Plane-wave Density Functional Theory Research Articles

Theoretical study of thermal stability of α″–Fe16N2 against other iron nitrides

α″–Fe16N2 has been investigated as one of promising candidates for environment-friendly magnets. While giant saturation magnetization has previously been experimentally observed in α″–Fe16N2, its magnetic anisotropy and structural stability leave room for improvement. Recent theoretical studies have considered alloying Fe16N2 with various elements to improve the magnetic properties and/or stability against decomposition. However, estimates of stability in particular are typically restricted to simple ground-state-energy comparisons, i.e. effectively taken at 0 K. For a more practical measure of stability, we therefore extend ground-state energies, obtained with the plane-wave density-functional theory code Quantum ESPRESSO, with appropriate empirical and/or statistical corrections to obtain free energies at arbitrary temperature. We then compare the stability of Fe16N2 against the neighboring phases in the Fe-N binary system, to estimate the range of temperatures at which it is stable. We compare against experimental observations of the Fe-N phase diagram.

An implicit electrolyte model for plane wave density functional theory exhibiting nonlinear response and a nonlocal cavity definition.

We have developed and implemented an implicit electrolyte model in the Vienna Ab initio Simulation Package (VASP) that includes nonlinear dielectric and ionic responses as well as a nonlocal definition of the cavities defining the spatial regions where these responses can occur. The implementation into the existing VASPsol code is numerically efficient and exhibits robust convergence, requiring computational effort only slightly higher than the original linear polarizable continuum model. The nonlinear + nonlocal model is able to reproduce the characteristic "double hump" shape observed experimentally for the differential capacitance of an electrified metal interface while preventing "leakage" of the electrolyte into regions of space too small to contain a single water molecule or solvated ion. The model also gives a reasonable prediction of molecular solvation free energies as well as the self-ionization free energy of water and the absolute electron chemical potential of the standard hydrogen electrode. All of this, combined with the additional ability to run constant potential density functional theory calculations, should enable the routine computation of activation barriers for electrocatalytic processes.

Adsorption of 4-(N,N-Dimethylamino)-4'-nitrostilbene on an Amorphous Silica Glass Surface.

Stilbenes are a compelling class of organic photoswitches with a high degree of tunability that sensitively depend on their environment. In this study, we investigate the adsorption properties of 4-(N,N-dimethylamino)-4'-nitrostilbene (DANS), a push-pull stilbene, on amorphous silica glass. Plane-wave density functional theory (DFT) calculations are used to understand how the trans and cis isomers of DANS interact with the amorphous surface and which are the most preferred modes of adsorption. Our calculations revealed that the O-H···O hydrogen bonds between the nitro group and hydroxyl groups of the silica surface dominate the intramolecular interaction. In addition to hydrogen bonding, O-H···π interactions with the aromatic ring and double bond play a critical role in adsorption, whereas C-H···O interactions are present, but contribute little. Therefore, both isomers of DANS favor parallel orientations such that not only the functional groups but also the aromatic parts can strongly interact with the glass surface.

Theoretical study of the photocatalytic activity of hBNC/MoSX (X = Se, Te) heterojunctions

The effects of the introduction of C-C bonds on the stability, electronic structure, work function, and optical properties of hBN/MoSX heterojunctions are investigated based on the density functional theory plane wave ultra soft pseudopotential method. The results show that the hBNC/MoSX heterojunctions have low binding energies (−4.066 eV to −4.777 eV) and lattice mismatch ratios (2.62 % and 1.44 %) and can be stabilized at room temperature of 300 K, indicating that they possess excellent stability. By comparing the electronic structure with that of the hBN/MoSX heterojunction, it is found that the introduction of C-C bonds can effectively adjust the band gap of the heterojunction. The bandgap of hBN/MoSX varies from 1.018 eV to 1.168 eV with small variations. The hBNC/SMoSe and hBNC/SMoTe heterojunctions have bandgaps of 0.309 eV and 0.494 eV, with significantly reduced bandgaps and orbital hybridization. The number of electrons in the excited state increases, which makes the electron leap easier and is beneficial to improve the response of the heterojunction to visible light. Due to the presence of the built-in electric field at the interface, the compounding of photogenerated electron-hole pairs can be effectively suppressed, which greatly enhances both the heterojunction photocatalytic ability and carrier migration. Compared with the hBNC monolayer system, both were red-shifted in the low-energy region. hBNC/SMoTe and hBNC/SMoSe showed the best absorption of light in the low-energy region, indicating excellent photocatalytic performance.

First principle study of enhanced CO adsorption on divacancy graphene-supported TM7 (TM = Fe, Co, Ni, Cu, Ag, and Au) clusters

The adsorption of CO on transition metal clusters TM7 (TM = Fe, Co, Ni, Cu, Ag, and Au) and their corresponding divacancy graphene-supported clusters has been investigated using plane-wave density functional theory with van der Waals D3 correction. The results reveal that all divacancy graphene-supported TM7 clusters show a stronger adsorption capacity for CO compared to the pure TM7 clusters. Orbital-resolved bonding interactions indicate, while the d-p orbital coupling between the substrate and CO dominates the magnitude of the absolute adsorption energy, the enhanced s-p orbital coupling between the substrate and CO determines the increase in the adsorption energy of CO on the divacancy graphene-supported TM7 clusters. On the other hand, the enhanced CO chemisorption energies on the divacancy graphene-supported TM7 clusters are caused by the induced electrostatic interaction between the positively charged TM7 on divacancy graphene and CO.

Elastic, electronic, and magnetic properties of MNb2O6 (M = Mn, Fe, Co, Ni) in the orthorhombic Pbcn structure, a computational study

The orthorhombic niobates containing magnetic transition ions such as Mn, Fe, Co, and Ni show excellent promise for materials in energy storage, electro-photoactivity, and low-dimensional magnetic character. We report on the computational studies focusing on the elastic, electronic, and magnetic properties of four compounds with chemical formulae MNb2O6 (M = Mn, Fe, Co, Ni) in orthorhombic Pbcn crystal structure using the plane wave density functional theory (DFT) computations. The fully optimized crystal structures were used to study the elastic stiffness, compliance, bulk, and young moduli, the Poisson ratios, and anisotropy constants in both single crystal and polycrystalline compounds. Moreover, the electronic structure including the bandgaps, density of states, and magnetic states was also determined. The bandgap values of 2.98 eV, 2.64 eV, 2.70 eV, and 1.86 eV were obtained for MNb2O6 (M = Mn, Fe, Co, Ni), respectively. The Young’s modulus in all samples showed anisotropic behavior. The sound velocity and Debye’s temperature showed a decrease across the series MNb2O6 (M = Mn, Fe, Co, Ni). All four compounds have an antiferromagnetic ground state and the spin-only moments of these ions were found to be 4.55 μB for Mn3+, 3.62 μB for Fe3+, 2.64 μB for Co3+, and 1.68 μB for Ni3+. Overall, this study provides a comprehensive theoretical investigation of the basic materials properties of MNb2O6 (M = Mn, Fe, Co, Ni) which will enable the researcher working on the application of these materials in devices to carefully tailor their properties according to the application requirements.

Competitive adsorption between sodium citrate and naphthenic acids on alumina surfaces: Experimental and computational study

The adsorption of naphthenic acids (NAs) on clay surfaces induces an increase in surface hydrophobicity, which is detrimental to crude oil recovery and froth quality. Sodium citrate (Na3Cit), with more functional groups than NAs, is expected to compete with NAs for adsorption on mineral surfaces. The adsorption of NAs on alumina surfaces was quantified in the presence and absence of Na3Cit at pH 8 ± 0.1. An increase in hydrophobicity of the alumina surface was observed after the adsorption of NAs without Na3Cit. However, adding Na3Cit significantly reduced NAs adsorption and protected the surface wettability. The viscoelasticity and electrokinetic properties of adsorbed materials indicate Na3Cit was preferentially covered on alumina surfaces and repelled NAs by its negative charges. The competition between Na3Cit and NAs for adsorption sites was explained using ab initio Car-Parrinello molecular dynamics (CPMD) simulation and periodic plane-wave density functional theory (DFT) to calculate inner-sphere adsorption and outer-sphere adsorption, respectively. Favourable adsorption of sodium citrate than NAs representatives on aluminum (hydr)oxide cluster and the basal and edge planes of gibbsite were observed from the perspective of free energy barrier and adsorption energy, respectively.

Oxidation Process of Ni Cathode in CO2 Electrolysis in SOEC: X-Ray and DFT Study

Solid oxide electrolysis cells (SOECs) can convert H2O and CO2 to syngas(H2/CO) at high temperature, promising a way in the power-to-gas process. A Ni electrode such as nickel/yttria-stabilized zirconia (Ni/YSZ) and nickel/scandia-stabilized zirconia (Ni/ScSZ) have widely used as a cathode for SOECs. Meanwhile, the cathode faced severe oxidizing condition due to a high steam and CO2 content. H2, a reduction agent, can inhibit the cathode oxidation; however, an additional H2 decreases a total energy efficiency. Oxidation tolerance is an important parameter for the material candidate as the cathode. A combination study of X-ray and DFT study is a promising way to understand oxidation mechanisms of Ni cathode, which provides an useful hint in the cathode design with oxidation tolerance.In this study, oxidation process of Ni cathode in CO2 electrolysis was studied using X-ray and density functional theory (DFT). Ni cathodes (Ni/YSZ, Ni/ScSZ) were used in H2/CO2 environment under the electrolysis mode. After CO2 electrolysis, Ni cathode surface was analyzed using SEM/EDS and X-ray diffraction (XRD). Influence of H2/CO2 ratio and operating condition on cathode oxidization was studied in the electrolysis mode. All calculations here are performed employing the plane wave density functional theory (DFT) method implemented in the Vienna Ab-Initio Simulation Package (VASP).XRD results showed that the cathode oxidation was progressed at H2/CO2 ratio of 0, whereas the cathode oxidation was inhibited when the H2/CO2 ratio was higher than 0.75 at 800 oC. In addition, it was suggested that the electrolysis mode, in which oxygen was moved from the cathode to the anode, had a role in inhibiting the cathode oxidation. In the DFT calculation, an activation energy of CO2 decomposition at the triple phase boundary (CO2→CO+O,vacancy) was much lower than that at the Ni surface (CO2→CO+O,Ni), indicating that CO2 electrolysis was more preferable than Ni oxidation. It was shown that the electrolysis mode can enhance the oxidation tolerance of Ni cathode. Moreover, Ni/YSZ and Ni/ScSZ cathode were also studied in terms of CO2 electrolysis and oxidation. The activation energy of CO2 decomposition of Ni/ScSZ was lower than that of Ni/YSZ mainly because O can easily enter the vacancy in the ScSZ. It was shown that the oxidation can be controlled from the electrolyte structure in the electrolysis mode.

Insights into Electrolyte Reactivity and Solid Electrolyte Interphase Formation at the Li Metal Anode Surface from DFT Simulations

Solid electrolyte interphase (SEI) is a critical component of lithium ion batteries, yet is poorly characterized and not well understood. We use plane wave density functional theory simulations to perform a comprehensive study of the degradation reaction mechanisms of four electrolyte solvents and additives, ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC), at the Li (001) metal anode surface. The calculations provide fundamental insights into electrolyte decomposition and spontaneous SEI formation. Utilizing automated workflows implemented in the Schrödinger Materials Science Suite, we compute the adsorption energies and calculate the reaction energies for each decomposition pathway. Furthermore, we employ nudged elastic band calculations to compute the decomposition reaction barriers and provide mechanistic insights into the onset of SEI formation. We analyze trends in decomposition reaction energies and the relationships between the reaction energies and the activation barriers for the various electrolyte molecules as a function of applied bias potential. We find that the preferred decomposition pathways are different for the solvent molecules (EC and PC) than for the additives (FEC and VC). Calculations also suggest that applied bias reduces decomposition reaction barriers thus enhancing SEI formation. Such a comprehensive dataset of reaction energies and activation barriers provides useful benchmarks for the development and validation of reactive machine learned force fields for modeling advanced battery chemistries at a larger scale.

All-electron APW+lo calculation of magnetic molecules with the SIRIUS domain-specific package.

We report APW+lo (augmented plane wave plus local orbital) density functional theory (DFT) calculations of large molecular systems using the domain specific SIRIUS multi-functional DFT package. The APW and FLAPW (full potential linearized APW) task and data parallelism options and the advanced eigen-system solver provided by SIRIUS can be exploited for performance gains in ground state Kohn-Sham calculations on large systems. This approach is distinct from our prior use of SIRIUS as a library backend to another APW+lo or FLAPW code. We benchmark the code and demonstrate performance on several magnetic molecule and metal organic framework systems. We show that the SIRIUS package in itself is capable of handling systems as large as a several hundred atoms in the unit cell without having to make technical choices that result in the loss of accuracy with respect to that needed for the study of magnetic systems.

A first-principles study of electronic and magnetic properties of 4d transition metals doped in Wurtzite GaN for spintronics applications.

We studied the electronic and magnetic properties of wurtzite GaN (w-GaN) doped with different concentrations of the 4d transition metal ions Nb, Mo, and Ru. We incorporated spin-polarized plane-wave density functional theory within an ultrasoft pseudopotential formalism. The 4d transition metals were doped at different geometrical sites to determine the geometry with the lowest total energy and the one that induced the largest magnetization. A spin-spin interaction study was performed to determine whether the doped compound was ferromagnetic or antiferromagnetic. The origin of magnetization in the transition metal-doped w-GaN compounds is due to the p-d hybridization of the nitrogen and 4d transition metals. From the bulk modulus results, we inferred that the structural integrity is preserved under compressive loads after doping w-GaN with these 4d transition metal ions. Our results indicate that these compounds can be used in spintronic applications.

An open database of computed bulk ternary transition metal dichalcogenides

We present a dataset of structural relaxations of bulk ternary transition metal dichalcogenides (TMDs) computed via plane-wave density functional theory (DFT). We examined combinations of up to two chalcogenides with seven transition metals from groups 4–6 in octahedral (1T) or trigonal prismatic (2H) coordination. The full dataset consists of 672 unique stoichiometries, with a total of 50,337 individual configurations generated during structural relaxation. Our motivations for building this dataset are (1) to develop a training set for the generation of machine and deep learning models and (2) to obtain structural minima over a range of stoichiometries to support future electronic analyses. We provide the dataset as individual VASP xml files as well as all configurations encountered during relaxations collated into an ASE database with the corresponding total energy and atomic forces. In this report, we discuss the dataset in more detail and highlight interesting structural and electronic features of the relaxed structures.

Finite-temperature effects on the x-ray absorption spectra of crystalline alumina from first principles

By including phonon-assisted transitions within plane-wave density functional theory methods for calculating the x-ray absorption spectrum (XAS), we obtain the Al K-edge XAS at 300 K for two crystalline Al2O3 phases. The 300 K XAS reproduces the pre-edge peak for α-Al2O3, which is not visible at the static lattice level of approximation. Configurations from Monte Carlo sampling of the γ-Al2O3 phase space at the 300 K XAS correctly describe two out of the three experimental peaks. We show that the second peak arises from 1s to mixed s-p transitions and is absent in the 0 K XAS. This letter serves as an insight into the electronic origins of the characteristic peaks in the Al K-edge XAS for alumina crystals.

Synthesis and Characterization of Xylazine Hydrochloride Polymorphs, Hydrates, and Cocrystals: A 35Cl Solid-State NMR and DFT Study

Xylazine HCl (X) is a veterinary analgesic with many known solid forms, making it an ideal system for studying the noncovalent interactions, such as hydrogen bonding, that provide stability to polymorphs, solvates/hydrates, and cocrystal of pharmaceuticals. Herein, we report methods for the reliable preparation and interconversion of polymorphs of X (including mechanochemical pathways), the discovery of a novel polymorph, and the synthesis of three cocrystals with coformers containing amide and carboxylic acid moieties. An understanding of ball milling protocols is essential for optimizing these reactions and ensuring clean and reproducible syntheses of the products in high yields. All materials were characterized using thermal analysis, powder and single-crystal X-ray diffraction (PXRD and SCXRD), and multinuclear solid-state NMR (SSNMR) spectroscopy. 35Cl SSNMR is highlighted for its versatility for fingerprinting polymorphs, hydrates, and cocrystals (including the detection of impurity phases that are not always evident from PXRD and offering an avenue for optimizing synthetic protocols) and providing molecular-level structural information. The 35Cl electric field gradient (EFG) tensor is extremely sensitive to the unique hydrogen-bonding network in each solid form of X, resulting in distinct powder patterns. Dispersion-corrected plane-wave density functional theory (DFT) structural refinements yield better models of the hydrogen-bonding environments of the chloride ions than is possible through XRD methods alone. Calculations employing the refined structures yield 35Cl EFG tensors that agree well with experiment. PXRD and 35Cl SSNMR, in tandem with reliable calculations of EFG tensors, are essential for the development of NMR crystallographic and crystal structure prediction protocols and crucial for future studies involving HCl salts and their concomitant solid forms.

Interfacial Chemistry in the Electrocatalytic Hydrogenation of CO2 over C-Supported Cu-Based Systems.

Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)-O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe-Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)-O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu-Zn system represents the optimal active ensembles with stabilized Cu(I)-O; DFT simulations rationalize this observation by indicating that Cu-Zn-O neighboring atoms are able to activate CO2, whereas Cu-Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies.

First-Principles Study of Electronic Structure and Magnetic Properties of SnO2 Co-doped with Transition Metals (Mo, Ru, Rh, and Pd) and Oxygen Vacancies (VO)

The formation energies, electronic structures, and magnetic properties of a series of SnO2 systems co-doped with transition metals (Mo, Ru, Rh, and Pd) and oxygen vacancies (VO) were investigated using plane-wave density functional theory with ultra-soft pseudopotentials. The results show that the formation energy of the Mo-VO-SnO2 system was the smallest among the doped systems, indicating that the system was the easiest to form. The net magnetic moments of the X-VO-doped SnO2 systems (X = Mo, Ru, and Rh) are not zero, indicating that the systems are all in the ferromagnetic state. Among them, the Mo-VO-doped SnO2 system had an enormous net magnetic moment, highest spin polarization rate, and best ferromagnetism, making it an excellent ferromagnetic candidate material. The ferromagnetism of the doping systems originates from the strong hybridization of the 4d orbitals of the unpaired electrons in the dopants. The VO-SnO2 does not exhibit any magnetic properties. Therefore, VO doesn’t play a role in the generation of magnetism

Elastic properties of disordered binary hcp-Fe alloys under high pressure: Effects of light elements

The effects of light elements on the elastic properties of disordered binary hcp-Fe alloys were investigated at high pressures using plane-wave density functional theory combined with the Monte Carlo special quasi-random structure method. We found that the increase in the O content in hcp-Fe had a more pronounced effect on the sound velocity than Si, S, and C. The longitudinal wave velocity was decreased by ∼ 6% with 2% O content, which was a much greater decrease than the values of 0.6% and 2% induced by the same content of Si and S, respectively, under high pressures. Compared with the other three light elements, the longitudinal wave velocity of the Fe-C alloy exhibited the most gradual decreasing with increasing C content. In addition, the effects of different O and S contents on the anisotropy of hcp-Fe alloys strongly depended on the variation in pressure, whereas the pressure only slightly affected the anisotropy of Fe-Si alloy systems.

A DFT Study of Ruthenium fcc Nano-Dots: Size-Dependent Induced Magnetic Moments.

Many areas of electronics, engineering and manufacturing rely on ferromagnetic materials, including iron, nickel and cobalt. Very few other materials have an innate magnetic moment rather than induced magnetic properties, which are more common. However, in a previous study of ruthenium nanoparticles, the smallest nano-dots showed significant magnetic moments. Furthermore, ruthenium nanoparticles with a face-centred cubic (fcc) packing structure exhibit high catalytic activity towards several reactions and such catalysts are of special interest for the electrocatalytic production of hydrogen. Previous calculations have shown that the energy per atom resembles that of the bulk energy per atom when the surface-to-bulk ratio < 1, but in its smallest form, nano-dots exhibit a range of other properties. Therefore, in this study, we have carried out calculations based on the density functional theory (DFT) with long-range dispersion corrections DFT-D3 and DFT-D3-(BJ) to systematically investigate the magnetic moments of two different morphologies and various sizes of Ru nano-dots in the fcc phase. To confirm the results obtained by the plane-wave DFT methodologies, additional atom-centred DFT calculations were carried out on the smallest nano-dots to establish accurate spin-splitting energetics. Surprisingly, we found that in most cases, the high spin electronic structures had the most favourable energies and were hence the most stable.

A comparative ab initio study of acetylene activation over Au9 cluster and Au9 supported over ZnO monolayer

A plane wave density functional theory has been employed to investigate acetylene activation over Au9 cluster and the same supported over single layer ZnO. Adsorption of C2H2 over gold clusters adopts the π mode and σ mode. The σ mode of adsorption is preferable as two C atoms bind to least coordinated Au atoms in Aun cluster. Adsorption energy of C2H2 over Au9 is increases from 0.47 eV to 0.87 eV when Au9 is supported on the ZnO single layer.Significantly lower activation energy barrier for acetylene activation (0.08 eV) has been found for Au9/ZnO in comparison to the same over Au9 cluster (2.14 eV). The lowering in the activation energy barrier is attributable to the reduction of Au9: The underlying ZnO monolayer transfers electronic charge to Au9 and thereby reduces it. It is supported by upshift in the Fermi level of the ZnO single layer by 0.47 eV towards higher energies upon hosting Au9. These results provide useful pointers in optimizing the catalytic processes via an appropriate choice of support for metal clusters serving as catalysts.

A coupled quantum-molecular mechanics approach for performance analysis of defective Silicon based photovoltaic solar cells

Although, molecular mechanics (MM) based approaches are capable of simulating the dynamic charge motion of atoms over time scales up to femto-seconds, the accuracy is an issue. As a result, quantum-mechanics (QM) based approaches are a choice to predict accurate dynamic motion of atomic charges. However, computational cost of QM simulations is significantly higher than that of MM simulations. In this study, a computationally efficient coupled QM/MM model is developed by combining the QM and MM approaches, enabling simulation of larger domains with accurate estimates. The proposed methodology is implemented with the help of QMMM package available in large-scale atomic/molecular massively parallel simulator (LAMMPS), to investigate the dynamic charge motion in the presence of cracks in Silicon. A Silicon domain of dimensions 48.87 Å × 48.87 Å × 5.43 Å is considered in the simulations. Furthermore, a small domain around the crack tip, with dimensions 4 Å × 4 Å is identified for carrying out QM analysis and denoted as embedded region (ER). Simulations are performed considering four different cases: (i) pristine Silicon, (ii) Silicon with an initial edge crack, (iii) pristine Silicon with Graphene deposition, and (iv) Graphene deposited Silicon containing an initial edge crack. In the coupled model, first, for the given load step, considering the minimum energy criteria molecular dynamics simulations are performed over the entire domain. This is followed by QM simulations over an identified ER based on first principle studies using the plane wave density functional theory. The latest atom positions from the QM simulations are updated in the MM domain before proceeding to the next load step. The electrical performance of Silicon solar cells is studied by estimating the effective Bader charge and total electric power. The effective Bader charge for atoms in QM domain is observed to be significantly higher indicating charge accumulation around the crack tip. This is further evidenced through the total electric power estimations, where pristine Silicon with Graphene deposition is observed to possess the highest power followed by cases iv, ii and i.