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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
${\rm{Ta}}_4{\rm{C}}_n^{-/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${\rm{Ta}}_4{\rm{C}}_n^{-} $ ( 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Ta 4C n( 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,${\rm{Ta}}_4^{-/0} $ 团簇为四面体结构,${\rm{Ta}}_4{\rm{C}}_1^{-/0} $ 团簇中碳原子覆盖在Ta 4四面体的一个面上方,${\rm{Ta}}_4{\rm{C}}_2^{-/0} $ 团簇则是两个碳原子分别覆盖在Ta 4四面体中的两个面上方.${\rm{Ta}}_4{\rm{C}}_3^{-/0} $ 团簇是一个缺角立方体结构.${\rm{Ta}}_4{\rm{C}}_4^{-/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${\rm{Ta}}_4{\rm{C}}_3^{-} $ 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${\rm{Ta}}_4{\rm{C}}_3^{-} $ 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,${\rm{Ta}}_4{\rm{C}}_n^{-/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${\rm{Ta}}_{4+n}^{-/0} $ ( n= 0—4)团簇. 中性Ta 4C 4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${\rm{Ta}}_4{\rm{C}}_n^{-/0} $ ( n= 0–4) clusters are investigated by combining anion photoelectron spectroscopy with theoretical calculations. The vertical detachment energy values of${\rm{Ta}}_4{\rm{C}}_n^{-} $ ( 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 Ta 4C n( 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${\rm{Ta}}_4^- $ cluster is a tetrahedron, and the most stable structure of${\rm{Ta}}_4{\rm{C}}_1^{-} $ has a carbon atom capping one face of the${\rm{Ta}}_4^- $ tetrahedron, while in the ground state structure of${\rm{Ta}}_4{\rm{C}}_2^{-} $ cluster, two carbon atoms cap two faces of the${\rm{Ta}}_4^- $ tetrahedron, respectively. The lowest-lying isomer of${\rm{Ta}}_4{\rm{C}}_3^{-} $ cluster holds a cube-cutting-angle structure. The ground state structure of${\rm{Ta}}_4{\rm{C}}_4^{-} $ is a 2 × 2 × 2 cube. The neutral Ta 4C n( n= 0–4) clusters have similar structures to their anionic counterparts and the neutral Ta 4C 4cluster can be considered as the smallest cell for α-TaC face-centered cube crystal. The analyses of molecular orbitals reveal that the SOMO of${\rm{Ta}}_4{\rm{C}}_3^{-} $ 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${\rm{Ta}}_4{\rm{C}}_n^{-/0} $ ( n= 0–4) clusters. The per-atom binding energy values of${\rm{Ta}}_4{\rm{C}}_n^{-/0} $ ( n= 0–4) clusters are higher than those of${\rm{Ta}}_{4+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 Ta 4C 4is about 7.13 eV, which is quite high, which may contribute to the high melting point of α-TaC as an ultra-high temperature ceramic material.-
Keywords:
- photoelectron spectroscopy/
- density functional theory/
- ${\rm{Ta}}_4{\rm{C}}_n^{-/0} $ clusters
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异构体 电子态 对称点群 ∆E/eV VDE/eV ADE/eV 理论值 实验值 理论值 实验值 ${\rm{Ta}}_4^{-} $ 0A C2 2B 0 0.94 1.16 0.92 1.10 0B C1 4A 0.30 1.32 1.16 0C D2h 2B2u 0.92 1.59 1.39 ${\rm{Ta}}_4{\rm{C}}_1^{-} $ 1A Cs 2A'' 0 1.23 1.35 1.22 1.31 1B C2v 2B2 0.27 1.07 1.03 1C C2v 2B2 0.46 1.18 0.76 ${\rm{Ta}}_4{\rm{C}}_2^{-} $ 2A Cs 2A'' 0 1.49 1.51 1.34 1.44 2B Cs 2A'' 0.29 1.22 1.18 2C Cs 4A'' 0.30 1.05 1.04 ${\rm{Ta}}_4{\rm{C}}_3^{-} $ 3A C3v 2A1 0 1.17 1.30 1.13 1.21 3B Cs 6A'' 1.03 1.66 1.65 3C C2v 2A1 1.41 1.35 1.29 ${\rm{Ta}}_4{\rm{C}}_4^{-} $ 4A D2d 4B2 0 1.70 1.86 1.69 1.80 4B C1 2A 0.09 1.61 1.39 1.60 1.35 4C D2d 6A2 0.21 1.75 1.74 
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n Eb ${\rm{Ta}}_4{\rm{C}}_n^{-} $ ${\rm{Ta}}_{4+n}^{-} $ Ta4Cn Ta4+n 0 4.40 4.40 4.35 4.35 1 5.10 4.78 5.43 4.65 2 5.90 4.99 6.16 4.93 3 6.56 5.30 6.81 5.22 4 6.98 5.44 7.13 5.37 下载:
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