Relaxed Atomic Structures Research Articles

AB-INITIO CALCULATIONS OF RHOMBOHEDRAL BaTiO<sub>3</sub> (111) SURFACE COMBINED WITH GRAPHENE FILMS

Thin films of ABO3 perovskite ferroelectrics are important for many industrial applications, i.e., high-capacity memory cells, catalysis, optical waveguides, and integrated optics. The use of BaTiO3 for these industries and products is due to the variety of its surface structure and, accordingly, its electronic and chemical properties. Calculations of the surface characteristics of BaTiO3 from the first principles are useful for understanding processes that play a crucial role, such as surface reaction chemistry, surface phenomena, and adsorption surfaces. This study examined theoretical calculations related to the relaxed atomic structures of the surface of BaTiO3 (111).

Data-driven discovery of 2D materials by deep generative models

Efficient algorithms to generate candidate crystal structures with good stability properties can play a key role in data-driven materials discovery. Here, we show that a crystal diffusion variational autoencoder (CDVAE) is capable of generating two-dimensional (2D) materials of high chemical and structural diversity and formation energies mirroring the training structures. Specifically, we train the CDVAE on 2615 2D materials with energy above the convex hull ΔHhull < 0.3 eV/atom, and generate 5003 materials that we relax using density functional theory (DFT). We also generate 14192 new crystals by systematic element substitution of the training structures. We find that the generative model and lattice decoration approach are complementary and yield materials with similar stability properties but very different crystal structures and chemical compositions. In total we find 11630 predicted new 2D materials, where 8599 of these have ΔHhull < 0.3 eV/atom as the seed structures, while 2004 are within 50 meV of the convex hull and could potentially be synthesised. The relaxed atomic structures of all the materials are available in the open Computational 2D Materials Database (C2DB). Our work establishes the CDVAE as an efficient and reliable crystal generation machine, and significantly expands the space of 2D materials.

Defect levels from SCAN and MBJ meta-GGA exchange-correlation potentials

For more than a decade the HSE06 hybrid exchange-correlation functional developed by Heyd, Scuseria and Ernzerhof has provided a tool for reliable defect level calculations in density functional theory for which postprocessing tools are not necessary, in contrast to previous calculations using semilocal density functionals. One of the main reasons for its reliability is the high precision of HSE06 for band gap calculations. Recently, other functionals from the meta-generalized gradient approximation (meta-GGA) class have been used extensively to calculate the electronic properties of solids. In particular, band gaps can be accurately evaluated with the modified Becke-Johnson (MBJ) potential, and relaxed atomic structures close to experimental findings can be obtained with the strongly constrained and appropriately normed (SCAN) exchange-correlation functional. Both approaches are computationally cheaper than HSE06, and we consider here their performance for defect level calculations. We compare results for the $\ensuremath{\epsilon}(+/0)$ transition levels of seven donors and $\ensuremath{\epsilon}(0/\ensuremath{-})$ transition levels of four acceptors in group IV semiconductors. We conclude that in certain situations where HSE06 cannot be applied because of excessive computational costs, SCAN and MBJ might provide a good alternative.

Influence of interactions between hydrogen and [formula omitted] twin boundary on hydrogen embrittlement in α-Ti

Ti and Ti alloys are used in several areas thanks to their good mechanical properties, however, the large affinity of hydrogen during industrial application would limit their service life. This study serves as a comprehensive investigation of the embrittlement effects induced by additional hydrogen in detail by means of first-principles calculations with the Pseudopotential Plane-wave method within the framework of the Rice-Wang thermodynamic model. The existence of the twin boundary markedly influences segregation energy, the activation barriers for diffusion between sites, and intergranular embrittlement. We demonstrate that segregation energy and the barriers in the region near the boundary are consistently lower than in bulk, indicating that the sites located at the boundary are more attractive to hydrogen than bulk. Further, the intergranular strength of a Ti grain boundary is reduced with hydrogen segregation. An analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the weakening effect of hydrogen is due to the breaking of the interfacial Ti-Ti bonds, which significantly decrease twin boundary adhesion and its resistance to cracking.

Shape change of submicron nickel particles under hydrogen and nickel chloride vapor

The shape of Ni particles is important. A spherical shape is favored for an electrode material in multilayer ceramic capacitor (MLCC) due to high packing density. A cubic shape is better for applications in catalysts and bioseparation due to its higher magnetic property than a spherical shape. It was found that cubic Ni particles which has a face-centered cubic (fcc) crystal structure, can be synthesized by chemical vapor synthesis (CVS). In this study, to examine whether the cubic shape of Ni particles synthesized by CVS is the growth or the equilibrium shape, the shape change of Ni particles by annealing under H2 and NiCl2 atmospheres was observed. It was confirmed by a density function theory (DFT) method that H2 and NiCl2 vapor favor respectively spherical and cubic shapes, which indicates that NiCl2 stabilizes the {1 0 0} faces of Ni. In this computational work, we provide gas adsorption energies and relaxed atomic structures of Ni(1 0 0) and Ni(1 1 1) surface models. Using this fact, suitable synthetic conditions for cubic and spherical Ni particles could be derived. It was confirmed that the high and low reduction rate favor respectively spherical and cubic shapes during CVS.

Characterization of Structural Iron in Smectites - An Ab Initio Based X-ray Absorption Spectroscopy Study.

Fe-bearing clay minerals are abundant in argillaceous rocks as their redox-active structural iron may control the sorption mechanism of redox sensitive elements on the surface of clay minerals. The extent and efficiency of the redox reactions depend on the oxidation state (Fe2+/Fe3+ ratio) and structural distribution of the substituting cations in the TOT-layer of clay minerals. Even smectites with similar structure originating from different locations might have a distinct arrangement of isomorphic substitutions (e.g., individual iron or Fe-Fe pairs). In this study, the proportion of different iron distribution in Milos-, Wyoming-, and Texas-montmorillonite was determined by combining X-ray absorption spectroscopy (XAS) with ab initio calculations. The relaxed atomic structures of the smectite models with different arrangement of individual Fe atoms and Fe-Fe/Fe-Mg clusters served as the basis for the calculations of the XAS spectra. The combination of simulation results and measured Fe K-edge XAS spectra of Wyoming-, Milos- and Texas-montmorillonites suggested that iron is present as Fe3+ in the octahedral sheet. Fe3+ in Texas-montmorillonite has a tendency to form clusters, while no definitive statement about clustering or avoidance of Fe-Fe and Fe-Mg pairs can be made for Milos- and Wyoming-montmorillonite.

An Ab Initio Study of the Structural and Electronic Properties of the Low-Defect TiC(110) Surface Simulating Oxygen Adsorption after Exposure to Laser Plasma

An ab initio simulation of the adsorption of atomic oxygen on the low-defect titanium carbide (110) surface reconstructed by laser radiation was performed. The relaxed atomic structures of the (110) surface of the O/TixCy system with Ti and C vacancies observed during the thermal treatment were studied in terms of the density functional theory. DFT calculations of their structural, thermodynamic, and electronic properties were performed. The bond lengths and adsorption energies were determined for various reconstructions of the atomic structure of the O/TixCy(110) surface. The effects of the oxygen adatom on the band and electronic spectra of the O/TixCy(110) surface were studied. The effective charges on the titanium and carbon atoms surrounding the oxygen atom in various reconstructions were determined. The charge transfer from titanium to oxygen and carbon atoms was found, which is determined by the reconstruction of the local atomic and electronic structures and correlates with chemisorption processes. The potential mechanisms of laser nanostructuring of the titanium carbide surface were suggested.

Crystal structures of Fe4C vs. Fe4N analysed by DFT calculations: Fcc-based interstitial superstructures explored

Knowledge of the thermodynamic and structural properties of iron carbide and nitride phases is crucial for understanding phase transformations and related microstructure formation in steels. While the existence and crystal structure of the primitive cubic fcc-based γ′-Fe4N1-z phase is experimentally well-established, there is no consensus in contemporary literature about an analogous γ′-Fe4C compound. Here, we present DFT calculations for all fcc-like Fe4C and Fe4N superstructures with up to two formula units per primitive unit cell, providing energy values and the relaxed atomic structures, which were analysed mathematically and by visual inspection of the atomic arrangement. Notably, all considered Fe4C and Fe4N superstructures are metastable with respect to α-Fe and cementite-Fe3C/ε-Fe3N. Unsurprisingly, we find the well-known γ′ compound's crystal structure to be most favourable among these metastable Fe4N superstructures. However, we find the equivalent superstructure to be quite unfavourable in Fe4C. The most favourable among these metastable Fe4C structures are stabilised by a partial Bain-like distortion into the direction of a body-centred cubic arrangement of Fe atoms. This makes the particular C-ordering interesting for comparison with the short-range order in Fe-C martensites. However, even the lowest-energy Fe4C structure releases about 0.056 eV/atom upon decomposition into α + Fe3C, much more than it is the case for Fe4N (0.019 eV/atom). That energy difference is difficult to overcome even at T > 0 K, in agreement with the lack of clear experimental evidence for existence of a Fe4C phase.

Effect of oxygen adsorption on structural and electronic properties of defective surfaces (0 0 1), (1 1 1), and (1 1 0) TiC: Ab initio study

A model of the oxygen adsorption on the defective surfaces (001), (111) and (110) of titanium carbide with different reconstructions is investigated by ab-initio calculations. In the framework of DFT calculations, the relaxed atomic structures of the surfaces (001), (111), and (110) of O/TixCy systems with titanium and carbon vacancies are studied. The structural and electronic properties of these systems are also investigated. The bond length and the adsorption energy for different reconstructions of the atomic structure of the surfaces (001), (111), and (110) of O/TixCy systems were established. The influence of the oxygen adatoms on the band structure and the electronic spectra of the O/TixCy system with different reconstructions was studied. Our calculations of effective charge per oxygen and nearest atoms show that the observed charge transfer from the titanium atoms to the oxygen and the carbon atoms is due to the reconstruction of local atomic and electronic structures and it correlates with processes of chemisorption. The physical nature and mechanisms of nanostructuring of the surfaces (001), (111) and (110) of titanium carbide are also discussed.

Adsorption of atomic oxygen, electron structure and elastic moduli of TiC(0 0 1) surface during its laser reconstruction: Ab initio study

We have performed ab initio simulation of oxygen atom adsorption on TiC(001) laser-reconstructed surface. Relaxed atomic structures of the O/TixCy(001) surface observed upon thermal impact have been studied. DFT calculations of their thermodynamic, electronic, and elastic properties have been carried out. For the first time we have established the bond length and adsorption energy for various reconstructions of the O/TixCy(001) surface atomic structure. We have examined the effects of the oxygen adatom upon the band and electron spectra of the O/TiC(001) surface in its various reconstructions. For the first time we have established a correlation between the energy level of flat bands (−5.4eV and −5.8eV) responsible for the doublet of singular peaks of partial densities of oxygen 2p electrons, and the adsorption energy of oxygen atom in non-stoichiometric O/TiCy(001) systems. Effective charges of titanium and carbon atoms surrounding the oxygen adatom in various reconstructions have been identified. We have established charge transfer from titanium atom to oxygen and carbon atoms determined by the reconstruction of local atomic and electron structures which correlate with atomic electronegativity values and chemisorption processes. Potential mechanisms for laser nanostructuring of titanium carbide surface have been suggested.

Atomic Structures of $$ [0\bar{1}10] $$ Symmetric Tilt Grain Boundaries in Hexagonal Close-Packed (hcp) Crystals

Abstract Molecular dynamics simulation and interface defect theory are used to determine the relaxed equilibrium atomic structures of symmetric tilt grain boundaries (STGBs) in hexagonal close-packed (hcp) crystals with a $$ [0\bar{1}10] $$ tilt axis. STGBs of all possible rotation angles θ from 0 deg to 90 deg are found to have an ordered atomic structure. They correspond either to a coherent, defect-free boundary or to a tilt wall containing an array of distinct and discrete intrinsic grain boundary dislocations (GBDs). The STGBs adopt one of six base structures, $$ P_{B}^{(i)} $$ , i = 1, …, 6, and the Burgers vector of the GBDs is related to the interplanar spacing of the base structure on which it lies. The base structures correspond to the basal plane (θ = 0 deg, $$ P_{B}^{(1)} $$ ); one of four minimum-energy, coherent boundaries, $$ (\bar{2}111),\;(\bar{2}112),\;(\bar{2}114) $$ , and $$ (\bar{2}116)\;\left( {P_{B}^{(2)} - P_{B}^{(5)} } \right) $$ ; and the $$ \left( {11\bar{2}0} \right) $$ plane (θ = 90 deg, $$ P_{B}^{(6)} $$ ). Based on these features, STGBs can be classified into one of six possible structural sets, wherein STGBs belonging to the same set i contain the same base boundary structure $$ P_{B}^{(i)} $$ and an array of GBDs with the same Burgers vector $$ b_{\text{GB}}^{(i)} $$ , which vary only in spacing and sign with θ. This classification is shown to apply to both Mg and Ti, two metals with different c/a ratios and employing different interatomic potentials in simulation. We use a simple model to forecast the misorientation range of each set for hcp crystals of general c/a ratio, the predictions of which are shown to agree well with the molecular dynamics (MD) simulations for Mg and Ti.

First-principles determination of the effect of boron on aluminum grain boundary cohesion

Despite boron being a common alloying element in aluminum, its segregation into the aluminum grain boundary and its effect on the grain boundary strength have not been studied. Here, the electronic structures of the boron-doped $\ensuremath{\Sigma}5(012)[100]$ symmetrical tilt grain boundary and (012) free surface systems for aluminum are investigated by means of first-principles calculations using the full-potential linearized augmented plane-wave method with the generalized gradient approximation, within the framework of the Rice-Wang thermodynamic model and the theoretical tensile test approach. We establish that boron has a large driving force to segregate from Al bulk to the symmetrical grain boundary hollow site, and its segregation significantly enhances the grain boundary strength. Through precise calculations on both the grain boundary and free surface environments, it is found that boron is a strong cohesion enhancer in aluminum with a potency of \ensuremath{-}0.19 eV/atom. An analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the aluminum-boron bond has mixed covalent and metallic character and is strong in both grain boundary and free surface environments. The strengthening effect of boron is due to creation of additional B--Al bonds across the grain boundary, which are as strong as existing Al--Al transgranular bonds and thus significantly increase grain boundary adhesion and its resistance to tensile stress and cracking.

First principles investigation of zinc-induced embrittlement in an aluminum grain boundary

Aluminum–zinc alloys have an extensive range of structural applications, but their susceptibility to intergranular stress corrosion cracking limits their wider use. In the present work the influence of zinc on a ∑5(0 1 2)[1 0 0] aluminum grain boundary was investigated by means of first principles calculations with the full potential linearized augmented plane wave method using two approaches: (i) within the framework of the Rice–Wang thermodynamic model; (ii) by the ab initio tensile test method. We determined the most energetically favorable segregation site of Zn along the Al grain boundary, its segregation energy from the Al grain boundary, the possible fracture paths of the grain boundary with Zn and the corresponding fracture energies. We established that Zn has a large driving force (−0.19 eV atom −1) for segregation from the Al bulk to the asymmetrical grain boundary site, and its segregation reduces the grain boundary strength slightly. Unusually, segregation of larger atomic size Zn leads to grain boundary contraction. Through precise calculations it was confirmed that zinc is a weak embrittler with a potency of +0.05 eV atom −1. Analysis in terms of the relaxed atomic and electronic structures and bonding characters showed that aluminum–zinc bonds have a metallic character in both grain boundary and free surface environments. This work provides a fundamental quantitative understanding of Zn-induced grain boundary embrittlement in Al alloys on the electronic level and establishes that Zn segregation is one of the factors contributing to stress corrosion cracking in Al alloys.

Sodium-induced embrittlement of an aluminum grain boundary

Primary aluminum produced industrially by electrolysis inevitably contains some sodium which is an undesired impurity element in aluminum alloys as it promotes intergranular fracture. However, the physical origins of Na-induced intergranular embrittlement in aluminum are still unclear. This work provides a comprehensive investigation of the nature of the Na-induced grain-boundary embrittlement in aluminum by means of first-principles calculations with the highly precise full-potential linearized augmented plane-wave method within the framework of the Rice-Wang thermodynamic model and within the method of ab initio tensile test. We introduce a free-surface slab model and determine the grain-boundary and free-surface energies, the most energetically favorable segregation site of Na along the Al grain boundary, its segregation energy to the Al grain boundary, and the possible fracture modes of the grain boundary with Na in the different sites and their corresponding fracture energies. We establish that Na has a large driving force $(\ensuremath{-}0.84\text{ }\text{eV}/\text{atom})$ to segregate from Al bulk to the symmetrical grain-boundary core site, and its segregation significantly reduces grain-boundary strength. We show that the method using the Rice-Wang thermodynamic model and the method of ab initio tensile test are essentially equivalent and both confirmed that Na is a strong intergranular embrittler with a potency of $+0.62\text{ }\text{eV}/\text{atom}$. Na segregation leads to grain-boundary expansion and a significant charge density decrease over the whole grain boundary. Analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the aluminum-sodium bond has ionic character and is weak in both grain-boundary and free-surface environments.

Structures and phonon properties of nanoscale fractional graphitic structures in amorphous carbon determined by molecular simulations

Although amorphous carbon (a-C) materials are being widely used, relaxed atomic structures of a-C have not yet been investigated in detail. In this study, a-C structures were relaxed in molecular simulations, and their structural properties and phonon properties were investigated. As a result, several nanoscale fractional graphitic structures were observed in the annealed a-C structures. Further, it was found that the fractional graphitic structures caused a peak in the fractional phonon density of states of the annealed a-C structures, which corresponded to the D peak. The main phonon mode in the fractional graphitic structure with phonon frequencies of the D peak position and that of the G peak position were the distortion mode of six-membered rings, and the stretching mode of the bonds between threefold coordinated atoms, respectively. Both the distortion mode of six-membered rings and the bond-stretching mode were observed in phonon frequencies between the D peak position and the G peak position.

Electronic and structural properties of Ti vacancies on the (001) surface of TiS2: Theoretical scanning tunneling microscopy images

Various defects--either bright or dark triangular defects--are observed on the (001) titanium disulfide surface by ultrahigh vacuum scanning tunneling microscopy. The experimental interpretations of the images available in the literature suggest that a fraction of Ti atoms could be displaced from the octahedral site they occupied to vacant sites of the crystal structure, leading to more or less correlated defects. In this paper, the authors have performed ab initio periodic linear combination of atomic orbitals-generalized gradient approximation (LCAO-GGA) calculations on (5x5) and (4x4) biperiodic supercells to model the electronic and geometrical involvements of Ti vacancy. The relaxed atomic structures of each system and the wave-function character of the defect states are carefully analyzed before the theoretical scanning tunneling microscopy images are generated within the Tersoff-Hamann approximation. The relaxed structure of the Ti vacancy shows an inward movement of the neighboring sulfur atoms at the surface. However, the occupied electronic states of the vacancy at the Fermi level are mainly developed on the atomic orbitals of the first sulfur neighbors at the surface, leading to bright triangular zones on the simulated image.

Electronic transport through Si nanowires: Role of bulk and surface disorder

We calculate the resistance and mean free path in long metallic and semiconducting silicon nanowires (SiNW's) using two different numerical approaches: a real-space Kubo method and a recursive Green's-function method. We compare the two approaches and find that they are complementary: depending on the situation a preferable method can be identified. Several numerical results are presented to illustrate the relative merits of the two methods. Our calculations of relaxed atomic structures and their conductance properties are based on density functional theory without introducing adjustable parameters. Two specific models of disorder are considered: Unpassivated, surface reconstructed SiNW's are perturbed by random on-site (Anderson) disorder whereas defects in hydrogen passivated wires are introduced by randomly removed H atoms. The unpassivated wires are very sensitive to disorder in the surface whereas bulk disorder has almost no influence. For the passivated wires, the scattering by the hydrogen vacancies is strongly energy dependent and for relatively long SiNW's $(L&gt;200\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ the resistance changes from the Ohmic to the localization regime within a $0.1\text{\ensuremath{-}}\mathrm{eV}$ shift of the Fermi energy. This high sensitivity might be used for sensor applications.

First Principles Modeling of Tunnel Magnetoresistance ofFe/MgO/FeTrilayers

We report ab initio calculations of nonequilibrium quantum transport properties of Fe/MgO/Fe trilayer structures. The zero bias tunnel magnetoresistance is found to be several thousand percent, and it is reduced to about 1000% when the Fe/MgO interface is oxidized. The tunnel magnetoresistance for devices without oxidization reduces monotonically to zero with a voltage scale of about 0.5-1 V, consistent with experimental observations. We present an understanding of the nonequilibrium transport by investigating microscopic details of the scattering states and the Bloch bands of the Fe leads.

Geometric Simulation of Perovskite Frameworks with Jahn-Teller Distortions: Applications to the Cubic Manganites

A new approach is presented for modeling perovskite frameworks with disordered Jahn-Teller (JT) distortions and has been applied to study the elastic response of the LaMnO3 structure to defects in the JT ordering. Surprisingly, antiphase domain boundary defects in the pattern of ordered JT octahedra, along the [110] and [110] bonding directions, are found to produce 1D stripe patterns rotated 45 degrees along a* directions, similar to stripe structures observed in these systems. Geometric simulation is shown to be an efficient and powerful approach for finding relaxed atomic structures in the presence of disorder in networks of corner-shared JT-distorted octahedra such as the perovskites. Geometric modeling rapidly relaxes large supercells (thousands of octahedra) while preserving the local coordination chemistry, and shows great promise for studying these complex systems.

Optical properties of Si and Ge nanocrystals: Parameter‐free calculations

AbstractAn overview is given on the electronic and optical properties of Si and Ge nanocrystals. We model free‐standing Si and Ge nanocrystals passivated with H, with up to 363 group‐IV atoms and 276 H and oxidized nanocrystals with additional silicon oxide shells, e.g. Si41(O60Si42)(O108Si64)H148. The results discussed are based on parameter‐free calculations for relaxed atomic structures. The electronic states and the many‐body effects are considered within the density‐functional theory and the supercell method. Energy gaps are described within the Kohn–Sham independent‐particle picture and within the independent‐quasiparticle picture, whereas electronic excitations are treated within the delta self‐consistent method by pair excitation energies. The projector‐augmented wave method allows the calculation of optical matrix elements, oscillator strengths, optical spectra, and radiative lifetimes. We discuss quantum confinement, structural relaxation, oxidation, and the role of defects in oxidized Si NCs. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)