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 P6(3)/mmc and P-6m2 of WN2, which are energetically much superior to previously proposed baddeleyite- and cotunnite-type structures and stable against decomposition into a mixture of W+N-2 or WN+1/2N(2). The calculated large bulk modulus (e.g., 411 GPa) and high hardness (36.8 GPa for P6(3)/mmc and 36.6 GPa for P-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.