Nanoscale periodic modulations on sodium chloride surface revealed by tuning fork atomic force microscopy

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 Cr₂₃C₆ with 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-CaCl₂ type (space group Pnnm). Many of these phases possess remarkable hardness, but only CrB₄ 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 Cr₂N, a WC-type phase of CrN, and a new compound CrN₄. Nitrogen-rich chromium nitride CrN₄ 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; CrN₄ becomes stable at ∼15 GPa (cf. 110 GPa for synthesis of pure polymeric nitrogen).

Adsorption of PTCDA and C60 on KBr(001): electrostatic interaction versus electronic hybridization

A plot of differential charge density shows opposite electron density variation in two slabs near PTCDA and KBr(001), revealing electrostatic attraction as the primary interaction between aromatic molecules and insulator substrates.