Ultrahard boron-rich tantalum boride: Monoclinic TaB₄

Highlights: • A novel boron-rich m-TaB₄ with monoclinic symmetry C2/m space group were predicted. • The m-TaB₄ is a superhard material with Vickers hardness of 29 GPa. • Phonon dispersions and elastic constant analysis suggest that m-TaB₄ is stable at ambient condition. • The high hardness of m-TaB₄ is mainly originated from unique boron six-membered ring unit in the structure of m-TaB₄. - Abstract: On the basis of the evolutionary methodology for crystal structure prediction, we have revealed a novel monoclinic C2/m structure TaB₄ with planar six-membered ring boron network along the crystallographic b axis. We name it as m-TaB₄. The calculations of elastic constants and phonon dispersion confirm that m-TaB₄ is mechanical and dynamical stable. Further calculations predicate that this structure is an ultrahard material with a large hardness (29 GPa). The presence of strong covalent boron–boron bonds and planar six-membered ring boron network in m-TaB₄ are crucial for its high hardness