No abstract

No 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 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 δ‐NbN is 23.4 GPa for the (111)true[true1¯true1¯2true] 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.

The structural, mechanical, and electronic properties of four orthorhombic noble-metal nitrides TMN 2 (TM ¼ Ru, Rh, Os, and Ir) (space group of Pnnm, No: 58) under 100 GPa were investigated through the first-principles calculation using the generalized gradient approximation within the plane-wave pseudopotential density-functional theory. The obtained equilibrium structures are in excellent agreement with other experimental and theoretical results. The calculated formation energy indicates that the nitrides are thermodynamically metastable but mechanically stable at zero pressure. The calculated B/G ratio indicated RhN 2 possesses a ductile nature at 0 GPa, while RuN 2 , OsN 2 , and IrN 2 are prone to brittleness under pressures of around 20.8, 20.0, and 4.3 GPa, respectively. Then, we calculated the partial and total densities of states. The results show that these four noble-metal nitrides are metallic and the order of metallicity from high to low is: RhN 2 > IrN 2 > RuN 2 > OsN 2 . Through the charge-density distributions, we find that strong covalency is present in the interstitial dinitrogen units for the four orthorhombic compounds. The calculated hardness of orthorhombic RuN 2 , RhN 2 , OsN 2 , and IrN 2 are 13. 66, 11.54, 16.33, and 17.92 GPa, respectively. Through the quasiharmonic Debye model, we also investigated the thermodynamic properties of these four compounds.