First Principles Calculations

Rising levels of carbon dioxide (CO 2) are of significant concern in modern society, as they lead to global warming and consequential environmental and societal changes. It is of importance to develop industries with a zero or negative CO 2 footprint. Electrochemistry, where one of the reagents is electrons, is an environmentally clean technology that is capable of addressing the conversion of CO 2 to value-added products. The key factor in the process is the use of catalytic electrode materials that lead to the desired reaction and product. Significant progress in this field has been achieved in the past two years. This review discusses the progress in the development of electrocatalysts for CO 2 reduction achieved during this time period.

The generalized gradient approximation (GGA) scheme in the first -principles calculations is used to study the effect of L21 and XA ordering on the phase stability, half-metallicity and magnetism of Co2FeAl (CFA) Heusler alloy. Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II are prepared under the conventional L21 and inverse XA phases by altering the atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of CFA is the most stable phase energetically among all the structures. The electronic structure calculations without U show the presence of half-metallic (HM) ground state only in L21-I structure and the other structures are found to be metallic. However, the electronic structures of CFA are significantly modified in the presence of U, although the total magnetic moments per cell remained the same and consistent with the Slater-Pauling (SP) rule. The metallic ground states of CFA in L21-II and XA-II structures are converted into the half-metallic ground states in presence of U, but remained the same (metallic) in XA-I structure. The results indicate that the electronic structures are not only dependent on the L21 and XA ordering of the atoms but also depend on the choice of U values. So experiments may only verify the superiority of GGA+U to GGA.

Topological edge states at the boundary of quantum spin Hall (QSH) insulators hold great promise for dissipationless electron transport. The device application of topological edge states has several critical requirements for QSH insulator materials, e.g. a large band gap, appropriate insulating substrates, and multiple conducting channels. In this paper, based on first-principles calculations, we show that Bi4Br4 is a suitable candidate. Single-layer Bi4Br4 was recently demonstrated to be a QSH insulator with sizable gap. Here we find that, in multilayer systems, both the band gaps and low-energy electronic structures are only slightly affected by the interlayer coupling. On the intrinsic insulating substrate of bulk Bi4Br4, the single-layer Bi4Br4 preserves its topological edge states well. Moreover, at the boundary of multilayer Bi4Br4, the topological edge states stemming from different single-layers are weakly coupled, and can be fully decoupled by constructing a stair-stepped edge. The decoupled topological edge states are very suitable for multi-channel dissipationless transport. Our work indicates that an ideal QSH insulator can be prepared by nano-fabricaton on the cleaved surface of layered Bi4Br4 single crystal.

Hydroxyapatite (HAp), a primary constituent of human bone, is usually nonstoichiometric with varying Ca/P molar ratios, with the well-known fact that Ca deficiency can cause marked reductions in its mechanical properties. To gain insights into the mechanism of this degradation, we employ first-principles calculations based on density functional theory and determine the effects of Ca deficiency on structure, vibrational, and elastic properties of HAp. Our simulation results confirm a considerable reduction in the elastic constants of HAp due to Ca deficiency, which was experimentally reported earlier. Stress-induced transformation of the Ca-deficient defected structure into a metastable state upon the application of stress could be a reason for this. Local structural stability of HAp and Ca-deficient HAp structures is assessed with full phonon dispersion studies. Further, specific signatures in the computed vibrational spectra for Ca deficiency in HAp can be utilized in experimental characterization of different types of defected HAp.

Aluminum nitride (AlN) is a very important industrial and technological material due to its properties, e.g. high melting point, thermal conductivity, electrical resistivity, mechanical strength, and corrosion resistance. This work represents a detailed study of the mechanical and elastic properties of AlN structures under compression. Theoretical modeling has been performed using quantum mechanical calculations and computed values were compared with the experimental results when available. Structural properties, volume change and strain (dilatation) under high pressure have been investigated for various AlN phases. Elastic constants Cijfor wurtzite, sphalerite, and rock salt structure were calculated under pressure. Important mechanical properties were investigated; bulk modulus B, shear modulus K, Young's modulus E, Vickers hardness Hv, anisotropy, stiffness, Poisson's ratio, brittleness/ductility, in order to investigate the influence of pressure on wurtzite, sphalerite, and rock salt-based AlN materials. Detailed analysis of anisotropic mechanical properties under compression has been performed, as well as the relationship between B, K, EandHv, in order to offer novel technological and industrial applications of AlN.

The generalized gradient approximation (GGA) to density functional theory (DFT) calculations indicate that the highly localized states derived from the defects of nitrogen doped carbon nanotube with divacancy (4ND-CN x NT) contribute to strong Sc and Ti bindings, which prevent metal aggregation. Comparison of the H 2 adsorption capability of Sc over Ti-decorated 4ND-CN x NT shows that Ti cannot be used for reversible H 2 storage due to its inherent high adsorption energy. The Sc/4ND-CN x NT possesses favorable adsorption and consecutive adsorption energy at the local-density approximation (LDA) and GGA level. Molecular dynamics (MD) study confirmed that the interaction between molecular hydrogen and 4ND-CN x NT decorated with scandium is indeed favorable. Simulations indicate that the total amount of adsorption is directly related to the operating temperature and pressure. The number of absorbed hydrogen molecules almost logarithmically increases as the pressure increases at a given temperature. The total excess adsorption of hydrogen on the (Sc/4ND) 10-CN x NT arrays at 300 K is within the range set by the department of energy (DOE) with a value of at least 5.85 wt%. Current echo political issues related to fossil-fuel energy resources have prompted broad interests in clean alternative energy. Biodiesel, solar energy, and wind farms are being pursued as supplements at commercially affordable scales. For automotive uses, hydrogen energy has been considered an ideal substitute for gasoline as it is recycla-ble and non-polluting. High-pressurized tank, liquefied form, or solid phase materials have been suggested and tested for hydrogen storage systems. Yet, none of the candidates suffices the requirements for commercial use in vehicles. Either the storage capacity or operating conditions are far short of the standard set by the Department of Energy (DOE) of the United States. Specific targets for automotive uses are summarized as gravimetric capacity of 5.5 wt% or higher for room temperature operations. Developing appropriate storage media is of the importance for practical application of hydrogen energy. Hydrogen has long been considered as a clean, abundant and efficient energy carrier. Carbon nanotubes (CNT) have attracted much interest due to their many exceptional properties 1–20 , for example an extremely large surface area. Finding efficient methods of metal dispersion will be of great practical importance for developing CNT-based hydrogen storage. Unfortunately, it was demonstrated before that the hydrogen storage capacity of porous CNT at ambient temperature is no more than 1 wt% 9. As an earth-abundant element, nitrogen is widely applied for hydrogen storage with its chemical hydrides and nanostructure forms 9–20. The formation of porphyrin-like (4ND) 9,10 vacancy structures can alter the chemical and physical properties of CNT. It was suggested as an origin for the acceptor like states which are very crucial for enhancing the metal binding to the defects. The 4ND vacancy is characterized by a four-nitrogen divacancy formed by removing two C atom among hexagons and replacing the four surrounding C atoms with four N atoms. The Nitrogen atoms inherently present in 4ND doping in graphitic carbon materials such as CNT have been reported to generate acceptor like states contrary to common perceptions 10–16 .

The I1 intrinsic stacking fault energy (I1 SFE) serves as an alloy design parameter for ductilizing Mg alloys. In view of this effect we
have conducted quantum–mechanical calculations for Mg15X solid-solution crystals (X = Dy, Er, Gd, Ho, Lu, Sc, Tb, Tm, Nd, Pr, Be, Ti, Zr, Zn, Tc, Re, Co, Ru, Os, Tl). We find that Y, Sc and all studied lanthanides reduce the I1 SFE and render hexagonal closed-packed (hcp) and double hcp phases thermodynamically, structurally and elastically similar. Synthesis, experimental testing and characterization of some of the predicted key alloys (Mg–3Ho, Mg–3Er, Mg–3Tb, Mg–3Dy) indeed confirm reduced I1 SFEs and significantly improved room-temperature ductility by up to 4–5 times relative to pure Mg, a finding that is attributed to the higher activity of non-basal dislocation slip.

We have investigated the structural, mechanical, electronic, and thermodynamic properties of AlFe 2 B 2 , FeB, AlB 2 and Al 2 Fe using first-principles calculations. The elastic constants imply the elastic stabilities of these structures. The elastic anisotropy has been depicted by three-dimensional iso-surface of Young's modulus. The electronic densities of states show that the Fed , Al-p and B-p states are hybridized at Fermi level. The phonon dispersion spectra manifest their dynamical stabilities. Furthermore, the free energy suggests the stabilities of these structures in finite temperature range 0-1500 K. Our results suggest the feasibility of the fabrication approach of AlB 2 + 8FeB + 2Al 2 Fe → 5AlFe 2 B 2. Our study provides insights on structure stabilities and might be helpful in the material design and fabrication of Al-Fe-B compounds.

First-principles calculations based on Density Functional Theory have
been done on an important amino acid – L-allo-isoleucine:D-leucine
which is also known as (S,R)-2-amino-3-methylpentanoic acid:R-2-
amino-4-methylpentaoic acid. Its structure has been simulated. The unit
cell is triclinic with lattice parameters a=5.236Å, b=5.425Å and
c=13.573Å showing a space group of P1. Bond lengths and bond angles
have been estimated. Electronic Density of States calculations show that
the material has a band gap of 4.16eV. Electronic band structure indi-
cates that the material can be effectively used for NLO applications. The
dielectric constant has been calculated and its value comes out to be
2.26, 2.22 and 2.13 along X, Y and Z axes respectively and its average
value comes out to be 2.20. Phonon modes at gamma point have also
been computed in L-allo-isoleucine:D-leucine and they range from 88cm -
1 to 5782cm -1 .

This paper will investigate the general class theory of polycalculii, and theory originally developed and introduced on the Quora platform as a kind of philosophy of logic and mathematics. The ostensible goal at minima will be to demonstrate the exclusion of an array of class types which edicate the theorem of the polycalculus. The polycalculus is a general method for deriving equivalent calculus’s other than the traditional intelligent calculus. The domain of this discussion is not only mathematics, but also logic, philosophy, and philosophy of science, as well as philosophy of the foundations of mathematics.

Silicon carbide is one of the most important semiconductors with wide bandgaps and various applications including power electronics, nuclear fuel particles, hostile-environment electronics, and blue light emitting diodes. We investigate the nonlinear mechanical properties of a proposed graphene-like planar hexagonal silicon carbide (g-SiC) monolayer using first-principles calculations. The strength of g-SiC is about half that of graphene. The ultimate strain of g-SiC is 0.2, 0.25, and 0.19, in the direction of in armchair, zigzag, and biaxial, respectively. The Poisson's ratio is 1.75 times of that of graphene. In the nonlinear elasticity regime, we obtain the high order elastic constants up to fifth order. The stiffness monotonically increases with pressure, has the same trend as that of second order elastic constants but opposes to that of Poisson's ratio. There is a minimum at −4 GPa in the speed-pressure curve of compressive sound wave, different from the monotonic increment of shear waves. These theoretical mechanical properties provide elasticity limits for various applications of g-SiC.

First principles calculations are performed to investigate the structural, electronic, optical and transport properties of the ternary semiconducting compounds AE2ZnN2 (AE=Ca, Sr, Ba) in the tetragonal crystal phase by using a modern and highly accurate full potential linearized augmented plane wave method. In the tetragonal ternary nitrides AE2ZnN2, Zn has a unusual linear coordination with nitrogen (N-Zn-N) along the c-axis. The band gap values for the AE2ZnN2 compounds are calculated with the modified Becke–Johnson (mBJ) approximation. The band gap calculation suggests that these materials are extremely attractive for excellent thermoelectric performance. Subsequently, semi-classic Boltzmann transport theory has been utilized to calculate the thermoelectric properties of the AE2ZnN2 (AE=Ca, Sr, Ba) compounds. The band gap of these compounds varies by replacing the cation AE and the band gap dependent optical parameters are predicted for experimental perspectives. In addition, the optical response suggests that the AE2ZnN2 materials are useful for optoelectronic devices. Furthermore, the figures of merit, thermo power, power factor, electrical and thermal conductivity are calculated for each compound.

Copper and silver are strategic metals in industry. However, corrosion in these metals severely limits their broad engineering applications. Organic molecules, such as thiadiazole (TD) derivatives have been effectively utilized as corrosion inhibitors for Cu and Ag metals and their alloys. Therefore, a fundamental understanding of their adsorption behavior on metal surfaces is key to the understanding of their effect on corrosion inhibition. In this work, we used first-principles calculations based on the density functional theory (DFT) to systematically study the adsorption behaviors of 1,3,4-thiadiazole (TD) and 2,5-dimercapto-1,3,4-thiadiazole (DMTD) on the low-index surfaces of pure Cu and Ag. Our results show that the average adsorption energy of DMTD is much lower than that of TD on both Cu and Ag surfaces. This is attributed to the removal of H atoms from DMTD in the DFT calculations as they easily dissociate in the aqueous solution, along with the presence of two more sulfur atoms in the DMTD structure as anchoring sites. It is also revealed that both S and N atoms in DMTD participate in the bond formation with Ag, while the three S atoms of DMTD prefer to form bonds with Cu. This is further validated from the calculated charge density difference before and after the adsorption and agrees well with the X-ray photoelectron spectroscopy measurements. Finally, the differences between the adsorption energies from DFT calculations and experimental extrapolations are discussed. They can be ascribed to the existence of H atoms in DMTD due to their plausible partial ionization in real experiments, as well as the presence of surface oxides and contact with the electrolyte in the experiments. This is further validated by simulating the adsorption of fully-, partially- and non-ionized DMTDs on the (111) surfaces of both Cu and Ag. Our work sheds light on the adsorption behaviors of TD derivatives on both Cu and Ag, which can potentially be used as a criterion to discover and determine appropriate corrosion inhibitors for metal protection.

Co2FeAl Heusler alloy was selected as a target to study L21 and XA ordering of atoms by first-principles calculations using WIEN2k code. It has been observed that Co2FeAl (CFA) Alloy tend to form L21 type structure, which is more energetically favorable and much stable compared to XA type structure. Further, we observed that the total magnetic moment (Mt) shows a strong dependence on site preferences of Fe in all available Wyckoff sites. However, this compound does not show a half-metallic character in both types of ordered structures still, it can be used as the spin-polarized material for spintronics application. Our results are opposite to usual site preference rule and approving the validity of the Slater-Pauling rule.

Chalcopyrite compounds are promising high efficient thermoelectric materials. However, the relatively high lattice thermal conductivity at modest temperatures limits their performance. Here, we investigate the lattice dynamics in a polycrystalline CuInTe 2 with a combined experimental and computational approach. The phonon dispersion and density of states are computed using the density functional theory. Raman scattering is performed to investigate the phonon dynamical properties. Together with the bulk modulus from X-ray diffraction, the mode-Grüneisen parameters are determined. The low energy B 1 1 and E 2 modes characterize the weak anharmonicity, thus responsible for the high lattice thermal conductivity. Meanwhile, B 1 1 and E 2 modes display energy redshift under pressure. The softening and enhanced anharmonicity of these modes naturally result in the reduction of the thermal conductivity. Our study suggests that pressure is a routine to reduce the phonon heat conduction at modest temperatures in chalcopyrites.

Experimental characterization and theoretical study of the interband transitions of self-assembled InAs quantum dots (QDs) grown on metamorphic pseudosubstrates of InxGa1-xAs (0.0<=x<=0.3) are reported. The effect of the varying underlying strain on the size distribution of InAs QDs and their photoluminescence emission wavelength is investigated by employing different substrate compositions. Atomic force microscopy images of the QDs show that the ratio of the height/lateral diameter of the QDs decreases with decreasing strain and the photoluminescence of the buried InAs QDs shows that the peak wavelength redshifts with increasing In mole fraction of the underlying pseudosubstrates. A theoretical model based on the Green's function technique is used to calculate the density of states (DOS) of the QDs for the different samples based on the measured dot geometries. From the DOS, the electron and hole energy levels can be obtained, yielding the possible interband transitions. Good agreement between the model and the experimental results is obtained by allowing for Ga incorporation, from the substrate and barrier layers, into the InAs QDs and it is found that the necessary Ga mole fraction varies linearly with the Ga mole fraction in the underlying InxGa1-xAs pseudosubstrate.

The morphology change is crucial to the catalysis performance of catalyst nanoparticles in heterogeneous cat-alysis. Iron and iron carbide nanoparticles are used as high temperature heterogeneous catalyst, such as Fischer-Tropsch synthesis and carbon nanotube growth. Here we have investigated the effect of temperature and entropy on the surface free energy and morphology of the iron and iron carbide (θ-Fe 3 C, χ-Fe 5 C 2 , and o-Fe 7 C 3) nano-particles using molecular dynamics. The free energies of all the bulk and most of the surface systems drop following a parabolic curve with elevating temperature due to entropic effect. The nanoparticles are covered by low index surfaces at low temperature. At low temperature, the surface free energies of all surfaces usually decrease with a similar slope with increasing temperature. However, a critical temperature exists at which the high-index surfaces starts to dominate the catalyst particles. Fe 7 C 3 shows an unusual minimum surface free energy at 400 K in all the surfaces. This study provides fundamental insights into the modulation of iron-based nanocatalysts morphologies with desired catalytic performance.

Elucidating the interactions between hydrogen and catalysts under complex realistic conditions is of great importance in rationally modulating the catalytic performance of hydrogenation processes. Herein, we have investigated the interaction between hydrogen and four typical surfaces, (1 0 0), (2 1 0), (2 1 1), and (3 1 1) of pyrite FeS 2 through density functional theory calculations. On (2 1 0) surface, the hydrogen dissociative adsorption on unsaturated-coordination sulfur atoms is favorable both in thermodynamics and kinetics. The hydrogen activation barrier is 0.83 eV with slight exothermic of 0.12 eV on (3 1 1). While on (1 0 0) and (2 1 1) surface, the hydrogen dissociation is unfavorable due to the high activation barriers and remarkable positive reaction energies. For high adsorption coverage, the pure molecule adsorption mode is favorable on (1 0 0) facet, opposed to the other surfaces which have temperature and pressure dependence. The saturated coverage sequence is (1 0 0) > (2 1 0) > (2 1 1) > (3 1 1) for a wide range of temperature and pressure. The remove of sulfur atoms most likely occurs on (2 1 0) surface. Our atomistic insights might be useful in engineering hydrogen-involved processes catalyzed by iron sulfide.

In this report, Fe2CoAl Heusler alloy is selected as a target to study site-preferences of atoms by first-principles calculations using WIEN2k code. It has been observed that Fe2CoAl (FCA) Alloy tend to form XA-I type structure (Hg2CuTi-type) which is energetically more favorable as compared to those of the others XA-II and XA-III structures. Further, we observed that the total magnetic moment per cell shows a strong dependence on-site preferences of Co in all available Wyckoff sites. However, this compound does not show a half-metallic character but a metallic behavior was observed for both spin direction at Fermi level (EF), still, it can be used as the spin-polarized material for spintronics application

In this paper we present first-principles calculations, based on both density functional theory and maximally localized Wannier functions, to study the electronic properties and interlayer coupling of twisted MoS 2 /NbSe 2 heterobilayers. We accurately investigate different stacking configurations and commensurate twist angles by including an in-depth analysis of the interlayer van der Waals interaction. The metallic character of the investigated heterostructures is dominated, at the Fermi energy, by the NbSe 2 atomic orbitals and shows a strong dependence on the twist angle. Notably, at the smallest considered twist angle, band structure flattening at the Fermi energy shows up, which should result in a lower conductivity of the metallic heterobilayer. The electrostatic potential analysis reveals no significant modification of the potential pattern with respect to the potentials of the isolated layers, with the exception of the interface region. A moderate electronic charge redistribution, compatible with electronically weakly coupled layers, is set up following the formation of the interface. The dependence of the electronic structure on the twist angle acts as a new degree of freedom for tuning properties relevant in electronic device applications.

Quantum spin Hall (QSH) insulators have gapless topological edge states inside the bulk band gap, which can serve as dissipationless spin current channels. The major challenge currently is to find suitable materials for this topological state. Here, we predict a new large-gap QSH insulator with bulk direct band gap of ?0.18 eV, in single-layer Bi4Br4, which could be exfoliated from its three-dimensional bulk material due to the weakly bonded layered structure. The band gap of single-layer Bi4Br4 is tunable via strain engineering, and the QSH phase is robust against external strain. Moreover, because this material consists of special one-dimensional molecular chain as its basic building block, the single layer Bi4Br4 could be torn to ribbons with clean and atomically sharp edges. These nanoribbons, which have single-Dirac-cone edge states crossing the bulk band gap, are ideal wires for dissipationless transport. Our work thus provides a new promising material for experimental studies and practical applications of the QSH effect.

Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-induced structural phase transformation, from a NaCl to a CsCl structure, at ∼7 GPa. In this paper, we use first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy to determine the structural stability of this transformation. Our results show that the stress required for this structural transformation is substantially lower (by more than an order of magnitude) when it is deviatoric in nature vis-à-vis that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. From the electronic structure calculations, we estimate the electrical conductivity of TiN in the CsCl structure to be about 5 times of that in NaCl structure, which should be observable experimentally.

Abstract First-principles calculations have been performed on lithium borohydride LiBH4 using the ultrasoft pseudopotential method, which is a potential candidate for hydrogen-storage materials due to its extremely large gravimetric capacity of 18 mass % hydrogen. We focus on an orthorhombic phase observed at ambient conditions and predict its fundamental properties; De-hydrogenation and electronic properties of doped Li1+xB1−xH4 by Li (with 0

Spin polarized density functional theory computations were performed to elucidate electronic effects based on first-row transition metal doped Fe(100) and Fe 5 C 2 IJ100) surfaces for CO dissociation. Both Mn and Cr doped Fe(100) and Fe 5 C 2 IJ100) surfaces can enhance the dissociation of CO, while the Co, Ni and Cu doped ones are unfavorable for the dissociation of CO. Besides the BEP relationship, a linear relationship between activation energy and the electronegativity of the dopant atom is established. It can be deduced that the metals with lower electronegativity are favorable for the activation of CO. The reason has been analyzed by density of states and crystal orbital Hamilton population. The metals with lower electronegativity relative to Fe could donate electrons to doped sites and then activate CO with a more delocalized O 2p orbital. The electronic effects revealed herein are helpful for the understanding of the CO activation process and for the design of catalysts with desired activity.

ABSTRACT Starting from theoretical calculations based on LSDA, the authors compute the lattice parameters, cohesive energies and formation enthalpies of monazite-type REPO4 compounds. The calculated values are satisfactory compared with the experimental results from the elastic constants obtained, the mechanical moduli are evaluated using the strain–stress method. The predicted bulk, Young’s and shear moduli are in good agreement with the experiments. It is shown that the mechanical moduli are low (&amp;amp;lt;200 GPa) and also increase from LaPO4 to GdPO4. The three-dimensional contours and their planar projections of Young’s modulus are plotted to illustrate the anisotropy in elasticity. It is found that Young’s moduli of all monazite-type REPO4 show strong dependence on direction. The linear thermal expansion coefficients are calculated using the empirical method, and the values are in the range 9 × 10−6–12 × 10−6 K−1. Using Clarke’s and Slack’s models, the thermal conductivities of REPO4 compounds obtained are close to the experimental profiles. The observed anomalies of experimental thermal properties of monazite-type GdPO4 are also explained based on the observed monazite to zircon-type transformation in experiment. Solving the Christoffel equation for monoclinic symmetry, the anisotropy in thermal conductivity is investigated. The results indicate that the total lattice thermal conductivities of monazite-type REPO4 show weak dependence on direction. Meanwhile, their sound velocities exhibit strong anisotropic properties.

Quantum-mechanical (so-called ab initio) calculations have achieved considerable reliability in predicting physical and chemical properties and phenomena. Due to their reliability they are becoming increasingly useful when designing new alloys or revealing the origin of phenomena in existing materials, also because these calculations are able to accurately predict basic material properties without experimental input. Due to the universal validity of fundamental quantum mechanics, not only ground-state properties, but also materials responses to external parameters can reliably be determined. The focus of the present paper is on ab initio approaches to the elasticity of materials. First, the methodology to determine single-crystalline elastic constants and polycrystalline moduli of ordered compounds as well as disordered alloys is introduced. In a second part, the methodology is applied on a-Fe, with a main focus on (i) investigating the influence of magnetism on its elasticity and phase stability and (ii) simulating extreme loading conditions that go up to the theoretical tensile strength limits and beyond.

Co2FeAl Heusler alloy was selected as a target to study L21 and XA ordering of atoms by first-principles calculations using WIEN2k code. It has been observed that Co2FeAl (CFA) Alloy tend to form L21 type structure, which is more energetically favorable and much stable compared to XA type structure. Further, we observed that the total magnetic moment (Mt) shows a strong dependence on site preferences of Fe in all available Wyckoff sites. However, this compound does not show a half-metallic character in both types of ordered structures still, it can be used as the spin-polarized material for spintronics application. Our results are opposite to usual site preference rule and approving the validity of the Slater-Pauling rule.

Quantum-mechanical (so-called ab initio) calculations have achieved considerable reliability in predicting physical and chemical properties and phenomena. Due to their reliability they are becoming increasingly useful when designing new alloys or revealing the origin of phenomena in existing materials, also because these calculations are able to accurately predict basic material properties without experimental input. Due to the universal validity of fundamental quantum mechanics, not only ground-state properties, but also ...

Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanoparticles (NPs). The NPs of size 23±10 nm are prepared using the co-precipitation method. X-ray and selected area electron diffraction patterns have confirmed the cubic Heusler phase of the NPs with the A2-disorder. These NPs are soft ferromagnetic, and exhibit a high saturation magnetization (Ms) along with a very high Curie temperature (Tc) of 1060 K. The observed Tc value matches closely with the theoretically calculated one following a model provided by Wurmehl et al. (2005). The high Ms and Tc make the present system a potential candidate for magnetically activated nano-devices working at high temperatures. The near-integral value 5.9 μB/f.u. of Ms at low temperatures indicates that the half-metallic ferromagnetism is preserved even in the particles even on the 20 nm length scale. Additionally, we have facilitated the existing HA-NP preparation method, which can be used in synthesizing other HA-NPs. The first-principles density functional theory computations complement the experimental results.

Surface feature and its variation along with complex atmosphere are of fundamental significance to understanding the functionality of applied materials especially in heterogeneous catalysis and corrosion prevention. Here we performed a unified theoretical study on the surface structure and morphology of iron borides and their evolution under dynamic gaseous conditions by combination of density functional theory, ab initio atomic thermodynamics and Wulff construction. In particular, thermodynamic stability of iron borides and corresponding surfaces varied from the boron chemical potential (μ Δ B) of certain atmosphere, which increases with decreasing pressure and increasing temperature and concentration of boron source. The stability of boron-rich surfaces has been improved with increasing μ Δ B , while all the Fe-rich facets of iron borides are favorable at low μ Δ B condition. Accordingly, the crystallite morphology of iron borides undergoes significant evolution upon dynamic condition. Finally, the surface properties of iron borides are carefully tested by CO adsorption which indicated the activation ability of CO is closely connected with boron triggered surface charge transfer between Fe and CO. This work was expected not only to help understand the surface structure and morphology of iron borides under realistic condition, but also provides fundamental insights into rational design of corrosion resistant and catalytic materials.

The properties of precipitates are important in understanding the strengthening mechanism via precipitation during heat treatment and the aging process in Al–Cu based alloys, where the formation of precipitates is sensitive to temperature and pressure. Here we report a first-principles investigation of the effect of temperature and pressure on the structural stability, elastic constants and formation free energy for precipitates of Al2Cu, as well as their mechanical properties. Based on the formation enthalpy of Guinier–Preston (GP(I)) zones, the size of the GP(I) zone is predicted to be about 1.4 nm in diameter, which is in good agreement with experimental observations. The formation enthalpies of the precipitates are all negative, suggesting that they are all thermodynamically stable. The present calculations reveal that entropy plays an important role in stabilizing θ-Al2Cu compared with θC′-Al2Cu. The formation free energies of θ′′-Al3Cu, θC′-Al2Cu, θD′-Al5Cu3 and θt′-Al11Cu7 increase with temperature, while those of θ′-Al2Cu, θO′-Al2Cu and θ-Al2Cu decrease. The same trend is observed with the effect of pressure. The calculated elastic constants for the considered precipitation phases indicate that they are all mechanically stable and anisotropic, except θC′-Al2Cu. θD′-Al5Cu3 has the highest Vicker's hardness. The electronic structures are also calculated to gain insight into the bonding characteristics. The present results can help in understanding the formation of precipitates by different treatment processes.

The first-principles calculations were used to study the hydrogen energetics on the 100 tungsten 2 2R45° surface. Two equilibrium sites for H at the surface are identified, with a low migration barrier from the energetically clearly higher long bridge site to the short bridge site. At low coverages, the majority of H surface diffusion events take place via the short bridge sites. The energetics for H penetration from the surface to the solute site in the bulk was defined, showing that the bulk H diffusion via neighboring tetrahedral sites takes place at depths beyond the second subsurface layer.