Ta4Cn^-/0(n=0-4)团簇的电子结构、成键性质及稳定性

Electronic structures,chemical bonds,and stabilities of Ta4Cn^-/0(n=0-4)clusters:Anion photoelectron spectroscopy and theoretical calculations

本文采用尺寸选择的负离子光电子能谱技术,结合密度泛函理论,对Ta4Cn^-/0(n=0-4)团簇电子结构、成键性质以及稳定性进行了研究.实验测得Ta4Cn-(n=0—4)团簇负离子基态结构的垂直脱附能分别为(1.16±0.08),(1.35±0.08),(1.51±0.08),(1.30±0.08)和(1.86±0.08)eV.中性Ta4Cn(n=0—4)团簇的电子亲和能分别为(1.10±0.08),(1.31±0.08),(1.44±0.08),(1.21±0.08)和(1.80±0.08)eV.研究发现Ta4^-/0团簇为四面体结构,Ta4C1-/0团簇中碳原子覆盖在Ta4四面体的一个面上方,Ta4C2^-/0团簇则是两个碳原子分别覆盖在Ta4四面体中的两个面上方.Ta4C3^-/0团簇是一个缺角立方体结构.Ta4C4^-/0团簇则是近似立方体结构,可以看成是α-TaC面心立方晶体的最小晶胞单元.分子轨道分析结果显示Ta4C3团簇的单电子最高占据轨道主要布居在单个钽原子周围,导致Ta4C3^-团簇的垂直脱附能明显低于其相邻团簇.理论研究显示随着碳原子数目的增加,Ta4Cn^-/0(n=0—4)团簇中的钽-钽金属键逐渐被钽-碳共价键取代,单原子结合能逐渐增加且明显高于Ta4+n^-/0(n=0-4)团簇.中性Ta4C4的单原子结合能高达7.13 eV,这说明钽-碳共价键的形成有利于提高材料的熔点,这与碳化钽作为高温陶瓷材料的特性密切相关.

The electronic structures,chemical bonds and stabilities of Ta4 Cn^-/0(n=0-4)clusters are investigated by combining anion photoelectron spectroscopy with theoretical calculations.The vertical detachment energy values of Ta4 Cn-(n=0-4)anions are measured to be(1.16±0.08),(1.35±0.08),(1.51±0.08),(1.30±0.08),and(1.86±0.08)eV,and the electron affinities of neutral Ta4 Cn(n=0-4)are estimated to be(1.10±0.08),(1.31±0.08),(1.44±0.08),(1.21±0.08),and(1.80±0.08)eV,respectively.It is found that the geometry structure of Ta4-cluster is a tetrahedron,and the most stable structure of Ta4 C1-has a carbon atom capping one face of the Taf tetrahedron,while in the ground state structure of Ta4 C2-cluster,two carbon atoms cap two faces of theTa4-tetrahedron,respectively.The lowest-lying isomer of Ta4 C3-cluster holds a cube-cutting-angle structure.The ground state structure of Ta4 C4-is a 2×2×2 cube.The neutral Ta4 Cn(n=0-4)clusters have similar structures to their anionic counterparts and the neutral Ta4 C4 cluster can be considered as the smallest cell forα-TaC face-centered cube crystal.The analyses of molecular orbitals reveal that the SOMO of Ta4 C3-is mainly localized on one tantalum atom,inducing a low VDE.Our results show that the Ta-Ta metal bonds are replaced by Ta-C covalent bonds gradually as the number of carbon atoms increases in Ta4 Cn^-/0(n=0-4)clusters.The per-atom binding energy values of Ta4 Cn^-/0(n=0-4)clusters are higher than those of Ta4+n^-/0(n=0-4)clusters,indicating that the formation of Ta-C covalent bonds may raise the melting point.The per-atom binding energy of neutral Ta4 C4 is about 7.13 eV,which is quite high,which may contribute to the high melting point ofα-TaC as an ultra-high temperature ceramic material.