Stoichiometric δ-NbN: The Most Incompressible Cubic Transition Metal Mononitride

We report the high-pressure synthesis and elastic properties of stoichiometric cubic delta-NbN investigated by a combination of experiments and first principles calculations. Using the high pressure solid-state ion-exchange reaction route, we have successfully synthesized polycrystalline delta-NbN at 5.5 GPa and 1673 K. The refined lattice parameter of as-synthesized sample is 4.3960(6) angstrom, corresponding to the stoichiometric niobium nitride. The determined bulk modulus of delta-NbN is B-0 = 319(2) GPa with B-0' = 4.4(2), which is one of the most incompressible cubic transition metal mononitrides. Theoretical calculations of the elastic constants, bulk modulus, shear modulus, Young's modulus, and Poisson's ratio agree well with experimental and previous theoretical results. The calculated minimum shear strength of delta-NbN is 23.4 GPa for the (111) [(1) over bar(1) over bar2] slip system, comparable to those of ZrN and HfN. In addition, a finite density of states at the Fermi level was revealed for delta-NbN, hence exhibiting metallic behavior.