Narrow Results

By first-principles study, we propose a hypothetical crystal (with space group P42/MNM) Krypton dioxide (KrO₂) which might be synthesized at a pressure over 95GPa. At a pressure around 95GPa noticeable charge transfer from Kr atom to O atom occurs and chemical bonds are formed. Mechanism of this phenomena is analyzed using the electron-hopping mechanism. The formed Kr–O bond at high pressure can exist with retreated pressure down to 5GPa. The electron hopping mechanism might be applicable to the fabrication of other noble gas compounds.

The elastic properties of rutile transition metal dioxides XO₂ (X = Ru, Rh, Os, and Ir) are investigated using first-principles calculations based on density functional theory. Elastic constants, bulk modulus, shear modulus, and Young's modulus as well as Possion ratio are given. OsO₂ and IrO₂ show strong incompressibility. The hardness estimated for these dioxides shows that they are not superhard solids. The obtained Debye temperatures are comparative to those of transition metal dinitrides or diborides.

The high-pressure behavior of perovskite (MgSiO₃) is studied based on density functional simulations within generalized gradient approximation (GGA). All calculations are performed by using the linear augmented plane waves plus local orbital (LAPW+lo) method to solve the scalar-relativistic Kohn-Sham equations. The static calculations predict a perovskite (pnma phase) — post-perovskite (Cmcm phase) transition occurring at 86 gigapascals (GPa). The similar bulk modulus values, differing only 3 GPa, are given by using three kinds of equation of states. The electronic structure and optical properties of MgSiO₃ at phase transition pressure are also discussed.

We have investigated the elastic and thermodynamic properties of ZrO₂ under pressure up to 120 Gpa by the plane wave pseudopotential density functional theory with the generalized gradient approximation (GGA) method. The elastic constants of ZrO₂ are calculated and meet the generalized stability criteria, suggesting that ZrO₂ is mechanically stable within this pressure range. The pressure effects on the elastic properties reveal that the elastic modulus B, shear modulus G and Young's modulus Y increase linearly with the pressure increasing, implying that the resistance to deformation is enhanced. In addition, by analyzing the Poisson's ratio ν and the value of B/G, we notice that ZrO₂ is regarded as being a ductile material under high pressure and the ductility can be improved by the pressure increasing. Then, we employ the quasi-harmonic Debye model considering the phononic effects to obtain the thermodynamic properties of ZrO₂. Debye temperature Θ_D, thermal expansion coefficient α, heat capacity C_p and Grüneisen parameter γ are systematically explored at pressure of 0–80 Gpa and temperature of 0–1000 K. Our results have provided fundamental facts and evidences for further experimental and theoretical researches.

In this paper, we employ first-principles methods based on electronic density functional theory (DFT) to investigate the phase stability, elastic and thermodynamic properties of Zr–Al binary substitutional alloys which are Zr₃Al, Zr₂Al, ZrAl, ZrAl₂ and ZrAl₃. By analyzing the elastic constants and enthalpy of formation, those phases both satisfy the generalized stability criteria and the results show that ZrAl₂ is the most stable. Due to high bulk modulus B, shear modulus G and Youngs modulus Y, ZrAl₂ also possesses excellent mechanical properties. Moreover, it is expected that there will be covalent bonding between Zr and Al atom in ZrAl₂ compound, which is confirmed by the electronic structure and the differences of charge density discussions. In the end, based on the calculated elastic modulus, the elastic wave velocity, Debye temperature Θ_D and specific heat C_V are discussed. As a result, ZrAl₃ possesses the highest Debye temperature and sound velocity, meaning a larger associated thermal conductivity and higher melting temperature.

The mechanical and thermodynamic properties of ZrAl₂ alloy under high pressure are investigated by first-principles based on the density functional theory. Due to all the elastic constants of ZrAl₂ alloy satisfy generalized stabilities criteria, ZrAl₂ is mechanically stable under pressure up to 100GPa. By analyzing the value of B/G and Poisson’s ratio $\nu$ which are correlated with the ductility and brittleness of material, we found that ZrAl₂ belongs to brittle material at pressure of 0–70GPa and will change from brittleness to ductility at 70GPa. Combining with high bulk modulus B and shear modulus G, the mechanical of properties will be improved under high pressure. Moreover, the thermodynamic properties, such as the Debye temperature ${\mathrm{\Theta }}_D$, heat capacity $C_p$ and thermal expansion $\alpha$, are discussed using the quasi-harmonic Debye model. We noted that the Debye temperature ${\mathrm{\Theta }}_D$ is mainly dependent on the pressure and the effect of temperature on the heat capacity $C_p$ is more important than the applied pressure.

The dissolution and diffusion behaviors of H in the four low-Miller-index W surfaces ((110), (112), (100) and (111)) are systematically studied by the density functional theory approach to understand the orientation dependence of the H bubble distribution on surface. The results show that H accumulation on surface is influenced by H diffusion barrier as well as vacancy and H formation. The barriers of diffusion towards surfaces are larger than that in bulk. It indicates that H is prone to diffuse into the deep in bulk once H dissolves in surface. H is preferred to accumulate on the W(111) surface due to the lower formation energies of vacancy and H comparing to that in bulk. However, W(110) is the resistant surface for forming H bubble due to the higher formation energies of vacancy and H. The results are helpful for understanding the orientation dependence of surface damages on W surface and designing new plasma-facing materials.

The phonon, elastic and thermodynamic properties of L1₂ phase Rh₃Zr have been investigated by density functional theory approach combining with quasi-harmonic approximation model. The relaxed lattice parameters of L1₂ phase Rh₃Zr at zero pressure are in good agreement with the experiment. To judge the stability of L1₂ phase Rh₃Zr under high pressure, the phonon band structure has been studied. The results show that L1₂ phase Rh₃Zr possesses dynamical stability in the pressure range from 0GPa to 80GPa due to the absence of imaginary frequencies. The pressure dependences of elastic constants $C_{ij}$ have been analyzed. All the elastic constants of Rh₃Zr in a wide pressure range (0–80GPa) meet general mechanical conditions, suggesting that L1₂ phase Rh₃Zr is mechanically stable under pressure up to 80GPa. L1₂ phase Rh₃Zr exhibits ductility under high pressure and the pressure can improve the ductility from the results of the value of $B∕G$ and Poisson’s ratio $\nu$. Hence, it is obvious that the mechanical properties of Rh₃Zr can be improved under high pressure. Moreover, we have obtained the thermodynamic properties using the quasi-harmonic Debye model. We note that the effect of the temperature on the Debye temperature ${\mathrm{\Theta }}_D$ is smaller than that of the pressure. We believe that our result will be a good guidance to future works and applications.

Understanding the effect of alloying elements on the retention and clustering behavior of Helium (He) in Aluminum (Al) is a great help to study the radiation damage process of Al-based nuclear structure materials under He irradiation. Based on the first-principles calculations, we investigated the influence of Gallium (Ga) dopant on the formation of ${\mathrm{\text{He}}}_{\mathrm{\text{n}}}$ and ${\mathrm{\text{He}}}_{\mathrm{\text{n}}}\mathrm{\text{V}}$ clusters in Al and Al-0.9at% Ga (Al-Ga) alloy. We found that Ga dopant could influence the formation and growth of He clusters. In addition, the presence of Ga reduces the binding energy of both ${\mathrm{\text{He}}}_{\mathrm{\text{n}}}$ and ${\mathrm{\text{He}}}_{\mathrm{\text{n}}}\mathrm{\text{V}}$ clusters, resulting in He dissociation from the clusters more easily. Moreover, Ga could serve as a trapping center by reducing the charge density in its vicinity to induce He nucleation in Al-Ga alloy compared to that in pure Al. Furthermore, the binding energy of He to vacancy around Ga is weaker than that around Al, suggesting that Ga will decrease the trapping capability of vacancy to He. We thus propose that Ga plays a key role in He clustering behavior. Our results are significant to understand the effect of dopant on He bubbling behavior and the distribution of damages in Al-Ga alloy under He irradiation.

Understanding the hydrogen (H) capacity, which represents the tritium capacity in Li₂TiO₃ crystal has become an important aspect of the tritium release process of nuclear fusion. In this work, a systematic density-functional-theory (DFT) study is performed to investigate the trapping and accumulation of H in Li₂TiO₃ crystal. In perfect crystal, the H adsorption properties are investigated and the maximum number of trapped H atoms are obtained. In the defect models, by calculating the trapping energy and Bader charge, we find that a Li vacancy can capture four H atoms while the capacity of a Ti vacancy is seven and then other H atoms tend to be trapped by interstitial sites outside the vacancy. Then the H capacity both inside and outside the vacancy in the defect models is studied and analyzed. According to our calculations, crystals containing a vacancy present stronger H trapping abilities than perfect crystals, especially for the crystal with a Ti vacancy. In addition, the increase of H atoms in the vacancy facilitates the formation of the neighboring vacancy so that more H atoms can be accommodated in the crystal with vacancy. Our results reveal the H capacity of different Li₂TiO₃ models, which provide theoretical support for related tritium release experiments.

The electronic structures of Co/Cu interface have been calculated by first-principles method based on local spin density approximation. Models 3Co/xCu (x=1-8 monolayers) with different Cu layer thickness are investigated. The results show that the oscillation of the density of states near the Fermi surface with the Cu spacer thickness has been observed and the period of oscillation is about 6 atom layers, which has a good agreement with the corresponding experiments. We also discuss the spin polarization and magnetic resistance with the change of Cu layers thickness. Further analysis shows majority spin states near the Fermi surface played a key role in giant magnetoresistance (GMR).

First-principles calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA) have been carried out to study the migration of helium in tungsten. The results show that helium interstitial jump directly between two tetrahedral sites with very low migration energy. The migration manner of interstitial He is related to the configuration of the host metal. Furthermore, the dissociation energy of a substitutional helium atom from a vacancy has been calculated from first-principles.

In this paper, the effect of Co and Ni in AerMet100 steel on the structure stability for Fe₃C was investigated using first-principles calculations based on generalized gradient approximation (GGA) density function theory. The internal positions of atoms within the unit cell were optimized and the ground state properties such as lattice parameter, the formation energy and density of states for Fe₃C structure without and with Co or Ni((Fe, M)₃C, M=Fe, Co, Ni) were calculated. The calculated results show that the equilibrium structural parameters of Fe₃C agree with the experimental ones. The formation energy of Fe₃C is negative, while that of Fe₃C with Co or Ni are positive. The Fe₃C with the substitution of Co or Ni for Fe at the 4c sites are metastable or unstable at ambient conditions. The effect of Co and Ni on the structural stability for Fe₃C is discussed in terms of their electronic structure and chemical bonding.

The electronic structure and ferromagnetism of Sn₂Co₂O₈ and Sn₂Co₂O₇ have been investigated based on the first-principles plane-wave pseudopotential method within the generalized gradient approximation. The calculated results reveal that the oxygen vacancy plays an important role in the electronic structure and ferromagnetism. The Sn₂Co₂O₈ shows half-metallic behavior, but by introducing single oxygen vacancy, the half-metallic transits to metallic behavior. At the same time, the spin magnetic moment of every Co atom and the total magnetic moment change greatly. For Sn₂Co₂O₈ and Sn₂Co₂O₇, the total spin magnetic moments are 1.99 and 3.49 u_B, respectively.

The structural phase transitions, mechanical properties and electronic structures of OsO₂ under high pressure are systemically investigated by the first-principles plane-wave basis pseudopotential calculations. The possible pressure-induced transition sequence in OsO₂ may be the rutile, pyrite and fluorite phases, and the stable CaCl₂ structure is not found. The fluorite phase has high bulk modulus (355.3 GPa), large shear modulus G (267.9 GPa), and huge elastic constant C₄₄ (292.7 GPa), and consequently is an excellent candidate of superhard materials. Crystal structures, valence electron densities, band structures, DOS and PDOS of the rutile, pyrite and fluorite phases of OsO₂ have also been carefully analyzed to elucidate their mechanical properties.

The all-electron full-potential linearized muffin-tin orbital method, by means of quasi-harmonic Debye model, is applied to investigate the elastic constant and thermodynamic properties of body-centered-cubic tantalum (bcc Ta). The calculated elastic constants of bcc Ta at 0 K is consistent with the previous experimental and theoretical results. Our calculations give the correct trends for the pressure dependence of elastic constants. By using the convenient quasi-harmonic Debye model, we refined the thermal equations of state. The thermal expansivity and some other thermal properties agree well with the previous experimental and theoretical results.

In this review article, we survey the relatively new theory of orbital magnetization in solids — often referred to as the "modern theory of orbital magnetization" — and its applications. Surprisingly, while the calculation of the orbital magnetization in finite systems such as atoms and molecules is straight forward, in extended systems or solids it has long eluded calculations owing to the fact that the position operator is ill-defined in such a context. Approaches that overcome this problem were first developed in 2005 and in the first part of this review we present the main ideas reaching from a Wannier function approach to semi-classical and finite-temperature formalisms. In the second part, we describe practical aspects of calculating the orbital magnetization, such as taking k-space derivatives, a formalism for pseudopotentials, a single k-point derivation, a Wannier interpolation scheme, and DFT specific aspects. We then show results of recent calculations on Fe, Co, and Ni. In the last part of this review, we focus on direct applications of the orbital magnetization. In particular, we will review how properties such as the nuclear magnetic resonance shielding tensor and the electron paramagnetic resonance g-tensor can be elegantly calculated in terms of a derivative of the orbital magnetization.

The hydration effects of water on the high-pressure elastic properties of three Mg₂SiO₄ polymorphs have been investigated using first-principles simulation density functional theory. The effect of water incorporation was simulated by considering Mg₂SiO₄ crystals with water contents of 0, 1.65 and 3.3 wt%. With increasing water content, the calculated bulk/shear modulus and sound velocities decrease and their pressure derivatives tend to increase. Thus the hydration effect becomes less obvious at higher pressure. The bulk velocity contrasts from olivine to wadsleyite and from wadsleyite to ringwoodite are found to decrease after water incorporation. Our theoretical results indicate that water content and partition around the phase boundaries have profound impact on the structural and seismic properties of the mantle.

Nonlinear optical (NLO) crystals are very important optoelectronic functional materials and their developments have significantly contributed to the progress of laser science and technology for decades. In order to explore new NLO crystals with superior performances, it is greatly desirable to understand the intrinsic relationship between the macroscopic optical properties and microscopic structural features in crystals. In this paper, the applications of density functional theory (DFT) method to the elucidation of the structure-property relationship and to the exploration on novel NLO materials in the ultraviolet and infrared spectrum regions are reviewed. The great success in the linear and NLO property predictions has been achieved using the first-principles computational simulations, and the mechanism understanding obtained by various analysis tools can give substantial guidance to the search and design of new NLO crystals.

The absorption edge shifted to long wavelength direction and short wavelength direction of two opposite experimental conclusions have been reported, when the band-gap and absorption spectra of Nb-doped anatase TiO₂ were studied. In order to solve this contradiction, the electronic structure and the optical property of Nb heavy doped anatase TiO₂ have been studied by the first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory with +U method modification. The calculated results indicate that the higher the Nb-doping is, the higher the total energy is, the worse the stability is, the higher the formation energy is, the more difficult the doping is, the wider the optical band-gap is, the more obvious the absorption edge shifting to short wavelength direction is, the lower the absorptivity and the reflectivity is, which is in agreement with the experimental results. The reasonable interpretation of the contradiction has been reported in this paper, too.