2H-PbI_2晶体的电子结构和力学性质

采用基于密度泛函平面波赝势方法(PWP)方法,计算了六角晶系2H-PbI2晶体的电子结构、力学性质和硬度.采用局域密度近似(LDA)方法计算的晶格常数、带隙、弹性常数与实验值和理论值符合较好.计算表明,2H-PbI2是一种直接带隙的半导体,带隙大约为2.38eV.运用复杂晶体硬度计算公式计算了六角晶系2H-PbI2晶体的硬度,硬度值大约为2.54GPa.还发现2H-PbI2晶体的各向异性非常明显.     

高压下β-BaCu2S2电子结构和光学性质的第一性原理研究

本文采用密度泛函理论方法对β-BaCu2S2的电子性质和光学性质进行了理论计算和分析。优化后的晶格常数与实验值符合良好。通过能带分析表明该晶体是一种直接带隙半导体,得出的带隙值为0.562 eV,与其他理论计算方法得出的结果接近。对介电函数的计算表明β-BaCu2S2是一种各向异性材料,随着压强的增大,介电函数虚部向高能区域移动,且峰值变高。计算所得静折射率的平方近似于静介电常数,与理论公式符合良好。随着压强的增加,晶体的光学常数如吸收系数、反射率、折射率和能量损失函数曲线均向光子能量增大的方向移动。研究表明加压是改变β-BaCu2S2晶体电子结构、调制光学性质的有效手段。     

高压下β-BaCu_2S_2电子结构和光学性质的第一性原理研究

本文采用密度泛函理论方法对β-BaCu_2S_2的电子结构和光学性质进行了理论计算和分析.优化后的晶格常数与实验值符合良好.通过能带分析表明该晶体是一种直接带隙半导体,得出的带隙值为0.562 e V,与其他理论计算方法得出的结果接近.对介电函数的计算表明β-BaCu_2S_2是一种各向异性材料,随着压强的增大,介电函数虚部向高能区域移动,且峰值变高.计算所得静折射率的平方近似于静介电常数,与理论公式符合良好.随着压强的增加,晶体的光学常数如吸收系数、反射率、折射率和能量损失函数曲线均向光子能量增大的方向移动.研究表明加压是改变β-BaCu_2S_2晶体电子结构、调制光学性质的有效手段.     

层状半导体WSe2的电子结构和各向异性光学性质研究

采用第一性原理密度泛函理论研究了六角层状晶体 WSe2 的电子结构和各向异性光学性质. 结果表明:WSe2 为间接带隙半导体, 带隙值为1 .44 eV, 略小于实验值(1 .51 eV) ; 价带和导带均主要由 W-5d 和Se-4p 电子构成, 在价带顶(0~2eV) 及导带底(1 .5 ~3.5 eV) , W-5d 和 Se-4p 电子杂化明显, 形成共价键. 介电函数的虚部和实部均表现出明显的各向异性,εi (xx ) 有一个明显的介电吸收峰, 而εi (zz ) 却有两个明显的介电吸收峰; WSe2 晶体对zz 光的低频透明区的能量范围几乎是xx 光的2 倍, 应用 WSe2 晶体的这一特性可以制备不同要求的偏振片.     

UO2的电子结构及光学性质的第一性原理研究

采用局域自旋密度近似 (LSDA)和有效库仑相关能 (U) 方法研究了UO₂的晶格参数、能带结构和光学常数. 计算得到的UO₂晶体的晶格常数为5.40 ?,带隙宽度为1.82 eV,正确预测了UO₂的反铁磁性半导体基态性质. 能带结构和介电函数的分析结果表明,铀的6d电子在晶体场中发生劈裂形成两个能级,与实验结果较为符合.     

LSDA+U方法研究PuO2电子结构和成键特征

采用LSDA(局域自旋密度近似) U(有效库仑相关能)方法计算研究了PuO2的电子结构和成键特征。计算的平衡体积和半导体带隙分别为0.03875 nm3和0.18 eV,与实验结果符合得很好。能态密度和电子密度的分析表明PuO2并不是纯粹的离子晶体,Pu5f和O2p轨道杂化形成共价键。计算结果有助于理解PuO2晶体中Pu5f电子的关联效应。     

α-BeH2电子结构与光学性质的第一性原理研究

基于密度泛函理论,采用赝势平面波方法研究了α-BeH2的结构、电子和光学性质。基态下,α-BeH2的晶格常数a和体积弹性模量B0计算值与实验值及其它理论值一致。根据能带理论研究了α-BeH2基态下的能带结构、总态密度(DOS)和分波态密度(PDOS)。经过分析发现α-BeH2为直接能隙半导体材料,能隙为5.44eV,与文献相比,本文计算的结果偏低,这主要是利用第一性原理中的局域密度近似(LAD)或广义梯度近似(GGA)交换关联能函数计算材料的带隙宽度或者磁耦合的理论结果均会偏低。通过对基态α-BeH2的Mulliken电荷分布和集居数的分析发现：α-BeH2属于离子键和共价键所形成的混合键化合物;α-BeH2的电荷总数分别来源于各自的H 1s轨道,Be 2s和2p轨道。同时本文还分析研究了α-BeH2的光学介电函数、吸收系数、复折射率、反射系数和能量损失等光学性质。     

α-BeH_2电子结构与光学性质的第一性原理研究

基于密度泛函理论,采用赝势平面波方法研究了α-BeH2的结构、电子和光学性质.基态下,α-BeH2晶格常数a和体积弹性模量B0计算值与实验值及其它理论值一致.根据能带理论研究了α-BeH2基态下的能带结构、总态密度(DOS)和分波态密度(PDOS).经过分析发现α-BeH2为直接能隙半导体材料,能隙为5.44eV,与文献相比,本文计算的结果偏低,这主要是利用第一性原理中的局域密度近似(LAD)或广义梯度近似(GGA)交换关联能函数计算材料的带隙宽度或者磁耦合的理论结果均会偏低.通过对基态α-BeH2的Mulliken电荷分布和集居数的分析发现:α-BeH2属于离子键和共价键所形成的混合键化合物;α-BeH2的电荷总数分别来源于H1s轨道,Be2s和2p轨道.同时本文还分析研究了α-BeH2的光学介电函数、吸收系数、复折射率、反射系数和能量损失等光学性质.     

α-Mg3Sb2的电子结构和力学性能

运用第一性原理研究了Mg-Sb合金中典型沉淀相α-Mg3Sb2的几何、电子结构和力学性能.结构优化得到的晶格常数和形成能与实验值符合很好.电子结构分析表明,具有半导体性质的α-Mg3Sb2带隙为0.303 eV,是间接带隙半导体.通过计算得到了α-Mg3Sb2的弹性常数,进而得到模量、泊松比等力学参数,对力学参数进行分析发现,α-Mg3Sb2有很好的延展性而塑性相对较差.通过对α-Mg3Sb2施加应变前后态密度的变化分析,发现对于六角结构的α-Mg3Sb2,与剪切模量相关的C11+C12,C33/2和与体模量相关的C11+C12+2C13+C33/2对体积变化不保守,而(C11-C12)/4和C44对体积变化保守.     

硅酸锌的电子结构

采用局域密度泛函理论和第一性原理的方法,计算四方结构和六角结构硅酸锌的平衡晶格常数、电子态密度和能带结构。计算结果表明,四方结构硅酸锌的平衡晶格常数为0.71048nm,六角结构为1.40877nm,两者与实验值的误差均在1%左右。态密度图显示,主要电子态分布在-7.18～0.00eV和2.79～10.50eV两个能量区域;同时,不同元素电子对导带和价带有不同贡献,其中氧的p态电子对价带顶贡献最大,锌的s态电子对导带底贡献最大。能带计算表明,四方与六角结构硅酸锌均为直接带隙半导体,禁带宽度分别为2.66,2.89eV。     

NbSi2奇异高压相及其热力学性质的第一性原理研究

采用基于粒子群优化算法的结构预测程序CALYPSO, 并结合第一性原理的VASP程序, 在175 GPa发现NbSi₂的奇异立方高压相. 在此结构中, Nb原子形成金刚石结构, 而Si原子则形成正四面体镶嵌在金刚石结构中. 声子谱计算结果表明该结构是动力学稳定的. 电子结构分析表明, 六角相和立方相NbSi₂均为金属, 对金属性贡献较大的是Nb原子, 而且Nb和Si原子之间存在明显的p-d杂化现象, 电荷更多地聚集在Si四面体中. 利用“应力应变”方法, 计算了NbSi₂的弹性常数, 分析了其体积模量、剪切模量、杨氏模量和德拜温度等热动力学性质随压力的变化并进行了详细的讨论. 根据剪切模量和体积模量的比值分析了NbSi₂两种相结构的脆性和延展性, 发现压力会导致六角相NbSi₂的延展性增加, 但对立方相结构的延展性影响较小; 采用经验算法计算了NbSi₂两种相结构硬度变化情况, 结合这一比值进行了详细的分析. 弹性各向异性计算结果表明, 随着压力增加, 六角结构的各向异性增强, 而立方结构的各向异性减小.     

Pressure-induced structural transition and thermodynamic properties of RhN₂ and the effect of metallic bonding on its hardness

The elastic constant,structural phase transition,and effect of metallic bonding on the hardness of RhN 2 under high pressure are investigated through the first-principles calculation by means of the pseudopotential plane-wave method.Three structures are chosen to investigate for RhN 2,namely,simple hexagonal P6/mmm(denoted as SH),orthorhombic Pnnm(marcasite),and simple tetragonal P4/mbm(denoted as ST).Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure.On the basis of the third-order Birch-Murnaghan equation of states,we find that the phase transition pressures from an SH to a marcasite structure and from a marcasite to an ST structure are 1.09 GPa and 354.57 GPa,respectively.Elastic constants,formation enthalpies,shear modulus,Young’s modulus,and Debye temperature of RhN 2 are derived.The calculated values are,generally speaking,in good agreement with the previous theoretical results.Meanwhile,it is found that the pressure has an important influence on physical properties.Moreover,the effect of metallic bonding on the hardness of RhN 2 is investigated.This is a quantitative investigation on the structural properties of RhN 2,and it still awaits experimental confirmation.     

Pressure-induced structural transition and thermodynamic properties of RhN2 and effect of metallic bonding on its hardness

The elastic constant, structural phase transition, and effect of metallic bonding on the hardness of RhN₂ under high pressure are investigated through the first principles calculation by means of the pseudopotential plane-waves method. Three structures are chosen to investigate for RhN₂, namely, simple hexagonal P6/mmm (denoted as SH), orthorhombic Pnnm (marcasite), and simple tetragonal P4/mbm (denoted as ST). Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure. On the basis of the third-order Birch-Murnaghan equation of states, we find that phase transition pressures from SH to marcasite structure and from marcasite to ST structure are 1.09 GPa and 354.57 GPa, respectively. Elastic constants, formation enthalpies, shear modulus, Young's modulus, and Debye temperature of RhN₂ are derived. The calculated values are, generally speaking, in good agreement with the previous theoretical results. Meanwhile, it is found that the pressure has an important influence on physical properties. Moreover, the effect of metallic bonding on the hardness of RhN₂ is investigated. This is a quantitative investigation on the structural properties of RhN₂, and it still awaits experimental confirmation.     

The stability and the nonlinear elasticity of 2D hexagonal structures of Si and Ge from first-principles calculations

By using the first-principles calculations based on the density-functional theory (DFT), we study the stability and the nonlinear elasticity of two-dimensional (2D) hexagonal structures of Si and Ge. The reproduced structure optimization and phonon-dispersion curves demonstrate that Si and Ge can form stable 2D hexagonal lattices with low-buckled structures, and provide a good agreement with the previous DFT calculations. The second- and third-order elastic constants are calculated by using the method of homogeneous deformation. The present results of the linear elastic moduli agree well with the previous results. In comparison with the linear approach, the nonlinear effects really matter while strain is larger than approximately 3.5%. The force-displacement behaviors and the breaking strength of 2D hexagonal Si and Ge are discussed using the nonlinear stress-strain relationship. By using the available results of graphene, we reasonably demonstrate that the radius of the atom increases and breaking strength of this element decreases for 2D hexagonal structures of group IV-elements.     

固态二氧化碳电子结构及性能的理论研究

采用基于赝势平面波理论的第一性原理计算方法,研究了9种结构的固态二氧化碳的结构和性质.经过结构的几何优化,得到α石英稳定结构晶格常数与他人的计算值基本一致.通过对平衡态能量的分析,我们得出β方石英(cristobalite)结构是能量最低的结构.这与文献的研究结果一致.对弹性性质的计算结果表明,除了超石英(stishovite)和立方黄铁矿(cubic-pyrite)结构之外,其他的结构都是弹性稳定的.利用基于Mulliken轨道重叠布居数的共价固体本征硬度计算方法,预测了各个结构的本征硬度值.结果表明, 关键词： 第一性原理计算 固态二氧化碳 电子结构 硬度     

A new quaternary semiconductor compound (Ba2Sb4GeS10): Ab initio study

Abstract

The newly synthesised Ba₂Sb₄GeS₁₀ compound is notable because of the interesting features of the quaternary Sb-containing materials. The first principle method has been used to determine the physical properties of this compound. In particular, the electronic structure has been analysed using both conventional GGA-PBE and HSE06 functional. The values of the band gap for PBE and HSE06 calculations were 1.324 and 1.84 eV, respectively. The calculated elastic constants were used to predict polycrystalline mechanical properties. The estimated Vickers hardness (2.7 GPa) values show that Ba₂Sb₄GeS₁₀ is soft matter. Moreover, the vibrational properties of the compound have been studied. The calculation of the elastic constants and phonon dispersion curves indicates that the Ba₂Sb₄GeS₁₀ compound is stable both mechanically and dynamically. Furthermore, the minimum thermal conductivity and optical properties, such as dielectric functions and energy loss function, have also been discussed in detail in this paper.     

Structural and mechanical stability of rare-earth diborides

Structural and mechanical properties of several rare-earth diborides were systematically investigated by first principles calculations. Specifically, we studied XB2 , where X=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Lu in the hexagonal AlB2 , ReB2 , and orthorhombic OsB2 -type structures. The lattice parameters, bulk modulus, bond distances, second order elastic constants, and related polycrystalline elastic moduli (e.g., shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature, sound velocities) were calculated. Our results indicate that these compounds are mechanically stable in the considered structures, and according to "Chen’s method", the predicted Vickers hardness shows that they are hard materials in AlB2 - and OsB2 -type structures.     

Electronic structure, phase stability, and hardness of the osmium borides, carbides, nitrides, and oxides: First-principles calculations

The chemical bonding, elastic behavior, phase stability, and hardness of OsB, OsB₂, OsC, OsO₂, OsN, and OsN₂ have been systematically studied using first-principles calculations. The calculation suggests that the chemical bonding in these compounds is a mixture of covalent and ionic components. The structural stability of OsB, OsC, and OsN can be understood in terms of the band filling of the bonding states, and the results indicate that the hexagonal tungsten carbide structure is more stable. The hardness of these osmium compounds is calculated using both ab initio and semiempirical model calculations. Analysis of the ab initio hardness suggested that the large occupations and high strength of the covalent bonds are crucial for a superhard material, and there is no clear connection between bulk modulus and hardness in these osmium compounds.     

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.     

Stability,elastic anisotropy,and electronic properties of Ca_2C_3

The systematic investigations of the mechanical, elastic, and electronic properties, and stability of the newly synthesized monoclinic C2/m-Ca_2C_3 are performed, based on the first-principles calculations. Ca_2C_3 is found to be mechanically and dynamically stable only from 0 GPa to 24 GPa. The elastic anisotropy studies show that Ca_2C_3 exhibits the elastic anisotropy increasing with the augment of pressure. Furthermore, using the HSE06 hybrid functional, the electronic properties of Ca_2C_3 under pressure are calculated. The structure can be regarded as a quasi-direct band gap semiconductor, and the pressure-induced direct-indirect band gap transition is studied in detail.