分子动力学模拟组分对Co1415-xAlx团簇结构特征的影响

本文采用分子动力学模拟方法结合镶嵌原子势,研究了在200 K时二元（CoAl）1415团簇的结构随Co原子浓度的变化情况。利用径向分布函数、对分析技术和键取向序参数方法研究了微观局部结构情况,研究结果表明： （CoAl）1415团簇的组分对最终冷却结构影响较大,Co原子浓度为100%~70%的团簇表现出不完全的六角密排结构特征;Co原子浓度为50%的团簇具有局部的体心立方体结构特征;Co原子浓度为30%~10%时,表现出部分区域的二十面体和缺陷二十面体结构特征。     

大尺寸核/层Co-Pd合金团簇稳定结构研究

纳米尺度合金团簇具有特殊的光学、电、磁和催化等性质,在基础研究和应用领域吸引了广泛的兴趣.使用多体Gupta势函数描述Co-Pd团簇原子间相互作用,应用基于内核构建的自适应免疫优化算法确定最稳定结构.研究结果显示:98原子Co-Pd团簇结构可分为面心立方结构、Mackay二十面体、双二十面体、由双二十面体面面相连构成的结构和十面体结构.序列参数显示Co原子位于内层,而Pd原子位于外层.原子半径和表面能进一步解释了Co和Pd原子的分布规律.原子数目为147的Co-Pd团簇均为完整二十面体结构.     

Cu-Co双金属团簇结构演化及其性质的分子动力学模拟

采用分子动力学结合嵌入原子方法对比研究了Co分布于Cu-Co团簇不同层的结构和性质.研究表明:Co原子分层掺杂可对团簇的结构转变点和熔点进行诱导控制;分层掺杂的Cu-Co团簇第一相变是一种扩散度较小的由立方八面体转变为二十面体的相变;Co原子易于向低能态团簇的亚表层(111)面偏析,从而诱导团簇结构紊乱,造成其熔点差异.     

Al12N、Al12B团簇结构和电子性质的密度泛函计算

利用密度泛函理论研究了Al12N和Al12B团簇的原子结构和电子性质,通过各种异构体的比较,发现两种掺杂团簇的最低能量结构都是完好的二十面体(Ih)结构,N(B)原子占据在二十面体的中心.高对称性团簇形成稀疏离散的电子态密度和大的电子能隙.在Al-N之间发生较大的电荷转移.因此我们建议把Al12N团簇看作是碱金属超原子,Al12B团簇看作是卤素超原子,用来构造团簇组装固体.     

Cu-Co双金属团簇结构演化及其性质的分子动力学模拟

采用分子动力学结合嵌入原子方法对比研究了Co分布于Cu-Co团簇不同层的结构和性质. 研究表明：Co原子分层掺杂可对团簇的结构转变点和熔点进行诱导控制;分层掺杂的Cu-Co团簇第一相变是一种扩散度较小的由立方八面体转变为二十面体的相变;Co原子易于向低能态团簇的亚表层（111）面偏析, 从而诱导团簇结构紊乱, 造成其熔点差异.     

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.     

Fe、Co、Ni混合团簇磁性的密度泛函研究

基于13原子二十面体结构,采用密度泛函方法系统计算研究了Fe、Co及Ni单质及二元混合团簇的磁性.发现有限温度下团簇磁性随结构畸变的敏感性随Fe、Co、Ni顺序逐渐减弱,同时发现二十面体结构Fe_(13)及Co_(13)均具有不同磁矩的近简并低能态.对FeNi及CoNi混合团簇、其磁矩随组分的变化不存在反常现象,但对于FeCo混合团簇、其磁矩随组分的演化行为存在个别反常现象.我们认为:这种反常现象能够对FeCo非晶合金中的实验观测结果提供一种可能的理论解释.     

利用Gupta势结合遗传算法研究ConCu55－n(n=0—55)混合团簇的结构演化及基态能量

采用半经验的Gupta多体势结合遗传算法对Co_nCu_55－n(n=0—55)混合团簇的基态结构和能量进行了研究,发现这些混合团簇的基态结构是在Co₅₅,Cu₅₅单质团簇(Mackay二十面体)的基础之上发生的畸变;从n=0(Cu₅₅)开始,Co原子从中心到表面,从棱到顶点依次、连续替换Cu原子;基态结构与键能较大键的数目及其平均键长有关;Co₁₃Cu₄₂具有最稳定的结构,13个Co原子全部位于团簇内部形成Mackay二十面体对整个团簇的稳定性有显著影响. 关键词： 团簇 结构和能量 Gupta势 遗传算法     

Co-Ag与Co-Au合金团簇核层结构模拟

钴基二元合金团簇具有特殊催化和电磁性能已成为研究的热点.探究Co-Ag与Co-Au团簇稳定结构是研究其性质的重要步骤.运用基于内核构建的自适应免疫优化算法对大尺寸Co_(55)Ag_n、Co_(55)Au_n(n=1-55, 60, 70, 80, 92)团簇结构进行优化.基于紧束缚势二阶矩近似的多体Gupta势函数被用于描述合金团簇原子间相互作用.计算结果显示Co_(55)Ag_n、Co_(55)Au_n(n=1-55, 60)最稳定结构的主要构型为Mackay二十面体.原子分布分析显示Co原子位于内层,而Ag、Au原子均趋于分布在外层,呈现出核层状态.对于大尺寸Co_(55)Ag_n、Co_(55)Au_n(n=80, 92)团簇,Co-Ag团簇采取无定形结构,而Co-Au团簇为规则的二十面体结构.     

Fe-Ni混合团簇基态结构的遗传算法研究

采用半经验的Gupta多体势结合遗传算法对不同组分的FenNim(N=n+m,N=13,38)混合团簇的基态结构和性质进行了模拟研究.结果显示:总原子数为13的混合团簇基态几何构型是基于单质团簇的基态二十面体结构,并且随着Fe原子数的增加表现出Fe原子首先占据中心位置的规律性;对总原子数为38的混合团簇,在轻混合(类掺杂)情形(n≤15,34≤n≤38)下基态几何构型为类似于纯单质团簇基态的截角八面体结构,而在重混合(n=16～33)时基态几何结构表现为不同于单质团簇的类截角二十面体构型;分析二级差分能表明Fe1Ni12、Fe7Ni31及Fe14Ni24具有相对高的稳定性,我们提出了基于有效键数的简化模型以解释此幻数结构序列.     

Hcp-icosahedron structural transformation induced by the change in Ag concentration during the freezing of （CoAg）₅₆₁ clusters

The synergy effect of alloy elements in bimetallic clusters can be used to tune the chemical and physical properties. Research on the influences of alloy concentration and distribution on the frozen structure of bimetallic clusters plays a key role in exploring new structural materials. In this paper, we study the influence of Ag concentration on the frozen structure of the (AgCo) 561 cluster by using molecular dynamics simulation with a general embedded atom method. The results indicate that the structure and chemical ordering of the (AgCo) 561 cluster are strongly related to Ag concentration. Hcp-icosahedron structural transformation in the frozen (CoAg) 561 cluster can be induced by changing Ag concentration. The chemical ordering also transforms to Janus-like Co–Ag from core-shell Co–Ag.     

Hcp-icosahedron structural transformation induced by the change in Ag concentration during the freezing of （CoAg）561 clusters

The synergy effect of alloy elements in bimetallic clusters can be used to tune the chemical and physical properties. Research on the influences of alloy concentration and distribution on the frozen structure of bimetallic clusters plays a key rolc in exploring new structural materials. In this paper, we study the influence of Ag concentration on the frozen structure of the （AgCo）561 cluster by using molecular dynamics simulation with a general embedded atom method. The results indicate that tt~e structure and chemical ordering of the （AgCo）561 cluster are strongly related to Ag concentration. Hcp-icosahedron structural transformation in the frozen （CoAg）561 cluster can be induced by changing Ag concentration. The chemical ordering also transforms to Janus-like Co Ag from core-shell Co-Ag.     

First-principles-based surface phase diagram of fully relaxed binary alloy surfaces

The combination of density-functional theory (DFT) calculations of geometrically fully relaxed binary alloy surfaces with concepts from statistical physics is applied to construct a DFT-based phase diagram for a binary alloy surface. As a first example, we studied the appearance of Co antisite atoms at CoAl(100) surfaces. The structural parameters as multilayer relaxations, surface buckling, lateral order, and segregation profile of the predicted stable surface phases are in excellent agreement with experimental structure determinations applying low-energy electron diffraction.     

Structures of CoAl(1 1 1) surface: A first principles study

The structures of the CoAl(1 1 1) surface are studied by first principles calculations. Our calculations show that the surface layer is always occupied by pure Al for all concentrations studied here, which indicates the dominant role of the Al segregation tendency. This is different from the CoAl(0 0 1) surface, where a number of Co anti-sites are found on the top most layer. The calculated surface phase diagram of ground states shows that there are three stable structures. The diffusion barriers of the metastable structure evolving to the stable structure are also calculated. The high diffusion barrier can explain the appearance of metastable structures at low temperature in experiment.     

CoAlN(N=2-11)团簇基态结构的稳定性和磁性

采用密度泛函理论中的广义梯度近似(GGA),对CoAlN(N=2-11)团簇进行构型优化和磁性计算.在考虑自旋多重度的情况下得到了团簇的平衡构型及基态结构,并重点讨论了Co原子的sp-d杂化效应对体系稳定性和磁性的影响.结果表明:N为偶数时,团簇的基态为自旋双重态,Co原子磁矩随N增加整体上呈减小趋势;N为奇数时(除N=5为自旋三重态外),团簇的基态为自旋单重态,Co原子磁矩为零.N≥8时,Co原子陷入主团簇内部,体系的对称性降低,稳定性增强;轨道杂化增强了体系的稳定性,但减小了双重态和三重态体系中Co原子的磁矩;N=3,8,10是团簇CoAlN(N=2.11)的幻数.     

Luminescence studies of Co implanted sapphires

Sapphire single crystals implanted with different fluences of cobalt and subjected to annealing treatments in vacuum and in air during 1 v h at 1000 v C were studied by optical and structural techniques. At room temperature the absorption mainly occurs in the ultraviolet (UV) being the luminescence dominated by a UV/blue broad emission band and a highly structured red emission. Structural analysis indicates that metallic precipitates, CoAl 2 O 4 and CoO phases are present in the implanted and annealed samples depending either on the fluence and on the annealing atmosphere. The observed luminescence cannot be correlated with intra-ionic Co 2+ and Co 3+ in an Al 3+ site in the sapphire lattice and a tentative assignment of the observed emission lines to the presence of aggregates is made.     

Transition from icosahedral to decahedral structure in a coexisting solid-liquid nickel cluster

We have used molecular dynamics simulations to construct a microcanonical caloric curve for a 1415 atom Ni icosahedron. Prior to melting, the Ni cluster exhibits static solid-liquid phase coexistence. Initially, a partial icosahedral structure coexists with a partially wetting melt. However, at energies very close to the melting point the icosahedral structure is replaced by a truncated decahedral structure that is almost fully wet by the melt. This structure remains until the cluster fully melts. The transition appears to be driven by a preference for the melt to wet the decahedral structure.     

Size and composition dependence of the frozen structures in Co-based bimetallic clusters

This Letter studies the size-dependent freezing of Co, Co-Ni, and Co-Cu clusters by using molecular dynamics with embedded atom method. Size effect occurs in these three types of clusters. The clusters with large sizes always freeze to form their bulk-like structures. However, the frozen structures for small sizes are generally related to their compositions. The icosahedral clusters are formed for Co clusters (for ?3.2 nm diameter) and also for Co-Ni clusters but at a larger size range (for ?4.08 nm). Upon the Co-Cu clusters, decahedral structure is obtained for small size (for 2.47 nm). The released energy induced the structural transformation plays a key role in the frozen structures. These results indicate that the preformed clusters with special structures can be tuned by controlling their compositions and sizes.     

Co@Au_n(n=1～8)团簇的几何结构和电子性质的理论研究

本文利用密度泛函PW91方法研究了Co@Au_n（n=1～8）团簇的平衡结构、稳定性和磁矩.结构优化显示Co原子在低能异构体中趋于占据最高配位位置,基态Co@Au_n（n=2～6）团簇为二维结构,Co@Au₇和Co@Au8转变为三维结构.原子平均结合能、二阶能量差分及HOMO-LUMO能级间隙分析表明掺杂Co原子提高了金团簇的稳定性,改变了金团簇能级间隙的奇偶振荡性,n=5为掺杂团簇的幻数.磁矩的计算揭示Co@Au_n团簇的磁性主要源于Co原子的3d轨道.