Electronic structures, chemical bonds, and stabilities of <inline-formula><tex-math id="Z-20210111103614">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-/0} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210111103614.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210111103614.png"/></alternatives></inline-formula> (<i>n</i> = 0–4) clusters: Anion photoelectron spectroscopy and theoretical calculations
Authors: Zhang Chao-Jiang , Xu Hong-Guang , Xu Xi-Ling , Zheng Wei-Jun ,   Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China,   University of Chinese Academy of Sciences, Beijing 100049, China,   Physical Science Laboratory, Huairou National Comprehensive Science Center, Beijing 101400, China
Published: January 2021

The electronic structures, chemical bonds and stabilities of <inline-formula><tex-math id="Z-20210109163924">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-/0} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109163924.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109163924.png"/></alternatives></inline-formula> (<i>n</i> = 0–4) clusters are investigated by combining anion photoelectron spectroscopy with theoretical calculations. The vertical detachment energy values of <inline-formula><tex-math id="Z-20210109163944">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109163944.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109163944.png"/></alternatives></inline-formula> (<i>n</i> = 0–4) anions are measured to be (1.16 ± 0.08), (1.35 ± 0.08), (1.51 ± 0.08), (1.30 ± 0.08), and (1.86 ± 0.08) eV, and the electron affinities of neutral Ta<sub>4</sub>C<i><sub>n</sub></i> (<i>n</i> = 0–4) are estimated to be (1.10 ± 0.08), (1.31 ± 0.08), (1.44 ± 0.08), (1.21 ± 0.08), and (1.80 ± 0.08) eV, respectively. It is found that the geometry structure of <inline-formula><tex-math id="Z-20210109164317">\begin{document}${\rm{Ta}}_4^- $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164317.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164317.png"/></alternatives></inline-formula>cluster is a tetrahedron, and the most stable structure of <inline-formula><tex-math id="Z-20210109164421">\begin{document}${\rm{Ta}}_4{\rm{C}}_1^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164421.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164421.png"/></alternatives></inline-formula> has a carbon atom capping one face of the <inline-formula><tex-math id="Z-20210122122205">\begin{document}${\rm{Ta}}_4^- $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210122122205.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210122122205.png"/></alternatives></inline-formula> tetrahedron, while in the ground state structure of <inline-formula><tex-math id="Z-20210109164437">\begin{document}${\rm{Ta}}_4{\rm{C}}_2^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164437.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164437.png"/></alternatives></inline-formula> cluster, two carbon atoms cap two faces of the<inline-formula><tex-math id="Z-20210109164454">\begin{document}${\rm{Ta}}_4^- $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164454.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164454.png"/></alternatives></inline-formula> tetrahedron, respectively. The lowest-lying isomer of <inline-formula><tex-math id="Z-20210109164509">\begin{document}${\rm{Ta}}_4{\rm{C}}_3^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164509.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164509.png"/></alternatives></inline-formula> cluster holds a cube-cutting-angle structure. The ground state structure of <inline-formula><tex-math id="Z-20210109164522">\begin{document}${\rm{Ta}}_4{\rm{C}}_4^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164522.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164522.png"/></alternatives></inline-formula> is a 2 × 2 × 2 cube. The neutral Ta<sub>4</sub>C<i><sub>n</sub></i> (<i>n</i> = 0–4) clusters have similar structures to their anionic counterparts and the neutral Ta<sub>4</sub>C<sub>4</sub> cluster can be considered as the smallest cell for <i>α</i>-TaC face-centered cube crystal. The analyses of molecular orbitals reveal that the SOMO of <inline-formula><tex-math id="Z-20210109164537">\begin{document}${\rm{Ta}}_4{\rm{C}}_3^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164537.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164537.png"/></alternatives></inline-formula> is mainly localized on one tantalum atom, inducing a low VDE. Our results show that the Ta-Ta metal bonds are replaced by Ta-C covalent bonds gradually as the number of carbon atoms increases in <inline-formula><tex-math id="Z-20210109164558">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-/0} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164558.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164558.png"/></alternatives></inline-formula> (<i>n</i> = 0–4) clusters. The per-atom binding energy values of <inline-formula><tex-math id="Z-20210109164613">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-/0} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164613.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164613.png"/></alternatives></inline-formula> (<i>n</i> = 0–4) clusters are higher than those of <inline-formula><tex-math id="Z-20210109164633">\begin{document}${\rm{Ta}}_{4+n}^{-/0} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164633.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210109164633.png"/></alternatives></inline-formula> (<i>n</i> = 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<sub>4</sub>C<sub>4</sub> is about 7.13 eV, which is quite high, which may contribute to the high melting point of <i>α</i>-TaC as an ultra-high temperature ceramic material.