Ⅷ簇元素基混合团簇的结构，稳定性和磁性研究

【作者】 温俊青；

【导师】 姜振益；

【作者基本信息】 西北大学 ， 理论物理， 2011， 博士

【摘要】 纳米团簇已经成为研制具有特殊性能的新型功能纳米材料的基础,金属团簇由于其在现代工业生产中的实用价值,越来越受到研究者的普遍关注,尤其是过渡金属团簇。对它们的结构和性质的研究是开发高新技术材料的基础。本论文主要采用基于第一性原理的密度泛函理论系统研究了纯Nin、Pdn、Ptn(n=1-9)团簇的结构、电子性质及磁性。并研究了Al元素掺杂小的Nin、Pdn、Ptn团簇的结构和磁性等性质。主要内容如下：1.第Ⅷ族Ni、Pd、Pt团簇的结构演化和磁性本章中我们运用密度泛函理论(DFT)方法系统研究了Nin、Pdn、Ptn小团簇的结构演化规律、稳定性和磁性。结果表明：通过在Nin-1、Pdn-1、Ptn-1团簇的稳定结构上增加相应的原子可以方便快捷地找到Nin、Pdn、Ptn团簇的一系列稳定结构。小的Ni-Pd-Pt系团簇的稳定结构具有相似的演化规律。n=2-6时,团簇的较稳定构型都是在三角和四角的基础上增加原子得到,且不能显著改变原有团簇的构型。n>6以后团簇的结构都是在八面体和十面体结构的基础上增加原子得到的,并逐渐具有笼的构型。Ni-Pd-Pt系团簇的平均键长和平均配位数都随团簇尺寸的增加而增加,表明团簇的结构是在键长和配位数共同作用下达到平衡的结果。随着团簇尺寸的增大Ni团簇的平均键长最小,而Pd和Pt团簇的平均键长较接近,除了n=3,4时Pd团簇的平均键长比Pt略小外,其余尺寸均大于Pt团簇。Ni-Pd-Pt系团簇的平均结合能都随团簇尺寸的增大而增大,在n=2-9的尺寸内,Pd团簇的平均结合能最小,Ni团簇的平均结合能比Pd团簇的略大,而Pt团簇具有较大的结合能。Ni-Pd-Pt系团簇的能量二阶差分、分裂能、HOMO-LUMO能隙随团簇尺寸的演化都没有表现出明显的奇偶振荡行为,但Nin团簇在n=5、7时有较大的值,说明相对应的团簇具有较高的稳定性、较低的化学活性。Pdn团簇的能量二阶差分、分裂能在n=4时有较大的值,说明相对应的团簇具有较高的稳定性。Ptn团簇的能量二阶差分、分裂能、HOMO-LUMO能隙和费米能级在n=2、5、8时均有较大的值,说明相对应的团簇具有较高的稳定性、较低的化学活性和较为封闭的电子壳层结构。对团簇磁性的研究表明随着团簇尺寸的增大,Ni和Pt团簇的平均每原子磁矩都大于Pd团簇的,Ni团簇的比Pt团簇的略大,平均每原子磁矩都随团簇尺寸的增大有减小的趋势,但Pd和Pt团簇减小较快。2.A1元素掺杂小的Nin、Pdn、Ptn团簇的结构和磁性本部分主要运用密度泛函理论(DFT)方法系统研究了A1元素掺杂第Ⅷ族基混合团簇NinAl、PdnAl、PtnAl(n=1-8)及其阳离子团簇的结构、稳定性、电子性质和磁性。结果表明：通过用Al原子替换Nin+1、Pdn+1、Ptn+1团簇的稳定结构中不同位置上的Ni、Pd、Pt原子可以方便快捷地找到相应掺杂团簇的稳定结构。除n=6外,Ni,,A1团簇的最稳定几何结构都具有与纯Nin+1团簇基态结构相似的框架。除n=3、6、8外,中性PdnA1团簇和纯Pdn+1团簇具有完全相同的基态构型。除n=2和8外,Pt,,A1团簇也得到了和Ptn+1团簇相同的基态构型。表明用一个Al原子置换第Ⅷ族基团簇相应原子后,对原有团簇构型的影响较小。得到的基态结构在n=3时都从平面结构转变为三维结构,且n较小时团簇的结构都是在三角和四角的基础上增加原子得到,随着团簇的增大,n>6以后团簇的结构都是在八面体和十面体的基础上增加原子得到的。阳离子NiAl+、PdnAl+、PtnA1＋(n=1－8)团簇的稳定结构均保持了中性团簇的框架,仅个别团簇的能量排序发生了变化,表明正电荷对这一簇团簇的结构影响较小。对团簇键长和配位数的分析表明团簇稳定结构的形成是强的Ni-Ni、Pd-Pd、Pt-Pt及Ni-Al、Pd-Al、Pt-Al键共同作用达到平衡的结果。强的Ni-Ni、Pd-Pd、Pt-Pt键决定了NinAl、PdnAl、PtnAl团簇的Ni、Pd、Pt框架结构,强的Ni-Al、Pd-Al、Pt-Al键使A1原子处于一个高的配位点。对混合团簇平均结合能的分析表明A1原子的加入增强了Ni和Pd团簇的稳定性,但降低了Pt团簇的稳定性,总体上没有改变Ni-Pd-Pt系团簇结合能的变化趋势。对团簇稳定性的分析表明Ni5Al和Ni7Al团簇,及阳离子Ni4Al+、Ni5Al+、Ni6Al+团簇的稳定性较其它团簇高,与Nin团簇的幻数为5和7不同。对PdnAl及其阳离子团簇稳定性的分析表明：Pd3Al及Pd3Al+团簇具有较高的稳定性,没有改变Pd4团簇的幻数特性。PdnAl及其阳离子团簇的能隙和电离势的分析也表明对于具有幻数特征的团簇Pd3Al及Pd3Al+团簇,具有较高的能隙,较高的稳定性。对A1元素掺杂Pt基混合团簇的稳定性的研究表明：Pt2Al、Pt4Al和Pt6Al团簇及阳离子Pt3Al+团簇具有较高的稳定性，对能隙及电离势的分析也表明Pt4Al和Pt3Al+团簇具有较高的稳定性。Pt4Al团簇具有较高的稳定性,也没有改变Pt5团簇的幻数特性。对混合团簇磁性的分析表明单个Al原子的加入对Nin、Pdn、Ptn团簇的平均每原子磁矩随尺寸的变化趋势没有影响,但总体上降低了团簇的平均磁矩,表明A1原子有电荷转移到相应Ni、Pd、Pt原子上。更多还原

【Abstract】 Nanocluster have became the foundation of developing new nanometer materials, while metal clusters are becoming a matter of concern for more and more people because the practical use in modern industry, especially for transition metal cluster. The base of high and new technological material is study for their structures and properties. In this paper we mainly perform an extensive study the structures and electronic properties as well as magnetism for Nin、Pdn、Ptn、clusters up to n=9, the neutral and cationic clusters of Al-doped Nin、Pdn、Ptnup to n=8 by using a density-functional-theory(DFT).1.Theoretical study of geometrical evolution and magnetism of group-ⅧclustersIn this chapter, we take up an extensive study on the evolution of structures, electronic properties and magnetism for Nin, Pdn, Ptn, clusters up to n=9 by using a density-functional-theory(DFT). The calculated results show that we can quickly find a series of stable structures of Nin, Pdn, Ptn clusters by adding corresponding atom on the stable structures of Nin-1, Pdn-1, Ptn-1 clusters. Nin、Pdn、Ptn clusters have the same evolution of structure.We can get the stable structures of clusters by adding atoms on triangle and tetragon when n=2-6, the stable structures of clusters can be found by adding an atom on octahedron and decahedron when n>6, and the structures have gradually the cage configuration with the increase of size.The average bond length and coordination number increase with the increase of size, this indicate that the stable structures of clusters get balance with the working together of bond length and coordination number. The average bond length of Ni is the smallest, and Pd and Pt are near with the increase of size, and the average bong length of Pd is greater than Pt in addition to n=3,4.The binding energies of Ni-Pd-Pt clusters increase with the increase of size. The binding energies of Pd cluster are lowest, Pt cluster is maximal, and Ni cluster is slightly larger than Pd. The second difference in energy, the first derivative of the energy and HOMO-LUMO gap of Ni-Pd-Pt clusters don’t show obvious parity oscillatory behavior with the increase of size. The stabilization energies reveal that Ni5 and Ni7, Pd2 and Pd4, Pt2,Pt5 and Pt8 are the relatively most stable in this series.The study of magnetism for Nin, Pdn and Ptn clusters indicates that the average magnetic moment/atom of Nin and Ptn are larger than Pdn, but that of Nin, is more than slightly larger than Ptn. Magnetic moment/atom of Ni-Pd-Pt clusters has been trending down with the increase of size, but that of Pdn and Ptn clusters reduces quickly.2.Structures, stabilities and magnetism of Al-doped Nin, Pdn, Ptn, clustersThe structural evolution, stabilities, electronic properties and magnetism of small neutral and cationic NinAl, PdnAl, PtnAl (n=1-8) clusters are investigated using DFT. Our study results predict the existence of a number of previously unknown isomers and structures. All structures may be derived from a substitution of a Ni (or Pd or Pt) atom by an Al atom in the stable structures Nin+1(Pdn+1 or Ptn+1) cluster. The most stable structures of NinAl cluster have the similar configurations with the ground state structures of Nin+1 cluster besides n=6. The most stable structures of the neutral PdnAl cluster have the same framework with the minimal-structures of Pdn+1 cluster besides n=3,6,8 and PtnAl cluster have the same structure with Ptn+1 cluster besides n=2,8. Compared with the neutral cluster, most of the corresponding structures of cationic clusters keep basically similar frameworks with the neutral clusters, while a few of the order of clusters energies has been changed. The findings indicate that the effect of the positive charge on NinAl, PdnAl, PtnAl are small.The study for bond length and coordination number of the impure clusters indicate that the stable structures of clusters get balance with the working together of Ni-Ni, Pd-Pd, Pt-Pt and Ni-Al, Pd-Al, Pt-Al bonds. The strong Ni-Ni, Pd-Pd, Pt-Pt bonds decide the framework of Ni, Pd, Pt, and the strong Ni-Al, Pd-Al, Pt-Al bonds make the Al atom in the high coordination number.The analysis for atomic averaged binding energies demonstrates that joining Al atom don’t change the evolution trend of averaged binding energies of Ni-Pd-Pt clusters, which indicate that the stabilities of the neutral and cationic NinAl, PdnAl, clusters have been proved simultaneously as compared with the corresponding pure clusters, but the stabilities have been reduced for the neutral and cationic PtnAl clusters. The stabilities of neutral Ni5Al, Ni7Al and cationic Ni4Al+, Ni5Al+,Ni6Al+ clusters are higher than other clusters, which are different from the magic numbers 5 and 7 of pure Nin cluster. Pd3Al and Pd3Al clusters have more stabilities than other clusters, which are same with the magic cluster Pd4. The study for the Al-doped Pt clusters indicate that the stabilities of Pt2Al, Pt4Al, Pt6Al and Pt3Al+ are higher than other clusters, and the higher stability cluster Pt4Al don’t change the magic number properties of Pt5.The study for magnetic properties of the impure clusters indicates that the Al-doped Nin, Pdn, Ptn clusters don’t change the evolution trend of atomic magnetic moment of pure clusters, but decrease the magnetic moment of clusters which originate the effective charge transferring from Al to Ni(Pd or Pt) atoms.更多还原