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

Abstract

We report the high‐pressure synthesis and elastic properties of stoichiometric cubic δ‐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 δ‐NbN at 5.5 GPa and 1673 K. The refined lattice parameter of as‐synthesized sample is 4.3960(6) Å, corresponding to the stoichiometric niobium nitride. The determined bulk modulus of δ‐NbN is B0 = 319(2) GPa with  = 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 δ‐NbN is 23.4 GPa for the (111) slip system, comparable to those of ZrN and HfN. In addition, a finite density of states at the Fermi level was revealed for δ‐NbN, hence exhibiting metallic behavior.