Aln(n=2～7)团簇的结构和能级分布

采用密度泛函B3LYP的方法研究了小原子团簇Al2～7的几何结构和能级分布,分析了随团簇原子数的增加,团簇的几何结构和费米能级的变化情况.研究结果表明:Al2～7的团簇的几何结构在5个原子以前为平面结构,而从六个原子开始为空间立体的稳定结构.电子壳层结构表明,在铝团簇中没有出现非常明显的象碱金属那样的稳定幻数结构.在Al2～Al7团簇中,能级结构呈现明显的分立特征,费米能级随着原子个数的增加而减小,到Al7时又有所增加,且团簇的能量间隙最小.     

低温下Cu13团簇负载于Cu(001)表面上结构变化的分子动力学研究

应用分子动力学方法研究温度为10和50 K时具有二十面体结构的Cu13团簇以不同接触条件与Cu(001)表面结合后的结构变化,原子间的相互作用势采用Johnson的嵌入原子方法模型.通过基于原子密度分布函数的分析表明,负载团簇与表面的结合能主要受团簇与载体相接触的最低层原子数及这些原子所具有的不同几何构型影响,同时更高层的原子呈现出不同的几何结构.温度为10 K时,负载团簇的初始位置对团簇几何结构和结合能影响较大.     

低温下Cu13团簇负载于Cu(001)表面上结构变化的分子动力学研究

应用分子动力学方法研究温度为10和50 K时具有二十面体结构的Cu₁₃团簇以不同接触条件与Cu（001）表面结合后的结构变化,原子间的相互作用势采用Johnson的嵌入原子方法模型.通过基于原子密度分布函数的分析表明,负载团簇与表面的结合能主要受团簇与载体相接触的最低层原子数及这些原子所具有的不同几何构型影响,同时更高层的原子呈现出不同的几何结构.温度为10 K时,负载团簇的初始位置对团簇几何结构和结合能影响较大. 关键词： 分子动力学 团簇 表面 计算机模拟     

Al_n(n=2～7)团簇的结构和能级分布

采用密度泛函B3LYP的方法研究了小原子团簇Al2～ 7的几何结构和能级分布 ,分析了随团簇原子数的增加 ,团簇的几何结构和费米能级的变化情况。研究结果表明 :Al2～ 7的团簇的几何结构在 5个原子以前为平面结构 ,而从六个原子开始为空间立体的稳定结构。电子壳层结构表明 ,在铝团簇中没有出现非常明显的象碱金属那样的稳定幻数结构。在Al2 ～Al7团簇中 ,能级结构呈现明显的分立特征 ,费米能级随着原子个数的增加而减小 ,到Al7时又有所增加 ,且团簇的能量间隙最小。     

双金属团簇Cu12Fe吸附CO和H2的理论研究

基于第一性原理密度泛函理论(DFT)方法研究了Cu₁₂Fe团簇的结构稳定性、热力学稳定性和反应活性.计算得出正二十面体Fe原子核心团簇Ih-core比正二十面体Fe原子壳层团簇Ih-shell的热力学稳定性更强.通过分析吸附能讨论了CO和H₂在Ih-core团簇上的吸附构型.计算结果表明,Ih-core团簇以顶角Corner位吸附CO时吸附能最大,吸附模型最稳定,H₂吸附过程中发生了解离,两个氢原子均形成表面Facet位吸附构型.最后,通过分析前线轨道得到吸附过程的轨道信息.     

X@Mg_8B_(14)(X=H,Li)团簇核壳结构与性质的密度泛函理论研究

运用杂化密度泛函B3LYP方法,在6-31G*水平上对X@Mg_8B_(14)(X=H,Li)两种团簇进行了几何结构优化,并计算了其电子结构、振动特性和成键特性.计算结果表明:优化后的X@Mg_8B_(14)(X=H,Li)团簇均为橄榄球状核壳结构,对称性点群均为D2h.用自然键轨道方法分析了成键性质,发现X@Mg8B14(X=H,Li)团簇中B原子主要是sp杂化轨道参与成键,Mg原子主要是s轨道参与成键.Mg原子和B原子之间发生了大量的电子转移,在B原子层堆积了大量的电子;尤其是封装Li原子后,B原子层得电子数量明显增加.     

核壳型量子点(ME)4@(ME)28(M=Cd/Zn，E=Se/S)核壳间相互作用研究

采用密度泛函理论和自然键轨道理论结合的方法对核壳型量子点团簇(ME)4@(ME)28(M=Cd/Zn,E=Se/S)的几何结构、范德华体积、分子轨道和核层与壳层间的轨道占据进行模拟,分析不同核壳团簇结构中核与壳之间的相互作用。结果显示,核层金属原子与壳层非金属间距离大于核层非金属原子与壳层金属原子间距离,相同壳层时,Zn-Se/S键长小于Cd/Zn-Se键长,孤立的核和壳体积之和大于相应核壳构型的体积;核壳间的相互作用主要源于壳层非金属原子Se的sp杂化轨道和核层金属原子Cd/Zn的s、p及少量的d轨道相互作用。研究表明,一方面,核内原子和壳上的原子之间存在很强的化学键的作用;另一方面,在核与壳之间存在着显著的边界,表明核、壳构型有差异,核壳构型中的核与壳之间存在相互作用且二者相互影响。     

Sc_(13)团簇的稳定性和磁性研究

使用梯度修正的密度泛函理论,对Sc_(13)团簇的稳定性和磁性进行了广泛的研究.与邻近钪团簇,或者与其它13-原子3d过渡金属团簇相比,Sc_(13)团簇的基态结构二十面体具有最高对称性,结构和磁性均表现出高的稳定性.由于二十面体的高对称性,Sc_(13)团簇的对称性分子轨道为晶体场分裂壳层9A_g(1),10T_(1u)(3),8H_g(5),10A_g(1),6T_(2u)(3),11T_(1u)(3),3G_g(4),3G_u(4),and 9H_g(5).20个价电子以类似于凝胶模型的1S~21P~61D~(10)2S~2壳层填充前四组壳层,其余19个价电子依据洪德法则填充后五组壳层,从而产生了19.0μB的磁矩.5个半填充的晶体场分裂壳层与离域价电子填充的1S~21P~61D~(10)2S~2类壳层导致了稳定的高磁矩Sc_(13)团簇.     

非晶Ag晶化过程中不同类型晶核结构的识别与跟踪

采用分子动力学模拟研究了非晶Ag的等温晶化过程,通过原子轨迹逆向追踪法分析了不同类型晶体团簇的结构遗传与组态演化.在团簇类型指数法的基础上,根据基本团簇种类与联结方式不同,提出了一种可区分fcc单晶、多晶与混晶团簇的分析方法.在非晶Ag等温晶化过程中,基于团簇结构的连续遗传性特征,发展了一种可区分fcc单晶、多晶与混晶晶胚与晶核的结构分析技术.结果发现:不论临界尺寸还是几何构型,不同类型的晶核结构都存在差异,其中fcc单晶临界尺寸最小,多晶次之,混晶最大; fcc单晶与多晶壳层原子中有少量hcp和bcc原子,而混晶壳层则全部为非晶类原子,并且fcc单晶、多晶与混晶的临界晶核都不是球型结构.     

体心立方固溶体合金中的"团簇+连接原子"结构模型

为体现固溶体合金中的溶质原子产生的化学短程序,文章提出了配位数为14的团簇在体心立方(bcc)点阵中的堆垛模式,并建立了基于bcc结构的"团簇+连接原子"结构模型,用团簇成分式[团簇](连接原子)_x表述. 此模型中,与基体组元具有相对大的负混合焓的溶质原子占据团簇心部,其他原子作为连接或者替代团簇壳层基体原子. 1 ∶1结构模型[团簇](连接原子)₁由于最大程度地保证了团簇与连接原子的近邻,构成了连接原子最有效的合金化方式. 在两个实用bcc固溶体合金体系中, 1 ∶1模型指导设计了低V含量的储氢合金 V₁和低弹性模量高强度的 Nb₁合金. 关键词： 体心立方固溶体成分设计 "团簇+连接原子"结构模型 Ti-Cr-V合金 Ti-Zr-Mo-Nb合金     

以团簇加连接原子模型解析Cr-C共晶成分

Cr-C体系材料是重要硬质防护涂层的代表,具有共晶特征.我们的前期工作指出,共晶合金满足双团簇近程序结构模型,由两种稳定液体亚单元构成,各自满足理想非晶团簇成分式,这里的第一近邻团簇来自相关共晶相.显然共晶成分解析的关键在于获得团簇,而相结构中往往存在多种团簇,进入到非晶/共晶团簇成分式的主团簇定义是关键环节.本文通过应用Friedel振荡理论及原子密堆,以团簇分布的球周期性及孤立度为判据,以Cr-C共晶相为例,进一步细化了共晶相中的主团簇选择流程,再搭配以2,4或6个连接原子,获得了描述共晶成分Cr_(86)C_(14)和Cr_(67.4)C_(32.6)的双团簇成分式:[Cr-Cr_(14)+C-Cr_9]Cr C_3和[C-Cr_9+C-Cr_8]C_6,其中四种团簇分别来自共晶相Cr,Cr_(23)C_6,Cr_7C_3和Cr_3C_2.该工作进一步证实了团簇加连接原子模型在共晶点解析中的普适性,并从理论上支持了相关的材料设计.     

NiZr,AlZr和BCr相局域原子短程序特征

晶体和非晶合金具有以近邻多面体团簇为特征的微结构,这些团簇包含有形成其母相的空间局域原子短程序结构信息.本文正是从局域原子团簇特征出发,详细分析了传统晶体相描述中具有相同结构的NiZr,AlZr和BCr相,结果表明它们却具有不同的局域原子特征.这种不同可能是由单胞大小及原子占位略有不同造成的.由于这种不同于传统的晶体学描述方法给出了微观结构特点,对晶体相的重新认知及材料设计会大有帮助.     

Optical switching in a Ξ system: A comparative study on DROP and EIT

Liquid Lennard-Jones clusters with magic number of atoms N = 55, 147, 309, 561 and 923 were cooled down in Monte Carlo simulations until freezing. Structural properties of the clusters, including the radial dependence of atomic concentration/density and the local regular structure in arrangement of atoms, just before freezing were analysed. Existence of spherical layers in atomic density around the centre of mass of liquid LJ clusters was confirmed. Formation of layers is explained by central net forces acting on every cluster atom and leading to positioning an atom close to the cluster centre of mass. The strong layering in small clusters of N = 55 and 147 affects atomic diffusion in radial and tangential directions inside the cluster, leading to easier movement of atoms on the layer surface. Analysis of radial profiles of four types of structural units detected in liquid clusters reveals that icosahedral units are the most numerous and are located mainly near cluster surface of all clusters and also in the centre of small clusters.     

Melting behavior of large disordered sodium clusters

The melting-like transition in disordered sodium clusters Na₉₂ and Na₁₄₂ is studied by performing density functional constant-energy molecular dynamics simulations. The orbital-free version of the density functional formalism is used. In Na₁₄₂ the atoms are distributed in two distinct shells (surface and inner shells) and this cluster melts in two steps: the first one, at ≈130 K, is characterized by the development of a high intrashell atomic mobility, and the second, homogeneous melting at ≈270 K, involves diffusive motion of all the atoms across the whole cluster volume. On the contrary, the melting of Na₉₂ proceeds smoothly over a very broad temperature interval, without any abrupt change in the thermal or structural indicators. The occurrence of two steps in the melting transition is suggested to be related to the existence of a grouping of the atoms in radial shells, even if those shells present a large degree of internal disorder. It then appears that a cluster can be considered fully amorphous (totally disordered) only when there are no radial regions of low atomic density separating shells. The isomer of Na₉₂ studied here fulfills this criterion and its thermal behavior can be considered as representative of that expected for fully amorphous clusters. Disordered Na₁₄₂, on the other hand, that has a discernible structure of an inner and a surface shell, should be considered as not fully disordered. The thermal behavior of these two clusters is also compared to that of icosahedral (totally ordered) sodium clusters of the same sizes. Received 5 February 2001 and Received in final form 21 May 2001     

Formation of argon cluster with proton seeding

ABSTRACT

We employ force-field molecular dynamics simulations to investigate the kinetics of nucleation to new liquid or solid phases in a dense gas of particles, seeded with ions. We use precise atomic pair interactions, with physically correct long-range behaviour, between argon atoms and protons. Time dependence of molecular cluster formation is analysed at different proton concentration, temperature and argon gas density. The modified phase transitions with proton seeding of the argon gas are identified and analysed. The seeding of the gas enhances the formation of nano-size atomic clusters and their aggregation. The strong attraction between protons and bath gas atoms stabilises large nano-clusters and the critical temperature for evaporation. An analytical model is proposed to describe the stability of argon-proton droplets and is compared with the molecular dynamics simulations.     

Cluster formulae for metallic glasses derived from devitrification phases

Bulk metallic glasses are known to have a composition formula [cluster](glue atom)_1,3 within the framework of the cluster-plus-glue-atom model. The key issue in applying the cluster formula is the determination of the right clusters and glue atoms. As examples, alloy phases in the glass-forming systems Al–Ni–Zr and B–Co–Si are analysed from the viewpoint of nearest coordination polyhedral clusters. These alloy phases are described by [effective cluster](glue atom) _x , where the effective cluster refers to true cluster composition after taking account of cluster-sharing in the phase structure. For each alloy phase, a principal cluster can be identified that features the local short-range order of that phase. It is pointed out that the principal clusters can express compositions with high glass-forming abilities, as verified by our experiments in Al–Ni–Zr and B–Co–Si–Ta.     

Packing efficiency of coordination polyhedra

Close-packing of hard spheres is a general structural stability criterion for alloy phases. This paper aims at explaining the close-packing geometries of coordination polyhedral clusters composed of non-equivalent spheres. The shell atoms are separately treated according to their site symmetries. Ideal close-packing curves as functions of atomic radius ratios between the central atom and the shell atoms have to be introduced to evaluate the packing efficiencies of the clusters with coordination numbers CN = 9–11, where the shell atoms are located at least at two non-equivalent sites. Close-packing features of clusters involved in amorphous alloys are confirmed.     

面心立方固溶体合金的团簇加连接原子几何模型及典型工业合金成分解析

工业合金牌号的成分选择体现了固溶体合金的化学短程有序结构,满足由最近邻两层原子组成的"团簇加连接原子"模型,例如对于置换型面心立方固溶体Cu-Zn,其合金牌号成分可以表述为[Zn-Cu_(12)]Zn_(1-6)和[Zn-Cu_(12)](Cu,Zn)_6,其中方括号内为第一近邻配位多面体团簇.基于此,本文赋予团簇式以具体原子结构的含义,对置换型面心立方固溶体结构中团簇的可能存在形式进行了穷尽,得出团簇式所对应的所有"团簇加连接原子"结构单元模型,给出团簇和连接原子之间的比例和空间排列的所有可能,并对Cu-Zn和Cu-Ni合金常用牌号对应的团簇式给出了三维结构模型,进一步验证了前期关于合金团簇式解析的正确性.用这些模型中原子化学序描述合金的成分,赋予团簇式以具体的原子结构意义,为进一步开发新的合金提供了理论依据.     

Magic clusters Na57 - and Na59 +

The electronic structure of positively charged sodium cluster with 59 atoms and negatively charged sodium cluster with 57 atoms (both having 58 electrons) have been studied using an ab initio molecular dynamics method. Although the geometries of both clusters are nearly spherical, the results show differences in the electronic shells.     

Different magic number behaviors in supported metal clusters

Two different magic number behaviors in supported metal clusters, which contain several to hundreds of atoms, are revealed on a series of fcc(001) metal surfaces based on the calculations with the tight-binding potential. The magic number sequence persists on some surfaces while gradually disappears on the others with the increasing cluster size. A theory is proposed to explain these behaviors in terms of atomic interactions. We find in surprise that the different magic number behaviors are triggered by the relatively weak adatom–adatom interactions between next nearest-neighbor (NNN) atoms, although the closed shell of the magic cluster is enhanced by nearest-neighbor interactions. For an attractive NNN interaction, the closed shell of the magic cluster is gradually destabilized and eventually broken, leading to the disappearance of the magic number sequence with increasing cluster size. For a repulsive one, the closed shell and magic number sequence persists. Besides, our theory also allows a good understanding of the equilibrium shape of Cu islands on the Cu(001) surface.