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

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Electronic structures, chemical bonds, and stabilities of &lt;inline-formula&gt;&lt;tex-math id="Z-20210111103614"&gt;\begin{document}${\rm{Ta}}_4{\rm{C}}_n^{-/0} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210111103614.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201351_Z-20210111103614.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; (&lt;i&gt;n&lt;/i&gt; = 0–4) …