First-principles Computational Study Research Articles

Enhancing electronic, magnetic, and elastic properties of SmFe2 by Mn substitution at Fe sites: A first-principle computational study

In this comprehensive theoretical investigation, we present the effects of manganese (Mn) substitution for iron (Fe) in samarium iron (SmFe2) on its electronic, magnetic, elastic, and magnetostriction properties, utilizing state-of-the-art Density Functional Theory (DFT) within the Generalized Gradient Approximation (GGA) complemented by the Full Potential Linearized Augmented Plane Wave (FP-LAPW) method. Our investigations are further enriched by Monte Carlo simulations framed within the Ising model, providing a comprehensive understanding of the material's behavior under Mn doping. Our findings reveal a significant shift in magnetic anisotropy from the [100] direction in pristine SmFe2 to the [111] direction following Mn substitution. This transition enhances the material's magnetic favorability and introduces new electronic states at the Fermi level, increasing the number of states available for conduction and enhancing its metallic behavior. Consequently, this transition results in altered elastic constants and magnetostriction values, highlighting a delicate interplay between the material's structural and magnetic properties that warrants further exploration. By developing a Hamiltonian model, we delve into the intricate coupling among magnetic atoms within SmFe2, offering insights into the underlying mechanisms governing the observed phenomena. Monte Carlo simulations further corroborate the robust ferromagnetic ordering of SmFe2, evidenced by a high critical temperature (Tc = 651.3 K).

First-principles computational study of Janus van der Waals layered VSiGeN4 as anode material for Li-ion battery

An emerging class of two-dimensional Janus materials has received increasing attention due to their unique non-reflective symmetry crystal structure. In order to find a potential anode material in this two-dimensional Janus materials, the kinetic and thermodynamic stability of VSi2N4, VGe2N4, VSiGeN4 structures were investigated. It is discovered that the stability of the VSiGeN4 structure is significantly better than VGe2N4 and is essentially similar to VSi2N4. Additionally, the addition of Ge atoms can significantly boost conductive performance and enhance charge and discharge rates. The usage of VSiGeN4 as an appropriate commercial anode material was demonstrated by its capacity of up to 1291.6 mAh/g and the relevant range of OCV fluctuations. As mentioned above, 2D VSiGeN4 will be a promising anode material for Li ion battery.

First-principles computational study of two-dimensional BeB2 monolayer as a material for negative high-rate sodium ion batteries

Sodium-ion batteries are of interest due to their low cost and abundance of natural resources. However, the exploration of suitable anode materials is a major challenge in advancing the technology of sodium-ion batteries. Herein, a novel type of Dirac cone two-dimensional material (BeB2) was systematically investigated based on first-principles calculations for its potential performance in sodium-ion batteries. The results show that BeB2 has good stability and conductivity, and the theoretical capacity of sodium atoms for monolayer BeB2 is 874.9 mAh g−1. The diffusion barrier is 0.231 eV, and the average open-circuit voltage is as low as 0.08 V. Encouragingly, the lattice change is only 1.2 %, and at the same time, the adsorption of sodium atoms also significantly improves the conductivity of BeB2. The above results indicate that BeB2 is a promising candidate as an anode material for high rate sodium-ion batteries.

Effect of point defects on water adsorption on the ThO2{111} surface: A first-principles computational study

H adatom and O vacancy point defects, and their effects on water adsorption, are studied computationally on the ThO2 {111} surface. Comparisons are made with the perfect {111} surface. For H adatoms, two different coverages (1/8 and ¼ ML) are investigated. H adatoms increasingly tend to bond at the nearby Osub position with increasing H coverage. The surface work function linearly increases with H coverage, resembling the classic catalytic transition metals Pt, W, Ni and Pd. For the O vacancies, those formed in the bulk are energetically favoured over those formed at the surface. Due to these point defects, the surface symmetry breaks, and hence water adsorptions at different sites are compared. The adsorption energy is linearly correlated with the bonded Th f band centre in the conduction band, similar to the d band theory for transition metal catalysts.

Investigations of Band Alignment at Interfaces of Au/Polypropylene and Au/Polyimide

As charge injection from metal into dielectric polymer causes degradation and breakdown of electronic and power devices, the band alignment at the metal/polymer interface should be revealed. To investigate the dominant factors of the band alignment at the metal/polymer interface, we carried out a combined experimental and first-principles computational study. We examined Au/polypropylene (PP) interface and Au/polyimide (PI) interface. PP and PI were chosen as a model system for nonpolar and polar polymers, respectively. The results imply that the band alignment can be determined under the assumption that vacuum levels of metal and polymer match at the interface regardless of the dipole moment of polymers.

First-principles computational study on adsorption and inhibition mechanism of kinetic hydrate inhibitors

Kinetic hydrate inhibitors (KHIs) have been used in the oil and gas industry as a cost-effective hydrate mitigation strategy, but the intrinsic inhibition mechanisms are less conclusive and require further verification. In this study, we have focus on the adsorption of KHIs on the metal surfaces to understand the inhibition performance by performing the first-principles computational method. The results show that KHIs are preferentially adsorbed onto the Fe surface than the Fe3O4 surface, and they accept electrons from the Fe surface, but donate electrons to the Fe3O4 surface. The hydrophobic interaction plays a dominate role in the adsorption affinity, and the electron transfer between KHIs and the surface enhances the adsorption stability. In addition, the adsorption affinity of four poly(N-vinyl lactam)s with 5–8-membered rings is compared, and PVCap that has a 7-membered ring shows the strongest adsorption strength on the Fe surface.

Accounting for super-, plateau- and mesa-rate burning by lead and copper-based ballistic modifiers in double-base propellants: a computational study.

We present a first-principles computational study to understand the action of lead and copper-based ballistic modifiers in the combustion of double-base propellants (DBPs). We show that lead oxide clusters are easily broken down upon addition of small amounts of carbon and the resulting graphitic matrix, dispersed with weakly bound and exposed Pb sites, acts as a Lewis acid to bind small molecule Lewis bases such as NO2 and CH2O that form in the combustion flame. This accounts for super-rate burning, where the fuel burn rate is enhanced. We also show how carbon availability accounts for the plateau- and mesa-rate burning effects, where the fuel burn rate is suppressed. In contrast, cluster integrity on binding carbon to copper oxide is retained, and interaction with NO2 and CH2O is essentially negligible. Carbon binds more strongly to copper oxide, however, and we therefore propose that when carbon levels start to fall this results in the lead oxide clusters being starved of carbon, which leads to plateau and mesa burning. Taken together, the calculations support a general model that accounts for the super-, plateau- and mesa-rate ballistic modifier burning effects.

First-Principles Computational Study of the Modification Mechanism of Graphene/Graphene Oxide on Hydroxyapatite

Due to its unique crystal structure and nano-properties, hydroxyapatite (HA) has become an important inorganic material with broad development prospects in electrical materials, for fire resistance and insulation, and in bone repair. However, its application is limited to some extent because of its low strength, brittleness and other shortcomings. Graphene (G) and its derivative graphene oxide (GO) are well known for their excellent mechanical properties, and are widely used to modify HA by domestic and foreign scholars, who expect to achieve better reinforcement and toughening effects. However, the enhancement mechanism has not been made clear. Accordingly, in this study, G and GO were selected to modify HA using the first-principles calculation method to explore the theory of interfacial bonding of composites and explain the microscopic mechanism of interfacial bonding. First-principles calculation is a powerful tool used to solve experimental and theoretical problems and predict the structure and properties of new materials with precise control at the atomic level. Therefore, the bonding behaviors of hydroxyapatite (100), (110) and (111) crystal planes with G or GO were comprehensively and systematically studied using first-principles calculation; this included analyses of the density of states and differential charge density, and calculations of interfacial adhesion work and elastic moduli. Compared to HA (100) and (111) crystal planes, HA (110) had the best bonding performance with G and with GO, as revealed by the calculation results. The composite material systems of HA (110)/G and HA (110)/GO had the smallest density of states at the Fermi level, the largest charge transfers of Ca atoms, the largest interfacial adhesion work and the most outstanding elastic moduli. These results provide a theoretical basis for the modification of HA to a certain extent, and are beneficial to the expansion of the scope of its application.

First-principles computational study on structural, elastic, magnetic, electronic, and thermoelectric properties of Co2MnGe: a potential Heusler ternary compound

First-principles computational study on structural, elastic, magnetic, electronic, and thermoelectric properties of Co2MnGe: a potential Heusler ternary compound

Quasiparticle self-consistent GW band structures and phase transitions of LiAlO2 in tetrahedrally and octahedrally coordinated structures

A first-principles computational study is presented of various phases of LiAlO$_2$.The $\beta$ and $\gamma$ tetrahedral phases are found to be very close in energy with the $\gamma$ phase having the lowest energy. The octahedral $\alpha$ phase is a high-pressure phase and the transition pressure from the $\gamma$ and $\beta$ phases to $\alpha$ is determined to be about 1 GPa. The electronic band structures are determined using the quasiparticle self-consistent (QS) $GW$ method. The effective masses of the band edges and the nature of the band gaps are presented. The lowest energy $\gamma$ phase is found to have a pseudodirect gap of 7.69 eV. The gap is direct at $\Gamma$ but corresponds to a dipole forbidden transition. The imaginary part of the dielectric function and the absorption coefficient are calculated in the long-wavelength limit and the random phase approximation, without local field or electron-hole interaction effects for each phase and their anisotropies are discussed. Si doping on the Al site is investigated as a possible $n$-type dopant in $\gamma$-LiAlO$_2$ using a 128 atom supercell corresponding to 3.125 \% Si on the Al sublattice in the generalized gradient approximation and a smaller 16 atom cell with 25 \% Si in the QS$GW$ approximation. The Si is found to significantly perturb the conduction band and lower the gap but a clearly separated deep donor defect level is not found. However, the donor binding energy is still expected to be relatively deep, of order a few 0.1 eV in the hydrogenic effective mass approximation.

(Invited) Small Polarons in Metal-Oxide Based Solar Absorber Materials

The key materials for photo-electrochemical (PEC) processes are the photoelectrocatalysts that are required to absorb sunlight, form electron-hole pairs, and transport the charge carriers efficiently towards the electrolyte-semiconductor interface. Metal-oxides are thought to be the most stable materials under intense interfacial reactive conditions. However, the overall efficiencies of these metal-oxide photoelectrocatalysts are not as high as expected, even for those with near-suitable bandgap energies required for efficient solar energy absorptions. One of the main reasons is attributed to the poor transport properties in metal-oxides with strongly correlated electrons. Typically, strongly correlated systems exhibit polaron conduction via electron-phonon interactions. Polarons act like a bottleneck for photoconductions in such metal-oxides. Recently, there have been significant efforts to characterize polaronic states and their effects on transport properties in photoactive materials. First-principles theoretical and computational studies can reveal the electronic origin of the polaron formation and, therefore, can suggest how to improve overall transport properties in such photoactive materials. In this presentation, representative metal-oxides photo-catalysts will be considered to demonstrate the small polaron formation mechanism. The polaronic states in metal-oxides will be depicted from detailed electronic structure calculations by density functional theory (DFT) based methods. We will show how these polaronic states affect the PEC cell's transport properties and photo-voltages. We will also discuss how these computational results compare with the recent experimental outcome. Some part of the work was funded by National Science Foundation, USA. Computations were performed at the Texas Advanced Computing Center (TACC).

Insights into LiMXO4F (M-X = Al-P and Mg-S) as Cathode Coatings for High-Performance Lithium-Ion Batteries.

Cathode coatings have received extensive attention due to their ability to delay electrochemical performance degradation in lithium-ion batteries. However, the development of cathode coatings possessing high ionic conductivity and good interfacial stability with cathode materials has proven to be a challenge. Here, we performed first-principles computational studies on the phase stability, thermodynamic stability, and ionic transport properties of LiMXO4F (M-X = Al-P and Mg-S) used as cathode coatings. We find that the candidate coatings are thermodynamically metastable and can be synthesized experimentally. The coating materials possess high oxidative stability, with the materials predicted to decompose above 4.2 V, suggesting that they have good electrochemical stability under a high-voltage cathode. In addition, the candidate coatings exhibit significant chemical stability when in contact with oxide cathodes. Finally, we have studied the Li-ion transport paths and migration barriers of LiMXO4F (M-X = Al-P and Mg-S) and calculated the low migration barriers to be 0.19 and 0.09 eV, respectively. Our findings indicate that LiMXO4F (M-X = Al-P and Mg-S) are promising cathode coatings, among which LiAlPO4F has been experimentally confirmed. The theoretical cathode coating computational methods presented here can be extended to the solid-state battery system.

First-principles computational studies on Na+ diffusion in Li-doped P3-type NaMnO2 as cathode material for Na-ion batteries

First-principles computational studies on Na+ diffusion in Li-doped P3-type NaMnO2 as cathode material for Na-ion batteries

B/C-doped MoS2 as promising materials for nitrogen removal from natural gas: A first-principles computational study

N2 removal is of great significance to the industrial production and usage of natural gas. It is necessary to find new methods and materials for separating nitrogen (N2) from methane (CH4) with high selectivity and efficiency. Molybdenum disulfide (MoS2) is a promising two-dimensional material. In this work, density functional theory (DFT) was applied to study the influences of non-mental (B, C, N, O and P) doping in 1H- or 1T'-MoS2 monolayer on the adsorption behaviors of N2 and CH4. The result shows that the interaction between either gas and pristine MoS2 is very weak, but that between N2 and MoS2 can be significantly improved by doping B or C. Nevertheless, the adsorption of CH4 is insensitive to such doping. The DFT computational results indicate that doping of B or C in MoS2 is a selective and efficient method to separate N2 from CH4. This study provides valuable information for designing novel approaches and advanced materials for N2 removal from natural gas with high efficiency.

Tuning the Electronic, Ion Transport, and Stability Properties of Li-rich Manganese-based Oxide Materials with Oxide Perovskite Coatings: A First-Principles Computational Study.

Lithium-rich manganese-based oxides (LRMO) are regarded as promising cathode materials for powering electric applications due to their high capacity (250 mAh g–1) and energy density (∼900 Wh kg–1). However, poor cycle stability and capacity fading have impeded the commercialization of this family of materials as battery components. Surface modification based on coating has proven successful in mitigating some of these problems, but a microscopic understanding of how such improvements are attained is still lacking, thus impeding systematic and rational design of LRMO-based cathodes. In this work, first-principles density functional theory (DFT) calculations are carried out to fill out such a knowledge gap and to propose a promising LRMO-coating material. It is found that SrTiO3 (STO), an archetypal and highly stable oxide perovskite, represents an excellent coating material for Li1.2Ni0.2Mn0.6O2 (LNMO), a prototypical member of the LRMO family. An accomplished atomistic model is constructed to theoretically estimate the structural, electronic, oxygen vacancy formation energy, and lithium-transport properties of the LNMO/STO interface system, thus providing insightful comparisons with the two integrating bulk materials. It is found that (i) electronic transport in the LNMO cathode is enhanced due to partial closure of the LNMO band gap (∼0.4 eV) and (ii) the lithium ions can easily diffuse near the LNMO/STO interface and within STO due to the small size of the involved ion-hopping energy barriers. Furthermore, the formation energy of oxygen vacancies notably increases close to the LNMO/STO interface, thus indicating a reduction in oxygen loss at the cathode surface and a potential inhibition of undesirable structural phase transitions. This theoretical work therefore opens up new routes for the practical improvement of cost-affordable lithium-rich cathode materials based on highly stable oxide perovskite coatings.

V4C3TX MXene: First-principles computational and separator modification study on immobilization and catalytic conversion of polysulfide in Li-S batteries

Many attempts have recently used rationally-designed Ti3C2Tx MXene-based materials to increase sulfur utilization and tackle the detrimental shuttle effect in Li–S batteries (LSBs) due to their merits of high electronic conductivity, considerable catalytic activity, and sulfur immobilization. Nevertheless, the investigation of applying other two-dimensional (2D) transition metal carbides in LSBs is comparatively rare. In this work, the first-principles computations predicted that V4C3Tx could boost the “adsorption-diffusion-conversion” process of lithium polysulfides (LiPSs) over that of most other metal carbides of the MXene family. Inspired by this, we prepared the V4C3Tx MXene by hydrofluoric acid (HF) etching and then used it as a functional material coating on a polypropylene (PP) separator for LSB. As expected, the V4C3Tx modified PP separator (V4C3Tx-PP) can effectively prevent the shuttle effect of LiPSs via physical blocking, chemical adsorption, and catalytic conversion, as confirmed by visual polysulfide adsorption and diffusion tests, XPS analysis, and a series of electrochemical evaluations. As a result, the LSB with a V4C3Tx-PP enabled a high capacity and enhanced cycling performance (927 mAh g−1 at 1 C and 516 mAh g−1 retained for over 800 cycles, 1 C = 1675 mA g−1). More encouragingly, the cell achieves a superior rate capability of 725 mAh g−1 at 2 C and 586 mAh g−1 at 4 C, respectively. In addition, the V4C3Tx-PP-based LSB shows a high areal capacity of 4.3 mAh cm−2, even with the sulfur loading up to 4 mg cm−2. This work expands the application types and scope of MXenes from theoretical and experimental points of view. The first use of the V4C3Tx MXene modified separator in Li-S batteries creates high potential for practical application.

Thermodynamics of carbon point defects in hexagonal boron nitride

We present a first-principles computational study of the thermodynamics of carbon defects in hexagonal boron nitride (hBN). The defects considered are carbon monomers, dimers, trimers, and larger carbon clusters, as well as complexes of carbon with vacancies, antisites, and substitutional oxygen. Our calculations show that monomers ($\text{C}_{\text{B}}$, $\text{C}_{\text{B}}$), dimers, trimers, and $\text{C}_{\text{N}}\text{O}_{\text{N}}$ pairs are the most prevalent species under most growth conditions. Compared to these defects, larger carbon clusters, as well as complexes of carbon with vacancies and antisites, occur at much smaller concentrations. Our results are discussed in view of the relevance of carbon defects in single-photon emission in hBN.

Delving into guest-free and He-filled sI and sII clathrate hydrates: a first-principles computational study.

The dynamics of the formation of a specific clathrate hydrate as well as its thermodynamic transitions depend on the interactions between the trapped molecules and the host water lattice. The molecular-level understanding of the different underlying processes benefits not only the description of the properties of the system, but also allows the development of multiple technological applications such as gas storage, gas separation, energy transport, etc. In this work we investigate the stability of periodic crystalline structures, such as He@sI and He@sII clathrate hydrates by first-principles computations. We consider such host water networks interacting with a guest He atom using selected density functional theory approaches, in order to explore the effects on the encapsulation of a light atom in the sI/sII crystals, by deriving all energy components (guest-water, water-water, guest-guest). Structural properties and energies were first computed by structural relaxations of the He-filled and empty sI/sII unit cells, yielding lattice and compressibility parameters comparable to experimental and theoretical values available for those hydrates. According to the results obtained, the He enclathration in the sI/sII unit cells is a stabilizing process, and both He@sI and He@sII clathrates, considering single cage occupancy, are predicted to be stable whatever the XDM or D4 dispersion correction applied. Our results further reveal that despite the weak underlying interactions the He encapsulation has a rather notable effect on both lattice parameters and energetics, with the He@sII being the most energetically favorable in accord with recent experimental observations.

Theoretical insights into interfacial stability and ionic transport of Li2OHBr solid electrolyte for all-solid-state batteries.

Li-rich antiperovskite materials are promising candidates as inorganic solid electrolytes (ISEs) for all-solid-state Li-ion batteries (ASSLIBs). However, the material faces several pressing issues for its application, concerning the phase stability and electrochemical stability of the synthesized material and the Li-ion transport mechanism in it. Herein, we performed first-principles computational studies on the phase stability, interfacial stability, defect chemistry, and electronic/ionic transport properties of Li2OHBr material. The calculation results show that the Li2OHBr is thermodynamically metastable at 0 K and can be synthesized experimentally. This material exhibits a wider intrinsic electrochemical stability window (0.80-3.15 V) compared with sulfide solid electrolytes. Moreover, the Li2OHBr displays significant chemical stability when in contact with typical cathode materials (LiCoO2, LiMn2O4, LiFePO4) and moisture. The dominant defects of Li2OHBr are predicted to be VLi- and Lii +, corresponding to lower Li-ion migration barriers of 0.38 and 0.49 eV, respectively, while the replacement of some of the OH- by F- is shown to be effective in decreasing migration barriers in Li2OHBr. These findings provide a theoretical framework for further designing high performance ISEs.

Calculated phonon modes, infrared and Raman spectra in orthorhombic α − MoO3 and monolayer MoO3

Orthorhombic α−MoO3 is a layered oxide with various applications and with excellent potential to be exfoliated as a 2D ultra-thin film or monolayer. In this paper, we present a first-principles computational study of its vibrational properties. Our focus is on the zone center modes, which can be measured by a combination of infrared and Raman spectroscopy. The polarization dependent spectra are simulated. Calculations are also performed for a monolayer form in which “double layers” of Mo2O6, which are weakly van der Waals bonded in the α-structure, are isolated. Shifts in phonon frequencies are analyzed.