Al3V化合物的内聚性质

用局域密度总能方法研究了两种不同(DO₂₂和L1₂)相结构Al₃V化合物的内聚性质。计算所得DO₂₂结构Al₃V化合物的点阵常数(a=3.72?,c=8.20?)与实验结果一致,体弹性模量(1.3Mbar)与Al₃Ti化合物的实验杨氏模最可相比较。计算所得生成热(29±1kcal/mol)与实验值(26.0kcal/mol)相符。 关键词：     

Titanium-functional group complexes for high-capacity hydrogen storage materials

Using first-principles density-functional electronic structure calculations, we propose functionalized organic molecules decorated with titanium atoms as high-capacity hydrogen storage materials. We study six kinds of functional groups which form complexes with Ti atoms and find that each complex is capable of binding up to six H₂ molecules. Among such complexes, Ti-decorated ethane-1,2-diol can store H₂’s with the maximum gravimetric density of 13 wt% and, under ambient conditions, a practically usable capacity of 5.5 wt%. We also present various forms of storage materials which are obtained by modifying well-known nanomaterials using Ti-functional group complexes.     

Study on the local structure of (CrO6) complex in garnet crystals by optical and EPR spectrum

The local lattice structure distortions for YAG and YGG systems doped with Cr³⁺ have been investigated by the d³ configuration complete energy matrices which contain the Zeeman energy besides the electron–electron interaction, the trigonal crystal field as well as the spin–orbit coupling interaction. The local lattice structure parameters R and θ of (CrO₆)⁹⁻ complex are determined for Cr³⁺ in YAG and YGG systems, respectively. The calculated results show that the local lattice structures have expansion distortions, which almost tend to the same after distortions. Meanwhile, the EPR parameter D, g factors (g_||, g) and optical spectrum of these systems have been interpreted uniformly by quantitative calculation. It is shown that the effect of the orbit reduction factor k on g factors (g_||, g) cannot be ignored.     

MoO_3/Si界面区钼掺杂非晶氧化硅层形成的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,通过模拟MoO_3/Si界面反应,研究了MoO_x薄膜沉积中原子、分子的吸附、扩散和成核过程,从原子尺度阐明了缓冲层钼掺杂非晶氧化硅(a-SiO_x(Mo))物质的形成和机理.结果表明,在1500 K温度下, MoO_3/Si界面区由Mo, O, Si三种原子混合,可形成新的稳定的物相.热蒸发沉积初始时, MoO_3中的两个O原子和Si成键更加稳定,同时伴随着电子从Si到O的转移,钝化了硅表面的悬挂键. MoO_3中氧空位的形成能小于SiO_2中氧空位的形成能,使得O原子容易从MoO_3中迁移至Si衬底一侧,从而形成氧化硅层;替位缺陷中, Si替位MoO_3中的Mo的形成能远远大于Mo替位SiO_2中的Si的形成能,使得Mo容易掺杂进入氧化硅中.因此,在晶硅(100)面上沉积MoO_3薄膜时, MoO_3中的O原子先与Si成键,形成氧化硅层,随后部分Mo原子替位氧化硅中的Si原子,最终形成含有钼掺杂的非晶氧化硅层.     

金红石TiO_2中本征缺陷扩散性质的第一性原理计算

基于密度泛函理论的爬坡弹性带方法,对金红石相二氧化钛晶体中钛间隙、钛空位、氧间隙、氧空位4种本征缺陷的扩散特征进行了研究.对比4种本征缺陷在晶格内部沿不同扩散路径的过渡态势垒后发现,缺陷扩散过程呈现出明显的各向异性.其中,钛间隙和氧间隙沿[001]方向具有最小的扩散势垒路径,激活能分别为0.505 eV和0.859 eV;氧空位和钛空位的势垒最小的扩散路径分别沿[110]方向和[111]方向,激活能分别为0.735 eV和2.375 eV.     

Na2TiGeO5: Crystal structure stability at low temperature and high pressure

The temperature evolution of the lattice parameters measured from 295 to 125 K exhibits a small instability below T_c≈278 K, indicating ferroelastic properties of Na₂TiGeO₅. The behavior is related to the specific crystal structure built of polyhedral layers with shared TiO₅ pyramids and GeO₄ tetrahedra, alternating with layers of Na⁺ cations. Antiparallel alignment of the short apical titanyl bond in adjacent rows of the polyhedral layer gives rise to spontaneous strain, when a distortion of the TiO₅ groups occurs. Single-crystal structures determined at room temperature and 120 K suggest that {1 1 0} domains, developing below T_c, entail a tetragonal-to-orthorhombic symmetry change. The mechanism is attributed to a shortening of the O–O distance between the polyhedral layers, and to minor shifts of the positions of the Ti atoms and the correlated oxygen atoms along the c-axis. The structure distortion, however, is too small to allow any unambiguous determination of the symmetry-breaking effects. The bulk modulus and its pressure derivative have been determined as B₀=89(2) GPa and . A pressure-induced phase transformation takes place at P_c≈12.5 GPa, presumably to an orthorhombic structure. The pressure effect on the transition temperature is given by ΔT_c/ΔP≈1.76 K/GPa.     

The vacancy defects and oxygen atoms occupation effects on mechanical and electronic properties of Mo_5Si_3 silicides

Improving brittle behavior and mechanical properties is still a big challenge for high-temperature structural materials.By means of first-principles calculations,in this paper,we systematically investigate the effect of vacancy and oxygen occupation on the elastic properties and brittle-orductile behavior on Mo_5Si_3.Four vacancies (Si_(–Va1),Si_(–Va2),Mo_(–Va1),Mo_(–Va2)) and oxygen occupation models (O_(–Mo1),O_(–Mo2),O_(–Si1),O_(–Si2)) are selected for research.It is found that Mo_(–Va2)vacancy has the stronger structural stability in the ground state in comparison with other vacancies.Besides,the deformation resistance and hardness of the parent Mo_5Si_3are weakened due to the introduction of different vacancy defects and oxygen occupation.The ratio of B/G indicates that oxygen atoms occupation and vacancy defects result in brittle-to-ductile transition for Mo_5Si_3.These vacancies and the oxygen atoms occupation change the localized hybridization between Mo–Si and Mo–Mo atoms.The weaker O–Mo bond is a contributing factor for the excellent ductile behavior in the O_(-Si2)model for Mo_5Si_3.     

含空位二维六角氮化硼电子和磁性质的密度泛函研究

基于密度泛函理论的第一性原理计算, 研究了含B原子空位(V_B), N原子空位(V_N), 以及含B–N键空位 (V_B+N)缺陷的二维氮化硼(h-BN)的电子和磁性质. 在微观结构上, V_B体系表现为在空位附近的N原子重构成等腰三角形, V_N体系靠近空穴的B 原子形成等边三角形, V_B+N体系靠近空穴处的B和N原子在h-BN平面上重构为梯形. 三种空位缺陷都使h-BN的带隙类型从直接带隙转变为间接带隙. V_B体系的总磁矩为1.0 μ_B, 磁矩全部由N原子贡献. 其中空穴周围的三个N原子磁矩方向不完全一致, 存在着铁磁性和反铁磁性两种耦合方式. 对于V_N 体系, 整个晶胞内的总磁矩也为1.0 μ_B, 磁矩在空穴周围区域呈现一定的分布. 关键词： 二维h-BN 空位 电子结构 磁性     

The Manning factor for direct exchange and ring diffusion mechanisms

Abstract

In this paper, we consider lattice-based diffusion kinetics for the direct exchange and ring mechanisms as possible proxy diffusion mechanisms for diffusion in liquid alloys. For these mechanisms, we assessed the Manning factor that arises from the Darken–Manning relation relating the interdiffusion coefficient and tracer diffusion coefficients and which can be obtained experimentally. The maximum values of the Manning factor for these two mechanisms occur when the exchange only takes place between the atoms of different type but not between the atoms of the same type. These values have strong composition dependence and reach a value of 2 (ignoring tracer correlation factors) for the direct exchange mechanism at equal compositions of the two components in binary alloys. But for the three atom ring mechanism, these values as a function of composition have a much more complicated form that sits below the direct exchange mechanism for compositions between 10 and 90%. When all exchanges (allowed by a mechanism) occur with approximately the same probability, then the Manning factor is about unity for all compositions.     

The structure properties of LaNi3.5Al1.5

The structure properties of LaNi_3.5Al_1.5 have been investigated by the density functional theory using the full-potential linearized augmented plane wave (FLAPW) method with the generalized gradient approximation (GGA). The elastic constants of LaNi_3.5Al_1.5 are determined by a least-squares fitting of the calculated binding energy with a generalized two dimensional cubic function of lattice parameter a and c. The results show that the calculated lattice parameters (a=b=0.5092 nm and c=0.4214 nm) are in good agreement with the corresponding experimental data, that Al atoms prefer to substitute Ni atoms in the 3g sites and tend to disperse in middle planes. For LaNi_3.5Al_1.5, the bulk modulus (123.1 GPa), the shear modulus (68.6 GPa) and the values of the elastic constants C₁₁+C₁₂ (296.2 GPa), C₁₃ (76.4 GPa) and C₃₃ (210.5 GPa) are worked out for the first time. The ratio of c/a tends to increase from 0.790 (LaNi₅) to 0.804 (LaNi₄Al) and 0.827 (LaNi_3.5Al_1.5), which indicates that the LaNi_3.5Al_1.5 compound is observed to change less anisotropic with the substitution of Al atoms than the host alloy.     

Interactions between vacancies and prismatic Σ3 grain boundary in α-Al_2O_3:First principles study

Interactions between vacancies and Σ3 prismatic screw-rotation grain boundary in α-Al_2O_3 are investigated by the first principles projector-augmented wave method.It turns out that the vacancy formation energy decreases with reducing the distance between vacancy and grain boundary(GB) plane and reaches the minimum on the GB plane(at the atomic layer next to the GB) for an O(Al) vacancy.The O vacancy located on the GB plane can attract other vacancies nearby to form an O–O di-vacancy while the Al vacancy cannot.Moreover,the O–O di-vacancy can further attract other O vacancies to form a zigzag O vacancy chain on the GB plane,which may have an influence on the diffusion behavior of small atoms such as H and He along the GB plane of α-Al_2O_3.     

Defects in ion-implanted hcp-titanium: A first-principles study of electronic structures

The electronic structures of hexagonal closed-packed (h.c.p) titanium containing a vacancy and krypton impurity atoms at various insertion sites are calculated by first-principles methods in the framework of the density-functional theory (DFT). The density of states (DOS) for titanium containing a vacancy defect shows resonance-like features. Also, the bulk electron density decreases from ∼0.15/ų to ∼0.05/ų at the vacancy centre. Electronic structure calculations have been performed to investigate what underlies the krypton site preference in titanium. The DOS of the nearest-neighbour (NN) titanium atoms to the octahedral krypton appears to be less distorted (relative to pure titanium) when compared to the NN titanium atoms to the tetrahedral krypton. The electronic density deformation maps show that polarization of the titanium atoms is stronger when the krypton atom is located at the tetrahedral site. Since krypton is a closed-shell atom, thus precluding any bonding with the titanium atoms, we may conclude that the polarization of the electrons in the vicinity of the inserted krypton atoms and the distortion of the DOS of the NN titanium atoms to the krypton serve to indicate which defect site is preferred when a krypton atom is inserted into titanium. Based on these considerations, we conclude that the substitutional site is the most favourable one, and the octahedral is the preferred interstitial site, in agreement with recent DFT calculations of the energetics of krypton impurity sites.     

Thermodynamics and defect structures of silver sulfide

Published measurements of sulfur vapor pressure and silver electromotive force were used to determine thermodynamic properties of silver sulfide above 379 K. They were Gibbs-Duhem integrated to estimate the formation properties of stoichiometric Ag₂S of fcc, bcc, and monoclinic crystal structures. Statistical thermodynamics was applied to estimate free energies and find possible atom arrangements in off-stoichiometric silver sulfide. Theoretical calculations show that silver vacancies and atoms may be in quasi-chemical equilibrium between tetrahedral and octahedral sites in the fcc structure and between two states of atoms within tetrahedral sites in the bcc structure and within octahedral sites in the monoclinic structure. A strong indication is that vacancy clusters should predominate, each containing four-atom vacancies in the fcc phase and three-atom vacancies in the bcc phase.     

Electronic conduction properties of Au/C60/p-Si and C60/Au/p-Si sandwich structures: I–V and transducer characteristics

Gold-fullerite [C₆₀]-silicon (p-type) sandwich structures have been fabricated in order to investigate intrinsic cross-sectional and planar electronic conductive properties, in particular the C₆₀/p-Si p–n heterojunction. The turn-on voltage of this p–n heterojunction lies in the range 0.25–0.27 V. The I–V characteristics of the Au/C₆₀/p-Si structure are mostly defined by the bulk specific resistance of the fullerite crystal film itself (6×10⁷ Ω cm). I–V curves in the C₆₀/Au/p-Si structure are shown to be ohmic. Au/C₆₀/p-Si sandwiches irradiated with swift (300 MeV) heavy ions, (⁸⁴Kr¹⁴⁺) to a total fluence 10¹⁰ ion/cm² yield structures which are sensitive to ambient air pressure, specifically in the case of a transverse contact configuration, and if one of the contacts is located on the irradiated part of the fullerite film. The sandwich-structure sensitivity to pressure is 5×10⁻⁶ Pa⁻¹. This exceeds the sensitivity of conventional silicon pressure transducers by almost three orders of magnitude.     

原子间相互作用势对中Al浓度Ni75AlxV25-x合金沉淀序列的影响

基于微观相场模型与反演算法,研究了中Al浓度及温度对Ni₇₅Al_xV_25-x合金沉淀过程的影响：在相同浓度下,L1₂与DO₂₂结构的第一近邻原子间相互作用势随温度升高呈线性增加,两者呈正比的关系;但在同一温度下,L1₂（DO₂₂）结构的第一近邻原子间相互作用势随Al原子浓度的增加而增加（减少）.同时将反演得出的原子作用势代入微观相场模拟中,探讨中Al浓度合金沉淀序列与原子作用势的关系,即当L1₂的第一近邻原子间相互作用势大于（小于）DO₂₂时,L1₂（DO₂₂）优先析出;当L1₂和DO₂₂的第一近邻原子间相互作用势相等时,两者同时析出.特别地,当Al原子的浓度等于0.0589时,发现L1₂和DO₂₂同时析出.利用微观相场法反演原子间相互作用势,为判断中Al浓度合金的沉淀序列增加了可信度.     

空位缺陷对β-AgVO_3电子结构和光吸收性能的影响

基于密度泛函的第一性原理研究了Ag空位、O空位和Ag-O双空位对β-AgVO_3的电子结构及光学性质的影响.采用广义梯度近似平面波超软赝势GGA+U方法,对不同缺陷体系的形成能、能带结构、电子态密度、差分电荷密度和吸收光谱进行了计算和分析.通过比较不同Ag空位和O空位的形成能,确定了β-AgVO_3中主要形成Ag3空位和O1空位,并且Ag空位较O空位更容易形成.Ag3空位和O1空位的存在都使得β-AgVO_3带隙有一定程度的减小;Ag3空位使β-AgVO_3呈现p-型半导体性质,而O1空位和Ag3-O1双空位使β-AgVO_3呈现n-型半导体性质.Ag3和O1空位对晶体在可见光范围内的光吸收影响较小.     

Adiabatic small polaron-hopping conduction in Ln0.85Ca0.15MnO3 (Ln=Nd, Pr and Sm) perovskites

Polycrystalline bulk ferromagnetic insulating (FMI) Ln_0.85Ca_0.15MnO₃ (Ln=Nd, Pr and Sm) samples are prepared by standard solid-state reaction route and characterized. Powder X-ray diffraction (XRD) data of the manganites show single-phase character. Existing theoretical models predict that the high temperature (T>θ_D/2, θ_D being the Debye temperature) dc conductivity (σ_dc) of these samples is due to adiabatic small polaron-hopping conduction. Greaves’ and Mott's variable range hopping (VRH) conduction mechanisms are not suitable to explain the σ_dc data at low temperature (T<θ_D/2).     

Chemical diffusion in non-stoichiometric cuprous oxide

The kinetics and thermodynamics of point defect diffusion in non-stoichiometric cuprous oxide, Cu_2−yO, has been studied as a function of temperature (1173–1373 K) and oxygen pressure (10²–7×10⁴ Pa) using microthermogravimetric reequilibration technique. It has been shown that the defect diffusion coefficient of cation vacancies, constituting the predominant point defects in this oxide does not change with their concentration, clearly indicating that these defects do not interact and are randomly distributed in the crystal lattice. Consequently, cation vacancy diffusion coefficient, D_V, being the direct measure of defect mobility, can be described by the following empirical equation:

     

Contribution a la theorie atomique de la diffusion chimique

Here we present an analysis and a development of the atomic theory of chemical diffusion as proposed by Manning for a binary system a/b.The general expression for the flux of a tracer in a concentration gradient is first established. This expression of the flux is identified with that deduced in the phenomenological theory. Thus a relationship between the partial correlation factors of vacancies with each of the a and b species is obtained.The effect of “vacancy flow” can be described in terms of these correlation factors. Thus the vacancy flow on species A leads to a correlation of the vacancy jumps with species B and vice versa.We shall see that the Nernst-Einstein equation can be extended to the case of chemical diffusion and that the ratio of the intrinsic diffusion coefficients is equal to the ratio of the mean jump frequencies W_A and W_B.Also, the activation energies of intrinsic diffusion coefficients are related very simply to the activation enthalpies of atomic jumps.In conclusion, we shall see that chemical diffusion in a binary system a/b can be completely described if either the thermodynamic factor and the coefficients of self diffusion, or the thermodynamic factor and the coefficients of intrinsic diffusion are known as functions of the concentration.     

Hf-N体系的晶体结构预测和性质的第一性原理研究

为了寻找具有优异力学性能的新型超高温陶瓷材料, 结合进化算法和第一性原理, 系统研究了Hf-N二元体系所有稳定存在的化合物及其晶体结构. 除了实验已知的岩盐结构的HfN之外, 本文还找到了Hf₆N(R-3), Hf₃N(P6₃22), Hf₃N₂(R-3m), Hf₅N₆(C2/m)和Hf₃N₄(C2/m)五种新结构, 基于准简谐近似原理计算了这些稳定结构的声子谱以验证其动力学稳定性, 常温甚至更高温度下的吉布斯自由能以验证其高温热力学稳定性. 结果表明, 这些结构是动力学稳定的, 且在1500 K以下都是热力学稳定的. 同时, 本文还列出了在搜索过程中出现的空间对称性较高、能量较低的亚稳态结构, 包括Hf₂N(P4₂/mnm), Hf₄N₃(C2/m), Hf₆N₅(C2/m), Hf₄N₅(I4/m), Hf₃N₄(I-43d)和Hf₃N₄(Pnma). 之后计算了上述所有结构的力学性质(弹性常数、体模量、 剪切模量、 杨氏模量、硬度), 随着N 所占比例的增加, 硬度呈现的整体趋势是先增大后下降, 在Hf₅N₆处取得最大值, 为21 GPa. 其中Hf₃N₂和Hf₄N₅也展现出了较高的硬度, 都为19 GPa. 最后, 计算了这些结构的电子态密度和晶体轨道汉密尔顿分布, 从电子结构的角度分析了力学性能的成因. 研究结果显示, 较强的Hf-N共价键和较低的结构空位率是Hf₅N₆具有优异力学性能的主要原因.