Extreme incompressibility and hardness of Re-N system

Abstract

Synthesized hexagonal phases, Re3N and Re2N, have always been considered to be the ground-state structures of ruthenium-nitrides systems, whereas theoretical studies proposed cubic (F-43m), tetragonal (P4(2)/mnm), orthorhombic (Pbca,Pbcn), hexagonal (P-6m2,P6(3)/mmc), and monoclinc (C2/m) crystal phases as the stable ground state structure of Re-N systems. However, the present state-of-the-art evolutionary technique combined with first-principles theory calculations revealed that cubicPm-3m ReN is the real ground-state-phase of ReN systems under ambient conditions. The proposed cubic-ReN is found thermodynamically, mechanically and dynamically stable. The present study evinces that ReN has huge elastic constantsC(11)(similar to 830 GPa), large bulk (similar to 415 GPa), and shear modulus (similar to 222 GPa). Theoretical Vickers hardness is estimated to be similar to 20 GPa and thus support that ReN is ultra incompressible and potential hard material. The analysis of the partial density of states reveals that the covalent-ionic bonding between 5d-rhenium and 2p-nitrogen states is the driving force of the fascinating mechanical properties of Re-N systems. This study could provide additional insight into the cubicPm-3m crystal phase of Re-N systems that are not readily apparent from experiments.