Previous Density Functional Theory Calculations Research Articles

Optical properties, energy band gap and the charge carriers’ effective masses of the R3c BiFeO3 magnetoelectric compound

Calculations based on collinear and non-collinear spin density functional theory (DFT) were used to investigate the complex dielectric function spectrum, energy band gap, electronic band structure and the effective mass of the charge carriers of the R3c BiFeO3 magnetoelectric compound. Exchange and correlation electronic effects were taken into account using the local spin density approximation (LSDA), including the effective Hubbard (Ueff) correction (LSDA + Ueff). With Ueff = 6.0 eV and using the non-collinear spin DFT, the calculated complex dielectric function spectrum agrees very well with the experimental data from the literature. The optical absorption edge is estimated to be 2.2 eV, which according to the calculated electronic band structure corresponds to an indirect energy band gap. The electronic band structures also revealed that the aforementioned material has a direct band gap of 2.6 eV, which agrees with some experimental predictions. In addition, the hole and electron effective masses of the R3c BiFeO3 compound were computed. The smallest hole effective mass eigenvalue (0.38 m0) agrees with previous DFT calculations (0.67 m0). However, the smallest electron effective mass eigenvalue (0.39 m0) was one order of magnitude smaller than that reported in an earlier study (3.06 m0).

Unveiling the Active Structure of Single Nickel Atom Catalysis: Critical Roles of Charge Capacity and Hydrogen Bonding.

A single nickel atom embedded in graphene is one of the most representative single-atom catalysts, and it has a high activity and selectivity for electrochemical CO2 reduction (CO2R) to CO. However, the catalytic origin, especially the coordination structure of Ni, remains highly puzzling, as previous density functional theory (DFT) calculations showed that all the possible structures should be inactive and/or nonselective. Here, using ab initio molecular dynamics (AIMD) and a "slow-growth" sampling approach to evaluate the reaction kinetic barriers, we show that the charge capacity (of the site) and hydrogen bonding (with the intermediates), which were neglected/oversimplified in previous DFT calculations, play crucial roles, and including their effects can resolve the catalytic origin. Particularly, a high charge capacity allows the catalytic site to carry more charges than required for the electrochemical step, lowering the electrochemical barrier, and hydrogen bonding promotes the reaction that produces polar intermediates by stabilizing the intermediates and facilitating the H transfer from water, explaining the high selectivity for CO2R over the hydrogen evolution reaction. Consequently, we find that a hybrid coordination environment (with one nitrogen and three carbon atoms) for the Ni-atom is most active and selective for CO2R. Our work not only explains a long-standing puzzle for an important catalyst but also highlights the crucial roles of charge capacity and hydrogen bonding, which can help elucidate the mechanisms of other heterogeneous electrocatalysts in aqueous solution and enable more effective catalyst design.

Application of Machine Learning to Predict Grain Boundary Embrittlement in Metals by Combining Bonding-Breaking and Atomic Size Effects.

The strengthening energy or embrittling potency of an alloying element is a fundamental energetics of the grain boundary (GB) embrittlement that control the mechanical properties of metallic materials. A data-driven machine learning approach has recently been used to develop prediction models to uncover the physical mechanisms and design novel materials with enhanced properties. In this work, to accurately predict and uncover the key features in determining the strengthening energies, three machine learning methods were used to model and predict strengthening energies of solutes in different metallic GBs. In addition, 142 strengthening energies from previous density functional theory calculations served as our dataset to train three machine learning models: support vector machine (SVM) with linear kernel, SVM with radial basis function (RBF) kernel, and artificial neural network (ANN). Considering both the bond-breaking effect and atomic size effect, the nonlinear kernel based SVR model was found to perform the best with a correlation of r2 ~ 0.889. The size effect feature shows a significant improvement to prediction performance with respect to using bond-breaking effect only. Moreover, the mean impact value analysis was conducted to quantitatively explore the relative significance of each input feature for improving the effective prediction.

Effect of lattice deformation on electronic and optical properties of CuGaSe2: Ab-initio calculations

In this study, we have investigated the effect of bi-axial, ϵab, and uni-axial, ϵc, strains on the optoelectronic properties of chalcopyrite semiconductor CuGaSe2 through first-principles full potential linearized augmented plane wave method. These materials have recently attracted much interest within the materials science community. The results are obtained in the framework of Density Functional Theory (DFT), using the Generalized Gradient Approximation based on the minimization of total energy, together with the modified Becke-Johnson exchange-correlation potential, as implemented in the WIEN2k code. Our results show that unstrained CuGaSe2 is a direct band gap semiconductor with a energy of 1.16 eV, thus improving the results of some previous DFT calculations, but still below the accepted experimental data. The incorporation of biaxial and uniaxial strain results in a monotonous decreasing behavior of the energy band gap when both ϵab and ϵc change between -8% and +8%, with unstrained value being, approximately, at the middle of the variation range. It is also found that strain causes modifications in the index of refraction of the material, with modifications of its static value that rank above 10% over the entire range of deformations considered.

The Predictive Power of Different Projector-Augmented Wave Potentials for Nuclear Quadrupole Resonance

The projector-augmented wave (PAW) method is used to calculate electric field gradients (EFG) for various PAW potentials. A variety of crystals containing reactive nonmetal, simple metal, and transition elements, are evaluated in order to determine the predictive ability of the PAW method for the determination of nuclear quadrupole resonance frequencies in previously unstudied materials and their polymorphs. All results were compared to experimental results and, where possible, to previous density functional theory (DFT) calculations. The EFG at the 14N site of NaNO2 is calculated by DFT for the first time. The reactive nonmetal elements were not very sensitive to the variation in PAW potentials, and calculations were quite close to experimental values. For the other elements, the various PAW potentials led to a clear spread in EFG values, with no one universal potential emerging. Within the spread, there was agreement with other ab initio models.

Anharmonic thermodynamics of vacancies using a neural network potential

Lattice anharmonicity is thought to strongly affect vacancy concentrations in metals at high temperatures. It is however non-trivial to account for this effect directly using density functional theory (DFT). Here we develop a deep neural network potential for aluminum that overcomes the limitations inherent to DFT, and we use it to obtain accurate anharmonic vacancy formation free energies as a function of temperature. While confirming the important role of anharmonicity at high temperatures, the calculation unveils a markedly nonlinear behavior of the vacancy formation entropy and shows that the vacancy formation free energy only violates Arrhenius law at temperatures above 600 K, in contrast with previous DFT calculations.

Imaging the local electronic and magnetic properties of intrinsically phase separated RbxFe2–ySe2 superconductor using scanning microscopy techniques

The discovery in 2008 of the iron-based superconducting pnictide and chalcogenide compounds has provided an entirely new family of materials for studying the crucial interplay between superconductivity and magnetism in unconventional superconductors. The alkali-metal-intercalated iron selenide (AxFe2−ySe2, A = alkali metal) superconductors are of particular interest owing to their relatively high transition temperatures over 30 K and the co-existence of the superconducting state with antiferromagnetic ordering. Intrinsic phase separation on the mesoscopic scale is known to occur in ‘single crystals’ of these compounds, adding to the complexity of interpretation of bulk property measurements. In this study, we investigate the local electronic structure and chemistry of RbxFe2−ySe2 crystals using scanning microscopy techniques. Nano-focussed angle-resolved photoemission spectroscopy has enabled the band structure of the minority superconducting phase and the non-superconducting matrix to be measured independently and linked to the surface chemistry from the same regions using core level spectroscopy. Valence band mapping reveals the characteristic microstructure of these crystals, but does not have sufficient spatial resolution to enable the precise morphology of the superconducting phase to be elucidated. Cryogenic magnetic force microscopy has shown that the superconducting phase has a fine-scale stripey morphology that was not resolved in the scanning photoemission spectroscopy experiment. The correlation of these findings with previous microstructural studies, bulk measurements and first-principles density functional theory calculations paves the way for understanding the intriguing electronic and magnetic properties of these compounds.

Accurate, First-Principle Study of Electronic and Related Properties of the Ground State of Li<sub>2</sub>Se

We present results from ab-initio, self-consistent calculations of electronic and related properties for the ground state of cubic lithium selenide (Li2Se). We employed a local density approximation (LDA) potential and performed computations following the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). This method verifiably leads to the ground state of materials without employing over-complete basis sets. We present the calculated electronic energies, total and partial densities of states, effective masses, and the bulk modulus. The present calculated band structures show clearly that cubic Li2Se has a direct fundamental energy band gap of 4.065 eV at the Γ point for the room temperature experimental lattice constant of 6.017 A. This result is different from findings of previous density functional theory (DFT) calculations that uniformly reported an indirect band gap, from Γ to X, for Li2Se. We predicted a direct band gap of 4.363 eV, at the computationally determined equilibrium lattice constant of 5.882 A, and a bulk modulus of 35.4 GPa. For the first time known to us, we report calculated electron and hole effective masses for Li2Se. The experimental confirmation of the large, direct gap we found will point to a potential importance of this material for ultraviolet technologies and applications. Due to a lack of experimental results, most of our calculated ones in this paper are predictions for Li2Se.

Crystallographic orientation effects of hydrogen diffusion in α-uranium from DFT: Interpreting variations in experimental data

Experimental values of hydrogen diffusion coefficients in bulk α-uranium obtained at elevated temperatures have significant scatter, leading to dramatic differences in extrapolated diffusion coefficients at room temperature. Previous density functional theory (DFT) calculations predicted higher diffusion barriers than experiments, suggesting hydrogen diffusion through undefected bulk is not possible. In this manuscript DFT calculations of hydrogen diffusion coefficients as a function of crystallographic orientation in α-uranium are reported. The findings explain previous discrepancies, show that diffusion of hydrogen through bulk uranium is consistent with experiments, and suggest a reasonable value for room temperature diffusion coefficient based on proposed pathway.

Adsorption of Ne on a planar solid Mg surface revisited

In this study, we reexamined the wetting behavior of Ne adsorbed on a solid Mg surface. In a recent study, Zhou and Zhang (J. Phys. Chem. Solids, 2017, 103, 123) reported an investigation of this system. They proposed an adsorption potential, ϕext(z), and studied this system based on augmented density functional theory (DFT) to determine the wetting temperature Tw and critical prewetting temperature Tcpw. The value obtained for Tcpw differed greatly from that produced by Grand Canonical Monte Carlo simulations (Phys. Rev. E, 1999, 59, 864) and previous DFT calculations (Phys. Rev. E, 2009, 79, 011603). In the present study, we calculated the adsorption isotherms with the Kierlik–Rosinberg DFT for two different external potentials, i.e., the aforementioned ϕext(z) and the proposed ab initio Chizmeshya–Cole–Zaremba (CCZ) potential. This study had two main aims. First, we aimed to identify the main sources of the discrepancies between the results obtained by Zhou and Zhang and those produced in other studies. Second, we analyzed the new data computed with the CCZ potential by applying the so-called arc-length continuation method. The new results supported the old ones, yielding more accurate values for both characteristic temperatures and indicating interesting new features of the system. We developed a novel alternative procedure for determining an appropriate value of Tcpw for moderately strong adsorbers. Assuming that Ne is liquid in the range of Tw≤T<Tt, then the DFT calculations were extended into this region. We showed that close to Tt and after the prewetting jump, the growth of the films exhibited a sequence of layering transitions, which originated coalescent prewetting lines.

Linkage Photoisomerization Mechanism in a Photochromic Ruthenium Nitrosyl Complex: New Insights from an MS-CASPT2 Study.

The N → O linkage photoisomerization mechanism in a ruthenium nitrosyl complex, [RuCl(NO)(py)4]2+, for which a quasicomplete photoconversion between the stable nitrosyl (N-bonded) and metastable isonitrosyl (O-bonded) isomers has been observed under continuous irradiation of the crystal at 473 nm ( Cormary et al. Acta Cryst. B 2009 , 65 , 612 - 623 ), is investigated using multiconfigurational second-order perturbation theory (CASPT2). The results support efficient intersystem crossing pathways from the initially excited singlet states to the lowest triplet excited state of metal-to-ligand charge transfer character (3MLCT). The topology of the involved potential energy surfaces corroborates a complex sequential two-photon photoisomerization mechanism involving nonadiabatic processes in agreement with experimental observations and previous density functional theory calculations.

Density Functional Theory Study of the Thermodynamic and Raman Vibrational Properties of γ-UO3 Polymorph

Gamma uranium trioxide, γ-UO3, is one of the most important polymorphs in uranium trioxide system, which is common throughout the nuclear fuel cycle and used industrially in the reprocessing of nuclear fuel and uranium enrichment. In this work, a detailed theoretical solid-state density functional theory study of this material was carried out. The computed lattice parameters, bond lengths, bond angles, and X-ray powder pattern were found in very good agreement with their experimental counterparts determined by X-ray diffraction. The equation of state of γ-UO3 was obtained, and therefore, the values of the bulk modulus and its derivatives, for which there are not experimental data to compare with, were predicted. The computed bulk modulus differs from that of a previous density functional theory calculation by only 4.4%. The thermodynamic properties of this material, including heat capacity, entropy, enthalpy, free energy, and Debye temperature were also determined as a function of temperature in the range...

Theoretical Study of Oxygen Vacancy Formation at LSC/GDC Interface

Introduction Doped lanthanum cobaltite and its derivatives are used as cathode materials in solid oxide fuel cell (SOFC). For a high catalytic activity, the improvement of properties governing the cathode performance is important. It is generally regarded that one of the important factors is the electronic structure of cathode material. Recently, the influence of the lattice strain on the kinetics of the oxygen reduction reaction (ORR) was reported [1]. In addition, the heterostructured oxide interface ((La,Sr)2CoO4±δ/La1-xSrxCoO3-δ (LSC)) and thin-film heterostructure interface (La0.6Sr0.4Co0.2Fe0.8O3-δ/gadolinium-doped CeO2 (GDC)) enhanced the surface oxygen exchange kinetics and ORR activity [2,3]. These results indicate the possibility that the structural change near interface changes the electronic structure of cathode materials. Detailed understandings of electronic structure of doped lanthanum cobaltite near the interface is important to design the high catalytic cathode materials. In this study, we analyzed the stability of oxygen vacancy in LSC of LSC/GDC interface by using the density functional theory (DFT) calculation. Computational Details We used LaO-terminated (001) surface of cubic LaCoO3 crystal structure. Half of La atoms were substituted by Sr atoms in LaO layers in LSC slab model (La6Sr6Co8O28). As a surface of CeO2, we used (111) surface model (Ce14O28) from the surface stability. GDC surface model was prepared based on the previous DFT calculation [4]. The interface of LSC and GDC was modeled from CoO2-terminated LSC(001) and GDC(110) surfaces. The initial spin configuration of Co3+ ion was set as intermediate spin state. GGA-PAW potentials were applied with cutoff energy of 600 eV and 1×1×1 k-points. We employed GGA+U to improve the description of Co 3d and Ce 4f electrons. All DFT calculations were performed by using VASP. Results and Discussion We first analyzed the charge distribution near LSC/GDC interface. In case of LSC/CeO2 interface, the electron transferred from CeO2 to LSC. We clearly observed the larger electron transfer from GDC to LSC. These results indicate that the LSC region near CeO2 or GDC interface shows the negatively charged structure. We analyzed the oxygen vacancy formation energy in LSC. The oxygen vacancy formation energy from surface at LSC/GDC interface was more stable than that of LSC model. Although the oxygen vacancy formation energy from 2ndlayer in LSC model was stable, unstable oxygen vacancy formation energy was obtained in the LSC/GDC interface model. One of the reasons of oxygen vacancy formation energy difference between LSC and LSC/GDC interface is an effect of electron transfer from GDC to LSC. The atomic position change at interface is also influenced the oxygen vacancy formation energy. To see the relation between oxygen vacancy formation energy and distance from interface, we prepared larger LSC/GDC interface models. Details are discussed in the presentation. Acknowledgement This work was supported by CREST, Japan Science and Technology Agency. All calculations were performed on the HA8000 computer systems in Research Institute for Information Technology, Kyushu University. The activities of Advanced Automotive Research Collaborative Laboratory in Hiroshima University are supported by Mazda Corporation. The activities of INAMORI Frontier Research Center in Kyushu University are supported by Kyocera Corporation.

Energetics of adsorbed formate and formic acid on Ni(111) by calorimetry

Surface carboxylates are common intermediates in many catalytic reactions and are also used for organo-functionalization of surface. Herein, adsorbed formate, the simplest carboxylate, was produced by dissociative adsorption of formic acid on the Ni(111) surface and the heats of adsorption were measured by single-crystal adsorption calorimetry (SCAC). At 240K, the integral heat of dissociative adsorption to make adsorbed bidentate formate and adsorbed hydrogen is 117.1kJ/mol at 1/7ML. The estimated accuracy of measured heats is within 3kJ/mol. Therefore, the standard enthalpy of formation of bidentate formate on Ni(111) is derived as −449.3kJ/mol and the bond enthalpy to the Ni(111) surface is 319.6kJ/mol. Comparison to our earlier results for Pt(111) shows that bidentate formate is 54kJ/mol more stable on Ni(111). This is the first experimental measurement of the energy of any molecular fragment bonded to any Ni surface through metal-oxygen bonds, so it is an important benchmark for validating the accuracy of quantum mechanical calculations of a wide range of adsorbed oxygenates on Ni catalysts. Compared to previous density functional theory (DFT) calculations, we found that DFT calculations systematically underestimate the bond energy of formate to Ni(111), but correctly predict that it is stronger to Ni(111) than to Pt(111). The integral heat of HCOOH adsorption to make molecularly adsorbed formic acid was also measured and found to be 63.2kJ/mol at 0.45ML and 120–155K.

Robust FCC solute diffusion predictions from ab-initio machine learning methods

We evaluate the performance of four machine learning methods for modeling and predicting FCC solute diffusion barriers. More than 200 FCC solute diffusion barriers from previous density functional theory (DFT) calculations served as our dataset to train four machine learning methods: linear regression (LR), decision tree (DT), Gaussian kernel ridge regression (GKRR), and artificial neural network (ANN). We separately optimize key physical descriptors favored by each method to model diffusion barriers. We also assess the ability of each method to extrapolate when faced with new hosts with limited known data. GKRR and ANN were found to perform the best, showing 0.15eV cross-validation errors and predicting impurity diffusion in new hosts to within 0.2eV when given only 5 data points from the host. We demonstrate the success of a combined DFT+data mining approach towards solving materials science challenges and predict the diffusion barrier of all available impurities across all FCC hosts.

Electronic structure investigation of biphenylene films.

Photoelectron Spectroscopy (PS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy have been used to investigate the occupied and empty density of states of biphenylene films of different thicknesses, deposited onto a Cu(111) crystal. The obtained results have been compared to previous gas phase spectra and single molecule Density Functional Theory (DFT) calculations to get insights into the possible modification of the molecular electronic structure in the film induced by the adsorption on a surface. Furthermore, NEXAFS measurements allowed characterizing the variation of the molecular arrangement with the film thickness and helped to clarify the substrate-molecule interaction.

Phthalocyanine adsorption on Au(1 1 0): 1D ordering and adaptive reconstruction

The adsorption of metal-free phthalocyanine molecules on an anisotropic Au(1 1 0)(1 × 2) surface has been studied with ultraviolet (UV) photoemission, low-energy electron diffraction and low-temperature scanning tunneling microscopy. In all cases, the molecules form rows in the [1 0] direction, i.e. along the troughs of the reconstructed substrates. However, depending on the exposure and adsorption temperature, the substrate maintains (1 × 2)- or transforms into a (1 × 3)-reconstruction, and the molecular separation along the rows shrink from six to five times the Au–Au interatomic distance. The results are in agreement with previous density functional theory (DFT) calculations.

Structure determination and stability for Pa-Si, Np-Si and U-X-Si (X = Mo, Th, Np) phases from first-principles

Density functional theory (DFT) calculations are performed for Pa-Si, Np-Si and uranium-based ternary silicide phases. Structure prediction calculations are used to search for competing phases in these systems. Results using the generalized gradient approximation (GGA), on-site Coulomb correction (GGA + U) and van der Waals interactions are presented. All Pa-Si compounds reported here are structurally analogous to those found in other actinide silicide systems. The electronic structure of Pa3Si2 shows the f-orbital electrons are largely unoccupied, which is in contrast to calculations for Np3Si2. For the Np-Si system, predicted stable structures using GGA differ from the experimentally observed structures, which, however, are energetically preferred in results using the GGA + U method. Structure searches for U2MoSi, U2ThSi2 and UNpSi reveal dynamically stable ternary compounds. The phonon dispersion curves, elastic constants and electronic density of states for the various phases are compared to those from previous DFT calculations for U-Si phases.

Stability of atomic oxygen chemisorption on Pt-alloy surfaces

A density functional theory calculation is used to investigate the atomic oxygen (O) stability over platinum (Pt) and Pt-based alloy surfaces. Here, the stability is connected with the preferential adsorption sites for O chemisorptions and the adsorption energy. Thus, the interaction mechanism between atomic O and metal surfaces is studied by using charge transfer analysis. In this present paper, atomic structure and binding energy of oxygen adsorption on the Pt(111) are in a very good agreement with experiment and previous density functional theory calculations. Furthermore, we obtained that the addition of ruthenium (Ru) and molybdenum (Mo) on the pure Pt surface enhances the adsorption energy. Our charge transfer analysis shows that the largest charge transfer contributing to the metal-O bonding formation is observed in the case of O/PtRuMo surface followed by O/PtRu surface. This is in consistency with metal d-orbital characteristic, where Mo has much more empty d-orbital than Ru in correspondence to accept electrons from atomic oxygen. Copyright © 2016 John Wiley & Sons, Ltd.

Boundary conditions for transition-metal dichalcogenide monolayers in the continuum model

We derive the boundary conditions for MoS$_2$ and similar transition-metal dichalcogenide honeycomb (2H polytype) monolayers with the same type of $\mathbf{k}\!\cdot\!\mathbf{p}$ Hamiltonian within the continuum model around the K points. In an effective 2-band description, the electron-hole symmetry breaking quadratic terms are also taken into account. We model the effect of the edges with a linear edge constraint method that has been applied previously to graphene. Focusing mainly on zigzag edges, we find that different reconstruction geometries with different edge-atoms can generally be described with one scalar parameter varying between 0 and $2\pi$. We analyze the edge states and their dispersion relation in MoS$_2$ in particular, and we find good agreement with the results of previous density functional theory calculations for various edge types.