自适应免疫优化算法研究大尺寸Cu-Au合金团簇稳定结构

合金团簇所具备的催化和光学等方面特性与团簇的尺寸、元素组成和元素序列密切关联,因而确定其稳定结构是研究纳米团簇合金性质的首要任务.本文利用基于内核构建的自适应免疫优化算法研究了完整元素组成的CunAum(n+m=61及79)二元合金团簇的稳定结构.应用多体Gupta势函数描述Cu-Au团簇原子间的相互作用.研究结果表明:对于CunAum(n+m=61)团簇,除了当n=12-15时为由三个双二十面体面面相连组成的环状结构外,其余均为二十面体结构.原子总数为79的Cu-Au合金团簇包括堆积缺陷的面心立方结构、双面心立方结构、二十面体、十面体和由四个双二十面体面面相连组成的环状结构.且当Au原子比例高和低时其主要构型分别为二十面体和十面体.此外,还分析了Cu-Au合金团簇结构势能量的分布情况及团簇的相对稳定性.原子分布规律显示Cu原子趋于占据内层,而Au原子趋向于分布在外层.     

Effect of the lattice structure of nickel nanoparticles on their practical magnetization

Magnetization of nickel nanoparticles with various lattice structures is studied. Nickel particles with the fcc structure are shown to be magnetically harder than the particles with the icosahedron structure. Easy magnetization axes in the particles are found.     

Magnetic properties of Co–Si alloy clusters

We have investigated the electronic structure and the magnetic properties of Co–Si alloy clusters using ab initio spin-polarized density functional calculations. The possible CoSi₂, CoSi, and Co₂Si phase clusters with oblique hexagon prism, icosahedron, and cuboctahedron structures are introduced. The CoSi phase cluster with icosahedron structure has the largest binding energy and amount of charge transfer. We found that HOMO-LUMO gap, magnetic moment, and spin polarization for the Co–Si alloy clusters with icosahedron structure increase with Co concentration. The Si atoms in the CoSi phase with icosahedron structure have negative magnetic moment.     

基于内核构建的Cu-Au-Pd团簇稳定结构优化

具有特殊催化、磁性和化学活性的三元合金团簇已成为基础科学研究的热点问题.确定其稳定结构是研究团簇性质的重要前提.针对大尺寸Cu-Au-Pd团簇结构优化,提出了内核构建的方法改进了自适应免疫优化算法的效率(称为AIOA-IC算法).采用基于紧束缚势二阶矩近似的多体Gupta势函数来描述三元合金团簇原子间相互作用.为测试算法效率优化了原子数为60的Ag-Pd-Pt团簇稳定结构.结果显示新得到的结构比文献报道的团簇结构势能量值更低,由此可知AIOA-IC算法具有更强的势能面搜索能力.运用该算法研究了38及55原子Cu-Au-Pd团簇的稳定结构.所研究的38原子Cu-Au-Pd团簇包含了五折叠、六折叠和截角八面体结构,并且原子成分比例影响了团簇的结构类型.而55原子Cu-Au-Pd团簇均为完整二十面体结构,序列参数显示Cu,Au和Pd原子分层现象明显.对于147原子Cu_(12)Au_(93)Pd_(42)团簇完整二十面体结构,中心原子为Au,内层和次外层分别被12个Cu原子和42个Pd原子占据,最外层则被92个Au原子占满.通过原子半径及表面能分析了Cu,Pd和Au原子分别倾向于分布在内层、次外层和最外层的规律.     

Theory of the liquid-solid transition

The liquid-solid transition is found by calculating the thermodynamic potential of a dense classical system as a function of order parameters which are proportional to the lattice Fourier components of the density. Properties of the fluid enter $\text{only}$ through the direct two-particle correlation function near freezing. Calculated parameters for freezing into bcc, fcc and hexagonal structures are in good agreement with experimental results.     

液态金属Ga急冷凝固中微观结构转变的模拟研究

采用分子动力学方法对液态金属Ga的快速凝固过程进行计算机模拟跟踪研究,运用HA键型指数法和原子团类型指数法分析了金属原子Ga的成键类型和形成的基本原子团结构.发现:与通常的液态金属(如Al)相反,随着温度的降低,二十面体及与二十面体相关的1551, 1541键型数目明显下降;与立方体(fcc,bcc),六角密集结构相关的1421,1422键型数目明显增加;而与菱面体相关的1321,1311键型的数目却显著增加,逐渐占据优势.最后形成一种新型的以菱面体结构为主、夹杂着立方体(fcc,bcc)、六角密集(hcp)等团簇结构所组成的非晶态结构.这正是非晶态金属Ga的g(r)曲线分裂的第二峰的顺序为前低后高,而与非晶态金属Al的g(r)曲线(其分裂的第二峰的顺序为前高后低)明显不同的微观结构上的物理根源.     

Structural evolution in the crystallization of rapid cooling silver melt

The structural evolution in a rapid cooling process of silver melt has been investigated at different scales by adopting several analysis methods. The results testify Ostwald’s rule of stages and Frank conjecture upon icosahedron with many specific details. In particular, the cluster-scale analysis by a recent developed method called LSCA (the Largest Standard Cluster Analysis) clarified the complex structural evolution occurred in crystallization: different kinds of local clusters (such as ico-like (ico is the abbreviation of icosahedron), ico-bcc like (bcc, body-centred cubic), bcc, bcc-like structures) in turn have their maximal numbers as temperature decreases. And in a rather wide temperature range the icosahedral short-range order (ISRO) demonstrates a saturated stage (where the amount of ico-like structures keeps stable) that breeds metastable bcc clusters. As the precursor of crystallization, after reaching the maximal number bcc clusters finally decrease, resulting in the final solid being a mixture mainly composed of fcc/hcp (face-centred cubic and hexagonal-closed packed) clusters and to a less degree, bcc clusters. This detailed geometric picture for crystallization of liquid metal is believed to be useful to improve the fundamental understanding of liquid–solid phase transition.     

Al-Mg合金熔体快速凝固过程中微观结构演化机理的模拟研究

采用分子动力学方法对Al₅₀Mg₅₀合金熔体的快速凝固过程进行了模拟,并采用双体分布函数、键型指数法和原子团类型指数法等方法,从微观结的不同层面对Al-Mg合金熔体快速凝固过程中微观结构的演化机理进行了深入的分析研究.结果表明：本模拟所获得的Faber-Ziman偏结构因子与实验结果符合较好.Al₅₀Mg₅₀合金熔体具有遗传性,在快速凝固过程形成了非晶态结构,其中二十面体短程序结构对非晶态结构的形成起决定性作用.基于原 关键词： Al-Mg合金熔体 快速凝固过程 分子动力学模拟 微观结构     

冷速对液态金属Pb凝固过程中微观团簇结构演变影响的模拟研究

采用分子动力学方法对六种不同冷却速率对液态金属Pb凝固过程中微观团簇结构演变的影响进行了模拟跟踪研究.采用双体分布函数、Honeycutt-Andersen键型指数法、原子团类型指数法(CTIM-2)、平均配位数等方法对凝固过程中微观团簇结构的演变进行了分析.结果表明：系统存在一个形成非晶态或晶态结构的临界冷速(介于5×10¹²与1×10¹²之间),大于这个临界冷速时,系统将形成以1551,1541和1431键型为主的非晶态结构;当小于这个临界冷速时,系统将先形成以1441和1661键型或以bcc基本原子团(14 6 0 8 0 0)为主的晶态结构,并稳定存在一段时间,然后又迅速转变为以1421和1422键型为主或以fcc基本原子团(12 0 0 0 12 0)和hcp基本原子团(12 0 0 0 6 6)以一定比例并存的部分晶态结构;同时发现,冷速对系统中fcc结构和hcp结构的相对比例有明显的影响,冷速越低,fcc结构所占的比例越多,越倾向于形成完美的fcc晶态结构. 关键词： 液态金属铅 凝固过程 团簇结构演变 分子动力学模拟     

室温下Fe62Ni27Mn11（wt%）合金的压致fcc-hcp相变

 本文采用Mao-Bell型金刚石对顶砧（DAC）及高压在位（in situ）粉末X光衍射照相方法研究了Fe₆₂Ni₂₇Mn₁₁（wt%）合金在0~43.2 GPa压力范围内的压致结构相变和等温压缩行为，实验结果表明，该合金在低压时为fcc结构，在19.4 GPa压力附近出现压致fcc→hcp结构相变，直到43.2 GPa一直保持fcc、hcp二相共存；相变过程中，二相的molar体积相同；高压hcp相得晶格参数比值c/a基本上不随压力而变，可以表示为c/a=1.630±0.006；在卸压过程中，hcp相可保持到5.8 GPa，当卸压到常压时，该合金完全恢复到fcc结构；用Murnaghan等温固体状态方程对其压缩数据进行最小二乘法拟合，得到B₀=(166±12) GPa，B₀'=5.2±0.5；本文还给出了该合金的压致fcc→hcp结构相变模型，并对存在很宽的二相共存区间问题进行了初步探讨。     

Island adsorption and adatom diffusion on 3D non-crystalline silver nanoclusters

We report a systematic study of island adsorption and single-adatom diffusion on free silver nanoclusters, and discuss the consequences on the growth. In our calculations, silver is modelled by semiempirical many-body potentials. We consider magic non-crystallographic structures at different sizes: icosahedra (Ih) at 55, 147 and 309 atoms; Marks-truncated decahedra (m-Dh) at 75, 146 and 192 atoms. We calculate the map of adsorption sites and the energy barriers for the different diffusion processes. We find that, due to purely geometrical reasons, medium-size (from 6–8 to 30–40 atoms depending on the cluster) islands on the cluster (1 1 1) facets prefer the hcp stacking on both Ih and Dh structures, while both smaller and larger islands are better placed on fcc stacking. Interfacet diffusion is easy on both Dh and Ih, indicating that large islands are easily grown; in particular, there are multi-atom diffusion processes which allow fast diffusion among the two caps of Dh clusters. For Dh clusters, islands on hcp stacking may lead to the appearance of new fivefold symmetry points, and to the transformation of the cluster into an icosahedron.     

Structural Dependence of Electronic and Magnetic Properties of the Cr13 Cluster

A tight-binding effective potential has been employed, in conjunction with a genetic algorithm, to fully optimize the Cr₁₃ cluster geometry without imposing any symmetry constraints. The minimum energy structure of this cluster is found to be a slightly distorted icosahedron. Based on the optimized structure and three assumed geometries (icosahedron,bcc-like and fcc-like), the structural dependence of electronic and magnetic properties of the Cr₁₃ cluster is discussed by using a d-band Hubbard-like Hamiltonian in the unrestricted Hartree-Fock approximation. Results are given for the average magnetic moment and local magnetic moments. It is found that for all considered geometries the Cr₁₃ cluster exhibits antiferromagnetic behavior. Results are also given for the cohesive energy, average coordination number, and local electronic densities of states. The results indicate that the average coordination number per atom in the cluster geometry is a significant factor to affect the magnetism.Also, the local density of states is a sensitive function of geometry.     

Computer simulation of crystal structure formation upon transition from amorphous state

The structure of a solid has been studied by the molecular dynamics technique upon transition from the amorphous state to the crystalline state. The influence of initial conditions in the simulation of an amorphous sample and the temperature of its heating on the resulting structure is examined. It is found that structures of two types can be formed in the sample: single crystals consisting of face-centered cubic (fcc) and hexagonal close-packed (hcp) cells with a small number of pentahedral cells in the boundary region of the sample and block crystals with an ordered pentahedral structure composed of fcc, hcp, pentahedral, and icosahedral cells. Linear chains of vacancies are revealed at the boundaries of blocks.     

Phase stabilities of fcc Ti nanocrystals

A model for the equilibrium transition size and temperature between hcp and fcc structure of nanocrystals Ti has been established in terms of the effect of surface stress on the internal pressure of nanocrystals. It was found that as size and temperature decreased, the relative stability of fcc structure in comparison with hcp structure increased. The obtained result is consistent with other theoretical and experimental data.     

Order-disorder transition in the solid phase of a charged hard sphere model

We investigate the solid phases of the restricted primitive model (RPM). Monte Carlo simulations show the existence of an order-disorder transition from a substitutionally disordered face centered cubic lattice (fcc) to a new ordered fcc structure which is proposed as the ground state of the RPM at the close packing density. Our results suggest that the new phase might turn out in a new triple point in the RPM phase diagram involving three solid phases: CsCl, fcc ordered and fcc disordered structures. The order-disorder transition is also studied using the cell theory. The theory shows good agreement with the simulation results and suggests that the transition is weakly first order.     

Structural and magnetic properties of Ni<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2–21) clusters

Extensive studies of structural and magnetic properties of pure Ni _n (n=2–21) clusters are carried out using density functional theory with generalized gradient approximation. A new structural growth pattern is reported and the icosahedron sequence is not the global minima structures for size n < 16. The evolution pattern of Ni₅–Ni₁₀ clusters is based on square pyramid. For cluster between n = 9 and n = 14, structures of them are trilayered stacking pattern. Especially for Ni₁₃, it seriously deviates from icosahedral pattern which was frequently assumed in previous studies. Ni₁₀, Ni₁₃ and Ni₁₄ are pieces of a double tetrahedron. For cluster size larger than 16, icosahedron based configurations are found to be the dominative structure pattern with exception of Ni₁₉. But trilayered structure is a strong competitor due to small difference in energy between this pattern and icosahedral family. The comparison of magnetic behavior between the calculations and experimental results are conducted. The average coordination mainly dominates on the magnetic behavior in the size range we considered.     

Density functional calculations on 13-atom Pd12M （M= Sc-Ni） bimetallic clusters

The geometric structures,electronic and magnetic properties of the 3d transition metal doped clusters Pd12M(M=Sc-Ni) are studied using the semi-core pseudopots density functional theory.The groundstate geometric structure of the Pd12M cluster is probably of pseudoicosahedron.The Ih-Pd12M cluster has the most thermodynamic stability in five different symmetric isomers.The energy gap shows that Pd12M cluster is partly metallic.Both the absolutely predominant metal bond and very weak covalent bond might exist in the Pd12M cluster.The magnetic moment of Pd12M varies from 0 to 5 μB,implying that it has a potential application in new nanomaterials with tunable magnetic properties.     

Local structure changes of 54-, 55-, 56-atom copper clusters on heating

The local structure changes among three types of Cu clusters containing 54-56 atoms at elevated temperatures have been studied by employing molecular-dynamics simulations. The simulations show sensitivities of structural changes to different structures of the three clusters with icosahedron-based geometries by removing or adding one atom from a complete icosahedron cluster, and how the structural changes can strongly cause internal energy to change accordingly. Because of the different structures of these clusters, the structure change processes of these types are quite different on heating.