Stability,electronic structures,and mechanical properties of Fe–Mn–Al system from first-principles calculations

The stability, electronic structures, and mechanical properties of the Fe–Mn–Al system were determined by firstprinciples calculations. The formation enthalpy and cohesive energy of these Fe–Mn–Al alloys are negative and show that the alloys are thermodynamically stable. Fe_3Al, with the lowest formation enthalpy, is the most stable compound in the Fe–Mn–Al system. The partial density of states, total density of states, and electron density distribution maps of the Fe–Mn–Al alloys were analyzed. The bonding characteristics of these Fe–Mn–Al alloys are mainly combinations of covalent bonding and metallic bonds. The stress-strain method and Voigt–Reuss–Hill approximation were used to calculate the elastic constants and moduli, respectively. Fe_(2.5)Mn_(0.5)Al has the highest bulk modulus, 234.5 GPa. Fe_(1.5)Mn_(1.5)Al has the highest shear modulus and Young's modulus, with values of 98.8 GPa and 259.2 GPa, respectively. These Fe–Mn–Al alloys display disparate anisotropies due to the calculated different shape of the three-dimensional curved surface of the Young's modulus and anisotropic index. Moreover, the anisotropic sound velocities and Debye temperatures of these Fe–Mn–Al alloys were explored.     

Elastic,mechanical, electronic properties and hardness of Nb2AsC from first principles

The study aims at the elastic, mechanical, electronic properties and hardness of Nb₂AsC using first principles based on the density functional theory method within the generalised gradient approximation. The calculated lattice parameters of Nb₂AsC are in good agreement with the experimental data. The five independent elastic constants are firstly calculated as a function of pressure, and our results indicate that it is mechanically stable in the applied pressure. The elastic anisotropy is examined through the computation of the direction dependence of Young's modulus. The pressure dependences of the bulk modulus, shear modulus, average velocity of acoustic waves and Debye temperature of Nb₂AsC are systematically investigated. The band structure and density of states are discussed, and the results show that the strong hybridisations C p–Nb d and As p–Nb d would be beneficial to the structure stability of Nb₂AsC. Based on the Mulliken population analysis, the hardness of Nb₂AsC is predicted.     

Electronic structure and thermodynamic properties of LiBC under high pressure

This paper investigates the electronic structure and thermodynamic properties of LiBC in the hexagonal structure by using the generalized gradient approximation （GGA） and local density approximation correction scheme in the frame of density functional theory. The geometric structure of LiBC under zero pressure, and the dependences of the normalized lattice parameters a/ao and c/co, the ratio e/a, the normalized primitive volume V/Vo on pressure are given. The thermodynamic quantity （including the heat capacity Cv, Debye temperature 6~D, thermal expansion a and Grfineisen parameter -y） dependences on temperature and pressure are obtained through the GGA method and the quasi-harmonic Debye model. The band structures and density of state of LiBC under different pressures have also been analysed.     

First-principles calculations for electronic, optical and thermodynamic properties of ZnS

The electronic, optical and thermodynamic properties of ZnS in the zinc-blende （ZB） and wurtzite （WZ） structures are investigated by using the plane-wave pseudopotential density functional theory （DFT）. The results obtained are consistent with other theoretical results and the available experimental data. When the pressures are above 20.5 and 27 GPa, the ZB-ZnS and the WZ-ZnS are converted into indirect gap semiconductors, respectively. The critical point structure of the frequency-dependent complex dielectric function is investigated and analysed to identify the optical transitions. Moreover, the values of heat capacity Cv and Debye temperature θ at different pressures and different temperatures are also obtained successfully.     

Exploring at nanoscale from first principles

Systems at the nanoscale can exhibit distinctive and unexpected properties in electrical, magnetic, mechanical, and chemical aspects. Understanding these properties not only is of importance from the fundamental scientific view but also offers great opportunities for future applications. Theoretical calculations can provide important information to interpret, modify, and predict the novel properties of objects at the nanoscale and therefore play a significant role in the process of exploring the nano world. In this review, six different areas are briefly presented, namely, prediction of new stable structures, modification of properties (especially the electronic structures), design of novel devices for applications, the structures and catalytic effects of clusters, the mechanical and transport properties of gold nanowires, and improvement of materials for hydrogen storage. Based on these examples, we show what can be done and what can be found in the investigations of nanoscale systems with participation of theoretical calculations.      

Pb-Sn合金及Pb的热力学性质的第一性原理计算

本文基于第一性原理的密度泛函理论(DFT)和密度泛函微扰理论(DFPT)方法,利用虚晶近似的计算方法研究了Pb-Sn合金的晶格结构、电子能带、声子能带及热力学性质,并用晶格能量差可与达到熔化温度时的振动能量相当的固-液相变机理研究了熔化温度,同时与所计算Pb的所有结果进行了对比.     

N掺杂钛酸盐的电子结构和磁性研究

基于密度泛函理论,从头计算了N掺杂立方结构钛酸盐的电子结构和磁学性质.结果表明,N掺杂钛酸盐在自旋极化状态下的总能量比自旋非极化状态下的总能量小,说明N掺杂钛酸盐的基态具有铁磁性.从态密度和自旋密度分析可知,其磁性源于掺杂的N 2p电子和价带顶O 2p电子间的p-p耦合作用. 关键词： 钛酸盐 电子结构 磁学性质     

MnTe电子结构和磁性的第一性原理研究

采用基于密度泛函理论的赝势投影缀加波方法, 对六种典型的二元晶体结构Rocksalt (RS), Cesiun-chloride (CC), Zinc-blende (ZB), Wurtzite (WZ), Iron-silicide (IS) 和Nickel-Arsenide (NA)的MnTe进行了计算研究. 通过比较六种结构的结合能, 确定了MnTe的基态结构是反铁磁的NA结构. 研究了这六种结构MnTe的电子结构、磁性, 并用Birch-Murnaghan状态方程拟合求得了各相结构的体弹性模量和相变压. 电子态密度表明, RS, CC和IS结构的MnTe为反铁磁导体, ZB, WZ和NA结构的MnTe均为反铁磁半导体.     

(Nb,N)共掺杂锐钛矿电子结构和光学性质的第一性原理研究

二氧化钛（TiO₂）作为一种性能优良的光催化剂已经受到越来越多的关注.本研究采用密度泛函理论的第一性原理和广义梯度近似+U方法,对锐钛矿结构TiO₂晶体三种可能的（Nb,N）共掺杂TiO₂的几何结构、形成能、能带结构、电子密度和光吸收系数进行了研究,并与单掺杂（Nb/N）体系进行了对比.对掺杂后体系的几何结构进行的计算表明杂质原子掺入后晶格发生了不同程度的畸变.此外,（Nb,N）共掺杂体系与纯TiO₂相比,其禁带宽度和吸收边较小.同时,与N掺杂TiO₂相比,N的2p态在共掺杂情形下变为完全占据,从而减少了电子空穴对的复合.而且共掺杂体系的形成能比N单掺杂体系低,因而更加稳定.因此,（Nb,N）共掺杂可以很好地提升锐钛矿型TiO₂在可见光波段的光催化性能.     

Mg, Sr掺杂CaF2电子结构及光学性质的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理方法对纯CaF2晶体和Mg、Sr掺杂CaF2体系的晶体结构、电学以及光学性质进行了详细的对比研究, 结果表明: 与纯CaF2晶体相比, 掺杂体系的带隙变窄且形成新的态密度峰, 费米面附近出现F与Mg、Sr原子间轨道杂化加强现象. 另外, 掺杂体系仅表现出介电性质, 其对紫外光的吸收强度大大减弱, 而Ca7SrF16掺杂体系在25.44 eV处产生新的小吸收峰. CaF2晶体掺入Mg、Sr原子后, 体系在紫外光区的消光系数减小且对紫外光的透过率增大. 此外, 掺杂体系的反射谱峰和损失函数峰均发生红移且峰值显著降低.     

Mg, Sr掺杂CaF2电子结构及光学性质的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理方法对纯CaF2晶体和Mg、Sr掺杂CaF2体系的晶体结构、电学以及光学性质进行了详细的对比研究, 结果表明: 与纯CaF2晶体相比, 掺杂体系的带隙变窄且形成新的态密度峰, 费米面附近出现F与Mg、Sr原子间轨道杂化加强现象. 另外, 掺杂体系仅表现出介电性质, 其对紫外光的吸收强度大大减弱, 而Ca7SrF16掺杂体系在25.44 eV处产生新的小吸收峰. CaF2晶体掺入Mg、Sr原子后, 体系在紫外光区的消光系数减小且对紫外光的透过率增大. 此外, 掺杂体系的反射谱峰和损失函数峰均发生红移且峰值显著降低.     

LiNH2 的晶格动力学、介电性质和热力学性质第一性原理研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论计算了LiNH₂的晶格动力学、介电性质和热力学性质,得到了布里渊区高对称方向上的声子色散曲线和相应的声子态密度,分析了 LiNH₂的红外和拉曼活性声子频率,同时给出它的介电张量和玻恩有效电荷张量. 研究表明,LiNH₂存在小的各向异性,计算所得结果与实验值和其他理论值符合较好.最后,利用得到的声子态密度进一步预测了LiNH₂的热力学性质 关键词： 密度泛函理论 晶格动力学 热力学性质 第一性原理计算     

Ni掺杂MgF2电子结构和光学性质的第一性原理研究

基于密度泛函理论(DFT) 的第一性原理平面波超软赝势方法,计算了过渡金属Ni不同比例（16.67%,12.5%,8.33%,6.25%）掺杂MgF2晶体的几何结构、电子结构和光学性质。通过对比发现,由于Ni原子的掺入, 体系的禁带宽度减小且能带中出现中间杂质带。另外, 介电函数虚部以及吸收光谱图中均出现双峰结构,结合前人的计算给出了详细的物理机制。上述现象,揭示了Ni掺杂MgF2 体系在光学元器件方面的潜在应用。     

Ni掺杂MgF2电子结构和光学性质的第一性原理研究

基于密度泛函理论(DFT) 的第一性原理平面波超软赝势方法,计算了过渡金属Ni不同比例（16.67%,12.5%,8.33%,6.25%）掺杂MgF2晶体的几何结构、电子结构和光学性质。通过对比发现,由于Ni原子的掺入, 体系的禁带宽度减小且能带中出现中间杂质带。另外, 介电函数虚部以及吸收光谱图中均出现双峰结构,结合前人的计算给出了详细的物理机制。上述现象,揭示了Ni掺杂MgF2 体系在光学元器件方面的潜在应用。     

Ta4Cn^-/0(n=0-4)团簇的电子结构、成键性质及稳定性

本文采用尺寸选择的负离子光电子能谱技术,结合密度泛函理论,对Ta4Cn^-/0(n=0-4)团簇电子结构、成键性质以及稳定性进行了研究.实验测得Ta4Cn-(n=0—4)团簇负离子基态结构的垂直脱附能分别为(1.16±0.08),(1.35±0.08),(1.51±0.08),(1.30±0.08)和(1.86±0.08)eV.中性Ta4Cn(n=0—4)团簇的电子亲和能分别为(1.10±0.08),(1.31±0.08),(1.44±0.08),(1.21±0.08)和(1.80±0.08)eV.研究发现Ta4^-/0团簇为四面体结构,Ta4C1-/0团簇中碳原子覆盖在Ta4四面体的一个面上方,Ta4C2^-/0团簇则是两个碳原子分别覆盖在Ta4四面体中的两个面上方.Ta4C3^-/0团簇是一个缺角立方体结构.Ta4C4^-/0团簇则是近似立方体结构,可以看成是α-TaC面心立方晶体的最小晶胞单元.分子轨道分析结果显示Ta4C3团簇的单电子最高占据轨道主要布居在单个钽原子周围,导致Ta4C3^-团簇的垂直脱附能明显低于其相邻团簇.理论研究显示随着碳原子数目的增加,Ta4Cn^-/0(n=0—4)团簇中的钽-钽金属键逐渐被钽-碳共价键取代,单原子结合能逐渐增加且明显高于Ta4+n^-/0(n=0-4)团簇.中性Ta4C4的单原子结合能高达7.13 eV,这说明钽-碳共价键的形成有利于提高材料的熔点,这与碳化钽作为高温陶瓷材料的特性密切相关.     

PuH2分子电子结构的DVM研究

关于钚的氢化物的分子结构和分子光谱公开解密的资料与数据甚少.基于密度泛函理论的全数值自洽场计算方法——离散变分方法(DVM)，数值解相对论的Dirac方程，在自由的钚原子和氢原子波函数的数值基及原子能级基础上计算了全电子的PuH2分子电子结构.得到PuH2分子基态最佳参数为键长Pu—H=0208617nm，键角θ°(H—H)=115.011°，轨道总能量为-19838.6630 a.u.，费米能级EF=-12.571eV. 比较了冻芯与非冻芯全电子计算结果.     

Mechanical and thermodynamic properties of the monoclinic and orthorhombic phases of SiC_2N_4 under high pressure from first principles

First principles calculations are preformed to systematically investigate the electronic structures, elastic and thermodynamic properties of the monoclinic and orthorhombic phases of Si C2N4 under pressure. The calculated structural parameters and elastic moduli are in good agreement with the available theoretical values at zero pressure. The elastic constants of the two phases under pressure are calculated by stress–strain method. It is found that both phases satisfy the mechanical stability criteria within 60 GPa. With the increase of pressure, the degree of the anisotropy decreases rapidly in the monoclinic phase, whereas it remains almost constant in the orthorhombic phase. Furthermore, using the hybrid density-functional theory, the monoclinic and orthorhombic phases are found to be wide band-gap semiconductors with band gaps of about 2.85 e V and 3.21 e V, respectively. The elastic moduli, ductile or brittle behaviors, compressional and shear wave velocities as well as Debye temperatures as a function of pressure in both phases are also investigated in detail.     

First-principles calculations of elastic,phonon and thermodynamic properties of W

We investigate the elastic properties, lattice dynamical, thermal equation of state and thermodynamic properties of bcc phase W under high pressure using density functional theory. The calculated high-pressure elastic constants of bcc phase W agree well with experimental and theoretical data. Under compression, the phonon dispersion curves of bcc phase W do not show any anomaly or instability. Our calculated zero-pressure phonon dispersion curves are in excellent agreement with experiments. Within the quasiharmonic approximation, we predict the thermal equation of state and other properties including the thermal expansion coefficient, adiabatic bulk modulus, specific heat at constant volume and entropy.     

Theoretical calculations of structural,electronic, and elastic properties of CdSe_(1-x)Te_x:A first principles study

The plane wave pseudo-potential method was used to investigate the structural, electronic, and elastic properties of Cd Se_(1-x)Te_x in the zinc blende phase. It is observed that the electronic properties are improved considerably by using LDA + U as compared to the LDA approach. The calculated lattice constants and bulk moduli are also comparable to the experimental results. The cohesive energies for pure Cd Se and Cd Te binary and their mixed alloys are calculated. The second-order elastic constants are also calculated by the Lagrangian theory of elasticity. The elastic properties show that the studied material has a ductile nature.     

第一性原理计算LiNBe(N=1～12)团簇的基态结构及其电子性质

从第一性原理出发利用密度泛函理论(DFT)计算了LiNBe(N=1-12)团簇的基态结构及其电子性质.计算结果表明:铍掺杂锂团簇LiNBe(N=1-12)的基态结构相当于Be原子取代LiN 1主团簇基态结构中一个Li原子的位置;当团簇尺寸N≥6时,杂质原子Be被束缚在主团簇笼子内;随着团簇尺寸增大,团簇的离解能和二阶能量差分均出现了奇-偶振荡;从结构稳定性上来看,Li6Be是个幻数团簇.