Ab initio study of the effects of helium on the mechanical properties of different erbium hydrides

Although rare-earth metals have increasingly received attention for use in the storage and transportation of the tritium used in nuclear fusion reactions, they still face great challenges, such as the effect of helium on the mechanical properties of different erbium hydrides. In this work, first principles are used to study the mechanical properties(elastic constants, Young's modulus, transverse shear modulus and bulk modulus) of different erbium hydrides exposed to helium. The Young's modulus, the transverse shear modulus and the bulk modulus are given based on the elastic constants calculated according to first principles. It is found that the mechanical properties of all three erbium hydrides decrease in the presence of helium, and the decline of the mechanical properties of ErH_3 is the most serious. To explain the reason for the decrease in the mechanical properties, the densities of the states of erbium hydrides are calculated. During the calculations, helium embrittlement is not found and the ductility of the erbium hydrides improves following the production of helium at the helium concentrations considered in this work.     

Ab initio study of the polymerisation of cyclopentasilane

The molecular structures and vibration modes of cyclopentasilane (Si₅H₁₀) have been examined by employing ab initio and density-functional methods. Three different structures of Si₅H₁₀ with different symmetries are analysed, and the results show that the envelope (C _s ) and the twist (C₂) forms have similar energies and that the planar form (D_5h ) is about 50 meV less stable than the C _s and C₂ forms. The excited-state potential energy surface of Si₅H₁₀ is performed using the CIS electronic energy calculation. The ring-open reaction of Si₅H₁₀ is investigated in detail by using the first-principles molecular-dynamics simulation for screening the reaction pathways. The formation of Si–H–Si is found to play an important role in the ring-open reaction.     

Ab initio investigation of the mechanical properties of copper

Employing the ab initio total energy method based on the density functional theory with the generalized gradient approximation, we have systematically investigated the theoretical mechanical properties of copper (Cu). The theoretical tensile strengths are calculated to be 25.3 GPa, 5.9 GPa, and 37.6 GPa for the fcc Cu single crystal in the [001], [110], and [111] directions, respectively. Among the three directions, the [110] direction is the weakest one due to the occurrence of structure transition at the lower strain and the weakest interaction of atoms between the (110) planes, while the [111] direction is the strongest direction because of the strongest interaction of atoms between the (111) planes. In terms of the elastic constants of Cu single crystal, we also estimate some mechanical quantities of polycrystalline Cu, including bulk modulus B, shear modulus G, Young’s modulus E p , and Poisson’s ratio ν.     

Ab initio study of palladium and silicon carbide

Ab initio methods have been used to investigate the properties of Pd as impurity in bulk SiC at five charged states within the framework of density functional theory using the local spin density approximation. It was found that Pd interstitials and substitutionals have similar energy to their intrinsic counterparts. In addition, Pd substitutes for a vacancy, di-vacancy, and tri-vacancy with similar energies. Pd diffuses through SiC via an interstitial mechanism employing the tetrahedral sites and Pd can substitute for Si and C at positive charged states. Removing electrons (p-type doping) from SiC lowers the formation and migration energies of Pd defects in SiC for most configurations.     

Thermophysical properties of Ni-5%Sn alloy melt

The surface tension and specific heat of Ni-5%Sn alloy melt were measured by the oscillating drop method and the drop calorimetric method using electromagnetic levitation, respectively. The temperature coefficient of surface tension is 6.43×10⁻⁴ N·m⁻¹K⁻¹ within the temperature regime of 1464–1931 K. The enthalpy change was measured in the temperature range from 1461 to 1986 K, and the average specific heat was obtained as 43.03 J·mol⁻¹K⁻¹. Some other thermophysical properties, such as viscosity, solute diffusion coefficient, density, thermal diffusivity and thermal conductivity of this alloy melt, were derived based on the experimentally measured surface tension and specific heat. Using these thermophysical parameters, the relation between solute trapping and undercooling in rapidly solidified α-Ni was calculated, and the theoretical prediction shows a good agreement with experimental data.     

氢吸附金刚石（001）表面的第一性原理研究

采用第一性原理方法研究氢吸附的金刚石（001）表面，计算了氢吸附金刚石表面构型.通过分析吸附前后空间电荷分布的变化，发现吸附H原子的金刚石（001）表面电荷向H原子转移，即表明氢吸附的金刚石表面带负电.分析了这种现象的微观机制，以及它对金刚石表面电学性质的影响. 关键词： 第一性原理计算 金刚石（001）面 表面吸附 电荷密度分布     

几种香豆素衍生物分子的二阶非线性光学性质的从头算研究

采用从头算ab initio Hartree_Fock/6_31G方法，对以3_苯基香豆素为母体的香豆素系列衍生物分子进行了几何优化，计算了它们的分子二阶非线性光学系数β值，分析了其分子结构、取代基类型和取代位置对β值的影响，并对计算结果所反映的规律性进行了探讨. 关键词： 3_苯基香豆素 非线性光学 二阶效应 从头算 电荷转移     

Ab initio potential energy surface for the nitrogen molecule pair and thermophysical properties of nitrogen gas

A four-dimensional potential energy hypersurface (PES) for the interaction of two rigid nitrogen molecules was determined from high-level quantum-chemical ab initio computations. A total of 408 points for 26 distinct angular configurations were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory and basis sets up to aug-cc-pV5Z supplemented with bond functions. The calculated interaction energies were extrapolated to the complete basis set limit and complemented by corrections for core–core and core–valence correlations, relativistic effects and higher coupled-cluster levels up to CCSDT(Q). An analytical site–site potential function with five sites per nitrogen molecule was fitted to the interaction energies. The PES was validated by computing second and third pressure virial coefficients as well as shear viscosity and thermal conductivity in the dilute-gas limit. An improved PES was obtained by scaling the CCSDT(Q) corrections for all 408 points by a constant factor, leading to quantitative agreement with the most accurate experimental values of the second virial coefficient over a wide temperature range. The comparison with the best experimental data for shear viscosity shows that the values computed with the improved PES are too low by about 0.3% between 300 and 700?K. For thermal conductivity large systematic deviations are found above 500?K between the calculated values and most of the experimental data.     

Study of structural and magnetic properties of Fe_(80)P-_9B_(11) amorphous alloy by ab initio molecular dynamic simulation

The structural and magnetic properties of Fe_(80)P_9B_(11) amorphous alloy are investigated through ab initio molecular dynamic simulation. The structure evolution of Fe_(80)P_9B_(11) amorphous alloy can be described in the framework of topological fluctuation theory, and the fluctuation of atomic hydrostatic stress gradually decreases upon cooling. The left sub peak of the second peak of Fe–B partial pair distribution functions(PDFs) becomes pronounced below the glass transition temperature, which may be the major reason why B promotes the glass formation ability significantly. The magnetization mainly originates from Fe 3d states, while small contribution results from metalloid elements P and B. This work may be helpful for developing Fe-based metallic glasses with both high saturation flux density and glass formation ability.     

Ab initio study of electronic structure and magnetic properties of copper (II) terephthalate

The first principles within the full potential linearized augmented plane wave (FP-LAPW) method was applied to study the compound of Cutp(OH₂)₂. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has a ferromagnetic interaction arising from the bridging water molecule. The spin magnetic moment 1.0μ_B per molecule mainly comes from the Cu ion with little contribution from O, C anion.     

Pressure-induced phase transition and electronic properties of AlN nanowires: an ab initio study

The structural stability of AlN nanowires have been analyzed in wurtzite (B4), zincblende (B3), rocksalt (B1) and CsCl (B2) type phases using density functional theory based ab initio approach. The total energy calculations have been performed in a self-consistent manner using local density approximation as exchange correlation functional. The analysis finds the B4 type phase as most stable amongst the other phases taken into consideration and observes the structural phase transition from B4?→?B3, B4?→?B1, B4?→?B2, B3?→?B1 and B3?→?B2 at 42.7, 76.54, 142, 30.4 and 108.9?GPa respectively. Lattice parameter, bulk modulus and pressure derivatives of AlN nanowires have also been calculated for all the stable phases. The electronic band structure analysis of AlN nanowires shows a semiconducting nature in its B4, B3 and B1 type phases, whereas the B2 type phase is found to be metallic.     

Lattice dynamical and thermodynamical properties of ReB₂,RuB₂,and OsB₂ compounds in the ReB₂ structure

Structural and lattice dynamical properties of ReB2,RuB2,and OsB2 in the ReB2 structure are studied in the framework of density functional theory within the generalized gradient approximation.The present results show that these compounds are dynamically stable for the considered structure.The temperature-dependent behaviors of thermodynamical properties such as internal energy,free energy,entropy,and heat capacity are also presented.The obtained results are in good agreement with the available experimental and theoretical data.     

蓝宝石在高压下光学性质及能带结构的从头算模拟

采用量子力学从头算和赝势平面波基组以及GGA-PBE交换相关函数对蓝宝石(α- Al2O3)窗口材料在0－1000GPa静水压力作用下光学性质及能带结构进行了研究。结果表明, 在静水压力作用下可以观察到蓝宝石一直保持良好的透明性,前沿带隙随着压力的增大先变大后变小,而且部分键长有明显变化,即蓝宝石单晶结构在加压过程中发生了原子位置移动而进行了位置重构。     

钽酸锂、蓝宝石在高压下的光学性质从头算研究

采用量子力学从头算和赝势平面波基组以及非局域广义梯度近似(PP-GGA)方法对两种光学窗口材料(单晶LiTaO3和蓝宝石α-Al2O3)在0~250 GPa静水压力范围内的光学透明性进行了研究.结果表明,随着压力的增加蓝宝石和钽酸锂的最大吸收峰位置均向低波长区移动(蓝移).蓝宝石在0~250 GPa压力下,可见光范围内一直保持其透明性,而LiTaO3则在高压下时就变得不透明了,其原因是钽酸锂的前沿能带闭合而导致其吸收带宽随外界压力的增大变宽所致.     

蓝宝石在高压下光学性质及能带结构的从头算模拟

采用量子力学从头算和赝势平面波基组以及GGA-PBE交换相关函数对蓝宝石(α- Al2O3)窗口材料在0－1000GPa静水压力作用下光学性质及能带结构进行了研究。结果表明, 在静水压力作用下可以观察到蓝宝石一直保持良好的透明性,前沿带隙随着压力的增大先变大后变小,而且部分键长有明显变化,即蓝宝石单晶结构在加压过程中发生了原子位置移动而进行了位置重构。     

Ab initio study of the optical properties of crystalline phenanthrene,including the excitonic effects

Using the ab initio methods for solving the Bethe–Salpeter equation on the basis of the FPLAPW method, optical properties of crystalline phenanthrene were calculated, in a comparison to its isomer, anthracene. It was found that despite the similarity of the structural, electronic, and the overall optical properties in a 40 eV energy range, phenanthrene and anthracene show significant differences in their optical spectra in the energy range below band gaps. Phenanthrene has two spin singlet excitonic features whereas anthracene shows one. The singlet and the lowest triplet binding energies of phenanthrene were found to be larger than anthracene. In this study, in addition, a comparison has been made between the optical spectra in RPA and the existing experimental data.     

Ab initio study on the anisotropy of mechanical behavior and deformation mechanism for boron carbide

The mechanical properties and deformation mechanisms of boron carbide under a-axis and c-axis uniaxial compression are investigated by ab initio calculations based on the density functional theory.Strong anisotropy is observed.Under a-axis and c-axis compression,the maximum stresses are 89.0 GPa and 172.2 GPa respectively.Under a-axis compression,the destruction of icosahedra results in the unrecoverable deformation,while under c-axis compression,the main deformation mechanism is the formation of new bonds between the boron atoms in the three-atom chains and the equatorial boron atoms in the neighboring icosahedra.     

Ab initio pseudopotential study of vacancies and self-interstitials in hcp titanium

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration. Using the nudged elastic band method (NEB), we determine a possible minimum energy path (MEP) for the diffusion of self-interstitial titanium atoms from an octahedral site to the nearest octahedral site. The energy barrier for this migration mechanism is shown to be about 0.20 eV.     

Ab initio predicted elastic and thermodynamic properties of Imm2-BN under high pressure

The structural parameters, elastic constants, thermodynamic properties of Imm2-BN under high pressure were calculated via the density functional theory in combination with quasi-harmonic Debye approach. The results showed that the pressure has the significant effect on the equilibrium lattice parameters, elastic and thermodynamic properties of Imm2-BN. The obtained ground state structural parameters are in good agreement with previous theoretical results. The elastic constants, elastic modulus, and elastic anisotropy were determined in the pressure range of 0–90?GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is evaluated and the elastic anisotropy of the Imm2-BN up to 90?GPa is studied in detail. Moreover, the pressure and temperature dependence of thermal expansion coefficient, heat capacity, Debye temperature, and Grüneisen parameter are predicted in a wide pressure (0–90?GPa) and temperature (0–1600?K) ranges. The obtained results are expected to provide helpful guidance for the future synthesis and application of Imm2-BN.