Nb掺杂对Mo2FeB2弹性、硬度和电子结构影响的第一性原理计算

Mo₂FeB₂具有耐高温、耐磨、高强度,是一种良好的硼基金属陶瓷材料,在模具领域有很广阔的应用前景.本文采用第一性原理计算的方法,研究了Nb元素掺杂Mo₂FeB₂合金的结构稳定性、弹性、硬度和电子结构.结合能和生成焓的计算结果表明,Nb在Mo₂FeB₂中更容易占据Fe位置,并且在Fe位掺Nb的Mo₂FeB₂比在Mo位处掺Nb具有更好的力学性能.此外,计算结果还表明,Nb掺杂可以提高Mo₂FeB₂的剪切模量、杨氏模量、体积模量和硬度,但塑性略有下降,合适的掺杂浓度应为2.5 at.%.电子结构计算结果表明,Nb掺杂Mo₂FeB₂力学性能的提高可归因于Nb-B共价键的形成.     

六角相ErAx (A=H,He)体系弹性性质的第一性原理研究

本文采用第一性原理研究了六角相ErA_x (A=H, He)的弹性性质, 其中x=0, 0.0313, 0.125, 0.25, 分别讨论了体系中不同浓度的氢和氦对体系弹性性质的影响. 计算结果表明, 六角相铒-氢体系晶体的弹性常数、体弹模量、剪切模量、杨氏模量基本上随着晶体中氢的浓度增加而增加, 然而, 铒-氦体系的弹性常数、体弹模量、剪切模量和杨氏模量几乎随着氦浓度的增加而降低. 从电荷转移方面分析了氢和氦与Er的相互作用, 发现六角相ErH_x的弹性性质随H浓度的变化机理与ErHe_x随He浓度的变化机理不同. 关键词： 第一性原理 弹性性质 x')" href="#">六角相ErH_x x')" href="#">六角相ErHe_x     

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. 关键词： 3Sb₂')" href="#">α-Mg₃Sb₂ 第一性原理 电子结构 力学性能     

新型石墨插层化合物HfC2结构与性质的第一性原理研究

在高压下,预测一种新型石墨插层化合物HfC₂.采用第一性原理方法对其在0 GPa下的结构和性质进行研究,分别采用GGA-PBESOL、GGA-PW91和LDA方法进行结构优化,得到的晶体学数据基本相同.弹性常数和声子谱计算证实其力学和晶格动力学稳定性,表明HfC₂在0 GPa下能够稳定存在.采用GGA-PBESOL方法计算得到HfC₂的体模量和剪切模量达到265 GPa和118 GPa,Pugh比k<0.57,是一种具有高体模量的韧性材料.HfC₂存在C-C、Hf-C共价作用,且具有金属特性和特殊层状结构,是其具有高体模量和韧性的原因.最后,对HfC₂在0~500 GPa内的键长、体模量、剪切模量、k值等进行研究,探索其力学性质随压力变化的规律.     

过渡金属Tc及其氮化物TcN，TcN$lt;sub$gt;2$lt;/sub$gt;，TcN$lt;sub$gt;3$lt;/sub$gt;与TcN$lt;sub$gt;4$lt;/sub$gt;低压缩性的第一性原理计算研究

采用基于密度泛函理论的第一性原理计算方法研究了Tc及其氮化物的弹性性质、电子结构、原子布局数等,并分析和计算了它的氮化物的理论硬度. 结果表明随着Tc中氮的掺入量的渐增,出现层状结构,它们的弹性模量并非单调增加,其中TcN的体弹模量最大而剪切模量最小;层状结构的TcN₃的剪切模量最大而体弹模量最小,TcN₄的理论计算硬度最大. 说明在Tc的氮化物中,其体弹模量与材料的晶体结构、平均每个原子上的电荷密度和材料的质量密度有关;化学键的共价性结构和氮元素的含量对理论计算硬度有正作用;而剪切模量的极大值则与其层状结构及体系中一定量的方向基本一致的N—N键相关. 关键词： 第一性原理 弹性性质 电子结构     

Fe-Si-Al合金结构的研究

本文叙述了利用电子显微镜对含10％Si和5％Al的Fe-Si-Al高导磁合金薄膜进行的结构研究。发现了基体为有序面心立方Fe₃(Al,Si)结构,其晶格参数为5.70±0.03?,该值在Fe₃Si和Fe₃Al晶格参数之间,说明部分Si原子为Al原子所置换。并获得三套基体和析出相合成的电子衍射图,相应确定了基体和析出相的取向关系:(001)_基‖(110)_相;[100]_基‖[00I]_相;[010]_基‖[I10]_相。对析出相的萃取碳复型,在电子显微镜中进行选区衍射,获得了立方Fe₃(Al,Si)C_x相的结构。提出:Fe原子占据面心;Al或Si原子占据顶角;C原子占据体心的看法。 关键词：     

压力及杂质氢对金属锂弹性特性的影响

针对六角密堆金属锂16个原子超晶胞(supercell)、填隙一个氢原子的周期单元，采用基于密度泛函理论的平面波-赝势方法，研究了零温条件下压力及填隙氢掺杂对体系弹性性质的影响.结果表明氢掺杂导致体系的体模量增加.常压下掺杂体系的弹性常数C₁₁，C₃₃，C₆₆和C₁₂高于单质体系，剪切模量C₄₄有所下降，而C₁₃则与单质体系持平.压力作用下C₁₁，C₃₃和C₆₆一直大于单质体系，但C₁₂的值低于单质体系.在2GPa—4GPa压力区间内，弹性常数C₁₃呈反常变化，小于单质体系；在高压区掺杂体系的C₄₄和C₁₃则高于单质体系的相应量值，压力导致掺杂体系和单质体系之间剪切模的偏离加剧.掺杂体系在压力作用下依然保持压缩模的各向同性，具有和单质体系相似的特性. 关键词： 第一性原理 压力效应 弹性常数 金属锂     

高压下ErNi2B2C弹性性质、电子结构和热力学性质的第一性原理研究

采用密度泛函理论中的赝势平面波方法研究了高压下超导材料 ErNi₂B₂C 的弹性性质、电子结构和热力学性质.分析表明, 弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显. 电子态密度(DOS)的计算结果显示, 在费米能级(E_F)处的 DOS 峰随外界压强的增大显著降低, 由于 ErNi₂B₂C 相对较高的超导温度(T_c)起因于E_F处的 DOS 峰, 因此推测压强增大可能会降低 ErNi₂B₂C 的 T_c.类似的现象在超导材料 MgB₂和 SrAlSi 中已被发现.此外, 基于准谐德拜模型, 对 ErNi₂B₂C 在高温高压下的热力学性质的研究表明, 在一定范围内, 温度和压强将对其热膨胀系数和热容产生明显的影响. 关键词： 高压 弹性性质 电子结构 热力学性质     

BaVS3晶格动力学研究

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研究了具有六角结构的BaVS₃化合物的晶格动力学性质,得到了整个声子谱.计算得到的Γ点拉曼频率和实验数据进行了比较,其中E²_2g模、A_1g模和实验测量值符合得比较好.对于E³_2g模,采用线性响应计算的结果与实验值差别较大.对该模应用冻结声子方法研究后认为差异主要是由于E³_2g模的较强的非谐性引起的.此外V原子在平面内的振动模E¹_2u出现了虚频.虚频的出现预示着六角相的BaVS₃结构的不稳定性,从而很好地解释了该材料由六角相到正交相的结构相变. 关键词： 晶格动力学 密度泛函 赝势方法 3')" href="#">BaVS₃     

Nb掺杂对TiO2/NiTi界面电子结构影响的第一性原理计算

采用基于密度泛函理论的第一性原理平面波超软赝势方法,计算了Nb掺杂对TiO₂/NiTi界面电子结构的影响.体系生成能的计算结果表明,4种TiO₂/NiTi界面结构中,NiTi中Ti原子和TiO₂中O原子相邻的界面,即Ti/O界面的生成能最大,结构最稳定.在Ti/O界面结构优化的基础上,态密度、电荷分布以及集居数的计算结果均表明：Nb原子取代界面上的Ti原子后,界面原子之间的结合力增强,且界面附近的基体和氧化层中原子之间的相互作用也增加,有利 关键词： NiTi金属间化合物 2/NiTi界面')" href="#">TiO₂/NiTi界面 电子结构 第一性原理计算     

The structural, elastic, and electronic properties of the pyrite-type phase for SnO2

We have studied some structural, thermodynamic, elastic, and electronic properties of pyrite-type SnO₂ polymorph by performing ab initio calculations within the LDA approximation. The basic physical properties, in particular lattice constant, bulk modulus, second-order elastic constants (C_ij), and the electronic structure, are calculated, and compared with the available experimental data. In order to gain some further information on the mechanical properties, we have also calculated the Young's modulus, Poison's ratio (ν), anisotropy factor (A), sound velocities, and Debye temperature for the same compound.     

Prediction of Pressure-Induced Structural Transition and Mechanical Properties of Mg Y from First-Principles Calculations

Using the particle swarm optimization algorithm on crystal structure prediction,we first predict that Mg Y alloy undergoes a first-order phase transition from Cs Cl phase to P4/NMM phase at about 55 GPa with a small volume collapse of 2.63%.The dynamical stability of P4/NMM phase at 55 GPa is evaluated by the phonon spectrum calculation and the electronic structure is discussed.The elastic constants are calculated,after which the bulk moduli,shear moduli,Young's modui,and Debye temperature are derived.The brittleness/ductile behavior,and anisotropy of two phases under pressure are discussed in details.Our results show that external pressure can change the brittle behavior to ductile at10 GPa for Cs Cl phase and improve the ductility of Mg Y alloy.As pressure increases,the elastic anisotropy in shear of Cs Cl phase decreases,while that of P4/NMM phase remains nearly constant.The elastic anisotropic constructions of the directional dependences of reciprocals of bulk modulus and Young's modulus are also calculated and discussed.     

Structural,elastic, and vibrational properties of phase H:A first-principles simulation

Phase H(MgSiO_4H_2), one of the dense hydrous magnesium silicates(DHMSs), is supposed to be vital to transporting water into the lower mantle. Here the crystal structure, elasticity and Raman vibrational properties of the two possible structures of phase H with Pm and P2/m symmetry under high pressures are evaluated by first-principles simulations. The cell parameters, elastic and Raman vibrational properties of the Pm symmetry become the same as the P2/m symmetry at～ 30 GPa. The symmetrization of hydrogen bonds of the Pm symmetry at ～ 30 GPa results in this structural transformation from Pm to P2/m. Seismic wave velocities of phase H are calculated in a range from 0 GPa to 100 GPa and the results testify the existence and stability of phase H in the lower mantle. The azimuthal anisotropies for phase H are A_(P0)= 14.7%,A_(S0)= 21.2%(P2/m symmetry) and A_(P0)= 16.4%, A_(S0)= 27.1%(Pm symmetry) at 0 GPa, and increase to A_(P30)= 17.9%,A_(S30)= 40.0%(P2/m symmetry) and A_(P30)= 19.2%, A_(S30)= 37.8%(Pm symmetry) at 30 GPa. The maximum V P direction for phase H is [101] and the minimum direction is [110]. The anisotropic results of seismic wave velocities imply that phase H might be a source of seismic anisotropy in the lower mantle. Furthermore, Raman vibrational modes are analyzed to figure out the effect of symmetrization of hydrogen bonds on Raman vibrational pattern and the dependence of Raman spectrum on pressure. Our results may lead to an in-depth understanding of the stability of phase H in the mantle.     

Shear induced phase transition in PbO under high pressure

We have studied the structural behavior of lead monoxide (PbO) as a function of pressure via angular dispersive X-ray diffraction employing two different pressure transmitting media that were quasi-hydrostatic (N₂) and non-hydrostatic (MgO), respectively. Besides litharge (-PbO) and massicot (β-PbO), which are both stable at ambient pressure, there is an orthorhombic γ-PbO phase which appears upon application of pressure to -PbO. We have found that the orthorhombic γ-PbO phase is favored by shear stress under non-hydrostatic conditions. -PbO shows strong anisotropy in compressibility. The a-axis is rather incompressible with a linear stiffness coefficient of K_a0=540(30) GPa whereas the c-axis stiffness is K_c0=25(1) GPa. The bulk modulus of -PbO is K₀=23.1(3) GPa and its derivative .     

High-pressure dynamic,thermodynamic properties,and hardness of CdP_2

Using a pseudopotential plane-waves method,we calculate the phonon dispersion curves,thermodynamic properties,and hardness values of α-CdP_2 and β-CdP_2 under high pressure.From the studies of the phonon property and enthalpy difference curves,we discuss a phase transform from β-CdP_2 to a-CdP_2 in a pressure range between 20 GPa and 25 GPa.Then,the thermodynamic properties,Debye temperatures,and heat capacities are investigated at high pressures.What is more,we employ a semiempirical method to evaluate the pressure effects on the hardness for these two crystals.The results show that the hardness values of both α-CdP_2 and β-CdP_2 increase as pressure is increased.The influence mechanism of the pressure effect on the hardness of CdP_2 is also briefly discussed.     

Site preference of Ti and Nb in L12-ordered Co-Al-W phase and their effect on the properties of the alloy: first-principles study

In this work,the equilibrium structure,electronic and elastic properties of L1₂-ordered Co-Al-W and Co-Al-W-X(X=Ti and Nb)phase were calculated,using first-principles calculations.Among six nonequivalent sites(Al₁,Al₂,Co₃,Co₄,W₅,W₆),Ti and Nb prefer to occupy the W₆site,since the formation enthalpy of the system is lowest when Ti and Nb occupy the W。site.Both Ti and Nb most affect the density of states of Al atoms.Compared with the Al₂ site,which is the sub-preference site of Ti and Nb,the density of states of Al atoms is higher with the addition of Ti and Nb in the W₆site,which means that the latter system is more stable.According to the bulk modulus B,shear modulus G,Young's modulus E,hardness Hv and Poisson's ratioσ,for Co₃(AI,W)alloy,the addition of Ti and Nb in the W₆site decreases its hardness hut increases its duetility.This work cnnfirms that Ti and Nh can stahilize the Cag(Al,W)alloy and have a positive effect in solving the relatively poor ductility of this alloy,which has important implications for the development of cobalt.based alloys.     

Mg-Sn-Si合金中Mg2(Si,Sn)复合相的结构与性能研究

研究了铸态Mg-Sn-Si合金中Mg₂(Si,Sn)复合相的结构、 特性以及该相对Mg-Sn-Si合金变质作用的影响. 结果表明: Sn原子能取代Mg₂Si中的部分Si生成Mg₂(Si,Sn)复合相, 该三元相与Mg₂Si, Mg₂Sn相的结构相同, 属于面心立方结构, Mg₂(Si,Sn)相的元素含量并不固定, 在Si富集区形成的Mg₂(Si,Sn)相中, Si元素含量高, 在Si贫乏区形成的Mg₂(Si,Sn)相中, Si元素含量低. Si含量较多的Mg₂(Si,Sn)相性能与Mg₂Si相接近, Sn含量较多的Mg₂(Si,Sn)相性能与Mg₂Sn相接近, 实验中发现Mg₂(Si,Sn)复合相的纳米硬度、 弹性模量与维氏硬度等物理性能介于Mg₂Si与Mg₂Sn之间, Mg₂(Si,Sn)相对汉字状Mg₂Si相的变质处理起到桥梁作用. 关键词： Mg-Sn-Si合金 2Si')" href="#">Mg₂Si 2Sn')" href="#">Mg₂Sn 2(Si,Sn)复合相')" href="#">Mg₂(Si,Sn)复合相     

Elastic properties of CaCO_3 high pressure phases from first principles

Elastic properties of three high pressure polymorphs of CaCO_3 are investigated based on first principles calculations.The calculations are conducted at 0 GPa–40 GPa for aragonite, 40 GPa–65 GPa for post-aragonite, and 65 GPa–150 GPa for the P2_1/c-h-CaCO_3 structure, respectively. By fitting the third-order Birch–Murnaghan equation of state(EOS), the values of bulk modulus K_0 and pressure derivative K~'_0 are 66.09 GPa and 4.64 for aragonite, 81.93 GPa and 4.49 for post-aragonite, and 56.55 GPa and 5.40 for P2_1/c-h-CaCO_3, respectively, which are in good agreement with previous experimental and theoretical data. Elastic constants, wave velocities, and wave velocity anisotropies of the three highpressure CaCO_3 phases are obtained. Post-aragonite exhibits 25.90%–32.10% V_P anisotropy and 74.34%–104.30% V_S splitting anisotropy, and P2_1/c-h-CaCO_3 shows 22.30%–25.40% V_Panisotropy and 42.81%–48.00% V_S splitting anisotropy in the calculated pressure range. Compared with major minerals of the lower mantle, CaCO_3 high pressure polymorphs have low isotropic wave velocity and high wave velocity anisotropies. These results are important for understanding the deep carbon cycle and seismic wave velocity structure in the lower mantle.     

Phase transition,elastic and electronic properties of topological insulator Sb_2Te_3 under pressure: First principle study

The phase transition, elastic and electronic properties of three phases(phase Ⅰ,Ⅱ, and Ⅲ) of Sb_2Te_3 are investigated by using the generalized gradient approximation(GGA) with the PBESOL exchange–correlation functional in the framework of density-functional theory. Some basic physical parameters, such as lattice constants, bulk modulus, shear modulus,Young's modulus, Poisson's ratio, acoustic velocity, and Debye temperature Θ are calculated. The obtained lattice parameters under various pressures are consistent with experimental data. Phase transition pressures are 9.4 GPa(Ⅰ→Ⅱ) and 14.1 GPa(Ⅱ→Ⅲ), which are in agreement with the experimental results. According to calculated elastic constants, we also discuss the ductile or brittle characters and elastic anisotropies of three phases. Phases Ⅰ and Ⅲ are brittle, while phaseⅡ is ductile. Of the three phases, phaseⅡ has the most serious degree of elastic anisotropy and phase Ⅲ has the slightest one.Finally, we investigate the partial densities of states(PDOSs) of three phases and find that the three phases possess some covalent features.     

Evolution of superconductivity and charge order in pressurized RbV3Sb5

The kagome metals AV₃Sb₅(A=K,Rb,Cs)under ambient pressure exhibit an unusual charge order,from which superconductivity emerges.In this work,by applying hydrostatic pressure using a liquid pressure medium and carrying out electrical resistance measurements for RbV₃Sb₅,we find that the charge order becomes suppressed under a modest pressure pc(1.4 GPa3Sb₅.Our findings point to qualitatively similar temperature-pressure phase diagrams in KV₃Sb₅ and RbV₃Sb₅,{and suggest a close link}between the second superconducting dome and the high-pressure resistance anomalies.