys

The possible presence of half-metallic properties in Fe2RhZ (Z = Al, Si, Ga, Ge, In, Sn) ferromagnetic Heusler alloys is ab initio studied. The calculation is carried out taking into account the exchange correlation effects using the generalized and meta-generalized gradient approximations. It is shown that the ground state of most investigated alloys is ferromagnetic ordering in the Тр model lattice. An analysis of the phase stability of the alloys showed that, in the considered approximations, most of them are stable in both the regular and inverse lattice, as well as in the three model lattices belonging to inverse-type Heusler structures with different atomic orders. It was predicted using the strongly constrained and appropriately normed (SCAN) functional that the Fe2RhSi alloy can exhibit the properties of half-metals, since it has an integer magnetic moment, a fairly high degree of polarization, and an energy gap at the Fermi level of spinup electrons.