Structure, mechanical, electronic, and thermodynamic properties of marcasite‐structure TMN2 (TM = Ru, Rh, Os, and Ir) under high pressure: A first‐principles study

The structural, mechanical, and electronic properties of four orthorhombic noble-metal nitrides TMN2 (TM = 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 RhN2 possesses a ductile nature at 0 GPa, while RuN2, OsN2, and IrN2 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: RhN2 > IrN2 > RuN2 > OsN2. 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 RuN2, RhN2, OsN2, and IrN2 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. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim