Computational study of Hf, Ta, W, Re, Ir, Os and Pt pernitrides

A comparative analysis of heavy-metal pernitrides using self-consistent computational methods is presented here. Three sets of prototype structures for MeN2 are considered, where Me is either of Hf, Ta, W, Re, Os, Ir or Pt. The first set consists of an ABC stacking of Me atoms, the second set have an AB stacking, and the third set an AA stacking. The ABC structure is cubic with N2-units centered in octahedral interstices of the Me sublattice. The AB and AA arrangements are hexagonal structures, with N2-units centered in trigonal antiprism and trigonal prism sites, respectively. The calculations reveal that the Hf, Os, Ir and Pt pernitrides have an enhanced stability with N2 in the octahedral sites, while the Ta, W and Re are energetically favored with N2 in trigonal sites. Starting from pure metal and molecular nitrogen precursors, HfN2, TaN2 and WN2 have negative formation energies; in contrast ReN2, OsN2, IrN2 and PtN2 have positive formation energies. Accordingly to our results, the highest bulk modulus is expected for WN2, then by ReN2, and then by the noble metal pernitrides.