Ultra-incompressible phases of tungsten dinitride predicted from first principles

Using ab initio evolutionary methodology for crystal structure prediction, we have found two ultra-incompressible hexagonal structures of $P{6}_{3}/mmc$ and $P\text{\ensuremath{-}}6m2$ of ${\text{WN}}_{2}$, which are energetically much superior to previously proposed baddeleyite- and cotunnite-type structures and stable against decomposition into a mixture of $\text{W}+{\text{N}}_{2}$ or $\text{WN}+1/2{\text{N}}_{2}$. The calculated large bulk modulus (e.g., 411 GPa) and high hardness (36.8 GPa for $P{6}_{3}/mmc$ and 36.6 GPa for $P\text{\ensuremath{-}}6m2$) reveal that they are ultra-incompressible and hard materials. The ultra-incompressibility is attributed to a staking of N-W-N ``sandwiches'' layers linked by strong covalent N-N single bonding. Thermodynamic study suggests that these phases are synthesizable at above 30 GPa.