Stable compounds in the V-N binary system are systematically investigated and four new phases are found: $Pbam\text{−}{\mathrm{V}}_5{\mathrm{N}}_2, Pnma\text{−}{\mathrm{V}}_2\mathrm{N}, P\overline{3}m1\text{−}{\mathrm{V}}_2{\mathrm{N}}_3$, and $I4/mcm\text{−}{\mathrm{VN}}_2$. 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 ${\mathrm{V}}_2\mathrm{N}$ under pressure is $\varepsilon {\text{-Fe}}_2\text{N}\text{type}\to \zeta {\text{-Fe}}_2\text{N}\text{type}\to {\text{Fe}}_2\text{C}\text{type}\to Pnma{\text{-V}}_2\text{N}$. 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 ($\sim 37\mathrm{GPa}$) and superior fracture toughness (4.3–6.1 MPa ${\mathrm{m}}^{1/2}$), 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 $I4/mcm\text{−}{\mathrm{VN}}_2$ 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.

• Received 14 February 2020
• Revised 8 July 2021
• Accepted 21 September 2021

DOI:https://doi.org/10.1103/PhysRevB.104.134111