The growth ofβphase in Mg-Gd-Y-Ni alloy by experimental and first-principles study

The paper reports on the atomic investigation aboutβphase in Mg_(96)Gd_(2)Y_(1)Ni_(1) alloy by using the first-principles study and the high-angle annular dark-field scanning transmission electron microscope(HAADF-STEM)corrected by atomic Cs.By using HAADF-STEM,the rectangularβphases were observed in the underage and peak aging stages in Mg_(96)Gd_(2)Y_(1)Ni_(1) alloy.Theβphase could be precipitated from the previously precipitatedβphase,and theβphase grew in steps when it was precipitated.A special transition structure of three atomic layer thicknesses was first observed at the edge of theβphase and the structure of this interface is probably as theβ/Mg_(1) interface for the analysis of thermodynamic characterization and electronic characterization.Theβ'phase and theβ_(H) structure were precipitated only at the edge of the length directions of theβphase.Theβ'phase continues to grow into aβphase directly without the formation ofβ_(1) phase,resulting in an increase in the length of theβphase,which is discovered for the first time.

First-principles study on the diffusion behavior of Cs and I in Cr coating

Cs and I can migrate through fuel-cladding interfaces and accelerate the cladding corrosion process induced by the fuel-cladding chemical interaction.Cr coating has emerged as an important candidate for mitigating this chemical interaction.In this study,first-principles calculations were employed to investigate the diffusion behavior of Cs and I in the Cr bulk and grain boundaries to reveal the microscopic interaction mitigation mechanisms at the fuel-cladding interface.The interaction between these two fission products and the Cr coating were studied systematically,and the Cs and I temperature-dependent diffusion coefficients in Cr were obtained using Bocquet’s oversized solute-atom model and Le Claire’s nine-frequency model,respectively.The results showed that the Cs and I migration barriers were significantly lower than that of Cr,and the Cs and I diffusion coefficients were more than three orders of magnitude larger than the Cr self-diffusion coefficient within the temperature range of Generation-IV fast reactors(below 1000 K),demonstrating the strong penetration ability of Cs and I.Furthermore,Cs and I are more likely to diffuse along the grain boundary because of the generally low migration barriers,indicating that the grain boundary serves as a fast diffusion channel for Cs and I.

A first-principles study on remote van der Waals epitaxy through a graphene monolayer on semiconductor substrates

To investigate the mechanism of remote epitaxy, where the overlayer can follow the same crystalline structure as the underlying semiconductor substrate through a thin two-dimensional interlayer, we systematically study the potential fluctuations of graphene covered Si, Ga As, and Ga N substrates from first-principles. We find that the uneven semiconductor surface, the distorted graphene, and the non-uniform interface charge transfer make significant contributions to the potential fluctuation. The semiconductor substrate with different surface reconstructions and orientations will generate different potential fluctuations through the graphene interlayer. We also calculate and compare the adsorption of adatoms on graphene covered substrates. The adsorption energies of adatoms not only depend on their distances to the underlying semiconductor surface, but are also sensitive to the direction of the charge transfer at the graphene/substrate interface. Changing the semiconductor reconstruction or orientation could even reverse the order of the adsorption energies of cation and anion adatoms by reversing the interface charge transfer direction, leading to a change in the growth orientation of the overlayer.Our study improves the understanding of the mechanism of remote epitaxy, and reveals that it is possible to control the initial nucleation and orientation of overlayers by changing the semiconductor reconstructions and/or orientations in remote epitaxy.

First-principles Study of Electronic Structural and Mechanical Properties of Mg_(x)La(x=1,2,3)Compounds under Pressure

The effects of pressure on structural,elastic and electronic properties of Mg_(x)La(x=1,2,3)compounds are investigated by using CASTEP program based on the density functional theory.The calculated equilibrium lattice parameters at zero pressure agree well with available experimental and theoretical values.The calculated DOS show that the structure of these compounds remains mechanically stable and structural phase transformation is not induced with increasing pressure from 0 to 30 GPa,and their structural stability increases with pressure.The ductility of MgLa can be improved by increasing pressure,which is the same as Mg_(2)La in 0-20 GPa,while brittle behavior turns into ductile behavior in 0-5 GPa for Mg_(3)La.The resistance to volume deformation of Mg_(x)La(x=1,2,3)compounds can be improved as the pressure increases.The shear deformation resistance and elastic stiffness of Mg_(3)La can be enhanced by rising pressure,but MgLa and Mg_(2)La increase first and then decrease when pressure is up to 25 GPa.In addition,the three compounds exhibit the elastic anisotropy with pressure.

Pressure-dependent electronic,optical,and mechanical properties of antiperovskite X_(3)NP(X=Ca,Mg):A first-principles study

Hydrostatic pressure provides an efficient way to tune and optimize the properties of solid materials without chang-ing their composition.In this work,we investigate the electronic,optical,and mechanical properties of antiperovskite X_(3)NP(X^(2+)=Ca,Mg)upon compression by first-principles calculations.Our results reveal that the system is anisotropic,and the lat-tice constant a of X_(3)NP exhibits the fastest rate of decrease upon compression among the three directions,which is different from the typical Pnma phase of halide and chalcogenide perovskites.Meanwhile,Ca_(3)NP has higher compressibility than Mg_(3)NP due to its small bulk modulus.The electronic and optical properties of Mg_(3)NP show small fluctuations upon compression,but those of Ca_(3)NP are more sensitive to pressure due to its higher compressibility and lower unoccupied 3d orbital energy.For example,the band gap,lattice dielectric constant,and exciton binding energy of Ca_(3)NP decrease rapidly as the pressure increases.In addition,the increase in pressure significantly improves the optical absorption and theoretical conversion effi-ciency of Ca_(3)NP.Finally,the mechanical properties of X_(3)NP are also increased upon compression due to the reduction in bond length,while inducing a brittle-to-ductile transition.Our research provides theoretical guidance and insights for future experi-mental tuning of the physical properties of antiperovskite semiconductors by pressure.

First-principles study of the bandgap renormalization and optical property ofβ-LiGaO_(2)

Theβ-LiGaO_(2)with an orthorhombic wurtzite-derived structure is a candidate ultrawide direct-bandgap semiconductor.In this work,using the non-adiabatic Allen-Heine-Cardona approach,we investigate the bandgap renormalization arising from electron-phonon coupling.We find a sizable zero-point motion correction of-0.362 eV to the gap atΓ,which is dominated by the contributions of long-wavelength longitudinal optical phonons.The bandgap ofβ-LiGaO_(2)decreases monotonically with increasing temperature.We investigate the optical spectra by comparing the model Bethe-Salpether equation method with the independent-particle approximation.The calculated optical spectra including electron-hole interactions exhibit strong excitonic effects,in qualitative agreement with the experiment.The contributing interband transitions and the binding energy for the excitonic states are analyzed.

Magnetic and electronic properties of bulk and two-dimensional FeBi_(2)Te_(4):A first-principles study

Layered magnetic materials,such as MnBi_(2)Te_(4),have drawn much attention owing to their potential for realizing twodimensional(2D)magnetism and possible topological states.Recently,FeBi_(2)Te_(4),which is isostructural to MnBi_(2)Te_(4),has been synthesized in experiments,but its detailed magnetic ordering and band topology have not been clearly understood yet.Here,based on first-principles calculations,we investigate the magnetic and electronic properties of FeBi_(2)Te_(4)in bulk and 2D forms.We show that different from MnBi_(2)Te_(4),the magnetic ground states of bulk,single-layer,and bilayer FeBi_(2)Te_(4)all favor a 120°noncollinear antiferromagnetic ordering,and they are topologically trivial narrow-gap semiconductors.For the bilayer case,we find that a quantum anomalous Hall effect with a unit Chern number is realized in the ferromagnetic state,which may be achieved in experiment by an external magnetic field or by magnetic proximity coupling.Our work clarifies the physical properties of the new material system of FeBi_(2)Te_(4)and reveals it as a potential platform for studying magnetic frustration down to 2D limit as well as quantum anomalous Hall effect.

Effects of oxygen concentration and irradiation defects on the oxidation corrosion of body-centered-cubic iron surfaces:A first-principles study

Oxidation corrosion of steels usually occurs in contact with the oxygen-contained environment, which is accelerated by high oxygen concentration and irradiation. The oxidation mechanism of steels is investigated by the adsorption/solution of oxygen atoms on/under body-centered-cubic(bcc) iron surfaces, and diffusion of oxygen atoms on the surface and in the near-surface region. Energetic results indicate that oxygen atoms prefer to adsorb at hollow and long-bridge positions on the Fe(100) and(110) surfaces, respectively. As the coverage of oxygen atoms increases, oxygen atoms would repel each other and gradually dissolve in the near-surface and bulk region. As vacancies exist, oxygen atoms are attracted by vacancies, especially in the near-surface and bulk region. Dynamic results indicate that the diffusion of O atoms on surfaces is easier than that into near-surface, which is affected by oxygen coverage and vacancies. Moreover, the effects of oxygen concentration and irradiation on oxygen density in the near-surface and bulk region are estimated by the Mc Lean’s model with a simple hypothesis.

First-Principles Study of Properties of Strained PbTiO_3/KTaO_3 Superlattice

The impacts of strain and polar discontinuities on the performance of superlattices have attracted widespread attention. Using first-principles calculation, we study the polarization and piezoelectricity of PbTiO3/KTa03 （PTO//KTO） superl^ttices with strain and polsr discontinuities. The strain caused by lattice mismatch between the superlattice and the substrate induces lattice distortion, the displacement of each atom and dynamical charge transfer between the Ti atom or Ta atom and the 0 atoms in the PTO//KTO superlattice. With more compressive or less tensile strain, the polarization value increases linearly, piezoelectric tensor e31 （e32） increases while e33 and e25 （e16） increase negatively. Polarity discontinuity caused by the interracial charge will produce large irreversible polarization. Proved by F-point phonons of PTO//KTO superlattices of different strain values, the polar discontinuity and the piezoelectric properties are just weakly dependent on temperature as found in PTO//KTO superlattices.

Lithium/Silver-Doped Cu_2ZnSnS_4 with Tunable Band Gaps and Phase Structures: a First-Principles Study

Doping is an effective approach for improving the photovoltaic performance of Cu2 ZnSnS4（CZTS）. The doping by substitution of Cu atoms in CZTS with Li and Ag atoms is investigated using density functional theory. The results show that the band gaps of Li（2 x）Cu2（1-x）ZnSnS4 and Ag（2 x）Cu2（1-x）ZnSnS4 can be tuned in the ranges of 1.30-3.43 and 1.30-1.63 eV, respectively. The calculation also reveals a phase transition from kesterite to wurtzite-kesterite for Li（2 x）Cu2（1-x）ZnSnS4 as x is larger than 0.9. The tunable band gaps of Li（2 x）Cu2（1-x）ZnSnS4 and Ag（2 x）Cu2（1-x） ZnSnS4 make them beneficial for achieving band-gap-graded solar cells.

First-principles study on nonlocal ferromagnetism in (Cu, N)-codoped ZnO

First-principles calculations based on spin density functional theory are performed to study the spin-resolved elec- tronic properties of ZnO codoped with Cu and N. （Cu, N）-codoped ZnO exhibits magnetism, and the total magnetic mo- ment mainly originates from the p--d hybridization of Cu-N and Cu-O as well as p--p coupling interaction between N and O at the Fermi level. The Zn34Cu2035N1 favors energetically a ferromagnetic ground state due to the existence of stable Cu-N-Cu complex. These results imply that the （Cu, N）-codoped ZnO is a promising dilute magnetic semiconductor free of magnetic precipitates, which could broaden the horizon of currently known magnetic systems.

Topological properties of Sb(111)surface:A first-principles study

First-principles calculations based on the density functional theory were performed to systematically study the electronic properties of the thin film of antimony in(111)orientation.By considering the spin-orbit interaction,for stoichiometric surface,the topological states keep robust for six-bilayer case,and can be recovered in the three-bilayer film,which are guaranteed by time-reversal symmetry and inverse symmetry.For reduced surface doped by non-magnetic Bi or magnetic Mn atom,localized three-fold symmetric features can be identified.Moreover,band structures show that the non-trivial topological states stand for non-magnetic substitutional Bi atom,while can be eliminated by adsorbed or substitutional magnetic Mn atom.

First-principles study of helium clustering at initial stage in ThO2

The clustering behavior of helium atoms in thorium dioxide has been investigated by first-principles calculations. The results show that He atoms tend to form a cluster around an octahedral interstitial site（OIS）. As the concentration of He atoms in ThO2 increases, the strain induced by the He atoms increases and the octahedral interstitial site is not large enough to accommodate a large cluster, such as a He hexamer. We considered three different Schottky defect（SD） configurations（SD1, SD2, and SD3）. When He atoms are located in the SD sites, the strain induced by the He atoms is released and the incorporation and binding energies decrease. The He trimer is the most stable cluster in SD1. Large He clusters, such as a He hexamer, are also stable in the SDs.

Electronic Structure and Visible-Light Absorption of Transition Metals(TM=Cr,Mn, Fe, Co) and Zn-Codoped SrTiO_3: a First-Principles Study

First-principles calculations are performed on the influence of transition metal（TM=Cr, Mn, Fe, Co） as codopants on the electronic structure and visible-light absorption of Zn-doped Sr TiO_3. The calculated results show that（Zn,Mn）-codoped Sr TiO_3 requires the smallest formation energy in four codoping systems. The structures of the codoped systems display obvious lattice distortion, inducing a phase transition from cubic to rhombohedral after codoping. Some impurity Cr, Mn and Co 3d states appear below the bottom of conduction band and some Fe 3d states are located above the top of valence band, which leads to a significant narrowing of band gap after transition metal codoping. The enhancement of visible-light absorption are observed in transition metals（TM=Cr, Mn, Fe, Co） and Zn codoped Sr TiO_3 systems. The prediction calculations suggested that the（Zn,Mn）-and（Zn,Co）-codoped SrTiO_3 could be the desirable visible-light photocatalysts.

First-principles study of the electronic and optical properties of the (Y,N)-codoped anatase TiO_2 photocatalyst

First-principles plane-wave pseudopotential calculations are performed to study the geometrical structures, for- mation energies, and electronic and optical properties of Y-doped, N-doped, and （Y, N）-codoped Ti02. The calculated results show that Y and N codoping leads to lattice distortion, easier separation of photogenerated electron-hole pairs and band gap narrowing. The optical absorption spectra indicate that an obvious red-shift occurs upon Y and N codoping, which enhances visible-light photocatalytic activity.

First-Principles Study of the Polar TiC/Ti Interface

The interface structure, work of adhesion, and bonding character of the polar TiC/Ti interface have been examined by the first-principles density functional plane-wave pseudopotential calculations. Both Ti- and C-terminated interfaces including six different interface structures were calculated, which present quite different features. For the Ti-terminated interface, the interfacial Ti-Ti bond has a strong metallic and weak covalent character; while for the C-terminated interface, the interfacial bond is a strong polar covalent interaction between the Ti-3d and C-2p orbital. The work of adhesion of C-terminated interface is nearly 9 J/m2 stronger than that of the Ti-terminated. It is found that each termination has relatively large work of adhesion, which is consistent with other polar interfaces.

First-principles study of interphase Ni_3Sn in Sn-Ni alloy for anode of lithium ion battery

This paper investigates the mechanism of Li insertion into interphase Ni3Sn in Ni-Sn alloy for the anode of lithium ion battery by means of the first-principles plane-wave pseudopotential. Compared with other phases, it is found that the Ni3Sn has larger relative expansion ratio and lower electrochemical potential, with its specific plateaus voltage around 0.3 eV when lithium atoms are filled in all octahedral interstitial sites, and the relative expansion ratio increasing dramatically when the lithiated phase transits from octahedral interstitial sites to tetrahedral interstitial sites. So this phase is a devastating phase for whole alloy electrode materials.

Methane adsorption on graphite(0001) films: A first-principles study

Using first-principles methods, we have systematically investigated the electronic density of states, work function, and adsorption energy of the methane molecule adsorbed on graphite（0001） films. The surface energy and the interlayer relaxation of the clean graphite（0001） as a function of the thickness of the film were also studied. The results show that the interlayer relaxation is small due to the weak interaction between the neighboring layers. The one-fold top site is found most favourable on substrate for methane with the adsorption energy of - 133 meV. For the adsorption with different adsorption heights above the graphite film with four layers, the methane is found to prefer to appear at about 3.21 A above the graphite. We also noted that the adsorption energy does not dependent much on the thickness of the graphite films. The work function is enhanced slightly by adsorption of methane due to the slight charge transfer from the graphite surface to the methane molecule.

First-principles study of atomic and electronic structures of kaolinite in soft rock

Kaolinite is a kind of clay mineral which often causes large deformations in soft-rock tunnel engineering and thus causes safety issues. To deal with these engineering safety issues, the physical/chemical properties of the kaolinite should be studied from basic viewpoints. By using the density-functional theory, in this paper, the atomic and the electronic structures of the kaolinite are studied within the local-density approximation （LDA）. It is found that the kaolinite has a large indirect band gap with the conduction band minimum （CBM） and the valence band maximum （VBM） being at the F and the B points, respectively. The chemical bonding between the cation and the oxygen anion in kaolinite is mainly ionic, accompanied by a minor covalent component. It is pointed that the VBM and the CBM of kaolinite consist of oxygen 2p and cation s states, respectively. The bond lengths between different cations and anions, as well as of the different OH groups, are also compared.

CO adsorption on small Au_n(n=1-7) clusters supported on a reduced rutile TiO_2(110) surface:a first-principles study

CO adsorption on small Aun （n = 1 - 7） clusters which are supported by a partially reduced rutile TiO2（110） surface has been investigated by the first-principles method. The low coordinated sites of Au clusters are favorable for CO adsorption. CO-Aun-TiO2 system displays surface magnetism. There is a strong interaction between the adsorbed CO molecule and the supported Au clusters.