Long-range Atomic Order Research Articles

Growth of AlGaN alloys with ∼ 90 % AlN mole fraction by plasma-assisted molecular beam epitaxy: Indication of phase-segregation effects

AlGaN alloys with very high AlN mole fraction (∼90 %) were grown by plasma-assisted molecular beam epitaxy (PA-MBE) and their alloy properties were investigated by high-resolution X-ray diffraction (HR-XRD) measurements. Growth was carried out employing a series of group III/V ratios and for samples grown under a high group-III regime, phase-segregation in the alloy was evident from a characteristic splitting of the AlGaN (0002) peak in the HR-XRD pattern. With decreasing excess group-III conditions the separation of peak positions continuously reduced, till a single peak was observed. However, for samples where such splitting was absent, spontaneous superlattice peaks were seen at lower incident angles of the XRD patterns indicating the presence of long-range atomic ordering (LRAO). Thus, for AlGaN alloys with extremely high Al content, effects of phase-segregation, or of LRAO were observed, depending on the kinetics of growth. These results on phase-segregation effects are expected to promote the development of high-efficiency deep ultraviolet emitters for skin-safe germicidal action by mitigating the detrimental effect of dislocations and related non-radiative recombination centers through carrier localization processes at potential minima.

Rare-Earth Lanthanum-Evoked Amorphization and Optimization to Boost Ambient Nitrogen Fixation over Single-Atom Catalysts.

Single-atom catalysts (SACs) have been widely studied in a variety of electrocatalysis. However, its application in the electrocatalytic nitrogen reduction reaction (NRR) field still suffers from unsatisfactory performance, due to the sluggish mass transfer and significant kinetic barriers. Herein, a novel rare-earth-lanthanum-evoked optimization strategy is proposed to boost ambient NRR over SACs. The incorporation of La with a large atomic radius tends to break the atomic long-range order and trigger the amorphization of SACs, endowing a greater density of dangling bonds that could modify affinity for reactants and adsorbates. Moreover, with unique 5d16s2 valence-electron configurations, its presence could further enrich the electron density and enhance the intrinsic activity of single-metal center via the valence orbital coupling. As expected, the La-modified catalyst presents excellent activity toward the electrochemical NRR, delivering a maximum ammonia yield rate of 33.91 μg h-1 mg-1 and a remarkable Faradaic efficiency of 53.82%.

Induced dipole moments in amorphous ZnCdS catalysts facilitate photocatalytic H2 evolution

Amorphous semiconductors without perfect crystalline lattice structures are usually considered to be unfavorable for photocatalysis due to the presence of enriched trap states and defects. Here we demonstrate that breaking long-range atomic order in an amorphous ZnCdS photocatalyst can induce dipole moments and generate strong electric fields within the particles which facilitates charge separation and transfer. Loading 1 wt.% of low-cost Co-MoSx cocatalysts to the ZnCdS material increases the H2 evolution rate to 70.13 mmol g−1 h−1, which is over 5 times higher than its crystalline counterpart and is stable over the long-term up to 160 h. A flexible 20 cm × 20 cm Co-MoSx/ZnCdS film is prepared by a facile blade-coating technique and can generate numerous observable H2 bubbles under natural sunlight, exhibiting potential for scale-up solar H2 production.

Long-Range Atomic Order on Double-Stepped Al2O3(0001) Surfaces.

The deterministic preparation of highly ordered single-crystalline surfaces is a key step for studying and utilizing the physical properties of various advanced materials. This paper presents the fast and straightforward preparation of vicinal Al2O3(0001) surfaces with micrometer-scale atomic order. Crisp electron-diffraction spots up to at least 20th order evidence atomic coherence on terraces with widths exceeding 1 μm. The unique combination of three properties of Al2O3(0001) underlie this remarkable coherence: its high-temperature stability; the differences in the ionic bonding systems of the surface as compared to the bulk; and the fact that the terraces are non-polar whereas the step edges have a polar character. The step edges are furthermore found to have alternating configurations, which drive a step-doubling transition. On double-stepped surfaces, the Al-rich ° surface reconstruction attains a singular in-plane orientation. These results set a benchmark for high-quality surface preparation and thus expand the scope for both fundamental studies on and the technological utilization of exciting materialsystems.

Room-temperature sub-100 nm Néel-type skyrmions in non-stoichiometric van der Waals ferromagnet Fe3-xGaTe2 with ultrafast laser writability

Realizing room-temperature magnetic skyrmions in two-dimensional van der Waals ferromagnets offers unparalleled prospects for future spintronic applications. However, due to the intrinsic spin fluctuations that suppress atomic long-range magnetic order and the inherent inversion crystal symmetry that excludes the presence of the Dzyaloshinskii-Moriya interaction, achieving room-temperature skyrmions in 2D magnets remains a formidable challenge. In this study, we target room-temperature 2D magnet Fe3GaTe2 and unveil that the introduction of iron-deficient into this compound enables spatial inversion symmetry breaking, thus inducing a significant Dzyaloshinskii-Moriya interaction that brings about room-temperature Néel-type skyrmions with unprecedentedly small size. To further enhance the practical applications of this finding, we employ a homemade in-situ optical Lorentz transmission electron microscopy to demonstrate ultrafast writing of skyrmions in Fe3-xGaTe2 using a single femtosecond laser pulse. Our results manifest the Fe3-xGaTe2 as a promising building block for realizing skyrmion-based magneto-optical functionalities.

The effect of composition and long-range order on the strength of defect-free faceted Cu-Au nanoparticles

Solid solution and long-range atomic order hardening represent two well-known strategies for increasing strength of bulk metallic materials. Here, we observed the opposite trends in mechanical behavior of defect-lean nanoparticles of Cu-Au alloys fabricated by solid state dewetting of Cu-Au bilayers deposited on sapphire substrate. In the fully disordered state, the nanoparticles of Cu3Au alloy exhibited the highest compressive strength reaching staggering 41 GPa for the smallest studied particles, followed by nanoparticles of pure Cu, and of CuAu alloy. Thus, taking pure Cu and Au nanoparticles as a reference, both solid solution hardening (Cu3Au) and solid solution softening (CuAu) were observed in nanoscale plasticity of the Cu-Au system. Additional annealings at the temperature of 350 °C, 40 °C below the critical point of A1-L12 transformation, were performed to induce different degrees of the long-range order in the Cu3Au nanoparticles. It was shown that the strength of fully ordered Cu3Au nanoparticles is very similar to that of their fully disordered counterparts, whereas the partially ordered Cu3Au nanoparticles larger than 500 nm in size exhibited a significant drop in strength. No effect of long-range order on compressive strength of smaller nanoparticles was observed. These findings were discussed in terms of plasticity mechanisms controlled by nucleation of new dislocations. Specifically, it was proposed that the presence of oxidation-induced Au-rich subsurface layer in the Cu3Au nanoparticles is responsible for their ultrahigh strength.

Revisiting Néel 60 years on: The magnetic anisotropy of L10 FeNi (tetrataenite)

The magnetocrystalline anisotropy energy of atomically ordered L10 FeNi (the meteoritic mineral tetrataenite) is studied within a first-principles electronic structure framework. Two compositions are examined: equiatomic Fe0.5Ni0.5 and an Fe-rich composition, Fe0.56Ni0.44. It is confirmed that, for the single crystals modeled in this work, the leading-order anisotropy coefficient K1 dominates the higher-order coefficients K2 and K3. To enable comparison with experiment, the effects of both imperfect atomic long-range order and finite temperature are included. While our computational results initially appear to undershoot the measured experimental values for this system, careful scrutiny of the original analysis due to Néel et al. [J. Appl. Phys. 35, 873 (1964)] suggests that our computed value of K1 is, in fact, consistent with experimental values, and that the noted discrepancy has its origins in the nanoscale polycrystalline, multivariant nature of experimental samples, that yields much larger values of K2 and K3 than expected a priori. These results provide fresh insight into the existing discrepancies in the literature regarding the value of tetrataenite’s uniaxial magnetocrystalline anisotropy in both natural and synthetic samples.

Atomic-Level Description of Chemical, Topological, and Surface Morphology Aspects of Oxide Film Grown on Polycrystalline Aluminum during Thermal Oxidation—Reactive Molecular Dynamics Simulations

Oxidation results in the formation of an oxide film whose properties and structure can be tailored by controlling the oxidation conditions. Reactive molecular dynamics simulations were performed to study thermal oxidation of polycrystalline Al substrates as a function of O2 density and temperature. The structural, chemical, and topological aspects of polycrystalline Al (poly-Al) substrates and oxide films formed upon oxidation were studied. The studies were supported by surface topography and morphology analyses before and after oxidation. An analysis of Al–O atomic pair distribution showed the development of long-range order in the oxide films grown upon exposure to low-density (0.005 g/cm3) and high-density (0.05 g/cm3) O2 gas. The long-range order was more apparent for the high-density environment, as the oxide films formed in low-density O2 gas did not fully cover the poly-Al surface. The dominance of over-coordinated polyhedral units in a tightly packed structure was indicative of medium- and long-range atomic order in the oxide films. The two-phase structure of the oxide was found in the films, with a crystalline phase at the metal/oxide interface and an amorphous phase at the oxide/O2 interface. The combination with topological analyses supported the conclusions of the chemical analysis and enabled us to capture an amorphous-to-crystalline phase transformation in the oxide films with increasing oxygen density and temperature. An important effect of Al surface roughness before oxidation on the behavior of the metal/oxide interface and on the oxide film structure was observed.

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.

Three coordinate nitrogen (N3c) vacancies from in-situ hydrogen bond breaking over polymeric carbon nitride for efficient photocatalysis

Polymeric carbon nitrides are limited by low electron mobility induced by hydrogen bonding between strands of polymeric melon units with NH/NH2 groups. In our work, an amorphous carbon nitride photocatalyst with rich three coordinate nitrogen (N3c) vacancy sites was prepared through an in-situ hydrogen bond breaking approach. The generation of a dense hydrogen bonding network and the disruption of the long-range atomic order to the carbon nitride layers provide for the selective production of the defective activity center. Compared to bulk carbon nitrides, the removal efficiency of tetracycline hydrochloride (TC-H) got a significant boost due to the excellent contaminant adsorption activity from nitrogen vacancy. Besides, it showed a triple lift of H2 production through releasing the coordination between amine and Pt occupied by hydrogen bonding. This work uses changes in hydrogen bonding content to regulate the active center of catalytic reactions, providing additional ideas for the study of defect engineering.

Hydrogen-Induced Order–Disorder Effects in FePd3

Binary intermetallic compounds, such as FePd3, attract interests due to their physical, magnetic and catalytic properties. For a better understanding of their hydrogenation properties, both ordered FePd3 and disordered Fe0.25Pd0.75 are studied by several in situ methods, such as thermal analysis, X-ray powder diffraction and neutron powder diffraction, at moderate hydrogen pressures up to 8.0 MPa. FePd3 absorbs small amounts of hydrogen at room temperature and follows Sieverts’ law of hydrogen solubility in metals. [Pd6] octahedral voids are filled up to 4.7(9)% in a statistical manner at 8.00(2) MPa, yielding the hydride FePd3H0.047(9). This is accompanied by decreasing long-range order of Fe and Pd atoms (site occupancy factor of Fe at Wyckoff position 1a decreasing from 0.875(3) to 0.794(4)). This trend is also observed during heating, while the ordered magnetic moment decreases up to the Curie temperature of 495(8) K. The temperature dependences of the magnetic moments of iron atoms in FePd3 under isobaric conditions (p(D2) = 8.2(2) MPa) are consistent with a 3D Ising or Heisenberg model (critical parameter β = 0.28(5)). The atomic and magnetic order and hydrogen content of FePd3 show a complex interplay.

Deformation of Nanoglass under High-pressure Torsion

Metallic glasses are amorphous solids with metallic atomic bonds. Different from traditional crystalline metals, they have no long-range atomic order. Only short-range order on the scale of 0.5–1 nm can be detected. Because of the lack of a lattice dislocation slip system, as in crystalline alloys, metallic glasses exhibit some outstanding properties, such as high elastic strain, high hardness and high strength.

Compositional Glass: A State with Inherent Chemical Disorder, Exemplified by Ti-rich Ni3(Al,Ti)1 D024 Phase

A compositional glass is a state with an unavoidable disorder in chemical compositions on each site, characterized by frustration and freezing of the compositional degrees of freedom at low temperature. From this state a full atomic long-range order is unachievable by a reasonable thermodynamic treatment. There is a similarity between a spin glass (a magnetic state with disorder in spin orientations) and a compositional glass (with disorder in site occupations by chemical elements): both have frustrated ground states and a frozen disorder at low temperatures T < Tf (here Tf is called the freezing temperature). While it is possible to perform a ground-state search in a compositional glass, the resulting set of the fully ordered structures does not adequately represent the real solid with an inherent atomic disorder. Compositional glasses constitute a class of materials, which is insufficiently understood, but is of high industrial importance. Some of the phases in the precipitated alloys (including steels, high-entropy alloys, and superalloys) might be compositional glasses, and their better understanding would facilitate materials design. Due to their strength at high operating temperatures, superalloys are used in combustion engines and particularly in jet turbine engines. Precipitation strengthening of nickel superalloys is an area of active research. Local phase transformations inside Ni3Al-based precipitates are of particular interest due to their impact on creep strength. In the Ni3(Al1−xTix)1 ternary system, the competing phases are Ni3Al-type L12 (γʹ) and Ni3Ti-type D024 (η), while D019 (χ) is higher in energy. These three phases differ by the stacking of atomic layers: locally, the last two look like the internal and external stacking faults in L12. We compute enthalpies of disordered and ordered Ni3(Al1−xTix)1 ternary structures, examine phase stability, investigate the ground states and competing structures, and predict that the Ti-rich Ni3(Al1−xTix)1 D024 phase is a compositional glass with the atomic disorder on the Al/Ti sublattice. To resolve apparent contradictions among the previous experiments and to confirm our prediction, we perform X-ray diffraction and scanning electron microscopy analysis of the cast Ni3(Ti0.917Al0.083)1 sample. Our measurements appear to confirm the ab initio computed results. Our results elucidate properties of compositional glasses and provide a better understanding of precipitation strengthening mechanisms in Ni superalloys.

Visible-light-driven photocatalytic activation of peroxymonosulfate by K+-reformed polymeric carbon nitride for effective sulfamethoxazole decomposition

The SO4•– radical based advanced oxidation process (SR-AOP) is the effective remediation strategy for addressing the concern of severe pollution risk caused by recalcitrant pollutants. In this study, we report the visible-light-driven photocatalytic activation (VL-PCA) of PMS by using K+-induced structure remodeling high crystallinity of (KPCN). The degradation of sulfamethoxazole (SMX) by KPCN+PMS+VL system (the corresponding kinetic constant is 0.1452 min–1) was enhanced remarkably compared with PCN+PMS+VL system (0.0263 min–1). •O2–, 1O2, •OH and SO4•– radicals were the dominant reactive species for the enhanced decomposition of SMX, evidently demonstrated by reactive species scavenger and electron paramagnetic resonance (EPR) experiments. The KPCN crystalline semiconductors maintain the properties of long-range atomic order configuration and nonlinear optical (NLO) response of pristine PCN. The unique NLO effects of KPCN enable it to produce more photogenerated carriers and minimize the electron-hole pair recombination, which exhibit superior VL-PCA performance of PMS toward SMX degradation. This study gives a proof-in-concept demonstration of the increased activity of the C3N4-based PMS photocatalytic-activator by regulating the disordered structure and phase defects of PCN, shedding light on the development of highly efficient VL-PCA catalysts based on carbon nitride.

Structural-Phase Сhanges During the D1a→A1 Phase Transition in the Ni4Mo Alloy

The results of studying the structural and thermal effects in the region of the D1a→A1 order-disorder phase transition in the Ni4Mo alloy are presented. It is found that the D1a→A1 transition occurs in a narrow temperature range of 16 °C while the long-range order parameter п in this range decreases from 1 to 0.85 and, upon reaching ТК, decreases abruptly to 0. Studying the thermal effects allows obtaining the temperature dependence of the specific heat Ср(Т) is obtained and determination of the enthalpy and entropy of the D1a→A1 order-disorder phase transition in the Ni4Mo alloy. It is found that the main contribution to the entropy of transformation is configurational. The nonconfiguration contribution is not large and contributes to the ordering processes. It is established that the beginning of the deviation on the temperature dependence of Cp(T) from the linear one correlates with the beginning of the formation of the two-phase state D1a +A1 and the beginning of a decrease in the atomic long-range order in the D1a. Based on calculations based on the first a principles of thermodynamic parameters of compounds, the enthalpy of formation in the Ni-Mo system is obtained. It is found that the most stable compound in this system is Ni3Mo. Theoretical calculations of the compound formation enthalpy give a value of -8.3 kJ/mol.

Advancing the Mechanical Performance of Glasses: Perspectives and Challenges.

Glasses are materials that lack a crystalline microstructure and long-range atomic order. Instead, they feature heterogeneity and disorder on superstructural scales, which have profound consequences for their elastic response, material strength, fracture toughness, and the characteristics of dynamic fracture. These structure-property relations present a rich field of study in fundamental glass physics and are also becoming increasingly important in the design of modern materials with improved mechanical performance. A first step in this direction involves glass-like materials that retain optical transparency and the haptics of classical glass products, while overcoming the limitations of brittleness. Among these, novel types of oxide glasses, hybrid glasses, phase-separated glasses, and bioinspired glass-polymer composites hold significant promise. Such materials are designed from the bottom-up, building on structure-property relations, modeling of stresses and strains at relevant length scales, and machine learning predictions. Their fabrication requires a more scientifically driven approach to materials design and processing, building on the physics of structural disorder and its consequences for structural rearrangements, defect initiation, and dynamic fracture in response to mechanical load. In this article, a perspective is provided on this highly interdisciplinary field of research in terms of its most recent challenges and opportunities.

Growth, interface structure, and magnetic properties of Fe/GaAs and Fe3Si/GaAs hybrid systems

Abstract Ferromagnetic Fe and Fe3Si films are grown on GaAs(001) substrates by molecular beam epitaxy. This review article summarizes the important aspects of the heteroepitaxial growth and the associated structural properties of the Fe/GaAs and Fe3Si/GaAs heterostructures. The magnetic anisotropy of the thin Fe epilayers is analyzed and correlated to the interface structure, which is determined by high-resolution transmission electron microscopy. The structural and electrical properties of Fe3Si alloys depend on the exact stoichiometry and the long-range atomic ordering that is controlled by the growth conditions during the molecular beam epitaxy. The interface reactivity and the thermal stability are critically discussed for both heterosystems.

Crystallinity modulation originates ferroelectricity like nature in piezoelectric selenium

Modern room temperature ferroelectrics/piezoelectrics significantly impact advanced nanoelectronics than conventional chemical compounds. Changes in crystallinity modulation, long-range order of atoms in metalloids permits the design of novel materials. The ferroelectric like nature of a single element (selenium, Se) is demonstrated via in-plane (E⊥ar to the Se helical chains in micro-rod (MR)) and out-of-plane (E ∥el to the Se helical chains in MR) polarization. Atomic electron microscopy shows large stacks of covalently bound Se atoms in a c-axis orientation for tip bias voltage-dependent switchable domains with a 180˚ phase and butterfly displacement curves. The single crystalline Se MR has a high in-plane piezoelectric coefficient of 30 pm/V relative to polycrystalline samples due to larger grains, crystal imperfections in MR, and tuned helical chains. The energy conversion of a single Se-MR demonstrated via d13, d12 (or d15) piezoelectric modes.

Intermetallic Compounds: Liquid-Phase Synthesis and Electrocatalytic Applications.

Characterized by long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure, intermetallics have attracted increasing attention in the area of chemical synthesis and catalytic applications. Liquid-phase synthesis of intermetallics has arisen as the promising methodology due to its precise control over size, shape, and resistance toward sintering compared with the traditional metallurgy. This short review tends to provide perspectives on the liquid-phase synthesis of intermetallics in terms of both thermodynamics and methodology, as well as its applications in various catalytic reactions. Specifically, basic thermodynamics and kinetics in the synthesis of intermetallics will be first discussed, followed by discussing the main factors that will affect the formation of intermetallics during synthesis. The application of intermetallics in electrocatalysis will be demonstrated case by case at last. We conclude the review with perspectives on the future developments with respect to both synthesis and catalytic applications.

Features of the disorder→order phase transition in non-stoichiometric Cu-56at%Au alloy

The evolution of the structure, electrical resistivity and microhardness of the non-stoichiometric Cu-56Au (at%) alloy during the disorder↔order phase transitions of samples with different thermo-mechanical histories has been studied. It was found that the rate of atomic ordering of the Cu-56Au alloy was much lower compared to the equiatomic Cu-50Au alloy that had been well studied earlier. It was shown that the deformation of the quenched alloy slowed down the rate of atomic ordering. It was established that annealing of the quenched alloy for 1 week at 250 °C led to the formation of the CuAuI phase with a degree of long-range atomic order S ≈ 0.8. The electrical resistivity of the alloy in this state is ρ = (7.75 ± 0.04) × 10 = (7.75 ± 0.04) × 10–8 Ωm, which is the lowest value known for this alloy. It was revealed that the ordered CuAuII phase forms as a result of annealing of the Cu-56Au alloy at high temperature followed by slow cooling to room temperature. During the experiments, we did not receive any data confirming the CuAuII→CuAuI transition both at heating and at cooling of the alloy. There was made a conclusion of a high thermal stability of the ordered orthorhombic CuAuII phase in the alloy under investigation.