Atomic Structure Properties Research Articles

Microstructural and mechanical characterization of quasicrystalline Al-Cu-Fe foams

Quasi-crystalline metal foams are considered a new class of materials, as they differ fundamentally from conventional materials in their atomic structure and mechanical properties. Their manufacturing methods are under constant development. In this article, the microstructural characterization and the mechanical behavior of quasicrystalline foams obtained via slow solidification and heat treatment with the presence of molten metal is presented. The studied Al63Cu28Fe9 alloy was thermally treated at 800 °C for short times from 5 to 15 min and for longer times of 180 and 360 min in order to promote the formation of a metal structure constituted by icosahedral phase with a high percentage of porosity. The microstructural characterization was performed by using the X-ray diffraction (XRD) technique, scanning electron microscopy (SEM) and optical microscopy (OM). The thermal properties of the as-cast alloy and the changes in the density of the thermally treated samples were characterized by a differential thermal analysis (DTA) and Archimedes method, respectively. The highest percentage of macroporosity (40%) was obtained in the heat-treated sample at 800 °C for 360 min. Under these heat treatment conditions, a completely porous structure (foam) mainly constituted by quasicrystalline icosahedral phase was formed. The maximum compressive stress value and average plateau stress reached for the alloy heat-treated during 360 min were 80 MPa and 30 MPa, respectively.

Особенности атомного строения и электронных свойств гибридных пленок, образованных одностенными углеродными нанотрубками и бислойным графеном

Особенности атомного строения и электронных свойств гибридных пленок, образованных одностенными углеродными нанотрубками и бислойным графеном

N‐Doped and P‐Doped Graphene on MgO (111): A First‐Principles Study

Herein, first‐principles density functional theory is used to investigate the atomic structure and electrical properties of the contact between primitive graphene, N‐doped graphene, P‐doped graphene, and MgO (111) surface. It has been found that the energy band of the graphene/Mg‐terminated MgO (111) interface exhibits p‐type characteristics, and the energy band of the graphene/O‐terminated MgO (111) interface exhibits n‐type characteristics. N‐doped or P‐doped can control the energy band structure of the interface. The interaction between the graphene and Mg‐terminated MgO (111) interface is a weak physical interaction, and the interaction between the graphene and O‐terminated MgO (111) surface is a strong chemical interaction. N‐doped or P‐doped can promote the charge transfer between the interfaces, and further enhancing the interfacial coupling strength. This result is helpful to understand the interaction between graphene and MgO interface and provides theoretical guidance for the practical application of graphene/MgO interface‐based devices.

Transition from high-entropy to conventional (TiZrNbCu)1−xCox metallic glasses

In this article, we describe the characterization of a newly fabricated amorphous alloy system (TiZrNbCu)1−xCox covering a broad composition range from high-entropy (HEA) to Co-rich alloys (x ≤ 0.43). We investigated thermal stability, atomic and electronic structure, and magnetic and mechanical properties as a function of chemical composition x. One of the important findings is that all studied properties change their dependence on concentration x within the HEA range. In particular, it has been found that the average atomic volume deviates from Vegard’s law for x > 0.2, the concentration for which the average atomic packing fraction suddenly changes. The valence band structure, studied with ultraviolet photoemission spectroscopy, shows a split-band shape with 3d-states of Co approaching the Fermi level on increasing x. Due to the onset of magnetic correlations, magnetic susceptibility rapidly increases for x > 0.25. Very high microhardness increases rapidly with x. The results are compared with those for similar binary and quinary metallic glasses and with those for Cantor type of crystalline alloys.

Determination of the atomic structure and radiative transition properties of atoms or ions under the dense and solid density magnetized plasmas

Simulating the magnetic field interactions with plasmas plays a paramount role in astrophysics, having significant impacts in several topics. Here, we propose a novel computational method within the framework of relativistic theory to the determination of the atomic structures and radiative properties of atoms or ions in presence of the dense and solid density magnetized plasma environments. In the method, the dense plasma effects on the atomic structure of highly charged ions are described by using the generalized b-potential model within the general framework of the ion sphere theory, which is somewhat simple and flexible and allows adjusting the b-parameter to consider finite temperature and density as well as solid density plasma environments. The weakly magnetic field effects are described using the perturbation term, which perturbs the Hamiltonian of the field-free case. Its special features include handling the effects of both electron correlation and interaction with the weakly magnetic field within the configuration interaction approximation. As a practical application, we present a systematic study of the plasma screening and magnetic field effects, separately and combined, on the structure and radiative properties of highly charged ions subjected to the magnetized plasma, using the atrophysically relevant He-like Mg10+ ion as an example. A comparison of our calculations with other theories, when available, is made. The present work not only originates a better understanding of the fundamentals of structure and radiative properties of ions in the presence of an external field, but also has some important relevance to applications in laboratory and space plasmas, particularly in solar corona, fusion, laser-produced plasmas, ect.

Effects of alloying elements on the atomic structure, elastic and thermodynamic properties of L12-Co3(V, Ti) compound

In this work, first-principles calculations were used to study the effects of alloying elements on the atomic structure, elastic property and thermodynamic property of L12 ordered γ′-Co3(V, Ti) at finite temperature. Twenty transition metals (TM), including Sc, Cr, Mn, Fe, Ni, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt, have been considered as the alloying element at different crystallographic sites. The results show that Mn, Fe, Ru, Os, Rh, Ir, Ni, Pd, and Pt tend to occupy the Co site, while other elements prefer the Ti site of γ′-Co3(V, Ti). In the ground state, all TMs except Cr, Mn, Fe, Y, Tc and Ru could stabilize the γ′-Co3(V, Ti) compound, and the addition of Sc, Fe, Y, Rh, Pd, Ir and Pt can remain the compound a stable phase after involving its competitive phases. The mechanical property of TM-substituted γ'-Co3(V, Ti) was found to be linearly related to both the value of the electron density and the molar volume of compound. However, at finite temperature, only Sc, Y, and Os enhance the thermodynamic stability of the γ'-Co3(V, Ti) with the increasing temperature, which is different from the cases in the ground state.

Unraveling the Structural Statistics and Its Relationship with Mechanical Properties in Metallic Glasses.

Metallic glasses exhibit excellent properties such as ultrahigh strength and excellent wear and corrosion resistance, but there is limited understanding on the relationship between their atomic structure and mechanical properties as a function of their structural state. In this paper, we bridge the processing-structure-property gap by utilizing molecular dynamics simulation for a model binary metallic glass, namely Ni80P20. The structural statistics including the fraction of Voronoi index, the distribution of Voronoi volume, and medium-range ordering are calculated to explain the observed changes in mechanical behavior and strain localization upon relaxation and rejuvenation. Our findings demonstrate that the evolution of mechanical properties can be linked to the atomic structure change in terms of short- and medium-range ordering. With the help of structural statistics, the mechanical properties are determined based on simple Voronoi analysis.

Continuous illumination picosecond imaging using a delay line detector in a transmission electron microscope

Progress towards analysing transitions between steady states demands improvements in time-resolved imaging, both for fundamental research and for applications in information technology. Transmission electron microscopy is a powerful technique for investigating the atomic structure, chemical composition and electromagnetic properties of materials with high spatial resolution and precision. However, the extraction of information about dynamic processes in the ps time regime is often not possible without extensive modification to the instrument while requiring careful control of the operation conditions to not compromise the beam quality. Here, we avoid these drawbacks by combining a delay line detector with continuous illumination in a transmission electron microscope. We visualize the gyration of a magnetic vortex core in real space and show that magnetization dynamics up to frequencies of 2.3 GHz can be resolved with down to ∼122ps temporal resolution by studying the interaction of an electron beam with a microwave magnetic field. In the future, this approach promises to provide access to resonant dynamics by combining high spatial resolution with sub-ns temporal resolution.

Transition from High-Entropy to Conventional Alloys: Which Are Better?

The study of the transition from high-entropy alloys (HEAs) to conventional alloys (CAs) composed of the same alloying components is apparently important, both for understanding the formation of HEAs and for proper evaluation of their potential with respect to that of the corresponding CAs. However, this transition has thus far been studied in only two types of alloy systems: crystalline alloys of iron group metals (such as the Cantor alloy and its derivatives) and both amorphous (a-) and crystalline alloys, TE-TL, of early (TE = Ti, Zr, Nb, Hf) and late (TL = Co, Ni, Cu) transition metals. Here, we briefly overview the main results for the transition from HEAs to CAs in these alloy systems and then present new results for the electronic structure (ES), studied with photoemission spectroscopy and specific heat, atomic structure, thermal, magnetic and mechanical properties of a-TE-TL and Cantor-type alloys. A change in the properties of the alloys studied on crossing from the HEA to the CA concentration range mirrors that in the ES. The compositions of the alloys having the best properties depend on the alloy system and the property selected. This emphasizes the importance of knowing the ES for the design of new compositional complex alloys with the desired properties.

Influence of Cation Size on the Local Atomic Structure and Electronic Properties of Ta Perovskite Oxynitrides.

Partial anion substitution in transition metal oxides provides rich opportunities to control and tune physical and chemical properties, for example, combining the merits of oxides and nitrides. In addition, the possibility of resulting anion sublattice order provides a means to target polar and chiral structures based on a wide array of accessible structural archetypes by design. Here, we investigate the local structures of a family of perovskite tantalum oxynitrides-ATaO2N (A = Ba, Sr, and Ca)-using a combination of experimental and theoretical approaches including neutron total scattering, density functional theory (DFT), and ab initio molecular dynamics (AIMD) simulations. We present the first experimental study of chemical short-range order (CSRO) in CaTaO2N, confirming local cis N ordering of the anion sub-lattice. Our systematic exploration of a local structure across the A cation size series (from the larger Ba to the smaller Ca) reveals a perovskite motif increasingly distorted with respect to long-range average structures. DFT and AIMD simulations support the observed trends. Overall, structures with cis ordering of the nitrogen anions in each TaO4N2 octahedron are favored over those with trans ordering. With diminishing A cation size, local cis ordering and Ta off-centering play decreasing roles in overall lattice stability, overshadowed by the stabilizing effects of octahedral tilting. The influence of these factors on local dipole formation and frustrated dipole ordering are discussed.

Structure and properties of alkali aluminosilicate glasses and melts: Insights from deep learning

Aluminosilicate glasses and melts are of paramount importance for geo- and materials sciences. They include most magmas, and are used to produce a wide variety of everyday materials, from windows to smartphone displays. Despite this importance, no general model exists with which to predict the atomic structure, thermodynamic and viscous properties of aluminosilicate melts. To address this, we introduce a deep learning framework, ‘i-Melt’, which combines a deep artificial neural network with thermodynamic equations. It is trained to predict 18 different latent and observed properties of melts and glasses in the K2O-Na2O-Al2O3-SiO2 system, including configurational entropy, viscosity, optical refractive index, density, and Raman signals. Viscosity can be predicted in the 100–1015 log10 Pa·s range using five different theoretical frameworks (Adam-Gibbs, Free Volume, MYEGA, VFT, Avramov-Milchev), with a precision equal to, or better than, 0.4 log10 Pa·s on unseen data. Density and optical refractive index (through the Sellmeier equation) can be predicted with errors equal or lower than 0.02 and 0.006, respectively. Raman spectra for K2O-Na2O-Al2O3-SiO2 glasses are also predicted, with a relatively high mean error of ∼25% due to the limited data set available for training. Latent variables can also be predicted with good precisions. For example, the glass transition temperature, Tg, can be predicted to within 19 K, while the melt configurational entropy at the glass transition, Sconf(Tg), can be predicted to within 0.8 J mol−1 K−1.Applied to rhyolite compositions, i-Melt shows that the rheological threshold separating explosive and effusive eruptions correlates with an increase in the fraction of non-bridging oxygens in rhyolite melts as their alkali/Al ratio becomes larger than 1. Exploring further the effect of the K/(K + Na) ratio on the properties of alkali aluminosilicate melts with compositions varying along a simplified alkali magmatic series trend, we observe that K-rich melts have systematically different structures and higher viscosities compared to Na-rich melts. Combined with the effects of the K/(K + Na) ratio on other parameters, such as the solubility, solution mechanisms and speciation of volatile elements, this could ultimately influence the eruptive dynamics of volcanic systems emitting Na-rich or K-rich alkali magmas.

Selective synthesis of Ni12P5 and Ni2P nanoparticles: Electronic structures, magnetic and optical properties

Nickel phosphide particles (Ni12P5 and Ni2P) of about 10 nm were successfully synthesized by a solvothermal route through controlling reaction time, and their chemical and atomic structure, magnetic and optical properties were respectively investigated through X-ray diffraction (XRD), transmission electron microscope (TEM), Vibration sample magnetometer (VSM) and UV–visible absorption spectra (UV–Vis). Furthermore, First-principles calculations based on density functional theory were used to conduct for systematically understanding the differences in magnetic, optical properties, chemical and atomic structure between the two kinds of Nickel phosphide particles. Therein, the electronic structures and optical properties of Ni12P5 and Ni2P are investigated in the theoretical framework of the pseudopotential plane-wave First-principles scheme. The electronic bonding characteristics are also analyzed according to the electron localization function and electron density difference. The optical properties including energy loss function, refractive index, extinction coefficient, reflection spectrum and absorption coefficient are evaluated from the calculations of complex dielectric spectra. The quite discrepant crystalline structures of two nickel phosphides lead to semi-metal and semiconductor band structures, respectively resulting in the continuous and cut-off characteristics of optical transition spectra and photocatalytic properties. The present results show that the nickel phosphides with different crystalline structures have different optical properties.

Advances in scientific literature mining for interpreting materials characterization

Using synchrotron light sources, such as the National Synchrotron Light Source II at Brookhaven National Laboratory, scientists in fields as diverse as physics, biology, and materials science, identify the atomic structure, chemical composition, or other important properties of varied specimens. x-ray spectroscopy from light sources is particularly valuable for materials research with vast information available about reference spectra in the scientific literature. However, as the technique is applicable to many science domains, searching for information about select x-ray spectroscopy spectra is impeded by the sheer number of publications. Moreover, useful information about the context of an experiment or figures presented in papers can be buried among the details, which takes time to assess. This work presents a scientific literature mining system that supports data acquisition, information extraction, and user interaction for referencing x-ray spectra identification and spectral interpretation. The goal is to provide efficient access to useful spectral data to researchers who may spend only a few days at a synchrotron light source. With this system, users browse a classification tree for papers arranged according to x-ray spectroscopic methods, chemical elements, and x-ray absorption spectroscopy edges. Relevant figures are extracted with sentences from the paper that explain them, known as ‘figure explanatory text.’ Notably, this system focuses on semantic aspects (logical analysis) to find figure explanatory text using deep contextualized word embeddings techniques and contains an interface to obtain labeled data from domain experts that is used to evaluate and improve the model.

Substrate effect on the electronic properties of graphene on vicinal Pt(1 1 1)

The atomic structure and electronic properties of monolayer graphene on a curved multi-nano-vicinal Pt (111) substrate are investigated with a low-energy electron diffraction, scanning tunneling microscopy and angle-resolved photoemission spectroscopy. Despite graphene grows continuously over the terraces and step bunching areas, the spatial periodicity also varies by varying the vicinal angle of the substrate. This superperiodicity has been evidenced by splitting of first order Pt spots in LEED and from STM. The photoemission spectroscopy unravels that the linearly dispersing π band of graphene is affected by the step periodicity as evidenced by the opening of bandgaps. The gaps opening occurs at the intersection of the main graphene band with Umklapp bands due to the superperiodicity of the one-dimensional nanostructured substrate. The energy and momentum locations of the minigaps change with the superperiodicity, which is related to the spatial periodicity and vicinal angle of the substrate. Our results show a simple way to tune the electronic properties of epitaxial graphene by tuning the substrate vicinality.

Band bending at heterovalent interfaces: Hard X-ray photoelectron spectroscopy of GaP/Si(0 0 1) heterostructures

GaP is a preferred candidate for the transition between Si and heterogeneous III-V epilayers as it is nearly lattice-matched to Si. Here, we scrutinize the atomic structure and electronic properties of GaP/Si(0 0 1) heterointerfaces utilizing hard X-ray photoelectron spectroscopy (HAXPES). GaP(0 0 1) epitaxial films with thicknesses between 4 and 50 nm are prepared by metalorganic vapor phase epitaxy on either predominantly single-domain (SD) or two-domain (TD) Si(0 0 1) surfaces. The antiphase domain content in the GaP films is in situ controlled, employing reflection anisotropy spectroscopy. Via the analysis of core level photoelectron intensities, we reveal core level shifts of the P 2p and Si 2p peaks near the interface as well as core level shifts in the Ga 3d peaks near the surface. We suggest an Inter-Diffused Layer (IDL) model of the GaP/Si(0 0 1) interfacial structure with SiP bonds at the heterointerface and residual P atoms in the Si substrate. Using a newly developed Parametrized Polynomial Function (PPF) approach, we derive a non-monotonic band bending profile in the heterostructures, correct experimental valence band offsets implying interfacial electronic barriers, and determine valence band discontinuities of ΔEV= 1.1 ± 0.2 eV (SD samples) and ΔEV= 0.8 ± 0.2 eV (TD samples) at GaP/Si(0 0 1) interfaces.

Investigation on Atomic Structure and Mechanical Property of Na- and Mg-Montmorillonite under High Pressure by First-Principles Calculations

Montmorillonite is an important layered phyllosilicate material with many useful physicochemical and mechanical properties, which is widely used in medicine, environmental protection, construction industry, and other fields. In order to a get better understanding of the behavior of montmorillonite under high pressure, we studied its atomic structure, electronic and mechanical properties using density functional theory (DFT), including dispersion corrections, as function of the interlayer Na and Mg cations. At ideal condition, the calculations of lattice constants, bond length, band structure, and elastic modulus of Na- and Mg-montmorillonite are in good agreement with the experimental values. Under high pressure, the lattice constants and major bond lengths decreased with increasing pressure. The calculated electronic properties and band structure show only a slight change under 20 GPa, indicating that the effect of pressure on the electronic properties of Na- and Mg-montmorillonite is weak. The bulk modulus, shear modulus, Young’s modulus, shear wave velocity and compression wave velocity of Na- and Mg-montmorillonite are positively correlated with the external pressure, and the other mechanical parameters have a little change. The calculated studies will be useful to explore experiments in the future from a purely scientific point of view.

Radiative properties of quantum emitters in boron nitride from excited state calculations and Bayesian analysis

Point defects in hexagonal boron nitride (hBN) have attracted growing attention as bright single-photon emitters. However, understanding of their atomic structure and radiative properties remains incomplete. Here we study the excited states and radiative lifetimes of over 20 native defects and carbon or oxygen impurities in hBN using ab initio density functional theory and GW plus Bethe-Salpeter equation calculations, generating a large data set of their emission energy, polarization and lifetime. We find a wide variability across quantum emitters, with exciton energies ranging from 0.3 to 4 eV and radiative lifetimes from ns to ms for different defect structures. Through a Bayesian statistical analysis, we identify various high-likelihood charge-neutral defect emitters, among which the native VNNB defect is predicted to possess emission energy and radiative lifetime in agreement with experiments. Our work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to characterize and identify quantum emitters in 2D materials.

Structural defects in transition metal dichalcogenide core-shell architectures

Curvature presents a powerful approach to design atomic structure and tailor material properties in atomically thin transition metal dichalcogenides (TMDs). The emerging TMD core-shell architecture, in which a multilayer TMD shell encapsulates a curved nanoparticle core, presents the opportunity to controllably induce defects into a TMD crystal by strategically constructing the shape of the underlying core. However, harnessing this potential platform first requires robust characterization of the unique structural features present in the core-shell architecture. To this end, transmission electron microscopy (TEM) and scanning TEM (STEM) are particularly powerful tools for direct structural characterization of 2D materials with a high spatial resolution and precision. Here, we reveal and describe defects inherently present in the TMD core-shell architecture. We develop a comprehensive framework to classify the observed defects and discuss potential origins and implications of structural variations. We utilize high resolution S/TEM to reveal the relationship between defects and their associated strain fields. Furthermore, we demonstrate that TMD shells often possess a wide range of interlayer spacings with varied spatial distribution. By exploring the rich array of structural defects inherently present in the TMD core-shell architecture, we provide an important foundation to ultimately induce exotic properties in TMDs through sophisticated defect engineering.

New frontiers in extreme conditions science at synchrotrons and free electron lasers

Synchrotrons and free electron lasers are unique facilities to probe the atomic structure and electronic properties of matter at extreme thermodynamical conditions. In this context, ‘matter at extreme pressures and temperatures’ was one of the science drivers for the construction of low emittance 4th generation synchrotron sources such as the Extremely Brilliant Source of the European Synchrotron Radiation Facility and hard x-ray free electron lasers, such as the European x-ray free electron laser. These new user facilities combine static high pressure and dynamic shock compression experiments to outstanding high brilliance and submicron beams. This combination not only increases the data-quality but also enlarges tremendously the accessible pressure, temperature and density space. At the same time, the large spectrum of available complementary x-ray diagnostics for static and shock compression studies opens unprecedented insights into the state of matter at extremes. The article aims at highlighting a new horizon of scientific opportunities based on the synergy between extremely brilliant synchrotrons and hard x-ray free electron lasers.

Effects of oxygen on local atomic order and diffusion properties in Al-Ni glass-forming liquids

Oxygen has long been a concern in metallic glass-forming liquids since a minor addition can have a significant influence on liquid properties and glass formation, especially by inducing crystallization. However, how exactly oxygen affects the atomic structure and liquid properties on atomic level remains unclear to date. Here, by using extensive reactive molecular dynamics simulations, we conduct a comparative study of the atomic structures and diffusion behavior in both Al-Ni and its oxygen-containing liquids. We find that the oxidation occurs with the formation of extremely stable oxide clusters with the bonding preference between Ni and O atoms. The presence of the oxide clusters results in an increase in free volumes around them. It is the covalent bonding as well as the local excess volumes around the oxides that lead to changes in diffusion coefficients. The diffusion coefficients follow the Arrhenius relation at high temperatures, but a dynamical crossover happens above the glass transition temperature. We show that the diffusivity still correlates well with the free volume as predicted by Cohen and Turnbull although the effect of oxide clusters is not considered. This fundamental discrepancy may challenge our current understanding of liquid properties.