An ab initio study of 5d noble metal nitrides: OsN2, IrN2, PtN2 and AuN2

The crystal structure, phonon stability, elasticity and electronic properties of four noble metal nitrides (PtN2, IrN2, CsN2 and AuN2) with three structural types (pyrite, marcasite and CoSb2 structure) were studied by first principles calculations. In agreement with experiments and previous theoretical predictions, it is found that the most stable structure for OsN2 is marcasite, for PtN2 is pyrite, and for IrN2 is the CoSb2 structure. It is found that these three compounds are thermodynamically metastable with respect to solid N-2 and the metal at zero pressure. The structures are mechanically and dynamically stable. The lowest energy structure of AuN2 is the CoSb2 structure. The formation energy of AuN2 is found to be very high compared to the other three nitrides studied here. This underlies the experimental difficulty in the synthesis of this compound. OsN2 is found to be metallic, while IrN2 and PtN2 are both semiconductors. (C) 2009 Elsevier Ltd. All rights reserved.