液态Ca7Mg3合金快速凝固过程中团簇结构的形成特性

采用分子动力学方法对液态Ca7Mg3合金凝固过程中团簇结构的形成特性进行了模拟研究. 采用双体分布函数、Honeycutt-Andersen(HA)键型指数法、原子团类型指数法(CTIM)以及遗传跟踪等方法对凝固过程中团簇结构的形成演变特性进行了分析. 结果表明: 在以冷速为1×1012 K·s-1 的快速凝固条件下, 系统形成以1551、1541、1431键型为主的非晶态结构; 二十面体基本原子团(12 0 12 0)在快速凝固过程中对非晶态结构的形成起决定性作用; 在合金凝固过程中, 团簇的稳定性不仅与构成团簇的基本原子团类型有关, 还与中心原子类型以及中心原子之间的连接方式有关. 由于(12 0 12 0)基本原子团能量较低并且在低温具有较好的遗传特性, 基本原子团之间很容易连接在一起组成更大的团簇. 所形成的团簇结构显著不同于那些由气相沉积、离子溅射等方法所获得的团簇结构.     

Ni3Al合金液态与非晶中的原子团簇

采用常温常压分子动力学模拟技术，模拟了液态Ni3Al中原子团簇在快速凝固条件下的演变过程，模型采用的是TB(tight binding)作用势.用偶分布函数、键对和多面体等结构参数来描述快速凝固条件下团簇种类和数量的变化，并将团簇结构可视化.在2 000 K下，液态Ni3Al中团簇数量较少，且都是由缺陷二十面体构成；在4×1013 K•s－1的冷速下，团簇的数量随温度的降低不断增加，且出现完整二十面体团簇，体系最终形成了由二十面体和缺陷二十面体团簇网络所组成的非晶结构.     

PSO-SA模拟铝团簇的几何结构与基态能量

介绍了采用基于模拟退火的粒子群算法(PSO-SA),应用Gupta多体势函数和Sutton-Chen势函数对铝团簇Aln(n≤13)的基态能量进行了计算,并给出其相应的稳定结构.同时研究了铝团簇的半径分布图、平均结合能和能量二阶差分与团簇尺寸变化的关系.结果表明:铝团簇的结构稳定性随团簇原子数n的增加而增大,并在原子数n为7,9和11处出现极值,表明其具有“幻数”效应和相对较高的结构稳定性,由半径的分布图得出Al6,Al7,Al13具有高度的对称性.     

自由表面的Ni原子团簇的熔化

采用分子动力学模拟技术研究了不同尺寸的Ni原子团簇的熔化过程.团簇的最初构型为FCC结构.研究结果表明,原子团簇的熔化温度与原子团簇中原子的个数有关,团簇的熔化首先从表面开始,当外层原子成为液态后,整个团簇的熔化从液态层开始,直至核心区域.该熔化过程可以被称为非均质熔化,自由表面充当非均质形核位置.作为对比,对无自由表面的大块固态Ni的熔化过程也进行了模拟,其熔化温度高于实验温度约400 K.表明对无自由表面的大块固态的熔化过程,液相形成无非均质形核位置,熔化的本质过程受均质形核机理控制.     

冷却速率对液态金属Zn快速凝固过程中微观结构的影响

用分子动力学模拟方法研究了六种不同冷却速率对液态金属Zn凝固过程微观结构的影响. 采用双体分布函数g(r)曲线、平均原子总能量、Honeycutt-Andersen(HA)键型指数法和原子团类型指数法(CTIM-2)对凝固过程中微观结构的变化进行了分析. 结果表明, 冷却速率对微观结构的转变有决定性影响, 当冷却速率为1×10¹⁴、5×10¹³、2×10¹³、1×10¹³、5×10¹² K·s-1时, 系统形成以1551、1541、1431键型为主体的非晶态结构; 当冷却速率为1×10¹² K·s-1时, 系统形成以1421、1422键型为主或以密排六方(hcp)基本原子团(12 0 0 0 6 6)和面心立方(fcc)基本原子团(12 0 0 0 1 2 0)共存的部分晶态结构. 同时发现, 在形成非晶的五个系统中,玻璃化转变温度Tg随着冷速的降低而降低.     

硅氧团簇(SiO2)nO2H4的密度泛函理论研究

提出硅氧团簇(SiO2)nO2H4的两种新构型: 基于笼状结构和环状结构的构型, 并与链状构型相比较, 用密度泛函理论的B3LYP方法在6-31G(d)基组水平上计算了三种构型n=2～22(n取偶数)的几何结构、平均结合能、能隙以及能量的二次差分. 分析计算结果发现, 笼状构型不但在n=4和8处存在幻数团簇(实验上已经观察到), 而且预测在n=14处也存在类似的幻数团簇; 此外, 与(SiO2)n团簇不同的是, (SiO2)nO2H4团簇的环状构型的稳定性从n=4开始大于链状构型, 意味着水的加成对硅氧团簇的稳定性有着重要的影响.     

Pdn(n=2～13)团簇的密度泛函理论研究

采用密度泛函理论B3LYP方法计算并讨论钯原子团簇Pdn(n=2～13)结构模型.通过对钯原子团簇进行几何构型优化和振动频率计算,找出团簇总能量最低的同分异构体.由于Jahn-Teller效应的存在,团簇的最稳定结构采取对称性较低的几何构型.在钯原子数相同时,往往存在多个能量极为相近的稳定构型.单位原子平均静态极化率呈奇偶变化.     

FMBen（FM=Fe,Co,Ni;n=1-12）团簇结构和电子性质

采用密度泛函理论中的广义梯度近似对FMBen(FM=Fe,Co,Ni;n=1-12)团簇的几何构型进行优化,并对能量、频率和磁性进行了计算,同时考虑了电子的自旋多重度.结果表明,纯铍团簇的幻数是由电子的壳层模型确定,而FMBen团簇的幻数主要由几何效应来解释;掺杂铁磁性的过渡金属(Fe,Co,Ni)提高了纯团簇的稳定性.二阶能量差分表明FMBen(FM=Fe,Co,Ni)的幻数分别为5,10;5,10;4,10.通过对磁性质的研究发现掺杂不同的过渡金属时,磁矩出现了不同的变化规律.     

[BxAl13-x]-(x=0～13)二元团簇的密度泛函理论研究

采用遗传算法（Genetic algorithm,GA）对B-Al二元团簇[B_xAl_13-x]^-（x=0～13）进行了全局结构搜索,并在密度泛函理论（Density Functional Theory,DFT）下对其进行了优化计算. 结果表明在二元团簇中,随着硼原子的数量逐渐增加,团簇的结构由二十面体（3D）逐渐转化为准平面（2D）且团簇结构由B与Al原子的比例所决定的. 当x=0～7时,Al原子占多数,团簇保持3D结构;反之团簇呈现2D构型;在x=7～8时,团簇的结构发生由3D向2D的转变. 能量决定着团簇的稳定性（例如相对能量）. 能量越小,其结构越稳定. 在计算相对能量E_rel,当x=1时,其相对能量最低,团簇结构最稳定. 为了进一步了解团簇的稳定性,计算了[B_xAl_13-x]^- 团簇的HOMO-LUMO能级差（E_H-L）和垂直电离能（Vertical Detachment Energies,VDE）,并且发现它们的值在整体上是随着B原子数量的增加而减小,表明其团簇的稳定性逐渐减弱. 在所有的团簇中,BAl₁₂^-的能级差最大,结构最稳定. 因此文章中对其进行了分子轨道分析,发现当一个B原子替代了Al₁₃^-团簇中的中心Al原子时,所得到的BAl₁₂^-的电子壳层结构的1s²和1p⁶几乎和Al₁₃^-团簇保持一致. 此外,对于x=13时的准平面全硼团簇B₁₃^-,文章中用适应性自然密度划分（Adaptive Natural Density Partitioning,AdNDP）对其化学成键进行了分析,结果显示B₁₃^-有8个π电子,具有π反芳香性.     

FMBen(FM=Fe,Co, Ni; n=1-12)团簇结构和电子性质

采用密度泛函理论中的广义梯度近似对FMBen(FM=Fe, Co, Ni; n=1-12)团簇的几何构型进行优化, 并对能量、频率和磁性进行了计算, 同时考虑了电子的自旋多重度. 结果表明, 纯铍团簇的幻数是由电子的壳层模型确定, 而FMBen团簇的幻数主要由几何效应来解释; 掺杂铁磁性的过渡金属(Fe, Co, Ni)提高了纯团簇的稳定性. 二阶能量差分表明FMBen(FM=Fe, Co, Ni)的幻数分别为5, 10; 5, 10; 4, 10. 通过对磁性质的研究发现掺杂不同的过渡金属时, 磁矩出现了不同的变化规律.     

氰化钾团簇的结构和形成机理研究

利用激光气化铁氰化钾或亚铁氰化钾并利用飞行时间质谱检测方法对氰化钾团簇的形成机理进行了研究，结果发现：团簇正离子可归属为［Ｋ（ＫＣＮ）＿ｎ］￣＋，ｎ＝０～３７，它们的幻数为ｎ＝４、１３、２２、３７；团族负离子可归属为［（ＫＣＮ）＿ｎＣＮ］￣－＝，ｎ＝０～１３，它们的幻数为ｎ＝４、１３。这些幻数与氯化钠等碱金属卤化物团簇体系完全一致。这表明：它们的团簇结构应该是相同的，即１×３×３（ｎ＝４）、３×３×３（ｎ＝１３）、３×３×５（ｎ＝２２）、３×５×５（ｎ＝３７）结构。因此氰化钾晶体的初期形成过程也应采取ＮａＣｌ型结构的增长途径。在激光气化产生的等离子体中，正、负离子与中性团簇的碰撞增长反应、正离子与负离子的复合反应以及团簇的亚稳态解离反应造成了团簇离子的不同丰度分布。氰化钾团簇中稳定立方体结构的产生决定了幻数的出现。     

铝原子簇上化学吸附的尺度效应及其理论模型

对铝原子簇Al_n(n=1～10,12,13)已报导过的理论预测几何构型进行合理选择, 用量子化学CNDO/2法研究了单分子一氧化碳在这些簇上取不同吸附位形时的吸附作用。结果表明吸附强度随簇尺度的变化呈“幻数”特性: Al_2、Al_6、Al_(12)簇具有特别高的吸附能, 与实验观测结果相符。采用作者建议的推广电子壳模型可合理解释这一尺度效应。对Al_(12)和Al_(13)簇电子结构的分析进一步支持了壳模型的观点。随着簇的增大, 尺度效应逐步减弱并趋向于体相铝的性质。     

B(14) (2+): A magic number double-ring cluster

B(20) is a "magic number" cluster with double-ring structure. Surprisingly, we also find that B(14) (2+) is a "magic number" cluster with double-ring structure, which has the largest HOMO-LUMO gap (3.31 eV) and the highest aromaticity in double-ring clusters. This double-ring B(14) (2+) cluster is energetically lower than the quasi-planar one by even ～1.2 eV using high level ab initio calculations. B(14) (2+) also has 40 valence electrons as in Al(13) (-) cluster. The reason leading to the unusual properties of B(14) (2+) may be the electronic shell closing as in Al(13) (-) cluster based on the jellium model, besides the double aromaticity in all double-ring clusters.     

Dynamic factors in the reactions between the magic cluster Al(13)⁻ and HCl/HI

The icosahedral Al is a "magic" cluster with remarkable stability due to its high symmetry and closed valence shells. Its reactivity has provided a molecular model for understanding oxidation and dissolution processes in bulk metals. By first principles calculations, we demonstrated the importance of dynamic factors in the Al + HX reactions, with HX being either HCl or HI. There was a barrier to the dissociative adsorption of HX on the surface of an Al cluster, which involved charge transfer from Al. Furthermore, the H atom could be bonded to the cluster in multiple ways, similar to the top, bridge and hollow adsorption sites on Al(111) surface. With a large amount of energy (～40 kcal mol(-1)) deposited during the formation of Al(13)HX(-), the H atom could easily migrate among these sites, similar to the diffusion of hydrogen on metal surfaces. These factors were therefore important considerations in the formation and dissociation of Al(13)HX(-), and more generally in reactions involving other metal clusters.     

Structure and energetics of equiatomic K-Cs and Rb-Cs binary clusters

The basin-hopping algorithm combined with the Gupta many-body potential is used to study the structural and energetic properties of (KCs)(n) and (RbCs)(n) bimetallic clusters with N=2n up to 50 atoms. Each binary structure is compared to those of the pure clusters of the same size. For the cluster size N=28 and for the size range of N=34-50, the introduction of K and Rb atoms in the Cs alkali metal cluster results in new ground state structures different from those of the pure elements. In the size range N>/=38 the binary and pure clusters show not only structural differences, but they also display different magic numbers. Most of the magic Rb-Cs and K-Cs clusters possess highly symmetric structures. They belong to a family of pIh structures, where a fivefold pancake is a dominant structural motif. Such geometries have not been reported for alkali binary clusters so far, but have been found for series of binary transition metal clusters with large size mismatch. Moreover, tendency to phase separation (shell-like segregation) is predicted for both K-Cs and Rb-Cs clusters with up to 1000 atoms. Our finding of a surface segregation in Rb-Cs clusters is different from that of theoretical and experimental studies on bulk Rb-Cs alloys where phase separation does not occur.     

A First-principles Calculation of Structures and Stability of Al_(13)I Cluster

Using first-principles pseudo-potential plane wave method, the energetics, geometrical and electronic structures of three Al13I cluster isomers were calculated. The calculation results of the binding energy indicate Al13I cluster is more stable than Al13 cluster although its electrons are not a magic number as in Al13 cluster, and among Al13I cluster isomers the "Bridge" structure is the most stable, the second is the "Ontop" structure, and the worst is the "Hollow" structure. By analyzing the geometrical structures of Al13I cluster isomers, it is found that after I atom and Al13 cluster combine the geometrical structures of Al13 moieties are changed besides Al13IHollow cluster, in which the Al13 moiety is still a regular icosahedron. For Al13IOntop cluster, the Al13 moiety has a shrinking trend to I, whereas in Al13IBridge cluster it is distorted. Mulliken population analysis shows for the interaction of electrons between Al(I atoms in Al13I cluster not only there exists an ionic bonding but there is a covalent bonding. Part of electrons in the Al13 cluster transfer to I as Al13 cluster and I atom combine. The order of the strength of covalent bonding between Al13 moiety and I in Al13I cluster isomers is Al13IBridge>Al13IHollow>Al13IOntop. Further analysis of electric structures of Al13 and Al13I clusters indicates a higher stability of Al13I cluster than Al13 cluster can be attributed to the s-p hybridization of 3s and 3p electrons of Al in Al13 moiety induced by I doped, which leads to fewer electrons N(EF) at EF in Al13I and a larger energy gap ΔEH-L between HOMO and LUMO levels in Al13I cluster. The distinguish of structural stability of Al13I cluster isomers mainly originates from their different magnitudes in decrease of N(EF) and increase of ΔEH-L relative to Al13 cluster. The fewest N(EF) and the largest ΔEH-L are responsible for the high stability of Al13IBridge cluster .     

First‐principles study of molecular hydrogen dissociation on doped Al12X (X = B,Al, C,Si, P,Mg, and Ca) clusters

Inspired by the concept of superatom via substitutionally doping an Al₁₃ magic cluster, we investigated the H₂ molecule dissociation on the doped icosahedral Al₁₂X (X = B, Al, C, Si, P, Mg, and Ca) clusters by means of density functional theory. The computed reaction energies and activation barriers show that the concept of superatom is still valid for the catalysis behavior of doped metal clusters. The hydrogen dissociation behavior on metal clusters characterized by the activation barrier and reaction energy can be tuned by controllable doping. Thus, doped Al₁₂X clusters might serve as highly efficient and low‐cost catalysts for hydrogen dissociation. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Formation,growth mechanism and packing sequences of binary alloy cluster anions from laser ablation of mixtures of lead and transition metals

By using laser ablation of the mixtures of a transition metal (M: Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pd, Ag) plus lead, M/Pb binary cluster anions were observed except for Zn, and the number of transition metal atoms contained in the binary clusters is at most 4. This behavior is different from that reported previously for M/Ge binary clusters. The experiments indicate that it is also very difficult to form Al/Pb clusters. The distribution patterns of M/Pb binary alloy cluster anions are remarkably similar to those of pure Pb clusters, consistent with a formation mechanism in which transition metal atoms are sequentially attached to [M(x-1)Pb(y)](-) clusters and thus form [M(x)Pb(y)](-) clusters by a simple condensation process. As the number of transition metal atoms increases, the intensities of binary clusters gradually decrease. It is proposed that [MPb(4)](-) and [MPb(5)](-) cluster anions might be the unit building blocks of M/Pb binary cluster anions, and the layer packing sequences for magic clusters are predicted on this basis. The [M(x)Pb(y)](-) binary clusters containing 13 atoms (x + y = 13; x not equal 0) are proposed to have an icosahedral structure.     

Studies on the Stability of On-top Al13Inm (n = 1～12,m =-1,0,+1) Clusters

Energetic and electronic structures of the on-top Al13Inm (n = 1 ～12,m = -1,0,+1)clusters have been investigated by employing a first-principles pseudo-potential plane wave method.Several parameters such as binding energies,second differences of energy and vertical-electron detachment energies have been adopted to characterize and evaluate the structure stability of Al13In (n = 1 ～ 12) clusters.The optimized models show that the Al13 moieties in the clusters can not retain the original regular icosahedron structure.Results from binding energy and second difference of energy show that Al13In and Al13In- clusters with even n are more stable than those with odd n in contrast with Al13In+ clusters.The calculation of vertical-electron detachment energies (VDE) of Al13In clusters indicates that Al13In and Al13In- clusters with even n are closer to the closed shell of the Jellium model.Further analysis of electron density of states and electron density differences reveals that the enhanced stability of Al13In- clusters is not only associated with the closed shell of valence electrons but also with the bonding type between I and associated Al atoms.     

硅、锗、锡、铅/磷二元原子团簇的形成、光解和结构

利用激光溅射产生了第IV主族 (硅、锗、锡、铅 ) /磷二元团簇正负离子 ,用飞行时间质谱研究了团簇离子的组成规律和激光光解产物 .研究表明二元团簇稳定性受团簇电子结构和几何结构的影响 ,但随着第IV主族元素自上而下 ,几何结构对团簇稳定性的作用越来越大 .在二元团簇离子中存在两类幻数团簇 :一类可以用Wade规则解释 ,其中磷原子或者充当给电子配体结合在第IV主族原子构成的团簇骨架外 ,或者直接参与团簇骨架的构成 ;另一类则与稳定的第IV主族中性团簇 (或磷中性团簇 )是等电子体 .利用从头计算和Wade规则对幻数团簇的结构和价键进行了分析 .