Vienna Ab Initio Simulation Package Research Articles

Adsorption of 2-Butanone on Pristine Graphene: A First-principles Study

The adsorption mechanism of 2-butanone (ethyl methyl ketone) on the surface of graphene is investigated by using Density Functionals Theory (DFT). A 2-butanone molecule is chosen as a selected example of main volatile organic compounds (VOCs) in exhaled breath. To describe the absorption of 2-butanone and graphene substrate, we have performed DFT simulations including van de Waals (vdW) interactions implemented in the Vienna Ab-initio Simulation Package (VASP). The global minimum energy configurations and binding energies for a 2-butanone molecule adsorbed on graphene are determined by using Computational DFT-based Nanoscope tool for imaging the binding possibility of the adsorbed molecules on the graphene surface. The adsorption energy profiles are calculated by three functionals of van der Waals interactions: revPBE-vdW, optPBE-vdW, and vdW-DF2. It is shown that the adsorption energy is highly sensitive to the vdW functionals. The fundamental insights of the interactions between 2-butanone and graphene through molecular doping, i.e., charge transfer are discussed in detail.

First-principles study of the electronic properties of graphene nanostructures for high current density cathodes

Graphene is a crystalline allotrope of carbon with 2D properties. Its carbon atoms are densely packed in a nanoscale hexagonal pattern. Graphene has many unusual properties. In this study, the authors study the electronic properties of graphene nanostructures using first-principles or ab initio calculations based on density functional theory as implemented in the Vienna ab initio simulation package in order to explore its applications in field-emission devices. The density of states and work function of graphene nanoribbons are calculated. The work function value is a key parameter in determining the field emission from a cathode surface according to the Fowler–Nordheim theory. For practical applications, the work functions of graphene nanoribbons with different widths and terminating edges, with and without passivation, have been investigated. Specifically, with the decoration of different alkali and alkaline earth metal species, the reduction of the work function has been systematically studied and determined for achieving higher current density emission.

Molecular and dissociative adsorption of CO and SO on the surface of Ir(111)

This study investigates the molecular and dissociative adsorption of CO and SO molecules on the perfect and a defective Ir(111) surface. It is aimed at providing a broad spectrum of adsorption sites in terms of coordination of Ir atoms and investigating the role of surface defects on the adsorption of small molecules on the surface Ir(111). First-principles density functional theory (DFT) simulation with the generalized gradient approximation as it is implemented in Vienna ab initio simulation package has been employed for this study. Preferred adsorption sites, adsorption energies, and surface electronic structures of CO and SO molecules on the perfect and defective Ir(111) surfaces were calculated to obtain a systematic understanding on the nature of adsorption and dissociative interactions. The DFT calculation reveals the possible molecular adsorption of CO on both perfect and defective Ir(111) surface by the end-on manner (CO bond perpendicular to the surface); the later surface is found to be energetically more favorable. However, no dissociative adsorption was obtained. For SO molecule, on the other hand, both molecular and dissociative adsorption was observed. The defective surface is now less favorable in terms of adsorption energy, but yields stronger activation of SO. The nudged elastic band method investigation also reveals that the Pt single-atom catalysis significantly reduces (up to 80% reduction) the energy barrier of the dissociative adsorption of SO. The electronic structure calculation reveals that all the adsorptions investigated in this study involve hybridization of different electronic states.

3DStructGen: an interactive web-based 3D structure generation for non-periodic molecule and crystal

BackgroundThe increasing number of organic and inorganic structures promotes the development of the “Big Data” in chemistry and material science, and raises the need for cross-platform and web-based methods to search, view and edit structures. Many web-based three-dimensional (3D) structure tools have been developed for displaying existing models, building new models, and preparing initial input files for external calculations. But few of these tools can deal with crystal structures.ResultsWe developed a user-friendly and versatile program based on standard web techniques, such as Hyper Text Markup Language 5 (HTML5), Cascade Style Sheet (CSS) and JavaScript. Both non-periodic organic molecule and crystal structure can be visualized, built and edited interactively. The atom, bond, angle and dihedral in a molecule can be viewed and modified using sample mouse operations. A wide range of cheminformatics algorithms for crystal structure are provided, including cleaving surfaces, establishing vacuum layers, and building supercells. Four displayed styles, namely “Primitive cell”, “Original”, “In-cell” and “Packing” can be used to visualize a unit cell. Additionally, the initial input files for Vienna Ab-initio Simulation Package (VASP) and Gaussian can be obtained by interacting with dialog boxes in 3DStructGen.Conclusions3DStructGen is a highly platform-independent program. It can provide web service independently or can be integrated into other web platforms. Other than local desktop software, it does not require any additional effort to install the system but a web browser supporting HTML5. 3DStructGen may play a valuable role in online chemistry education and pre-processing of quantum calculations. The program has been released under MIT open-source license and is available on: https://matgen.nscc-gz.cn/Tools.html.

Enhanced Reaction of Renewable Hydrogen Energy Production Using Platinum-based Nanoclusters

Objectives It would be essential for solving the global warming problem to use renewable energy as well as increase the efficiency of energy production. One of the renewable and efficient energy sources is hydrogen energy, and the electrochemical hydrogen evolution reaction (HER) is the most important reaction in the hydrogen energy production. For HER, inexpensive catalyst development is indispensible because the precious metal Pt is being used as the most efficient catalyst for HER. Methods Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) to calculate atomic and electronic structures and total energies using the projector-augmented wave (PAW) method. Results and Discussion The HER efficiencies of Pt(111), Pt nanoclusters, and Pt-based core-shell nanoclusters were examined based on the hydrogen adsorption energies (Eads) by using the density functional theory (DFT) calculations. The DFT results demonstrate that Pt nanoclusters (0.5 ~ 2.1 nm in diameter) showed stronger hydrogen adsorption (Eads = -0.65 ~ -1.25 eV) than that of Pt(111) (Eads = -0.61 eV), implying a lower HER efficiency of Pt nanoclusters than Pt(111). On the other hand, a Rucore-Ptshell nanocluster (Eads = -0.48 eV) showed weaker hydrogen adsorption than Pt(111), indicating an increase in the HER efficiency. The weaker hydrogen adsorption on Rucore-Ptshell was attributed to the d;-band shift of Pt atoms further away from the Fermi level in negative direction due to the core Ru atoms. Conclusion By evaluating the HER efficiency using a descriptor of the hydrogen atom adsorption strength, a Ru-Pt core-shell (Rucore-Ptshell) nanocluster was expected to exhibit the highest HER efficiency among Pt(111), Pt55 nanocluster, and Mcore-Ptshell (M = Ni, Co, Mn, Cu, Ru, Ir and Au) nanoclusters. Key words: Hydrogen Evolution Reaction (HER), Platinum (Pt) Catalysts, Nanocluster, Core-shell, Density Functional Theory (DFT), d-Band Center Theory

Czochralski growth, electronic structure, luminescence and scintillation properties of Cs2Mo3O10: A new scintillation crystal for 0νββ decay search

A new molybdenum (Mo) based Cesium tri-molybdate (Cs2Mo3O10) single crystal scintillator has been grown successfully by the Czochralski technique. The crystal growth conditions such as pulling, rotation and cooling rate were optimized for crack free high-quality single crystals. The electronic band structure has been calculated with density functional theory using projector augmented wave potentials as implemented in the Vienna ab initio Simulation Package. The major contribution in the formation of valence band near the fermi level is observed from Mo and O p orbitals while the bottom of the conduction band mainly consists from Mo-d and O-p states. The grown crystal possesses good transparency above the fundamental absorption edge in the wavelength range of 400–900 nm. Optical band gap of 3 eV is estimated from the absorption spectra measured at room temperature. Luminescence and scintillation properties were measured in the temperature range of 10–300 K under the excitation of UV light at 4.42 eV, X-rays and beta rays. The emission spectrum measured at 230 K is comprising of a single band peaking at 645 nm, which intensity is enhanced with the cooling of crystal. A single exponential photoluminescent decay time of 43 μs is obtained at 220 K which further slows down upon cooling of the crystal and found to be 1.64 ms at 10 K. The scintillation light yield of Cs2Mo3O10 crystal at 10 K is found to be 185% of Li2MoO4 (LMO) crystal. The thermally stimulated luminescence (TSL) kinetic parameters such as trap depth and frequency factor from the glow curve measured after X-ray irradiation at 10 K, have been determined using general order kinetics. A possibility of crystal growth with large volume, good optical, luminescence and scintillation properties reveal that this scintillation material can be a good candidate for the neutrinoless double beta (0νββ) decay search of 100Mo.

Implicit self-consistent electrolyte model in plane-wave density-functional theory.

The ab initio computational treatment of electrochemical systems requires an appropriate treatment of the solid/liquid interfaces. A fully quantum mechanical treatment of the interface is computationally demanding due to the large number of degrees of freedom involved. In this work, we develop a computationally efficient model where the electrode part of the interface is described at the density-functional theory (DFT) level, and the electrolyte part is represented through an implicit solvation model based on the Poisson-Boltzmann equation. We describe the implementation of the linearized Poisson-Boltzmann equation into the Vienna Ab initio Simulation Package, a widely used DFT code, followed by validation and benchmarking of the method. To demonstrate the utility of the implicit electrolyte model, we apply it to study the surface energy of Cu crystal facets in an aqueous electrolyte as a function of applied electric potential. We show that the applied potential enables the control of the shape of nanocrystals from an octahedral to a truncated octahedral morphology with increasing potential.

Graphene Acoustic Phonon-Mediated Pseudo-Landau Levels Tailoring Probed by Scanning Tunneling Spectroscopy.

Graphene has attracted great interests in various areas including optoelectronics, spintronics, and nanomechanics due to its unique electronic structure, a linear dispersion with a zero bandgap around the Dirac point. Shifts of Dirac cones in graphene creates pseudo-magnetic field, which generates an energy gap and brings a zero-magnetic-field analogue of the quantum Hall effect. Recent studies have demonstrated that graphene pseudo-magnetic effects can be generated by vacancy defects, atom adsorption, zigzag or armchair edges, and external strain. Here, a larger than 100 T pseudo-magnetic field is reported that generated on the step area of graphene; and with the ultrahigh vacuum scanning tunneling microscopy, the observed Landau levels can be effectively tailored by graphene phonons. The zero pseudo-Landau level is suppressed due to the phonon-mediated inelastic tunneling, and this is observed by the scanning tunneling spectroscopy spectrum and confirmed by the Vienna ab initio simulation package calculation, where graphene phonons modulate the flow of tunneling electrons and further mediate pseudo-Landau levels. These observations demonstrate a viable approach for the control of pseudo-Landau levels, which tailors the electronic structure of graphene, and further ignites applications in graphene valley electronics.

Lattice dynamical and thermo-elastic properties of M2AlB (M = V, Nb, Ta) MAX phase borides

The structural, electronic, dynamic, and thermo-elastic properties of M2AlB (X = V, Nb, Ta) MAX phase borides were investigated using first principle calculations as implemented in the Vienna Ab-initio Simulation Package (VASP) with the generalized gradient approximation (GGA). The obtained structural properties and formation energies showed the thermodynamic stability and synthesizability of M2AlB. The electronic band structures were determined and they revealed that these compounds had a metallic character. The dynamic stability of M2AlB compounds were investigated with phonon dispersion curves and these compounds were found to be dynamically stable. The elastic constants were also calculated to determine the mechanical stability and to obtain the polycrystalline properties such as bulk modulus, shear modulus, etc. The thermo-elastic properties (thermal expansion coefficient, heat capacity, entropy, and free energy) were studied in a temperature range in between 0 and 1000 K and a pressure range in between 0 and 30 GPa. In addition, the direction dependent sound wave velocities were studied in three dimensions. Moreover, the minimum thermal conductivities and the diffusion thermal conductivities of these compounds were determined. This work is the processor study for the investigation of the main physical properties of M2AlB (M = V, Nb, Ta) ceramic compounds to date.

First-Principles Calculation of Physical Tensors of α-Diisopropylammonium Bromide (α-DIPAB) Molecular Ferroelectric Crystal

We report accurate calculations of tonsorial elements of α-Diisopropylammonium bromide (α-DIPAB) molecular ferroelectric crystal. In particular, elastic, piezoelectric and dielectric tensors were calculated using Vienna ab initio simulation package (VASP) within the framework of density functional (DFT) theory. The determination of above parameters allows an accurate description of the energy landscape for modeling of realistic devices at finite temperatures. We determine the major physical tensors in energy expansion of total energy per volume of un-deformed crystal to provide experimentalists with valuable information for designing and fabrication of pyroelectric detectors, capacitors, piezoelectric ferroelectric memories, display devices, electro-optic devices, and ultrasound transducers based on α-DIPAB. The spontaneous polarization P_s was calculated using Berry phase approach and found to be 22.64 μC⁄〖cm〗^2 in agreement with reported theoretical value. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of α-DIPAB to get a deeper insight into the bonding network. The neighboring layers of DIPA molecules were found to be strongly crenelated due to the strong short-ranged electrostatic repulsion between Br sites in the DIPAB crystal structure. The organization of species in DIPA molecular layer as well as in the bromine “stitching” layer is essential for accurate calculation of DIPAB elastic properties. Having understood the actual network bonding in α-DIPAB, we calculated the components of the elastic moduli tensor. Our results indicate that a Young’s modulus of 10-180 GPa and a shear modulus of 4-26 GPa were found. Thus, α-DIPAB phase has a great potential to be a terrific candidate for flexible electronic device applications. The value of the principle component of electronic contribution to the static dielectric tensor of α-DIPAB is found to be ≈ 2.5, i.e. 50% smaller than that in typical perovskite-based ferroelectrics. Therefore, α-DIPAB is anticipated to exhibit creative materials' innovations. It offers highly customized features for specific applications, including insulation on polymer thick film conductive inks in the manufacture of membrane switches and flex circuits. Furthermore, clamped-ion piezoelectric tensor is calculated. Our results indicate a reasonable piezoelectric response of this polar crystal making it a low cost attractive candidate for piezoelectric applications.

Evolution of carbides and performance of knives made of aged 8Cr13MoV steel

The dissolution and precipitation behaviours of M7C3 and M23C6 carbides in 8Cr13MoV steel were calculated using Thermo-Calc software. The mechanical properties of carbides were calculated using the Vienna Ab-initio Simulation Package. Results showed that M7C3 carbides were ductile and M23C6 carbides were brittle. High holding temperature provided favourable conditions for M7C3 carbides to precipitate and promoted the dissolution of bulky M23C6 carbides. Two heat treatment sequences were designed. During aging treatment, dispersive rodlike nanoscale M7C3 carbides precipitated from matrix, which improved wear resistance and the retention ability of the cutting edge of knives by 56 and 22%, respectively.

The research of data retention of HfO2 as trapping layer: First-principles study

The influence of the Charge Trapping Memory storage feature was researched by doping the substitutional impurity Al and introducing oxygen vacancy within HfO2. Materials studio and Vienna Ab-initio Simulation Package were used to investigate the influence of doping Al on the formation of the oxygen vacancy in HfO2 as trapping layer. At the same time, the interaction energy of two defects under different distances was calculated. The calculation results show that doping the substitutional impurity Al reduces the formation energy of oxygen vacancies in the HfO2, and the reduced formation energy of the three-fold-coordinated O vacancy is larger than that of the four-fold-coordinated O vacancy. After having studied three different defect distances between substitutional impurity Al and three-fold-coordinated O vacancy, the results indicate that the system acquires the largest charge trapping energy, the most quantum states when the distance of the defects is 2.107Å. As a conclusion, the data retention in the trapping layer of monoclinic HfO2 can be improved by doping the substitutional impurity Al. This work will provide a theoretical guidance for the performance improvement with respect to the data retention of Charge Trapping Memory.

Simulation for the correlation of positron annihilation rate with charge density near defects in iron

Positron annihilation technology is a nondestructive detection technique to study the microscopic defects in materials. It is well known that the micro defects will redistribute the charge in materials. In this paper, the charge density of helium in iron was calculated by the Vienna Ab initio Simulation Package (VASP) and the positron annihilation parameters were obtained by Doppler program. The results demonstrated that different types of defects in materials will cause a great variety in charge density, which lead to the change of positron annihilation parameters. In a vacancy cluster containing 6 monovacancies, the positron annihilation lifetime decreases rapidly once the number of helium is greater than the number of vacancies.

Structural, electronic and magnetic properties of MxPt1-X, (M= Co, Ni and V) binary alloys

The structural, optical and magnetic properties of ordered MxPt1-x (M = Co, Ni and V) binary alloys have been investigated using Vienna ab initio Simulation Package (VASP) within the framework of Density Functional Theory (DFT) and the Generalized Gradient Approximation (GGA). Ab initio calculations have been performed to obtain the most stable structure for each of the three binary systems. In addition, the optical and electrical properties such as electronic band structure, density of states and partial density of states of MxPt1-x binary alloys have been investigated. Specifically, total energy minimization has been performed to calculate the equilibrium in-plane, ao, out-of-plane, co, and volume, Vo, structural lattice parameters of MxPt1-x binary alloys. We found that ao, co and Vo for CoPt, NiPt and VPt3 equal to (ao = 3.806 A, co = 3.707 A and Vo = 53.7 A3) (ao = 3.84 A, co = 3.62 A and Vo = 53.64 A3) and (ao = 3.88 A, co = 7.88 A and Vo = 118.71 A3) respectively. Furthermore, the magneto-crystalline anisotropy energies (MAE) have been calculated to get a deeper insight into magnetic characteristics of the MxPt1- x binary alloys. We found that MAE values for CoPt, NiPt and VPt3 binaries are equal to 1.60, 0.231 and 0.0116 meV/unit cell respectively. These MAE values correspond to magneto-crystalline anisotropy constant K values equal to 4.8 ×107, 6.9 ×106 and 1.46 × 105 erg/cm3. The obtained results reveal that CoPt and NiPt binary systems exhibit attractive optical and magnetic properties, which make both systems potential candidates for magneto-optical and optical-electronic devices. Our results are in good agreement with the previous experimental and theoretical findings.

Controlled Oxygen Redox for Excellent Power Capability in Layered Sodium-Based Compounds

Inducing anionic redox reaction of transition metal oxides in sodium ion batteries is one of the most reliable way to enhance capacity. Unfortunately, although the delivered capacity obtained from the transition-metal redox and oxygen redox is one of highest among sodium cathodes, the system suffered from not only serious capacity fading but also poor rate capability because of the sluggish kinetics of the oxygen redox. Given the overlapping in the density of states (DOS) between the oxygen 2p and cobalt 3d (Co4+/3+) orbitals in LiCoO2, which is one of the most representative materials showing good cyclability and acceptable rate performances, we applied this phenomenon to P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2). The P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2) compounds were synthesized using a spray pyrolysis method. NaNO3, Mn(NO3)2·4H2O, Mg(NO3)2·6H2O, Co(NO3)2·6H2O, citric acid, and sucrose were used as starting materials for the spray pyrolysis. Calcination was conducted at 870 °C in air to obtain the P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2). The compounds were stored in an Ar-filled glove box to minimize the moisture contact during storage. Powder X-ray diffraction (XRD; PANalytical, Empyrean) using Cu Kα radiation was employed to identify the crystalline phases of the as-prepared P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2) powders. The produced powders were examined using scanning electron microscopy (SEM; SU-8010, Hitachi) to analyze morphology of P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2) powders. Differential scanning calorimetry (DSC; Pyris 1, Perkin–Elmer) measurements were performed to analyze the thermal stability of desodiated electrodes. R2032 coin-type cells using Na metal as the negative electrode were assembled for electrochemical cell tests. The electrolyte solution was 0.5 M NaPF6 in a mixed solvent of propylene carbonate (PC) and fluoroethylene carbonate (FEC) in a volume ratio of 98:2. The fabricated cells were charged and then discharged between 1.5 and 4.6 V at a rate of 0.1C (26 mA g− 1), and the current was further raised to 5C (1.3 A g− 1) for a long-term cycle test at 25 °C. X-ray absorption near edge structure (XANES) spectroscopy was employed to understand the variation in the oxidation states of the transition metals and oxygen. Transmission electron microscopy (TEM; JEM-F200, JEOL) was used to analyze structural factors of the pristine P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 electrode and the electrode after 1000 cycles. Density functional theory (DFT) calculations were performed using the projector augmented wave method implemented in the Vienna ab initio simulation package (VASP). Among studied compounds P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2), P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 electrode delivered high discharge capacity of about 214 mAh g− 1, which is attributed to the reactions of Co4+/3+, O1 − /2 −, and Mn4+/3+, with approximately 87% (186 mAh g− 1) of the capacity retention over 100 cycles at a rate of 0.1 C (26 mA g-1). More importantly, the present P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 was able to deliver discharge capacities of approximately 108 mAh g− 1 at a rate of 5C (1.3 A g− 1), showing capacity retention of over 72% (78 mAh g− 1) after 1000 cycles (Figure 1). This outstanding electrode performance was rationalized by theoretical calculations; namely, the incorporation of Co in the crystal structure was shown to dramatically reduce the band-gap energies. Detailed information will be presented at the conference. Figure 1. Long–term cyclability at 5 C (1.3 A g− 1) for x = 0.2 in P2 Na0.6[Mg0.2Mn0.8 − xCox]O2. Figure 1

Designing Corrosion Resistant High Entropy Alloys Via Chlorine Susceptibility Index

In this poster presentation, first principle density functional theory (DFT) implemented in the Vienna Ab-initio Simulation Package (VASP) is coupled with Chlorine Susceptibility Index (CSI) to guide the design of high entropy alloys (HEA) for materials employed in marine environment. On the atomic scale, elements within HEAs are randomly distributed throughout the alloy. This disordered crystalline state renders the investigation of all possible combinations impossible for DFT calculations. The CSI methodology developed by Dr. Huibin Ke in the Taylor group can be applied here to circumvent this issue [1]. The first successful CSI model for alloy systems was derived based on the special quasi-random structure approach[2] utilized for the study of Ni-Cr 22 alloy[3] and uses the first-principles thermodynamics approximation that allows the prediction of preferred chemisorption states in mixed media as a function of pH, electrochemical potential and chloride concentration[4]. Herein, we apply this method in search for combinations of elements that can provide the best corrosion resistance, under constraints of satisfying the high entropy alloy predictors of configurational entropy, valence electron concentration, atomic size difference[5] as well as cost efficiency (cost should be on par with Stainless Steel 304). Performance predictions will be compared with a parallel experimental program involving alloy synthesis, structural characterization and electrochemical characterization of the corrosion performance [6]. Reference: [1] H. Ke and C.D. Taylor, Chloride Susceptibility Index (CSI): An ab initio based corrosion resistance indicator, Research in Progress, NACE (2019), Nashville, TN [2] S. Wei, L.G. Ferreira, J.E. Bernard, A. Zunger, Electronic properties of random alloys: Special quasirandom structures, 42 (1990). [3] A.J. Samin, C.D. Taylor, First-principles investigation of surface properties and adsorption of oxygen on Ni-22Cr and the role of molybdenum, Corros. Sci. 134 (2018) 103–111. doi:10.1016/j.corsci.2018.02.017. [4] C.D. Taylor, S. Li, A. Samin, S. Li, A. Samin, Oxidation versus salt-film formation: Competitive adsorption on a series of metals from first-principles, Electrochim. Acta. (2018). doi:10.1016/j.electacta.2018.02.150.This. [5] M.C. Gao, J.-W. Yeh, P.K. Liaw, Y. Zhang, eds., High-Entropy Alloys: Fundamentals and Applications, Springer, 2016. doi:10.1007/978-3-319-27013-5_12. [6] A. Panindre, G.B. Viswanathan, S. Li, C. D. Taylor, G.S. Frankel, The effect of intermediate temperature anneal on the corrosion properties of a High Entropy Alloy, NACE Conference 2019, Nashville, TN, USA.

Simulating Polarization Curves of Commonly Identified Intermetallic Particles from High Strength Aluminum Alloys Via First Principles

In this talk, first principles density functional theory (DFT) implemented in the Vienna Ab-initio Simulation Package (VASP) is coupled with micro-kinetic modeling to simulate the polarization curves of some commonly identified intermetallic particles (IMPs). First principles DFT calculation is used to characterize the adsorption energies for the cathodic reaction intermediates on intermetallic surfaces. The species include atomic oxygen (O) and hydroxyl (OH) as intermediates for the oxygen reduction reaction. We will show that when coupling the ground state energy calculations with an appropriate micro-kinetic model[1–4], we can simulate simple polarization curves of the IMPs. One of the most desirable properties obtained from the simulated polarization curve is corrosion potential of the IMPs. Corrosion potential can indicate the relative nobilities of IMPs with respect to the alloy matrix. Experimentally the corrosion potential of IMPs on the range of microns are measured via a micro-capillary cell technique. On the simulation side, however, a method that can be directly compared to experimental data has not yet been fully realized. Whereas attempts to estimate corrosion potentials from the surface work functions obtained by DFT calculations have been made in recent literature [7]. we contend here that a model founded in electrode kinetics will be more successful in bridging simulation and experiment. We started with Al7FeCu2, one of the most commonly identified cathodic IMP in high strength aluminum alloy. Our simulated for this IMP is -336.7 mV vs NHE, which is in good agreement with experimental data: -307 mV vs NHE[5]. We also aim to extend our current model to more reactive IMPs that contain Mg, such as Mg2Si and MgZn2, since these particles are measured to be more anodic than the aluminum matrix. For Mg containing particles, the cathodic reaction is the hydrogen evolution reaction (HER), and the reaction mechanism on the atomic scale has been proposed by Taylor[6]. Results for both anodic and cathodic particles based on the above DFT plus microkinetic model will be presented in this presentation. Reference: [1] C.D. Taylor, S. Li, A. Samin, S. Li, A. Samin, Oxidation versus salt-film formation: Competitive adsorption on a series of metals from first-principles, Electrochim. Acta. (2018). doi:10.1016/j.electacta.2018.02.150.This. [2] J.K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. Jónsson, Origin of the overpotential for oxygen reduction at a fuel-cell cathode, J. Phys. Chem. B. 108 (2004) 17886–17892. doi:10.1021/jp047349j. [3] H. Ma, X.Q. Chen, R. Li, S. Wang, J. Dong, W. Ke, First-principles modeling of anisotropic anodic dissolution of metals and alloys in corrosive environments, Acta Mater. 130 (2017) 137–146. doi:10.1016/j.actamat.2017.03.027. [4] H.A. Hansen, V. Viswanathan, J.K. Nørskov, Unifying Kinetic and Thermodynamic Analysis of 2 e – and 4 e – Reduction of Oxygen on Metal Surfaces, J. Phys. Chem. C. 118 (2014) 6706–6718. doi:10.1021/jp4100608. [5] N. Birbilis, R.G. Buchheit, Electrochemical Characteristics of Intermetallic Phases in Aluminum Alloys, J. Electrochem. Soc. 152 (2005) B140. doi:10.1149/1.1869984. [6] C.D. Taylor, A First-Principles Surface Reaction Kinetic Model for Hydrogen Evolution under Cathodic and Anodic Conditions on Magnesium, J. Electrochem. Soc. 163 (2016) C602–C608. doi:10.1149/2.1171609jes. [7] Y. Zhu, J. Poplawsky, S. Li, R. Unocic, C. D. Taylor, J. Locke, E. Marquis, G. S. Frankel, Probe to the localized corrosion on/around nm-scale hardening precipitates in Al-Cu-Li alloys, in preparation.

Study on impact of Cr and Mo on diffusion of H in 2.25Cr1Mo steel using first-principle calculations

Chrome-molybdenum steel (2.25Cr1Mo steel) is one of the main materials of a steam generator (SG) in high-temperature reactor-pebblebed modules (HTR-PM). It is essential to analyze the source term of tritium in this material, because the behavior of tritium in a SG is important for performing source term analysis in normal and accident conditions. In this article, the diffusion behavior of H atom in 2.25Cr1Mo steel was calculated to estimate the diffusivity of tritium using first-principles density functional theory. To develop and simplify the model of hydrogen diffusion in 2.25Cr1Mo steel, the impact of Cr and Mo on the diffusion of hydrogen in bcc-Fe were first calculated, all the possible diffusion paths were considered, and the minimum energy path was obtained. The diffusion activation energy and pre-exponential factor of the diffusion coefficient were obtained from Vienna Ab initio Simulation Package combined with the climbing image-nudged elastic band method. The results indicate that the minimum energy path for the impurity H atom is from one tetrahedral interstitial site to an adjacent tetrahedral interstitial site. The function of the diffusion coefficient of H in 2.25Cr1Mo steel with temperature T can be expressed as D=1.486×10−7×(−16.350kJ/molRT) (m2/s). The diffusion coefficient of our calculation and some of the previous experiments have an excellent quantitative agreement, which indicates the reliability of our crystalline model and the practicability of the present theoretical approach. More importantly, the computational results in this work can be treated as a good screening method to collect reasonable experimental data, which will provide a good reference for tritium source term evaluation in the SG of the HTR-PM.

Properties of BaYO3 perovskite and hydrogen storage properties of BaYO3Hx

In this study, Density Functional Theory (DFT) calculations have been performed for BaYO3 perovskite with the generalized gradient approximation (GGA) as implemented in Vienna Ab-initio Simulation Package (VASP). The structural optimization of BaYO3 perovskite have been studied for the five possible phases: cubic, tetragonal, hexagonal, orthorhombic and rhombohedral to determine the most stable phase of BaYO3 perovskite. It has been found that the cubic phase is the most stable one and electronic and mechanical properties of this phase have been investigated. Moreover, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young's and Shear moduli for cubic phase. Then, hydrogen bonding to BaYO3 perovskite has been conducted and hydrogen storage properties of BaYO3Hx (x = 3 and 9) such as: formation energy, cohesive energy and gravimetric hydrogen storage capacity have been analyzed. Having no study about BaYO3 perovskite and hydrogen bonding in the literature makes this study the first considerations of BaYO3 perovskite. Hence, this work could enlighten the possible future studies.

Structure, stability and electronic properties of bimetallic atomic chains of Au–Ag and Au–Pt

Quantum confinement of electrons in atomic chains provides the most powerful and versatile means to control electronic, optical, magnetic and thermoelectric properties of materials needed to make diodes, spin valves and optical labels. Furthermore, the alloying of metallic atoms in different compositions produces novel mechanical, electronic and chemical behaviours in bimetallic chains as well as in other structures. This motivated us to perform theoretical investigations on the structure, stability, magnetic and electronic properties of bimetallic atomic chains of Au–Ag and Au–Pt, by using Vienna ab-initio simulation package (VASP), which is based on the density functional theory (DFT) within generalised gradient approximation. We have used tension and cohesive energy criteria to assess the stability of the Au–Ag and Au–Pt atomic chains. A comparison between the computed cohesive energies of various possible structures are made to suggest the most probable chain structures that can occur in break junction experiments. Our computed results suggest that the ground state of the Au–Ag and Au–Pt atomic chains should have zig-zag geometry. Furthermore, the most favoured chain structures that can be formed at the last stage of nanowires stretching are: (i) an atomic chain with alternate arrangement of equal number of Au and Ag / Pt atoms and (ii) an atomic chain where two Ag / Pt atoms are separated by one Au atom. Our results on the electronic band structure and optical properties suggest that the Au–Ag atomic chain could be of semiconducting nature, while the most stable Au–Pt chain is metallic in nature. A spin-polarised calculation with the inclusion of spin–orbit coupling shows that the Au–Pt atomic chains are magnetic, if the number of Au atoms is not more than the number of Pt atoms.