Theoretical investigation on vanadium dinitrides from first-principles calculations

Motivated by successful synthesis of the first early transition metal dinitride TiN2 under high pressure, the possible crystal structures of another family member vanadium dinitride (VN2) have been systematically explored up to 300 GPa by using a well-developed CALYPSO (Crystal structure Analysis by Particle Swarm Optimization) method for crystal structural prediction. Three theoretical phases with P63/mmc, Cmc21, and I4/mcm space group have been determined at different pressures. And their dynamical and mechanical stabilities have been confirmed by the phonon dispersion and elastic constants calculations. Among three predicted phase, the tetragonal I4/mcm-VN2 exhibits highly improved Young's, bulk and shear moduli relative to the traditional fcc-VN, indicating its potential mechanical applications. In addition, the elastic anisotropy of each phase was fully investigated by the dependence of Young's modulus on the crystal orientation. To further study the mechanical property, the ideal tensile, shear, and compressive strengths are calculated. Our results imply the tetragonal phase of VN2 is not an intrinsic superhard material. The electronic structures are analyzed to study the chemical bonding nature so as to confirm the driving force of the enhancement of the mechanical property.