Theoretical study of the crystal structure, stability, and properties of phases in the V-N system

Stable compounds in the V-N binary system are systematically investigated and four new phases are found: Pbam-V5N2, Pnma-V2N, P (3) over bar m1-V2N3, and I4/mcm-VN2. All the predicted high-pressure vanadium nitrides are dynamically stable at ambient pressure. Moreover, the thermodynamic stability of vanadium nitrides in the temperature range of 0-1500 K at different pressures (0, 20, 40, 60, and 120 GPa) was also evaluated within the harmonic approximation. The sequence of phases of V2N under pressure is epsilon-Fe2N type -> zeta-Fe2N type -> Fe2C type -> Pnma-V2N. In addition, relative stability and lattice dynamics properties of several vanadium mononitrides are systematically calculated and discussed. Structural features, mechanical properties, electronic structures, and chemical bonding of all the V-N compounds are analyzed at 0 GPa. Among these vanadium nitrides, WC-type VN has the highest Vickers hardness (similar to 37 GPa) and superior fracture toughness (4.3-6.1 MPa m(1/2)), which mainly originate from its strong V-N bonding as well as its strong three-dimensional V-N covalent bond network. The configuration of the strong and short N-N covalent bonds enables the new phase I4/mcm-VN2 to exhibit good mechanical properties. Our results also reveal that the formation of a strong covalent-bond network topology in a crystal is a fundamental principle for designing a hard or superhard structure.