Ultrahard boron-rich tantalum boride: Monoclinic 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 (29GPa). The presence of strong covalent boron–boron bonds and planar six-membered ring boron network in m-TaB4 are crucial for its high hardness.