Effect of Ti Additions on Mechanical and Thermodynamic Properties of W-Ti Alloys: A First-principles Study

The mechanical and thermodynamic properties of W-Ti alloys(including W_(15)Ti_(1),W_(14)Ti_(2),W_(12)Ti_(4) and W_(8)Ti_(8) alloys)were investigated by the first-principles approach based on density functional theory.The results indicate that W-Ti alloys except W_(8)Ti_(8) are thermodynamically stable.The modulus and hardness of W-Ti alloys are smaller than those of pure tungsten and gradually decrease with increasing Ti concentration.However,their B/G ratios and Poisson's ratios exceed those of pure tungsten,suggesting that the introduction of Ti decreases the mechanical strength while enhancing the ductility of W-Ti alloys.The thermal expansion coefficients for W-Ti alloys all surpass those of pure tungsten,indicating that the introduction of titanium exacerbates the thermal expansion behavior of W-Ti alloys.Nevertheless,elevated pressure has the capacity to suppress the thermal expansion tendencies in titanium-doped tungsten alloys.This study offers theoretical insights for the design of nuclear materials by exploring the mechanical and thermodynamic properties of W-Ti alloys.

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 stability and superconductivity of ternary hydride LaYH_(x)(x=2,3,6 and 8)

Recent studies have shown that the La-and Y-hydrides can exhibit significant superconducting properties under high pressures.In this paper,we investigate the stability,electronic and superconducting properties of LaYH_(x)(x=2,3,6 and 8)under 0-200 GPa.It is found that LaYH_(2) stabilizes in the C2/m phase at ambient pressure,and transforms to the Pmmn phase at 67 GPa.LaYH_(3) stabilizes in the C2/m phase at ambient pressure,and undergoes phase transitions of C2/m→P2_(1)/m→R3m at 12 GPa and 87 GPa,respectively.LaYH_(6) stabilizes in the P4_32_12 phase at ambient pressure,and undergoes phase transitions of P4_(3)2_(1)2→P4/mmm→Cmcm at 28 GPa and 79 GPa,respectively.LaYH_(8) stabilizes in the Imma phase at 60 GPa and transforms to the P4/mmm phase at 117 GPa.Calculations of the electronic band structures show that the P4/mmm-LaYH_(8) and all phases of LaYH_(2) and LaYH_(3) exhibit metallic character.For the metallic phases,we then study their superconducting properties.The calculated superconducting transition temperatures(T_c)are 0.47 K for C2/m-LaYH_(2) at 0 GPa,0 K for C2/m-LaYH_(3) at 0 GPa,and 55.51 K for P4/mmm-LaYH_(8) at 50 GPa.

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.

First-Principles Study of the New Layered Ternary Metal Telluride,Eu_(2)InTe_(5)

In this study,we performed first-principles calculations using the VASP(Vienna Ab initio Simulation)software package to investigate the crystal structure,electronic structure,and optical properties of a new layered ternary metal chalcogenide,Eu_(2)InTe_(5).Our results show that Eu_(2)InTe_(5) is a non-zero-gap metal with a layered structure characterized by strong intra-layer atomic bonding and weak inter-layer interaction,which suggests its potential application as a nanomaterial.We also studied the optical properties,including the absorption coefficient,imaginary and real parts of the complex dielectric constant,and found that Eu_(2)InTe_(5) exhibits strong photoresponse characteristics at the junction of ultraviolet and visible light as well as blue-green light,with peaks at wavelengths of 389 nm and 477 nm.This suggests that it could be used in the development of UV(ultraviolet)detectors and other optoelectronic devices.Furthermore,due to its strong absorption,low loss,and low reflectivity,Eu_(2)InTe_(5) has the potential to be used as a promising photovoltaic absorption layer in solar cells.

First-principles Study on Neutral Nitrogen Impurities in Zinc Oxide

The atomic geometries, electronic structures, and formation energies of neutral nitrogen im- purities in ZnO have been investigated by first-principles calculations. The nitrogen impuri- ties are always deep acceptors, thus having no contributions to p-type conductivity. Among all the neutral nitrogen impurities, nitrogen substituting on an oxygen site has the lowest formation energy and the shallowest acceptor level, while nitrogen .substituting on a zinc site has the second-lowest formation energy in oxygen-rich conditions. Nitrogen interstitials are unstable at the tetrahedral site and spontaneously relax into a kick-out configuration. Though nitrogen may occupy the octahedral site, the concentrations will be low for the high formation energy. The charge density distributions in various doping cases are discussed, and self-consistent results are obtained.

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 phase stability and electronic properties of RhZr

First-principles calculations were carried out to investigate the structural stabilities and electronic properties of RhZr.The plane wave based pseudopotential method was used,in which both the local density approximation（LDA） and the generalized gradient approximation（GGA） implanted in the CASTEP code were employed.The internal positions of atoms in the unit cell were optimized and the ground state properties such as lattice parameter,density of state,cohesive energies and enthalpies of formation of ortho-RhZr and cubic-RhZr were calculated.The calculation results indicate that ortho-RhZr can form more easily than cubic-RhZr and the ortho-RhZr is more stable than cubic-RhZr.The density of states（DOS） reveals that the strong bonding in the Rh-Zr and Rh-Rh or Zr-Zr interaction chains accounts for the structural stability of ortho-RhZr and the hybridization between Rh-4d states and Zr-4d states is strong.

First-principles study of stacking fault energies in Ni_3Al intermetallic alloys

The first-principles method based on the projector augmented wave method within the generalized gradient approximation was employed to calculate the superlattice intrinsic stacking fault（SISF） and complex stacking fault（CSF） energies of the binary Ni3Al alloys with different Al contents and the ternary Ni3Al intermetallic alloys with addition of alloying elements,such as Pd,Pt,Ti,Mo,Ta,W and Re.The results show that the energies of SISF and CSF increase significantly with increase of Al contents in Ni3Al.Addition of Pd and Pt occupying the Ni sublattices does not change the SISF and CSF energies of Ni3Al markedly in comparison with the Ni-23.75Al alloy.While addition of alloying elements,such as Ti,Mo,Ta,W and Re,occupying the Al sublattices dramatically increases the SISF and CSF energies of Ni3Al.The results suggest that the energies of SISF and CSF are dependent both on the Al contents and on the site occupancy of the ternary alloying element in Ni3Al intermetallic alloys.

A first-principles study on elastic properties and stability of Ti_xV_(1-x)C multiple carbide

The structure,stability and elastic properties of di-transition-metal carbides TixV1-xC were investigated by using the first-principles with a pseudopotential plane-waves method.The results show that the equilibrium lattice constants of TixV1-xC show a nearly linear reduction with increasing addition of V.The elastic properties of TixV1-xC are varied by doping with V.The bulk modulus of Ti0.5V0.5C is larger than that of pure TiC,as well as Ti0.5V0.5C has the largest C44 among TixV1-xC（0≤x≤1）,indicating that Ti0.5V0.5C has higher hardness than pure TiC.However,Ti0.5V0.5C presents brittleness based on the analysis of ductile/brittle behavior.The Ti0.5V0.5C carbide has the lowest formation energy,indicating that Ti0.5V0.5C is more stable than all other alloys.

First-principles study of intrinsic defects,dopants and dopant-defect complexes in LiBH_4

A first-principles study was reported based on density functional theory of hydrogen vacancy,metal dopants,metal dopant-vacancy complex in LiBH4,a promising material for hydrogen storage.The formation of H vacancy and metal doping in LiBH4 is difficult,and their concentrations are low.The presence of one kind of defect is helpful to the formation of other kind of defect.Based on the analysis of electronic structure,the improvement of the dehydrogenating kinetics of LiBH4 by metal catalysts is due to the weaker bonding of B—H and the new metal-like system,which makes H atom diffuse easily;H vacancy accounts for a trace amount of BH3 release during the decomposing process of LiBH4;metal dopant weakens the strength of B—H bonds,which reduces the dehydriding temperature of LiBH4.The roles of metal and vacancy in the metal dopant-vacancy complex can be added in LiBH4 system.

A first-principles study on electronic structure and elastic properties of Al_4Sr, Mg_2Sr and Mg_(23)Sr_6 phases

The electronic structures and mechanical properties of Al4Sr, Mg2Sr and Mg23Sr6 phases were determined by the use of first-principles calculations. The calculated heat of formation and cohesive energy indicate that Al4Sr has the strongest alloying ability as well as the highest structural stability. The elastic parameters were calculated, and then the bulk modulus, shear modulus, elastic modulus and Poisson ratio were derived. The ductility and plasticity were discussed. The results show that Al4Sr and Mg2Sr phases both are ductile, on the contrary, Mg23Sr6 is brittle, and among the three phases, Mg2Sr is a phase with the best plasticity.

First-Principles Study of Magnetism in Transition Metal Doped Na0.5Bi0.5TiO3 System

The origins of magnetism in transition-metal doped Na0.5Bi0.5TiO3 system are investigated by ab initio calculations. The calculated results indicate that a transition-metal atom sub- stitution for a Ti atom produces magnetic moments, which are due to the spin-polarization of transition-metal 3d electrons. The characteristics of exchange coupling are also calculated, which shows that in Cr-/Mn-/Fe-/Co- doped Na0.5Bi0.5TiO3 system, the antiferromagnetic coupling is favorable. The results can successfully explain the experimental phenomenon that, in Mn-/Fe- doped Nao.sBio.sTiO3 system, the ferromagnetism disappears at low tem- perature and the paramagnetic component becomes stronger with the increase of doping concentration of Mn/Fe/Co ions. Unexpectedly, we find the Na0.5Bi0.5Ti0.67V0.33iO3 sys- tem with ferromagnetic coupling is favorable and produces a magnetic moment of 2.00 P-B, which indicates that low temperature ferromagnetism materials could be made by intro- ducing V atoms in Na0.5Bi0.5TiO3. This may be a new way to produce low temperature multiferroic materials.

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.

A first-principles study on interfacial properties of Ni(001)/Ni_3Nb(001)

The Ni （001） surface, Ni3Nb （001） surface and Ni （001）/Ni3Nb （001） interfaces were studied using the first-principles pseudopotential plane-wave method. The adhesion work, thermal stability and electronic structure of Ni/Ni3Nb （001） interfaces were calculated to expound the influence of atom termination and stacking sequence on the interface strength and stability. Simulated results indicate that Ni and Ni3Nb （001） surface models with more than eight atomic layers exhibit bulk-like interior. The （Ni＋Nb）-terminated interface with hollow site stacking has the largest cohesive strength and critical stress for crack propagation and the best thermal stability among the four models. This interfacial Ni and the first nearest neighbor Nb atoms form covalent bonds across the interface region, which are mainly contributed by Nb 4d and Ni 3d valence electrons. By comparison, the thermal stability of Ni/Ni3Nb （001） interfaces is worse than Ni/Ni3A1 （001） interface, implying that the former is harder to form. But the Ni/Ni3Nb interface can improve the mechanical properties ofNi-based superalloys.

First-principles Study on Enhanced Optical Stability of BaMgAl10O17：Eu^2＋ Phosphor by SiN Doping

Using density functional theory, we studied band structure, density of states, optical proper- ties and Mulliken population of the pure and SiN doped BaMgAl10017：Eu^2＋ （BAM：Eu^2＋） phosphors. Calculation results showed that the bands of BAM：Eu2＋ were of low band energy dispersion, indicating large joint density of states, hence high performance of optical absorption and luminescence. BAM：Eu^2＋ showed stronger absorption intensity while Eu^2＋ occupied the BR sites instead of the mO sites. The concentration of Eu^2＋ at BR sites increased while that at mO sites decreased after Si-N doping. The influence of the variation of Eu^2＋ distribution on the spectra was stronger than the influence of the decrease of Eu^2＋ PDOS when SiN concentration was lower than 0.25, therefore the absorption and luminescence intensity of BAM：Eu^2＋ were enhanced. Mulliken population of Si-N bond was higher than A1-O bond, while that of Eu-N bond was higher than Eu-O bond as well, indicating that Si-N bonds and Eu-N bonds possessed higher covalence than Al-O bonds and Eu-N bonds respectively. The existence of Si-N bonds and Eu-N bonds enhanced the local covalence of Eu^2＋, hence the optical stability of BAM：Eu^2＋.