Evolution Of Atomic Structure Research Articles

Evolution of atomic structure in phase-separated Cu50Ag50 glass under compression deformation

Although phase-separated metallic glass has been reported in the literature, its plastic deformation mechanism is still open to debate. In this paper, molecular dynamics simulations were performed to explore the plastic deformation mechanism of phase-separated Cu50Ag50 metallic glass during compression deformation. Our findings indicate that homogeneous atom pairs show stronger interaction than heterogeneous atom pairs in the Cu50Ag50 glass, which also shows higher plastic deformation ability demonstrated by stress–strain curves. The degree of phase separation increases with increasing deformation. Ag atoms show higher Voronoi volume and potential energies than Cu atoms. The Ag-rich regions, which yield preferentially, also exhibit larger local shear strain than the Cu-rich regions. Voronoi volume and potential energy of Ag atoms remain nearly unchanged during plastic deformation because of the atom arrangement and release of elastic energy; on the other hand, the same parameters of Cu atoms increase because of their resistance to shear transformation.

Atomic-scale structural evolution in selective laser melting of Cu50Zr50 metallic glass

Selective laser melting (SLM) provides a promising alternative to overcome the size and geometry limitation in the fabrication of metallic glasses (MGs). Understanding the change of atomic-scale structure is vital to glass formation and the properties of SLM-fabricated MGs. In this work, atomic structural evolution of Cu50Zr50 MG fabricated by SLM was investigated using molecular dynamics simulations. Voronoi tessellation analysis indicates that icosahedron-like clusters and Frank-Kasper polyhedra are dominant in cladding and deposited layers. In addition, the fractions of populous icosahedron-like clusters decrease in molten pool while slightly increase in heat affected zone during SLM process, and the fractions of populous Frank–Kasper polyhedra remain virtually unchanged after laser melting and heat affecting. This work provides a necessary understanding and novel guidance to the fabrication of MGs using SLM.

In-situ synchrotron X-ray diffraction study of dual-step strain variation in laser shock peened metallic glasses

Atomic-structure evolution is significant in understanding the deformation mechanism of metallic glasses. Here, we firstly find a dual-step atomic strain variation in laser-shock-peened (LSPed) metallic glasses during compression tests by using in-situ synchrotron X-ray diffraction. Under low compressive load, LSP-deformed zone's atomic-structure shows low Young's Modulus (E); with load increase, atomic-structure are re-hardened, showing high E. An atomic deformation mechanism is proposed by using flow unit model, that LSP could induce interconnected flow units and homogenize the atomic-structure. These interconnected flow units are metastable and start to annihilate during compressive loading, causing the dual-step atomic strain variation.

Structural evolution in a metallic glass pillar upon compression

The in-situ observation of structural evolution of a metallic glass pillar during deformation is carried out in a high energy synchrotron X-ray source. The changes of the first maximum in structure factor, S(q), reveal the evolution of atomic structure upon stress. The width of the first maximum in S(q) increases as stress increasing during elastic deformation. After the elastic deformation, the serrated flow occurs, in which the width of the first maximum of S(q) in the loading stage of the serration event also increases. The broadening of the first maximum in S(q) means that the stress induces disordering of the glassy phase, which is because the densely packed clusters is separated into many loosely packed ones. This creates the excess free volumes.

First principle study on atomic structure of La65X35(X=Ni, Al) metallic glasses

The atomic-level structure of metallic glasses (MGs) is one of the most fundamental and challenging topics in condensed matter physics. Unlike crystalline metals or alloys, the MGs are lacking in a well-defined description of structure order, which is a major obstruction for relating its structure to physical properties. Obviously, it is vitally important to have an in-depth understanding of the atomic packing scheme in MGs. Due to the limitations of experimental characterization methods, it is hard to obtain the atomic packing scheme of MGs in experiment. Computational simulation on an atomic scale has become an important method of characterizing the atomic structure of MGs. The La-based LaNiAl glass forming system is well-known for its good glass-forming ability, distinctive relaxation peak that is well separated from relaxation, and liquid-liquid transition at a temperature around 1000 K. Many efforts have been made to investigate these novel properties. However, the atomic structure of this system is rarely studied. In this paper, the atomic structure evolution from liquids to glass states in La-based binary MGs La65Ni35 and La65Al35 are studied via ab initio molecular dynamics based on the density functional theory. The local structures are systematically analyzed by the radical distribution function, partial radical distribution function (PRDF), Voronoi tessellation method, and bond-type method in Honeycutt-Andersen. The results indicate that the PRDF of NiNi is much weaker than that of AlAl, which indicates the NiNi avoidance in La65Ni35. The major peaks of PRDFs are always smaller than the sum of efficient radius of the two kinds of atoms, especially for LaNi pairs. Atomic structure of the two systems are coincident with dense atomic packing scheme and the difference between major Voronoi polyhedron types (0, 3, 6, 0 for La65Ni35 and 0, 2, 8, 1, 0, 2, 8, 0 for La65Al35) in local structures is controlled by their ratio of solute to solvent atomic size. The high five-fold local symmetry structure gradually increases in both systems with the decrease of temperature, which validates its pivotal part in hindering crystallization. The electronic structure is studied with the partial density of states. It is found that the significant bond-shortening between La and Ni is due to the strong hybridization between Ni-3d and La-5d electrons and this result may play a key role in understanding composition related structure and properties in MGs.

Temperature dependent structural evolution in liquid Ag50Ga50 alloy

The temperature dependence of atomic structural evolution in liquid Ag50Ga50 alloy has been studied using an in situ high energy x-ray diffraction (XRD) experiment combined with first-principles molecular dynamics (FPMD) simulations. The experimental data show a reversible structural crossover at the temperature of about 1050 K. Changes in both electrical resistivity and absolute thermoelectric power at about 1100 K strongly support the XRD results. Additionally, FPMD simulations reveal the abnormal temperature dependent behavior of partial coordination number and atomic diffusivity at about 1200 K, elucidating that the partition experimentally observed changes in structure and properties could be linked with the repartition between Ag and Ga atoms in the liquid at around 1050–1200 K. This finding will trigger more studies on the structural evolution of noble-polyvalent metals in particular and metallic liquids in general.

Evolution of atomic and electronic structures of TaP under high pressure

As one of the first known Weyl semimetals, TaP exhibits intriguing topological properties and has received intensive attention. We investigated the evolution of atomic and electronic structure of TaP in a wide pressure range from 0 GPa to 200 GPa via first-principles calculations combined with intelligent particle swarm optimization. Three pressure-induced phase transformations were identified, following the transition sequence of I41md (α-phase) → Pmmn (β-phase) → P21/c (γ-phase) → Pm-3m (δ-phase) with transition pressure of 67 GPa, 90 GPa, and 125 GPa, respectively. The coordination number of Ta in α-phase, γ-phase, and δ-phase is 6, 7, and 8, respectively, indicating a denser packing under compression. All these newly pressure-stabilized structures were dynamically stable at both high and ambient pressure. Interestingly, we observed a semimetal to metal transition in the β phase. This work would shed light on further studies and applications of TaAs-class Weyl semimetals under compression or extreme condition.

Graphene modification via cobalt and silicon intercalation

Cobalt and silicon intercalation under graphene formed on the Co(111) surface has been investigated. The thickness of deposited films was varied within the range of up to 2 nm and the temperature of their annealing was 400-450°C. The evolution of atomic and electronic structure of the surface due to intercalation was studied by low-energy electron diffraction and high-resolution photoelectron spectroscopy with synchrotron radiation. It is shown that annealing of deposited Si layer leads to formation on the sample surface of both intercalated and nonintercalated areas. On the intercalated areas, the Co-Si solid solution is formed covered with segregated Si. The presence of a silicon phase strongly affects the electronic structure of graphene and substantially weakens its interaction with the substrate. Subsequent intercallation of Co leads to the restoration of strong coupling between graphene and the substrate.

The chemical states and atomic structure evolution of ultralow-energy high-dose Boron implanted Si(110) via laser annealing

Further scale down the dimension of silicon-based integrated circuit is a crucial trend in semiconductor fabrication. One of the most critical issues in the nano-device fabrication is to confirm the atomic structure evolution of the ultrathin shallow junction. In this report, UV Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) and reflective second harmonic generation (RSHG) are utilized to monitor the pulse laser induced atomic structure evolution of ultralow-energy high-dose Boron implanted Si(110) at room and cold substrate temperature. A peak feature around 480 cm−1 resolved in UV Raman spectra indicates the formation of Si-B bond after the laser irradiation. The red shift of binding energy of Si element (~99 eV) in XPS and the evolution of absorption peak (~196.2 eV) in XANES reveal that the changes in the chemical states of ultra shallow junction strongly correlate to the activation process of Boron implantation, which is confirmed by RSHG measurement. The substrate temperature effect in the recrystallization of Boron implanted region is also realized by cross-section high-resolution TEM (HRTEM). The phenomena of Si-B bond formation and ultra-shallow junction recrystallization can be traced and applied to improve the reliability of Si ultra shallow junction in the future.

Compression-induced phase transition of GaN bulk from wurtzite phase to five-fold coordination hexagonal phase

The phase transformation of GaN bulk from the Wurtzite phase (WZ) to the hexagonal phase (HX) is studied by molecular dynamics simulation. The mechanical response and atomic structural evolution of transition are analyzed in detail. In addition, the loading rate effect on the phase transition is determined, that is, the phase transition ratio declines with a decrease of the strain rate. The WZ GaN bulk completely transforms into the HX phase in the case of compression at an ultrahigh strain rate. However, at a relatively slower strain rate, the HX phase of GaN partly nucleates and the untransformed regions are proved to be elastic deformed regions. Combined with an energy analysis, two atomic movement modes are recognized as the inducements for the phase transition and formation of elastic deformed regions. The first mode, which is responsible for the formation of elastic deformed regions, is an atomic sliding motion along the c {0001} planes. The second mode is a radial stretching atomic motion. The radial stretching motion, which requires a lot of energy, induces the WZ-HX phase transition. Moreover, the phase transition is affected drastically by the rise of temperature.

Structural properties and energy analysis of ZrxCu92−xAl8 ternary metallic glasses

In this work, classic molecular dynamics simulations were employed to study ZrxCu92−xAl8 (x=55, 60, 65, and 70) ternary alloys. When Cu atoms were gradually replaced by Zr atoms, the local atomic structural evolution and energy landscape were analyzed. Pair distribution functions were used to characterize the structural features. The results showed that Zr-Al, Zr-Cu and Cu-Al atom pairs showed a strong short range order (SRO) within the first shell and they exhibited a weak sensitivity to atomic concentrations while Zr-Zr and Cu-Cu atom pairs presented a strong sensitivity to atomic concentrations. The analyses of the coordination number and coordination polyhedra indicated that substituting Zr atoms for Cu atoms led to the reduction of high coordination polyhedra and precluded the formation of the icosahedral structure, resulting in a more loosely packed atomic structure of the metallic glass. Finally, the energy landscape revealed that, with the increase of Zr composition and the decrease of Cu composition, the energy distribution of the Zr-centered clusters shifted towards a higher energy significantly and this originated from the increase of the number of Zr-Zr atom pairs and the growth of the Zr-Zr atom pair energy. The simulation results revealed that, from both consideration of the structure and energy distributions, the high Zr content Zr-Cu-Al metallic glasses were composed of more weakly bonded regions and packed more loosely, in line with the experimental results that the high Zr content glassy alloys exhibited high plasticity and low glass-forming ability.

Structural evolution in liquid calcium under pressure

The atomic structure evolution of liquid Ca at 3500K as a function of pressure up to 100GPa has been studied by ab initio molecular dynamics simulations. The pair correlation function, structure factor and respective peak positions reveal two crossovers at ~10GPa and ~50GPa in liquid Ca while the coordination number shows four pressure regions: about 0.1–5GPa, about 5–20GPa, about 20–50GPa and above ~50GPa and heat capacity shows two different pressure dependences separated at about 30GPa, i.e., below about 30GPa a negative one and a positive value above about 30GPa. It is also observed below 10GPa the first nearest neighbor CaCa distance decreases more sharply as compared to above 10GPa, this behavior is attributed to the gradual occupation of the interstitial sites. Comparisons of the pressure-dependent bond angle distribution and bond orientation order data of liquid Ca with solid crystalline phases imply that pressure-dependent atomic structural evolution in liquid calcium is, to some extent, similar to that of solid crystalline calcium reported by Teweldeberhan and Bonev [Phys. Rev. B 78 140101(R) (2008)]. These sluggish structural crossovers that have been observed in liquid Ca under pressure will trigger more studies in other disordered systems.

Local atomic structure evolution in YSZ solid solutions upon Mn doping

Abstract Local atomic structure in Mn doped Yttria Stabilized Zirconia was investigated using X-ray absorption spectroscopy at cation K-edges. Both XANES and EXAFS results show that the most significant changes in the local oxygen environment caused by doping appear in the Mn neighborhood. First shell FEFF modeling shows that both vacancies and distortions occur in Mn local environments. Single scattering fits of the EXAFS spectra of all the cations reveal distortions in the first oxygen coordination shells that are most evident for Mn and less pronounced for Zr and Y cations. Mn EXAFS spectra also confirm the presence of Mn 3 O 4 precipitations at high doping.

Size effect on brittle and ductile fracture of two-dimensional interlinked carbon nanotube network

The mechanical properties of two-dimensional (2D) interlinked carbon nanotube (CNT) network are investigated using ab initio calculation and molecular dynamics simulations (MD) with Reaxff force field. The simulation results show that bulk 2D interlinked CNT network has good mechanical properties along the axial direction which can be comparable to that of single-walled CNT and graphene, but has better ductility along the radial direction than single-walled CNT and graphene. In addition, the mechanical properties of 2D interlinked CNT network ribbon along the radial direction depend strongly on the size of the ribbon. The Young's modulus and Poisson's ratio decrease as the size increases while the fracture strain increases with the size increasing. By analyzing the atomic structural (both bond length and atomic von Mises stress) evolution of the ribbons, the mechanism of a brittle-to-ductile transition is revealed. The exploration of the mechanical properties of the 2D interlinked CNT network paves the way for application of the relevant devices that can benefit from the high Young's modulus, high tensile strength, and good ductility.

Atomic origin for rejuvenation of a Zr-based metallic glass at cryogenic temperature

The temperature-dependent atomic structural evolution of a Zr64.13Cu15.75Ni10.12Al10 metallic glass is studied by in-situ high-energy X-ray diffraction in the temperature range of 79–553 K. The interatomic distance in the first nearest neighbor shell increases with decreasing temperature, while the interatomic distance in other atomic shells decreases. This decrease in the interatomic distances causes volume shrinkage and an increase in the strength of the metallic glass. The volume shrinkage can be quantitatively described by the Varshni function. The structural information is helpful for deeply understanding of the origin of the rejuvenation of metallic glasses at the cryogenic temperatures.

Engineering a tubular mesoporous silica nanocontainer with well-preserved clay shell from natural halloysite

The in situ synthesis of mesoporous nanotubes from natural minerals remains a great challenge. Herein, we report the successful synthesis of mesoporous silica nanotubes (MNTs) with a varying inner-shell thickness and a preserved clay outer shell from natural-halloysite nanotubes (HNTs). After the enlargement of the lumen diameter of the tubular aluminosilicate clay by acid leaching, uniform mesopores were introduced by a modified pseudomorphic transformation approach, while the clay outer shell was well-preserved. Using density functional theory calculations, the atomic structure evolution and the energetics during Al leaching and Si–OH condensation were studied in detail. After the leaching of Al ions from the HNTs, local structural changes from Al(Oh) to Al(V) at a medium leaching level and to Al(Td) at a high leaching level were confirmed. The calculated hydroxylation energy of two kinds of silica components in the acid-leached HNTs (the distorted two-dimensional silica source in the inner shell and the intact aluminosilicate structure in the outer shell) was 0.5 eV lower or 1.0 eV higher than that of bulk silica, which clarifies the different behavior of the silica components in the hydrothermal process. The successful synthesis of reactive MNTs from HNTs introduces a new strategy for the synthesis of mesoporous nanocontainers with a special morphology using natural minerals. In particular, MNT samples with numerous reactive Al(V) species and a specific surface area up to 583 m2/g (increased by a factor of 10) are promising drug-loading nanocontainers and nanoreactors.

Atomic Insight into the Layered/Spinel Phase Transformation in Charged LiNi0.80Co0.15Al0.05O2 Cathode Particles

Layered LiNi0.80Co0.15Al0.05O2 (NCA) holds great promise as a potential cathode material for high energy density lithium ion batteries. However, its high capacity is heavily dependent on the stability of its layered structure, which suffers from a severe structure degradation resulting from a not fully understood layered → spinel phase transformation. Using high-resolution transmission electron microscopy and electron diffraction, we probe the atomic structure evolution induced by the layered → spinel phase transformation in the NCA cathode. We show that the phase transformation results in the development of a particle structure with the formation of complete spinel, spinel domains, and intermediate spinel from the surface to the subsurface region. The lattice planes of the complete and intermediate spinel phases are highly interwoven in the subsurface region. The layered → spinel transformation occurs via the migration of transition metal (TM) atoms from the TM layer into the lithium layer. Incomplete mi...

Hydrogen-induced atomic structure evolution of the oxygen-chemisorbed Cu(110) surface.

Using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) modeling, we determine the mechanism of the atomic structural evolution of the oxygenated Cu(110) surface induced by the reaction of adsorbed hydrogen with chemisorbed oxygen in the Cu(110)-c(6 × 2)-O structure. Our STM observations show that the reconstructed Cu(110)-c(6 × 2)-O surface undergoes a phase transition to the (2 × 1)-O reconstruction in the course of oxygen loss induced by the reaction with H2 gas. Using DFT modeling, we find that the surface phase transition is initiated via the adsorption of molecular hydrogen on the chemisorbed oxygen, which results in the formation of H2O molecules that desorb spontaneously from the surface. The loss of chemisorbed oxygen induces the c(6 × 2) → (2 × 1) transition that involves the diffusion of Cu-O-Cu chains along the ⟨1¯10⟩ direction.

<italic>In-situ</italic> study of electron irradiation on two-dimensional layered materials

High precision fabrication of nanostructure is a primary limiting factor to devices miniaturization. Electron beam is expected to promote the process on the accuracy of fabrication. For instance, electron beam lithography has been developed to replace optical lithography to transfer patterns and has produced line width on the order of 10 nm or smaller. Furthermore, electrons can directly interact with the sample to cause changes in structure and properties, which is expected to be used for fabrication of specialized nanostructures with nanometer even sub-nanometer precision. Transmission electron microscopy (TEM) has become an indispensable tool for the study of the interaction between energetic electrons and sample. A large number of experimental studies have been carried out inside TEM where electron beam can not only be used for atomic resolution imaging but also for irradiation. These studies are conductive to understanding phenomena induced by irradiation in essence, which are the theoretical and experimental basis for controllable manipulation via electron irradiation. However, TEM images are parallel projections of three-dimensional (3D) structures onto the image plane, which tremendously increases the difficulty of atomic-structure analysis. Two-dimensional (2D) layered materials (graphene, hexagon boron nitride, transition metal dichalcogenides and so on) emerged and boomed in the past decade, which provide ideal systems to study electron irradiation effects on matter. As mono- or bi-atomic sheets, their atomic structures are accessible to image directly with atomic resolution in the advent of spherical aberration corrected TEM. The lateral resolution in TEM image, which is smaller than the bond length, leaves almost no room for uncertain interpretation. Meanwhile, the reconstruction of the lattice occurs in the 2D plane can be seen without any projection artifacts. Besides, structural changes, as they are generated under irradiation with energetic electrons, can be monitored in real time. On this basis, novel mechanisms of lattice reconstruction, which did not appear in other materials, have been discovered, such as transformations between hexagons and pentagon-heptagon pairs by in-plane 90˚ rotation of C–C bonds in graphene sheet. In addition to a detailed understanding of radiation-induced structure evolution and their influence on the properties of 2D sheet, the design of new structures has become feasible. Especially, electron beams inside modern TEM can be focused onto spots less than 1 A, which allows use of the beam as tweezers to manipulate materials even at the single atom level. In this way, sub-nanometer quasi one dimensional structures, such as nanowires, nanoribbons, nanotubes and atomic chains, have been fabricated in 2D nanosystems. Sub-5 nm nanopores can also be sculpted in 2D sheets, which is particularly suitable for chemical and biological molecule detection. Besides, 2D materials can serve as substrates to study radiation-induced structural evolution of substance on their surface. This review gives a brief summary of our current knowledge about electron irradiation in the aspect of elastic scattering and inelastic scattering, and a summary of the most recent experimental results on in situ irradiation in 2D layered material systems, including atomic structural evolution under electron irradiation and controllable fabrication of nanostructures by TEM.

On the role of Sm in solidification of Al-Sm metallic glasses

During the solidification of Al-Sm metallic glasses the evolution of the supercooled liquid atomic structure has been identified with an increasing population of icosahedral-like clusters with increasing Sm concentration. These clusters exhibit slower kinetics compared to the remaining clusters in the liquid leading to enhanced amorphous phase stability and glass forming ability (GFA). Maximum icosahedral-ordering and atomic packing density have been found for the Al90Sm10 and Al85Sm15 alloys, respectively, whereas minimum cohesive energy has been found for the Al93Sm7 which is consistent with the range of compositions (from Al92Sm8 to Al84Sm16) found experimentally with high GFA.