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></inline-formula> (<i>n</i> = 0–4) clusters: Anion photoelectron spectroscopy and theoretical calculations

The electronic structures, chemical bonds and stabilities of <inline-formula><tex-math id="Z-20210109163924">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-/0} $\end{document}</tex-math></inline-formula> (<i>n</i> = 0–4) clusters are investigated by combining anion photoelectron spectroscopy with theoretical calculations. The vertical detachment energy values of <inline-formula><tex-math id="Z-20210109163944">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-} $\end{document}</tex-math></inline-formula> (<i>n</i> = 0–4) anions are measured to be (1.16 ± 0.08), (1.35 ± 0.08), (1.51 ± 0.08), (1.30 ± 0.08), and (1.86 ± 0.08) eV, and the electron affinities of neutral Ta₄C<i>_n</i> (<i>n</i> = 0–4) are estimated to be (1.10 ± 0.08), (1.31 ± 0.08), (1.44 ± 0.08), (1.21 ± 0.08), and (1.80 ± 0.08) eV, respectively. It is found that the geometry structure of <inline-formula><tex-math id="Z-20210109164317">\begin{document}${\rm{Ta}}_4^- $\end{document}</tex-math></inline-formula>cluster is a tetrahedron, and the most stable structure of <inline-formula><tex-math id="Z-20210109164421">\begin{document}${\rm{Ta}}_4{\rm{C}}_1^{-} $\end{document}</tex-math></inline-formula> has a carbon atom capping one face of the <inline-formula><tex-math id="Z-20210122122205">\begin{document}${\rm{Ta}}_4^- $\end{document}</tex-math></inline-formula> tetrahedron, while in the ground state structure of <inline-formula><tex-math id="Z-20210109164437">\begin{document}${\rm{Ta}}_4{\rm{C}}_2^{-} $\end{document}</tex-math></inline-formula> cluster, two carbon atoms cap two faces of the<inline-formula><tex-math id="Z-20210109164454">\begin{document}${\rm{Ta}}_4^- $\end{document}</tex-math></inline-formula> tetrahedron, respectively. The lowest-lying isomer of <inline-formula><tex-math id="Z-20210109164509">\begin{document}${\rm{Ta}}_4{\rm{C}}_3^{-} $\end{document}</tex-math></inline-formula> cluster holds a cube-cutting-angle structure. The ground state structure of <inline-formula><tex-math id="Z-20210109164522">\begin{document}${\rm{Ta}}_4{\rm{C}}_4^{-} $\end{document}</tex-math></inline-formula> is a 2 × 2 × 2 cube. The neutral Ta₄C<i>_n</i> (<i>n</i> = 0–4) clusters have similar structures to their anionic counterparts and the neutral Ta₄C₄ cluster can be considered as the smallest cell for <i>α</i>-TaC face-centered cube crystal. The analyses of molecular orbitals reveal that the SOMO of <inline-formula><tex-math id="Z-20210109164537">\begin{document}${\rm{Ta}}_4{\rm{C}}_3^{-} $\end{document}</tex-math></inline-formula> is mainly localized on one tantalum atom, inducing a low VDE. Our results show that the Ta-Ta metal bonds are replaced by Ta-C covalent bonds gradually as the number of carbon atoms increases in <inline-formula><tex-math id="Z-20210109164558">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-/0} $\end{document}</tex-math></inline-formula> (<i>n</i> = 0–4) clusters. The per-atom binding energy values of <inline-formula><tex-math id="Z-20210109164613">\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-/0} $\end{document}</tex-math></inline-formula> (<i>n</i> = 0–4) clusters are higher than those of <inline-formula><tex-math id="Z-20210109164633">\begin{document}${\rm{Ta}}_{4+n}^{-/0} $\end{document}</tex-math></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 Ta₄C₄ 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.