1 research outputs found
Computational Search for Novel Hard Chromium-Based Materials
Nitrides, carbides,
and borides of transition metals are an attractive
class of hard materials. Our recent preliminary explorations of the
binary chemical compounds indicated that chromium-based materials
are among the hardest transition metal compounds. Motivated by this,
here we explore in detail the binary Cr–B, Cr–C, and
Cr–N systems using global optimization techniques. Calculated
enthalpy of formation and hardness of predicted materials were used
for Pareto optimization to define the hardest materials with the lowest
energy. Our calculations recover all numerous known stable compounds
(except Cr<sub>23</sub>C<sub>6</sub> with its large unit cell) and
discover a novel stable phase <i>Pmn</i>2<sub>1</sub>-Cr<sub>2</sub>C. We resolve the structure of Cr<sub>2</sub>N and find it
to be of anti-CaCl<sub>2</sub> type (space group <i>Pnnm</i>). Many of these phases possess remarkable hardness, but only CrB<sub>4</sub> is superhard (Vickers hardness 48 GPa). Among chromium compounds,
borides generally possess the highest hardnesses and greatest stability.
Under pressure, we predict stabilization of a layered TMDC-like phase
of Cr<sub>2</sub>N, a WC-type phase of CrN, and a new compound CrN<sub>4</sub>. Nitrogen-rich chromium nitride CrN<sub>4</sub> is a high-energy-density
material featuring polymeric nitrogen chains. In the presence of metal
atoms (e.g., Cr), polymerization of nitrogen takes place at much lower
pressures; CrN<sub>4</sub> becomes stable at ∼15 GPa (cf. 110
GPa for synthesis of pure polymeric nitrogen)