First principles calculations were performed to investigate the structural, elastic, and electronic properties of $\mathrm{Ir}{\mathrm{N}}_2$ for various space groups: cubic $Fm\text{−}3m$ and $Pa\text{−}3$, hexagonal $P{3}_{2}21$, tetragonal $P{4}_2∕mnm$, orthorhombic $Pmmn$, $Pnnm$, and $Pnn2$, and monoclinic $P{2}_1∕c$. Our calculation indicates that the $P{2}_1∕c$ phase with arsenopyrite-type structure is energetically more stable than the other phases. It is semiconducting (the remaining phases are metallic) and contains diatomic N-N with the bond distance of $1.414\mathrm{\AA }$. These characters are consistent with the experimental facts that $\mathrm{Ir}{\mathrm{N}}_2$ is in lower symmetry and nonmetallic. Our conclusion is also in agreement with the recent theoretical studies that the most stable phase of $\mathrm{Ir}{\mathrm{N}}_2$ is monoclinic $P{2}_1∕c$. The calculated bulk modulus of $373\mathrm{GPa}$ is also the highest among the considered space groups. It matches the recent theoretical values of $357\mathrm{GPa}$ within 4.3% and of $402\mathrm{GPa}$ within 7.8%, but smaller than the experimental value of $428\mathrm{GPa}$ by 14.7%. Chemical bonding and potential displacive phase transitions are discussed for $\mathrm{Ir}{\mathrm{N}}_2$. For $\mathrm{Ir}{\mathrm{N}}_3$, cubic skutterudite structure $\left(Im\text{−}3\right)$ was assumed. Our calculation indicated that it is also promising to be superhard due to the large bulk modulus of $358\mathrm{GPa}$ and shear modulus of $246\mathrm{GPa}$. The diatomic N-N bond distance is even shorter $\left(1.272\mathrm{\AA }\right)$.

• Received 7 November 2006
• Corrected 23 August 2007

DOI:https://doi.org/10.1103/PhysRevB.76.054115