Family of noble metal nitrides: First principles calculations of the elastic stability

By calculating the elastic stability using the full-potential linearized augmented plane waves (FLAPW) method, three noble metal nitrides (IrN₂, AgN₂, and AuN₂) were found to be stable with the fluorite structure. Together with PtN₂, they form a new family of transition metal nitrides, where nitrogen atoms occupy all the tetrahedral interstitials of the metal sublattice, instead of larger octahedral interstitials. Using the topological analysis of charge density (atoms in molecules theory), it was shown that the stability of the new nitrides can be understood based on the simple size effect. The filling of all the small tetrahedral interstitials makes the phases difficult to compress, resulting in large bulk moduli. The frequencies of the Raman-active phonon modes were also given.