Novel superhard B–C–O phases predicted from first principles

Abstract

We explored the B–C–O system at pressures in the range 0–50 GPa by ab initio variable-composition evolutionary simulations in the hope of discovering new stable superhard materials. A new tetragonal thermodynamically stable phase B₄CO₄, space group I, and two low-enthalpy metastable compounds (B₆C₂O₅, B₂CO₂) have been discovered. Computed phonons and elastic constants show that these structures are dynamically and mechanically stable both at high pressure and zero pressure. B₄CO₄ is thermodynamically stable at pressures above 23 GPa, but should remain metastable under ambient conditions. Its computed hardness is about 38–41 GPa, which suggests that B₄CO₄ is potentially superhard.