title: Computational Search for Novel Hard Chromium-Based Materials.
abstract: 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 Cr23C6with its large unit cell) and discover a novel stable phase Pmn21-Cr2C. We resolve the structure of Cr2N and find it to be of anti-CaCl2type (space group Pnnm). Many of these phases possess remarkable hardness, but only CrB4is 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 Cr2N, a WC-type phase of CrN, and a new compound CrN4. Nitrogen-rich chromium nitride CrN4is 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; CrN4becomes stable at ∼15 GPa (cf. 110 GPa for synthesis of pure polymeric nitrogen).
authors.affiliation: Skolkovo Institute of Science and Technology , Skolkovo Innovation Center, 3 Nobel Street, Moscow 143026, Russian Federation.|Moscow Institute of Physics and Technology , 9 Institutsky Lane, Dolgoprudny 141700, Russian Federation.
authors.forename: Alexander G
authors.identifier: http://orcid.org/0000-0002-0718-6691
authors.initials: AG
authors.lastname: Kvashnin
authors.affiliation: Skolkovo Institute of Science and Technology , Skolkovo Innovation Center, 3 Nobel Street, Moscow 143026, Russian Federation.|Moscow Institute of Physics and Technology , 9 Institutsky Lane, Dolgoprudny 141700, Russian Federation.|Department of Geosciences and Center for Materials by Design, Institute for Advanced Computational Science, State University of New York , Stony Brook, New York 11794-2100, United States.|International Center for Materials Design, Northwestern Polytechnical University , Xi'an 710072, China.
authors.forename: Artem R
authors.initials: AR
authors.lastname: Oganov
authors.affiliation: Skolkovo Institute of Science and Technology , Skolkovo Innovation Center, 3 Nobel Street, Moscow 143026, Russian Federation.
authors.forename: Artem I
authors.initials: AI
authors.lastname: Samtsevich
authors.affiliation: Skolkovo Institute of Science and Technology , Skolkovo Innovation Center, 3 Nobel Street, Moscow 143026, Russian Federation.|Moscow Institute of Physics and Technology , 9 Institutsky Lane, Dolgoprudny 141700, Russian Federation.
authors.forename: Zahed
authors.initials: Z
authors.lastname: Allahyari