First-principles study of the elastic and thermodynamic properties of thorium hydrides at high pressure

The high pressure behaviors of Th_4H_(15) and ThH_2 are investigated by using the first-principles calculations based on the density functional theory(DFT). From the energy–volume relations, the bct phase of ThH_2 is more stable than the fcc phase at ambient conditions. At high pressure, the bct ThH_2 and bcc Th_4H(15) phases are more brittle than they are at ambient pressure from the calculated elastic constants and the Poisson ratio. The thermodynamic stability of the bct phase ThH_2 is determined from the calculated phonon dispersion. In the pressure domain of interest, the phonon dispersions of bcc Th_4H(15) and bct ThH_2 are positive, indicating the dynamical stability of these two phases, while the fcc ThH_2 is unstable. The thermodynamic properties including the lattice vibration energy, entropy, and specific heat are predicted for these stable phases. The vibrational free energy decreases with the increase of the temperature, and the entropy and the heat capacity are proportional to the temperature and inversely proportional to the pressure. As the pressure increases, the resistance to the external pressure is strengthened for Th_4H_(15) and ThH_2.     

First-principles study of He point-defects in HCP rare-earth metals

He defect properties in Sc, Y, Gd, Tb, Dy, Ho, Er and Lu were studied using first-principles calculations based on density functional theory. The results indicate that the formation energy of an interstitial He atom is smaller than that of a substitutional He atom in all hcp rare-earth metals considered. Furthermore, the tetrahedral interstitial position is more favorable than an octahedral position for He defects. The results are compared with those from bcc and fcc metals.     

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

He原子掺杂铝材料的第一性原理研究

采用基于第一性原理的平面波超软赝势方法,结合广义梯度近似(GGA),计算了铝及铝晶胞间隙位置掺入He原子后体系的几何结构、电子结构、总体能量和电荷布居值.计算结果表明:随着氦在金属铝中逐渐形成,铝晶胞体系会发生晶格畸变,但总的趋势是He在铝体系的八面体位置的晶格畸变小于其在四面体位置的晶格畸变.He在铝晶胞八面体和四面体间隙的杂质形成能分别为1.3367 eV和2.4411 eV.由此可知,He在铝晶胞中最稳定位置是八面体间隙位置.同时,文中还从原子尺度层面分析了He原子在铝晶胞中的占位及其键合性质,讨论 关键词： 铝材料 第一性原理 形成能     

First-principles study on superconductive properties of compressive strain-engineered cryogenic superconducting heavy metal lead(Pb)

As one of the main materials in the practical application.of superconductor,lead(Pb)has been used to manufacture superconducting AC power cable,and some weak current fields.With the development of manufacturing,technology,more and more researchers focus on exploring the physical and chemical properties of cryogenic superconducting materials,instead of blindly pursuing the improvement of the superconducting transition temperature(T_c).In this paper,the structural properties and superconducting transition temperature under high pressure of Pb have been studied by first-principles calculations.It has shown that Pb can withstand the compressive strain up to 10%while the lattice structure remains stable,indicated by the calculations of phonon band structures.From 0%to 10%compressive strain,there is neither a band-gap nor changing of the band structure.The.changing of electronic DOS.at the Fermi level leads to a decreasing of T_c.Our calculations show that Pb is a stable elemental metallic superconductor even under high pressure,which explains the reason why it has been used in practical productions.     

金属钨中氦行为的分子动力学模拟

采用分子动力学方法模拟了氦在金属钨中的扩散聚集行为. 首先,建立了氦与钨原子间相互作用势,短程部分采用ZBL势形式,长程部分采用从头算法数据,实现了两者之间的平滑连接. 通过计算氦在钨中不同间隙位的形成能发现,单个氦原子更易存在于金属钨中的四面体间隙位,这与最新的研究成果是一致的. 在400-1200 K的温度范围内,考察了氦原子在金属钨中的扩散行为,获得了扩散迁移能,其值介于实验值和从头算法结果之间. 最后,研究了氦的聚集行为,从能量的角度考察了氦团簇形成初期的生长机理. 研究发现,在氦团簇形成初期,氦团簇对氦的结合能随着氦团簇的生长有逐渐增大的趋势,说明氦团簇吸收氦的能力逐渐增强. 关键词： 氦扩散 氦团簇 辐照损伤 分子动力学模拟     

Comparative study of Mo_2Ga_2C with superconducting MAX phase Mo_2GaC: First-principles calculations

The structural, electronic, optical and thermodynamic properties of Mo_2Ga_2C are investigated using density functional theory(DFT) within the generalized gradient approximation(GGA). The optimized crystal structure is obtained and the lattice parameters are compared with available experimental data. The electronic density of states(DOS) is calculated and analyzed. The metallic behavior for the compound is confirmed and the value of DOS at Fermi level is 4.2 states per unit cell per e V. Technologically important optical parameters(e.g., dielectric function, refractive index, absorption coefficient, photo conductivity, reflectivity, and loss function) are calculated for the first time. The study of dielectric constant(ε1) indicates the Drude-like behavior. The absorption and conductivity spectra suggest that the compound is metallic.The reflectance spectrum shows that this compound has the potential to be used as a solar reflector. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats, and thermal expansion coefficient of Mo_2Ga_2C MAX phase are derived from the quasi-harmonic Debye model with phononic effect also for the first time. Analysis of T c expression using available parameter values(DOS, Debye temperature, atomic mass,etc.) suggests that the compound is less likely to be superconductor.     

Si基外延Ru2Si3电子结构及光学性质研究

采用基于第一性原理的赝势平面波方法,对异质外延关系为Ru₂Si₃ （100）//Si（001）,取向关系为Ru₂Si₃［010］//Si［110］正交相的Ru₂Si₃平衡体系下的能带结构、态密度和光学性质等进行了理论计算.计算结果表明：当晶格常数a取值为1093 nm时,正交相Ru₂Si₃处于稳定状态并且是具有带隙值 关键词： 外延 第一性原理 电子结构 光学性质     

3d过渡金属掺杂锐钛矿相TiO2的第一性原理研究

采用基于密度泛函理论的平面波超软赝势方法研究了纯锐钛矿相TiO₂及掺杂3d过渡金属TiO₂的几何、电子结构及光学性质. 计算结果表明掺杂能级的形成主要是掺杂过渡金属3d轨道的贡献，掺杂能级在禁带中的位置是决定TiO₂吸收带边能否出现红移的重要因素. Cr，Mn，Fe，Ni，Co，Cu掺杂使TiO₂的吸收带边产生红移，并在可见光区有一定的吸收系数； Sc，Zn掺杂使TiO₂的吸收带边产生蓝移，但在可见光区有较大的吸收系数；掺V不但使TiO₂的吸收带边产生红移，增强了在紫外光区的光吸收，而且在可见光区有非常大的吸收系数.     

First-principles calculations of optical properties of Mg2Pb

Based on the density functional theory(DFT),the electronic structures and optical properties of Mg2Pb are calculated by using the local density approximation(LDA)and plane wave pseudo-potential method.The calculation results show that the indirect band gap width of Mg2Pb is 0.02796 eV.The optical properties of Mg2Pb have isotropic characteristics,the static dielectric function of Mg2Pb is1(0)=10.33 and the refractive index is n0=3.5075.The maximum absorption coefficient is 4.8060×105cm 1.The absorption in the photon energy range of 25–40 eV approaches to zero,shows the optical colorless and transparent behaviors.     

TiO$lt;sub$gt;2$lt;/sub$gt;纳米管电子结构和光学性质的第一性原理研究

运用基于密度泛函理论的第一性原理方法, 系统研究了小尺寸锐钛矿相(n,0)型TiO₂纳米管(D<16 Å)的几何构型、电子结构和光学性质. 结果表明: 随着管径增大, 体系单位TiO₂分子的形成能降低, 体系趋于稳定; 在管径14 Å左右, (n,0)型TiO₂纳米管会发生一次构型的转变. 能带分析显示, TiO₂纳米管的电子态比较局域化, 小管径下(D<14 Å)其导电性更好; 随着构型的转变, TiO₂纳米管由直接带隙转变为间接带隙, 并且带隙值随着管径的增大而增大, 这是由于π轨道重叠效应的影响大于量子限域效应所导致的结果. 两种效应的竞争, 使得TiO₂纳米管的介电函数虚部ε₂ (ω)谱的峰值位置随管径增大既可能红移也可能蓝移, 管径大于9 Å (即(8, 0)管)之后, TiO₂纳米管的光吸收会出现明显的增强. 关键词： 2纳米管')" href="#">TiO₂纳米管 第一性原理 电子结构 光学性质     

金属钛中氦团簇融合的分子动力学模拟

运用分子动力学方法研究了金属钛中氦的扩散聚集行为.在300—800K的温度范围内,模拟了钛基底中氦团簇之间的融合过程.研究发现,温度的升高会加快氦团簇的融合.在300—800K,融合后的氦团簇在所模拟的时间尺度内三维结构保持不变.模拟结果还表明,常温下氦团簇之间的吸引力是导致氦团簇融合的重要因素. 关键词： 氦团簇 团簇融合 分子动力学模拟     

PtN2的结构和力学性质的第一性原理计算

采用平面波赝势密度泛函理论方法计算了PtN₂的坐标、平衡态的晶格常数、体弹模量、剪切模量和弹性常数,计算结果与已有的实验值和理论值符合较好.通过能量与体积曲线,可知ST_AA结构比黄铁矿结构具有更低的能量.根据计算结果和Pugh提出的经验判据,PtN₂是易脆的硬物质,随着压强增加PtN₂的脆性逐渐过渡到延性.两种结构的能带结构和态密度表明了黄铁矿结构的PtN₂是半导体而ST_AA 关键词： 2')" href="#">PtN₂ 第一性原理 力学性质     

ZrV2结构、弹性和热力学性质的第一性原理计算

采用基于密度泛函理论的第一性原理平面波赝势方法研究ZrV₂的晶体结构和弹性,利用准谐Debye模型计算在不同温度(T=0~1200 K)和不同压强(P=0~20 GPa)下ZrV₂的热力学性质,包括弹性模量与压强,热熔与温度,以及热膨胀系数与温度和压力的关系.结果表明:计算的ZrV₂晶格常数与实验值符合较好,晶体材料的弹性常数随着压力增加而增加;在一定温度下,相对体积、热熔随着压强的增加而减小,德拜温度、弹性模量随着压强的增加而增加,且高压下温度对ZrV₂热膨胀系数的影响小于压强的影响.     

硅纳米团簇与石墨烯复合结构储锂性能的第一性原理研究

本文采用第一性原理计算方法, 研究了不同晶向硅纳米团簇与石墨烯复合结构稳定性及其储锂性能. 计算了不同高度、大小硅团簇与石墨烯复合结构的结合能, 复合结构中嵌锂吸附能和PDOS. 分析表明, 硅团簇和石墨烯之间形成较强的Si—C键, 其中[111]晶向硅团簇与石墨烯作用的形成能最高, 结构最为稳定. 进一步计算其嵌锂吸附能, 发现硅团簇中靠近石墨烯界面处的储锂位置更加有利于锂的吸附, 由于锂和碳、硅之间有较强电荷转移, 其吸附能明显大于其他储锂位置. 同时在锂嵌入过程中, 由于石墨烯的引入, 明显减小了界面处硅的形变, 有望提高其作为锂电池负极材料的可逆容量.     

金属钛中氦团簇生长行为的分子动力学研究

运用分子动力学方法模拟了常温下金属钛中氦团簇的生长过程.从能量的角度考察了氦团簇的生长机理.研究发现,随着氦团簇的生长,在氦团簇周围逐渐形成位错环缺陷,氦团簇与氦原子的结合能有逐渐下降的趋势,当氦团簇生长到一定尺寸时会通过发射周围缺陷以使得结合能上升,从而增强了进一步吸收氦原子的能力.模拟还发现,随着氦团簇的继续生长,氦团簇的形状由原来的不规则结构逐渐变成了较为规则的棱柱形结构,在随后的生长过程中其生长仅在（001）平面进行,沿［001］轴的厚度几乎不变. 关键词： 氦团簇 缺陷发射 分子动力学模拟     

黑索金电子结构和光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理方法对黑索金晶体的电子结构和光学性质进行了计算. 结果表明: 黑索金是能隙值为3.43 eV的绝缘体, 价带主要由C, N和O的2s与2p态构成, 而导带主要由N-2p和O-2p态构成; 静态介电函数ε₁(0)=1.38, 介电常数的虚部有5个峰值, 其中最大峰值在光子能量4.59 eV处, 并对造成这些峰值的可能的电子跃迁做了详细分析. 利用能带结构和态密度分析了黑索金的光反射系数、吸收系数及能量损失函数等光学性质, 发现黑索金是对光吸收、反射及能量损失不敏感的材料. 关键词： 黑索金 第一性原理 电子结构 光学性质     

氢修饰石墨烯纳米带压电性质的第一性原理研究

采用第一性原理计算方法, 系统研究了不同宽度、不同边缘修饰模式的间隔氢吸附锯齿型石墨烯纳米带的压电性质. 结构优化和结合能计算表明, 氢修饰石墨烯纳米带结构稳定. 氢原子间隔排列的吸附使得纳米带中的相邻碳原子成键及电荷状态不同, 导致拉伸时纳米带中六元碳环的正负电荷中心不再重合, 产生宏观电极化. 纳米带宽度越宽, 包含六元碳环数目越多, 则拉伸时纳米带长度方向上电偶极矩密度越大, 其压电性能越强. 另外, 边缘原子电荷状态决定了无拉伸时纳米带的初始电偶极矩密度, 其大小可以通过改变边缘氢原子的修饰模式来有效调控. 关键词： 石墨烯纳米带 第一性原理 修饰改性 压电性质     

铝氮共掺杂氧化锌纳米管电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算, 对(6,0)单壁氧化锌纳米管、铝掺杂、氮掺杂和铝氮共掺杂纳米管的能带结构、态密度和差分电荷密度进行了研究. 结果表明, 氮掺杂可以在纳米管禁带中引入受主能级, 实现纳米管的p型掺杂, 但是受主能级局域性较强, 导致氮溶解度低. 引入铝元素可以有效降低氮形成受主能级局域性, 激活氮元素, 铝氮共掺杂有望成为氧化锌纳米管一种更为有效的p型掺杂方法. 关键词： 氧化锌纳米管 电子结构 共掺杂 第一性原理计算     

第一性原理计算和Monte Carlo算法耦合对共掺杂稀磁半导体磁学性质的研究

将第一性原理计算和Monte Carlo算法耦合,对共掺杂稀磁半导体进行模拟,预测其磁学性质,并与同种过渡金属原子单掺杂稀磁半导体相比较进行分析.首先用第一性原理计算掺杂原子间的磁相互作用强度,作为后面Monte Carlo模拟的输入参数;然后利用Monte Carlo方法结合海森堡自旋模型和磁相互作用强度来模拟体系热力学磁化强度和磁化率,由此得到居里温度;用同样的方法模拟掺杂磁性体系的磁滞回线.计算结果显示,对应于掺Co、Al-Co、Fe、Al-Fe的ZnO基稀磁半导体的居里温度分别为346 K、450 K、743 K、467 K,与实验值很吻合;模拟这四种掺杂情况得到的磁滞回线也很符合理论分析和实验结果,且呈现和居里温度相同的变化规律.