Evolution Of Atomic Structure Research Articles

Structural Evolution of α-Fe2O3(0001) Surfaces Under Reduction Conditions Monitored by Infrared Spectroscopy.

The precise determination of the surface structure of iron oxides (hematite and magnetite) is a vital prerequisite to understand their unique chemical and physical properties under different conditions. Here, the atomic structure evolution of the hematite (0001) surface under reducing conditions was tracked by polarization-resolved infrared reflection absorption spectroscopy (IRRAS) using carbon monoxide (CO) as a probe molecule. The frequency and intensity of the CO stretch vibration is extremely sensitive to the valence state and electronic environments of surface iron cations. Our comprehensive IRRAS results provide direct evidence that the monocrystalline, stoichiometric α-Fe2O3(0001) surface is single Fe-terminated. The initial reduction induced by annealing at elevated temperatures produces surface oxygen vacancies, where the excess electrons are localized at adjacent subsurface iron ions (5-fold coordinated). A massive surface restructuring occurs upon further reduction by exposing to atomic hydrogen followed by Ar+-sputtering and annealing under oxygen poor conditions. The restructured surface is identified as a Fe3O4(111)/Fe1−xO(111)-biphase exposing both, Fe3+ and Fe2+ surface species. Here the well-defined surface domains of Fe3O4(111) exhibit a Feoct2-termination, while the reduced Fe1−xO(111) is Fe2+(oct)-terminated. These findings are supported by reference IRRAS data acquired for CO adsorption on magnetite (111) and (001) monocrystalline surfaces.

Atomic structure evolution in metallic glasses under cyclic deformation

Because of the limited experimental capability as well as computing power, the understanding of atomic structure evolution in metallic glasses (MGs) under cyclic loading is still lacking, which in turn makes the fatigue mechanism of MGs remaining largely elusive. Here we perform molecular dynamics simulations of tension-compression fatigue tests in Cu50Zr50 MGs under strain-controlled cyclic loading. When the applied strain lies in the elastic regime, no noticeable changes in the atomic structure of MGs in terms of pair distribution function and the fraction of various Voronoi polyhedra have been found during fatigue tests. When the applied strain lies in the plastic regime, however, shear band (SB) thickening is the intrinsic fatigue mechanism for MGs. It is shown that after its formation, the SB does not propagate along the shearing plane or develop into fatigue cracks with further cyclic loading. Instead the SB expands spatially and eventually evolves into the soft second phase with increased free volume within. The present study provides an in-depth atomic understanding of the structural evolution in MGs under low-cycle fatigue deformation.

An evidence of local structural disorder across spin-reorientation transition in DyFeO3: an extended x-ray absorption fine structure (EXAFS) study

The present work is aimed at exploring the local atomic structure modifications related to the spin reorientation transition (SRT) in DyFeO3 orthoferrite exploiting x-ray absorption fine structure (XAFS) spectroscopy. For this purpose we studied by XAFS the evolution of the local atomic structure around Fe and Dy as function of temperature (10–300 K) in a DyFeO3 sample having the SRT around 50–100 K. For sake of comparison we studied a YFeO3 sample having no SRT. The analysis of the extended region has revealed an anomalous trend of Fe–O nearest neighbour distribution in DFO revealing (i) a weak but significant compression with increasing temperature above the SRT and (ii) a peculiar behavior of mean square relative displacement (MSRD) [] of Fe–O bonds showing an additional static contribution in the low temperature region, below the SRT. These effects are absent in the YFO sample supporting these anomalies related to the SRT. Interestingly the analysis of Dy L3-edge data also reveal anomalies in the Dy–O neighbour distribution associated to the SRT, pointing out a role of magnetic Re ions across . These results point out micro-structural modification at both Fe and Dy sites associated to the magnetic transitions in DFO, it can be stated in general terms that such local distortions across and magnetic Re3+ may be present in other orthoferrites exhibiting multiferroic nature.

Rejuvenation saturation upon cyclic elastic loading in metallic glass

Rejuvenation is a desirable structural excitation process that could improve the deformability of metallic glasses (MGs). Here we evaluate the rejuvenation processes of MG models under cyclic elastic loading by molecular dynamics simulations. Interestingly, the simulation result shows rejuvenation of the MG models enhanced effectively with increased cyclic number and eventually reached a saturated regime. To interpret the two-regime rejuvenation, a new method, atomic computer tomography (ACT) was proposed herein to evaluate the atomic packing density and its distribution of the simulated MGs. Different from those well-documented models proposed previously, three distinctive kinds of regions with various atomic packing densities were identified via ACT evaluation. The atomic packing density transitions among the three kinds of regions upon cyclic elastic loading were proposed to be responsible for both the accumulated and saturated rejuvenation processes, which consisted with the simulated tensile deformation behavior. Our findings may help connecting atomic structure evolution and rejuvenation process in MGs and demonstrating the existence of rejuvenation threshold under cyclic elastic loading.

Atomic Structural Competition in the Al85.5Ni9.5La5 Alloy During Liquid-to-Solid Transition

The atomic structural evolution of the molten Al85.5Ni9.5La5 alloy is investigated during cooling using an in situ high-energy X-ray diffraction method following indirect analysis by a reverse Monte Carlo simulation and Voronoi tessellation. The competition between crystal-like and non-crystal-like clusters is insignificant above liquidus. However, below liquidus, a sudden increase in the crystal-like clusters was found at slow cooling rates while at high cooling rates all clusters were frozen into a glassy state directly.

Relationship between Ta Oxidation State and Its Local Atomic Coordination Symmetry in a Wide Range of Oxygen Nonstoichiometry Extent of TaOx

Evolution of electronic and atomic structure of amorphous Ta2O5 during sputtering by Ar+ ions was investigated by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine...

Evolution of local atomic structure during solidification of U116Nb12 liquid: An ab initio molecular dynamics study

By means of ab initio molecular dynamics (AIMD), the evolution of local atomic structures of the U116Nb12 liquid alloy during solidification from 2600 K to 600 K was systemically investigated in terms of the pair correlation function, structure factor, bond angle distribution, coordination number, the Honeycutt-Anderson (HA) and Voronoi index as well as diffusivity and viscosity. The distinctly linear dependence of the mean square displacements (MSDs) as a function of time indicates that the equilibrium configurations are derived from our AIMD simulations. The equations of the volumetric expansion, thermal diffusion and viscosity of the U116Nb12 liquid alloy are presented, respectively. The most abundant structures in the U116Nb12 liquid alloy are those having HA indices of 1551, 1541, 1431, 1441 and 1661. The Voronoi analysis reveals that the distributions of local atomic structures at high temperature are homogeneous, whereas Voronoi polyhedra trend to form <0,2,8,2>, <0,0,12,0>, <0,1,10,2>, <0,1,10,3>, <0,2,8,4>, <0,3,6,5> and <0,3,8,3> with the larger fraction at low temperature. The analysis of chemical short-range order and structural properties suggests that the icosahedra and bcc-type clusters play the dominant roles upon cooling and, the addition of Nb in the U116Nb12 liquid alloy is indeed beneficial to occurrence of the local atomic structural evolution.

Atomic structure evolution of high entropy metallic glass microwires at cryogenic temperature

The atomic structure evolution of Gd36Tb20Co20Al24 high entropy metallic glass microwire from room to cryogenic temperature has been studied by in-situ high energy synchrotron X-ray diffraction. During the cooling process, the atomic volume decreases. For short-range order, the coordination numbers of all atomic pairs increase. As the temperature decreases, bond lengths of large atom-small atom and small atom-small atom pairs keep decreasing whereas that of large atom-large atom pairs unexpectedly increases continuously. The mechanism of atomic structure evolution is proposed, which might be helpful for better understanding the low temperature magnetocaloric effect of high entropy metallic glasses.

Atomic Scale Study on the Tension-Compression Asymmetry of Single Crystalline and Nanocrystalline NiTi Shape Memory Alloy

The tension-compression asymmetry of single crystalline and nanocrystalline NiTi shape memory alloys (SMAs) is investigated at atomic scale utilizing molecular dynamics (MD) simulation. This study is based on the second nearest neighbor modified embedded-atom method interatomic potential for Ni-Ti binary systems. First of all, multiple variants of martensite are simulated via temperature-induced martensitic transformation in single crystal and nanocrystalline. The characteristics of forward and reverse martensitic transformation is derived by means of the atomic structural evolution. Secondly, it is observed that the tension-compression asymmetry for single crystalline NiTi is attributed to types of stress induced martensitic variants and martensitic reorientation, which respectively leads to stress asymmetry and strain asymmetry. However, the phenomenological tension-compression asymmetry in nanocrystalline is mainly credited to stress-induced martensitic variants, no obvious martensitic reorientation occurs under tension and compression loads. The corresponding atomic-scale structural evolution also explains why the length of tensile stress plateau in nanocrystalline is shorter than that of the single crystal. Finally, the nanocrystalline tension-compression asymmetric behavior is simulated under a broad range of temperature to obtain the critical stress-temperature relation. The simulation results are compared with the corresponding results obtained by experiment reported in the literature. The tension-compression asymmetry mechanism of single crystalline and nanocrystalline NiTi shape memory alloy is derived based on numerical results at atomic scale.

Lithium‐Ion Batteries: Atomic‐Scale Observation of Electrochemically Reversible Phase Transformations in SnSe2 Single Crystals (Adv. Mater. 51/2018)

In situ transmission electron microscopy is a powerful tool to capture spatiotemporal evolution of dynamic phenomena. In article number 1804925, Vinayak P. Dravid, Kai He, and co-workers implement this method to uncover atomic structural evolution of electrochemically reversible phase transformations during lithiation and delithiation of SnSe2, which is promising for lithium-ion batteries.

Evolution of local atomic structures during rapid solidification of liquid metal W

Molecular dynamics (MD) simulations were performed to investigate the evolution of local atomic structure during rapid solidification of liquid metal W. The microstructures were characterized by means of potential energy (PE) per atom, pair distribution function and largest standard cluster analysis (LSCA). Instead of [12/555] perfect icosahedra, the [8/555 2/544 2/433] distorted icosahedra dominated in W monatomic metallic glasses (MGs). With the decrease of temperature, these clusters in metal W systems aggregated steeply into icosahedral medium-range orders (IMROs), especially in the super-cooling liquid region. Moreover, the maximal size [Formula: see text] of IMROs played a control role in the formation of W monatomic MGs rather than the number [Formula: see text]. The mean local fivefold symmetry [Formula: see text] of IMROs was demonstrated to be an effective indicator to characterize the glass transition of monatomic metal W liquids.

Structural relaxation in a Fe-based metallic glass: Changes in local structure and role of heterogeneity

Atomic structure evolution induced by thermal relaxation is investigated in Fe80P11C9 metallic glass by a suite of techniques. The topological structure of Fe80P11C9 metallic glasses characterized by the Voronoi polyhedrons varies with relaxation proceeding. Meanwhile, the results of partial pair distribution functions and local composition distribution indicate that the degree of the chemical order for different species changes and the solvent-solute correlation significantly enhances. Moreover, the Mössbauer spectra measurements suggest that significant element redistribution occurs during the structural relaxation. Hence, only topological structure change is insufficient to describe the structural relaxation. These results manifest that the structural relaxation can be described via heterogeneity degeneration, which is also supported by the small angle X-ray scattering measurements.

Comparative study of free-volume nanostructurization in glassy and crystalline As2S3 re-examined with annihilating positrons

Evolution of free-volume atomic scale structure under crystal-to-glass transition was re-examined in arsenic trisulfide As2S3 in glassy and crystalline state, employing mutually correlated positron annihilation lifetime studies treated in a framework of two-state simple positron trapping model. In crystalline state represented by mineral orpiment As2S3 supplied from Wanshan mine (China), the trapping rate is restricted to ~1.14 ns−1, associated with 0.240 ns defect-free bulk and 0.402 ns defect-specific lifetimes due to free volumes stabilized preferentially within inter-layer spaces. The positron trapping rate is notably enhanced to ~1.51 ns−1 in glassy state presented by melt-quenched As2S3, the positron traps being smaller with decreased bulk (0.278 ns) and defect (0.380 ns) lifetimes due to nanostructurized free volumes stabilized within corrugated quasi-layers. In general, vitrification in As2S3 produces depressing and time-enhancing trend in the recorded PAL spectra peaks. The ratio of bulk positron lifetimes for glassy and crystalline states serving as criterion of vitrification completeness reaches 1.16, meaning that some additional volume is available for annihilation in a glass. The ratio of mean positron lifetimes approaching 1.12 reflects a degree of realized crystal-to-glass transformation. Vitrification-related changes in As2S3 can be imagined as fragmentation of existing free-volume voids due to crumpling of intrinsic inter-layer spaces.

Atomic structure evolution of (CuZr)100-xAgx glass under compression deformation

Molecular dynamics simulations are performed to investigate the evolutions of strain localizations, short-range orders as well as the stability of the interpenetrating connection of icosahedra in (CuZr)100-xAgx (x = 0,10,20,33) glasses during compressive deformation based on the embedded atom method (EAM). Our finding indicates that (CuZr)100-xAgx (x≠0) undercooled liquid exhibits faster dynamics than Cu50Zr50, but stronger chemical heterogeneity. The addition of Ag atoms to the CuZr glass shows better plastic deformation, and the local shear strains distribute more homogeneously with increasing Ag content. Further research indicates that the Ag-centered full icosahedra (FI) are structurally less stable, although the total number of FI and connectivity of FI in multicomponent glasses become larger with increasing Ag content, the network formed by Ag-centered FI are also prone to be destroyed during deformation. These findings may suggest a link between the deformation and structural heterogeneity in ternary glasses.

Second nearest-neighbor modified embedded-atom method interatomic potentials for the Pd-M (M = Al, Co, Cu, Fe, Mo, Ni, Ti) binary systems

Palladium (Pd) has attracted attention as one of the major components of noble metal catalysts due to its excellent reactivity to a wide range of catalytic reactions. It is important to predict and control the atomic arrangement in catalysts because their properties are known to be affected by it. Therefore, to enable atomistic simulations for investigating the atomic scale structural evolution, we have developed interatomic potentials for Pd-M (M = Al, Co, Cu, Fe, Mo, Ni, Ti) binary systems based on the second nearest-neighbor modified embedded-atom method formalism. These potentials reproduce various fundamental properties of the alloys (the structural, elastic and thermodynamic properties of compound and solution phases, and order-disorder transition temperature) in reasonable agreements with experimental data, first-principles calculations and CALPHAD assessments. Herein, we propose that these potentials can be applied to the design of robust bimetallic catalysts by predicting the shape and atomic arrangement of Pd bimetallic nanoparticles.

Phonon Instability and Broken Long-Ranged p Bond in Ge-Sb-Te Phase-Change Materials from First Principles

The evolution of atomic structure in phase-change materials is an essential ingredient for designing next-generation nonvolatile memory devices and memristors, yet has remained elusive for decades. This first-principles study offers quantitative analysis to understand the atomic bonding and dynamics in phase-change materials. Phonon softening initiates the dynamic instability, which triggers atomic disorder, and drastic softening of a specific optical mode with temperature destroys the $p$-bonding network. This deeper understanding of the phase-change process is expected to help in improving device performance, and in developing functional memristors.

Transient metal-centered states mediate isomerization of a photochromic ruthenium-sulfoxide complex

Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. Understanding the coupled evolution of atomic and molecular structure during isomerization is paramount for control and rational design in molecular science. Here we report transient X-ray absorption studies of the photo-induced linkage isomerization of a Ru-based photochromic molecule. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metal-centered excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. These results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization. Global temporal and spectral data analysis combined with time-dependent density functional theory reveals a complex mechanism for photoisomerization with atomic details of the transient molecular and electronic structure not accessible by other means.

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.